Environmental Review Toolkit
Water, Wetlands, and Wildlife

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4. COMMERCIALLY AVAILABLE EQUIPMENT

4.1. Data Loggers

4.1.1. Description

Data loggers are used to monitor signals from various pieces of equipment and store the impulses that they generate. When data loggers are combined with software to measure and route signals between instruments and analyze data, they are referred to as "data acquisition systems" and are often used as the execution center of a monitoring station. Most data loggers have several input ports and can accommodate a variety of sensory devices, such as a probe or transducer (e.g., flow meters, rain gauges, etc.). While specific design characteristics vary between instruments, overall data logger design is relatively standard. Some water quality samples have data loggers built into them; however, they are usually more limited in terms of capability (e.g., programmability, communication options, etc.) than independent data loggers.

Data loggers suitable for stormwater monitoring applications are typically constructed of weather-resistant materials capable of protecting the internal circuitry from water and dust hazards. They are designed to operate at extreme temperatures, from as low as -55o C to as high as 85o C ( 67o F to 185o F). In addition, most models can be securely mounted in remote locations, providing protection from wind and rain, wildlife, and vandalism.

Data Logger with Weatherproof Housing
Data Logger with Weatherproof Housing
(Vaisala Inc., Handar Business Unit)

Typical data loggers for field use consist of the following components: a weatherproof external housing (a "case"), a central processing unit (CPU), or microprocessor, a quantity of random-access memory (RAM) for recording data, one or several data input ports, a data output port, at least one power source, and an internal telephone modem. In addition, most data loggers have an input panel or keyboard and a display screen for field programming. The CPU processes the input data for storage in RAM, which usually has a backup power source (such as a lithium battery) to ensure that data are not lost in the event of a failure of the primary power. Data stored in RAM may be retrieved by downloading to a portable personal computer (PC) or to a host PC via modem.

Data loggers vary in size from 0.2 to 9 kilograms (0.5 to 20 pounds) or more. Both portable and fixed data-logging systems are available. For long-term, unattended monitoring projects, a fixed instrument capable of serving as a remote transmitting unit (RTU) may be preferable to a portable one. Manufacturers of data loggers suitable for stormwater monitoring include:

Data Logger without Housing
Data Logger without Housing
(Campbell Scientific)

Campbell Scientific, Logan, Utah; Global Water Instrumentation, Fair Oaks, California; Handar, Inc., Sunnyvale, California; In-Situ, Inc., Laramie, Wyoming; ISCO, Inc., Lincoln, Nebraska; Logic Beach, Inc., La Mesa, California; and Sutron Corporation, Sterling, Virginia.

4.1.2. Programmability

Most data loggers can be programmed to record data at user-selected intervals. The user may select a sampling interval from a range of time intervals available for each device. For example, a particular model may be designed to permit a user to select a data recording frequency from once every two seconds to once every 48 hours, with the choice of frequencies varying by two-second intervals. The minimum and maximum intervals vary from vendor to vendor, and often vary among models offered by the same vendor. In addition, some data loggers have the ability to record event-related data, such as minimum and maximum discharge rates and event timing and duration.

Most data loggers are field programmable, meaning that the software is equipped with an interface that permits on-site manipulation. However, some less expensive models may only be programmed at the factory. These models provide the advantage of cost savings but provide limited versatility, especially if project requirements change over time.

In addition to being field programmable, most data loggers possess the capability of remote programming via telephone modem. These models offer a significant advantage over factory programmed and field programmable data loggers because they allow the user to manipulate the program or monitor its effectiveness remotely. A network of data loggers used in a multi-site monitoring effort can be reprogrammed more efficiently than by traveling from site to site. An example where this would be very useful is if a predicted storm rainfall depth changes after sites are set up, the sampling interval could be adjusted remotely.

Although many vendors offer data loggers with the capability of remote manipulation via modem and PC, the user-friendliness of the various models may vary greatly between vendors. Most vendors have developed software packages that are provided free of charge with the purchase of their data logging systems. These software packages allow for remote data logger programming, and provide for data manipulation, analysis, and presentation at the host PC location. The interface environments used by these packages vary from DOS-like command lines to menu-driven point-and-click environments.

Most data loggers that are provided with vendor-developed software packages require an IBM-compatible PC with WindowsTM to run the packages. Therefore, this additional cost should be considered when evaluating a particular model. Another point of consideration is the format in which a particular model logs the data it receives. Some models log data in a format that can be converted from ASCII files to any of several readily available spreadsheet or word processing files, while others require the use of their particular vendor-developed software for data analysis and manipulation.

4.1.3. Data Capacity

Memory type and capacity vary greatly between instruments. Standard capacity varies between models and vendors from less than 8K to more than 200K. In general, one data point uses 2 bytes of information; therefore, a data logger with 64K of memory could be expected to have a maximum data point capacity of 32,000 data points before data downloading or additional memory would be required. However, some types of data require as much as 4 bytes of memory per point. It should be noted that when recording sets of data related to storm events, memory might be exhausted more quickly than expected.

The type of memory used by a particular model is also an important consideration. Most data loggers use non-volatile RAM (memory that is not lost in case of a power failure). Although this provides insurance that essential data will not be lost, the use of non-volatile memory may not be necessary if the data logger is equipped with a backup power source. A backup power source is automatically activated when the primary power source is lost. Typically, a lithium battery supplies backup power, with protection varying from 1 to 10 years.

Most models are programmed to stop recording data upon exhaustion of available memory ("stop when full"). However, some models are equipped with wraparound or rotary memory, which rewrites over the oldest data when available memory becomes exhausted. When using rotary memory, it is therefore important to realize that data may be lost if they are not downloaded regularly.

Data loggers separate from water quality samplers increase the flexibility of the system because of their increased programmability over those loggers on samplers. Memory capacity is often an issue (even with the current inexpensive memory) and requires that careful attention be paid to downloading data before it is overwritten.

4.1.4.Communications

Models vary in their ability to accept input from more than one source. Some data loggers are designed with a single analog input channel, while others are designed with up to 16 channels. In addition, some of the newer models accept digital input data. The choice of a particular model should be based upon the number of sensors or probes from which the instrument will be required to accept data.

Data loggers can accept information from many different types of sensors and transducers. This allows for versatile use of most data logging systems. Some vendors offer probes and transducers with built-in data loggers; however, these systems typically cannot accept input data from other sensory devices and their ability to communicate output data is often limited.

With regard to output communications, all data loggers interface with the standard RS-232 interface type, and some possess the capability to communicate with other interface types. In most cases, data can be downloaded on-site to a laptop PC or a unit may be transported to a lab or office so that the data can be downloaded to a desktop PC. Data loggers also can be equipped with an internal modem for telecommunications, allowing a user to download data from a remote host PC without having to mobilize to the field site.

In most cases, use of a telephone modem requires an IBM-compatible PC as the host and the vendor's software. Typically, the user can select baud rates. However, some models are capable of only a few baud rates, a limitation that should be considered when choosing a specific model. Some machines also possess the capability to transmit data via line-of-sight, UHF/VHF, or satellite radio. These options also allow for remote manipulation of programming and downloading of data.

4.1.5. Power Requirements

In general, data loggers are energy efficient devices. An internal battery, with the option of using external electrical power, powers most data loggers. Some also can be equipped to use solar power.

Data loggers powered by internal batteries typically offer a choice of cell type. That is, whereas some models offer the option of using either rechargeable cells or standard 12-volt alkaline cells, others offer the option of either alkaline or lithium. The choice of power source, and potentially model selection, depends on several factors, including site accessibility, distance, amount of data to be recorded, total cost, and operating temperature. Especially in remote locations, solar power with battery backups should be considered. The sunlight can be used to recharge batteries and the batteries can be used during periods of no or weak sunlight.

Alkaline cells are less expensive than lithium or rechargeable batteries, but they have a shorter life and must be replaced more often. While alkaline cells offer a potential power life of several months, lithium cells offer a potential power life of several years. However, since lithium batteries are considered a hazardous material, data loggers using lithium batteries are subject to more stringent shipping requirements than models using standard alkaline cells. Since alkaline batteries must be replaced and discarded frequently, the use of alkaline batteries may actually be more expensive than rechargeable batteries. However, whereas rechargeable batteries offer less battery waste and a potential cost savings, the time and cost required to recharge the batteries should be considered when evaluating power options.

Operating temperature range is another important factor to consider when choosing a power supply. Lithium expands both the minimum and maximum temperatures at which a data logger can be used. Under extreme conditions, it may not be feasible to use a data logger powered by alkaline batteries.

4.1.6. Data Logger Summary

The selection of a particular data logger depends on several factors. The first factor to consider is the expected number and type(s) of input data. For example, if a stormwater monitoring task requires recording three different parameters (e.g., surface water temperature, flow, and pH), then a data logger with at least three data input channels is required. Similarly, if a user intends to use a transducer that conveys information as digital data, then the user must select a data logger with the capacity to accept and store data in this form.

The next factors to consider are programmability and memory capacity. In considering these factors, it is important to identify the frequency at which data will be recorded and evaluate the possibility that conditions may arise that would require a change in this frequency. In addition, a user should consider how frequently data will be downloaded (e.g., once per month, once per quarter), and identify the preferred method of interrogation (i.e., downloading at the site or downloading via modem). Programmability and memory capacity requirements should be easy to identify once these considerations have been evaluated.

The selection of a power source partially depends on the frequency at which data will be collected, since power life varies greatly between power source types. Each available power source (AC power, solar power, or alkaline, lithium, or rechargeable batteries) offers advantages and disadvantages. The choice of one source over the others depends on specific project requirements, including cost. AC power generally provides the most flexibility. Solar power has the advantage of potentially not needing to replace the battery for long periods as part of the available operating power is diverted to recharging onsite batteries; however, in practice they are often much more problematic than AC power. Solar power may be the only viable option for stations that are located in remote locations. The characteristics of certain battery types should be understood when selecting a DC power source as each battery type has properties that will affect its performance (e.g., heat/cold, storage capacity, etc).

The final consideration influencing data logger selection is cost. However, once individual project needs have been identified, it is likely that several data acquisition systems will accommodate the needs within a close range of costs. In this case, the final selection of a particular data logger may depend on a user's preferred communication software, availability of a power source, or the range of programming options for flexibility in future project applications. In general, independent devices (separate from water quality sampling equipment) offer the most flexibility and features; however, they place greater demands on the abilities of the sampling team.

4.2. Flow Measurement Methods and Equipment

4.2.1. Methods for Measuring Discharge Rate

Natural channels, engineered open channels, and pipes are used as stormwater conveyances. In each case, hydraulic considerations dictate the mathematical relationships that can be used to describe the discharge rate at a given point in time. One of the primary hydraulic considerations is whether the flow configuration represents an "open" or "closed" channel. Open channel flow has a free water surface, and because the flow is driven by gravity, it varies with depth. Closed channel flow, in which the flow fills a conduit, is caused by and increases with the hydraulic pressure gradient. Some stormwater conveyance system pipes may function as open channels during periods of low storm runoff and as closed channels when the runoff volume becomes sufficiently large or when water is backed up due to downstream flow conditions (e.g., tide, river flooding, etc).

In general, the discharge rate in an open channel depends on the depth of flow and several other factors (Chow, 1959) including:

  • Geometric shape and changes in shape and slope along the length of the channel (affects potential for development of turbulence and/or varied flow and therefore the choice of methods and instruments used for measurement of discharge).
  • Hydraulic roughness of the conveyance surface, whether natural or manmade (affects the energy losses of the flow).
  • Rate at which the depth of flow changes over time (steady vs. unsteady flow).
  • Spatial scale over which the discharge rate changes (uniform vs. varied flow).

The measurement of the discharge rate in an open channel is more difficult to attain than that of a full pipe, because the free surface will change with respect to time.

Typically, stormwater collection systems for transportation sites will fit the open channel flow configuration. However, many highways are drained by piped systems that may be flowing full at times. Therefore, methods used for measuring discharge in full pipes will also be discussed.

Table 4.1 summarizes available discharge measurement methods, the requirements for their use, typical highway use, and required equipment. Each of these methods is discussed in more detail in the following sections.

TABLE 4.1 DISCHARGE MEASUREMENT METHODS
Method Major Requirements For Use Typical Highway Use Required Equipment
Volume-Based
  • Low discharge rates
  • Calibrating equipment
  • Manual sampling
Container and stopwatch
Stage-Based Empirical Equations
  • Open flow
  • Known channel/pipe slope
  • Channel slope, geometry, roughness consistent upstream
  • Manual or automatic sampling
Depth Measurer
Stage-Based Weir/Flume
  • Open flow
  • Constraint will not cause flooding
  • Manual or automatic sampling
Weir/flume and depth measurer
Stage-Based Variable Gate Meter
  • 4-, 6-, or 8-inch pipes only
  • Not typically used for highways
ISCO Variable Gate Meter
Velocity-Based
  • None
  • Automatic sampling
Depth measurer and velocity meter
Tracer Dilution
  • Adequate turbulence and mixing length
  • Calibrating equipment
Tracer and concentration meter
Pump-Discharge
  • All runoff into one pond
  • Not typically used for highways
Pump

Volume-Based Methods

The concept behind volume-based flow measurement is simple-all the discharge is collected over a short period of time, the volume is measured, and the collected volume is divided by the length of the time period.

Q = V/T
where,
Q = discharge, m3/s (ft3/s)
V = volume, m3 (ft3)
T = time, s

A stopwatch can be used to measure the period required to fill a receptacle of known quantity to a predetermined level. The receptacle must be large enough that it requires some accurately measurable period of time to fill. The receptacle could be a bucket, a drum, or a larger container such as a catch basin, holding tank, or some other device that will hold water without leakage until the measurement is made.

This method is easy to understand, requires relatively simple equipment, and can be very accurate at low rates of discharge. At higher rates of discharge, collecting all the runoff from typical highway conveyances (an essential component of the method) may become infeasible. This method is most useful for conducting limited research and for calibrating equipment.

Stage-Based Empirical Equations

Discharge rate can be estimated from the depth of flow (i.e., water level or stage) using well-understood, empirically-derived mathematical relationships. That is, for a set hydraulic configuration, the relationship between stage and discharge is known. The most commonly used empirical relationship, the Manning equation, is appropriate for open channels in which flow is steady-state (i.e., the discharge rate does not vary rapidly over time) and uniform (the depth of flow does not vary over the length of the channel) (Gupta, 1989). In automated stormwater sampling, the Manning equation is commonly used to estimate the discharge rate of the flow stream.

Q=1/nAR2/3 S1/2
where,
Q=discharge, m3/s
n=Manning roughness coefficient (dimensionless)
A=cross sectional area, m2
R=hydraulic radius, m=A/(wetted perimeter)
S=slope of the channel, m/m

Q=1.486/nAR2/3S1/2
where,
Q=discharge, ft3/s
n=Manning roughness coefficient (dimensionless)
A=cross sectional area, ft2
R=hydraulic radius, ft=A/(wetted perimeter)
S=slope of the channel, ft/ft

Manning Equation

The variables required for the Manning equation are the slope of the energy grade line (usually assumed to be the slope of the channel bottom), the cross-sectional area of the flow, the wetted perimeter, and an empirical roughness coefficient that takes into account channel material, age, and physical condition.

The Manning equation truly applies only to steady and uniform flow but can provide a fairly accurate estimate of discharge rates if certain conditions are met. The channel slope and cross-sectional geometry must be constant for some distance upstream of the site, the exact distance varying with overall system hydraulics (a rule of thumb is a length of 20 channel diameters upstream). Flow conditions at the site should not be affected by downstream features (i.e., no backflow effects). The cross-sectional area and wetted perimeter are both geometric functions of the channel shape and the depth of flow. The "roughness" of the conveyance walls can be described by a roughness coefficient. Additional information on applicability and values for Manning's roughness coefficients for common channel types are provided in most hydraulics texts (Chow, 1959 and Gupta, 1989).

Use of the Manning equation assumes that the slope of the channel bottom is accurately known. Monitoring studies using this technique to estimate discharge rate often rely on as-built drawings to determine channel slope. Because these drawings vary in accuracy, direct measurement of the slope of the channel bottom and verification of hydraulic conditions is recommended.

The discharge rate from stormwater runoff tends to be unsteady. This is due to changes in the intensity of precipitation and the dynamic nature of overland flow, which causes the discharge rate to vary with time, either gradually or rapidly. Depending on the frequency with which the depth of flow is measured, rapid fluctuations in discharge rate will be missed and the total runoff volume from a storm event will be miscalculated.

Other Empirical Stage-Discharge Relationships

Another empirical relationship used to estimate discharge is the Chézy equation (Gupta, 1989):

Q/A=C&√(RS)

where,
Q = discharge, m3/s (ft3/s)
A = cross sectional area, m2 (ft2)
R = hydraulic radius, m (ft)
S = slope of the energy grade line, m/m (ft/ft)
C = discharge coefficient, m1/2/s (ft1/2/s)

Under open channel flow, the coefficient "C" can be defined as:

C =R1/6/n

where,
n = Manning roughness coefficient (dimensionless).

When "C" is substituted into Chézy's equation, the resulting equation is identical to the Manning equation.

A failure of both the Manning and Chézy equations is that they imply that the Manning "n" value is constant for a given channel. However, it is known that for natural channels "n" may vary greatly with respect to discharge (Ponce 1989). Therefore, when consideration is given to applying these equations to a natural channel, the alluvial material in the channel and expected flow magnitude should be evaluated first. It may be desirable to select another discharge measurement approach for natural channels with highly varied surfaces and discharge rates.

Stage-Based Weirs and Flumes

The accuracy with which discharge is estimated can be improved by using a weir or flume to create an area of the channel where hydraulics are controlled (control section). Each type of weir or flume is calibrated (i.e., in the laboratory or by the manufacturer) such that the stage at a predetermined point in the control section is related to the discharge rate using a known empirical equation. (For examples, see Stevens, 1991.)

Weirs

A weir is an obstruction (usually a vertical plane) built or placed across an open channel (or within a pipe under open channel flow) so that water flows over the weir's top edge (or through a well defined opening in the plane). Many types of weirs can be used to measure discharge; the three most commonly used are the rectangular, trapezoidal (also called a Cipolletti weir), and triangular. The weir opening (i.e., the rectangular, trapezoidal, or triangular opening) is called the "notch." Specific discharge equations are used for each type of weir.

Weirs are simple, inexpensive, and relatively easy to install. A weir can be used to regulate flow in a natural channel with irregular geometry, a situation where the Manning equation, for example, would not provide reliable estimates for the discharge rate. However, a weir will back water up in channels by creating a partial dam. During large storm events, backed-up water could cause or worsen flooding upstream, particularly in a closed conduit. Some jurisdictions prohibit the use of weirs for this reason. When evaluating the suitability of a monitoring site for a weir, it is important to determine whether the system was "over designed." That is, will the conveyance be able to move the design capacity after weir installation? In the case where the downstream depth of flow is greater than the crest of the weir, a different stage-discharge relationship for the weir will apply.

Another potential problem that weirs introduce to a channel is that sediments and/or debris may accumulate behind the weir, which can alter the hydraulic environment. By altering the hydraulic environment, these materials also change the empirical relationship between depth of flow and discharge rate. Therefore, weirs must be inspected regularly and all the accumulated sediment and debris removed.

Flumes

A flume is a specially built reach of channel (sometimes a prefabricated insert) with a converging entrance section, a throat section, and diverging exit section.

Parshall Flume Parshall Flume (Plasti-Fab, Inc.)

The area or slope (or both) of the flume is different from that of the channel, causing an increase in water velocity and a change in the level of the water flowing through the flume (Grant, 1989). Stage-discharge relationships have been established for a variety of flume configurations (USGS, 1980; Gupta, 1989; Stevens, 1991). The USGS has developed and tested a modified Palmer-Bowlus flume (USGS, 1985) for use in circular pipes carrying highway stormwater runoff, where the flow can be under both open and pressurized channel flow. This flume has been designed to measure the discharge under pressurized flow by using two bubbler sensors (discussed later in this section) to detect the hydraulic pressure change between an upstream and downstream location on the flume. This system has been found to be one of the most accurate available after calibration is performed.

Because the velocity of water accelerates as it passes through a flume, the problem of sedimentation associated with weirs is avoided; however, problems with debris accumulation may still occur. Another benefit is that flumes introduce a lower headloss than weirs, resulting in a reduced backwater effect. A flume may be more expensive and difficult to install than a weir due to its more complex design.

an H flume H-Flume (Tracom, Inc.)

Staged-Based Variable Gate Metering Inserts

A relatively new development in flow metering technology is ISCO Inc.'s (Lincoln, Nebraska) Variable Gate Metering Insert. Discharge flows through the insert and under a pivoting gate, creating an elevated upstream level that is measured with a bubbler system (discussed later in this section). The meter uses an empirical relationship to calculate the discharge rate based on the angle of the gate and the depth of flow upstream of the gate. This approach can be used only under conditions of open channel flow in circular pipes. Currently the system is only available for pipe diameters of 10.16, 15.24, and 20.32 cm (4, 6, and 8 inches). The Variable Gate Metering Insert was designed to measure the discharge rate under fluctuating flows and should be effective at both very high and very low flow rates. Its main limitation is the size of the conveyance for which it is designed. Most stormwater conveyances used for monitoring are at least 24 inches in diameter. The insert may be useful for sampling very small catchment areas. Again, problems with debris accumulation can occur.

Velocity-Based Method

The continuity method is a velocity-based technique for estimating discharge rate. Each determination requires the simultaneous measurement of velocity and depth of flow. These parameters may be measured using any of the methods discussed in Section 4.2.4 (Equipment for Measuring Velocity).

Discharge rate is calculated as the sum of the products of the velocity and the cross-sectional area of the discharge at various points across the width of the channel:

Q = AiVi

where,
Q = discharge, m3/s (ft3/s)
Ai = cross-sectional area of the flow at section i, m2 (ft2)
Vi = mean velocity of the flow at section i, m/s (ft/s)

The sections i = 1-n are planar segments of a cross-section of the flow where n is the number of points across the width of the channel. In stormwater runoff applications, the conveyance is small enough that a single cross-sectional area and estimate of average velocity is typically used to estimate discharge rate. Typically, it is not necessary to segment the cross-sectional area of the flow. The accuracy of this method is dependent on the ability of a sensor to measure velocity over a range of discharges.

Although this method is useful for calibrating equipment, it is more sophisticated and expensive than the stage-discharge relationships previously discussed. In addition, this method is suitable only for conditions of steady flow. That is, water level must remain essentially constant over the period required for obtaining velocity measurements. This is not generally a problem in small conveyance systems when instruments that make measurements rapidly are employed.

Additional relationships, developed for pipes that are flowing full, are the Darcy-Weisbach equation and the Hazen-Williams equation. These equations are used in systems where pressurized flow (i.e., pipes flowing full, no free water surface) is present (Gupta, 1989).

Tracer Dilution Methods

Tracer dilution methods can be used where the flow stream turbulence and the mixing length are sufficient to ensure that an injected tracer is completely mixed throughout the flow stream (USGS, 1980; Gupta, 1989). Tracers are chosen so that they can be distinguished from other substances in the discharge. For example, chloride ion can be injected into fresh water; dyes or fluorescent material can be used if turbidity is not too high.

Dilution studies are well-suited for short-term measurements of turbulent flow in natural channels and in many manmade structures such as pipes and canals. However, these methods are better suited to equipment calibration than to continuous monitoring during a storm event. Two dilution methods can be used to determine discharge rate as described below.

Constant Injection Rate Tracer Dilution Studies

A known concentration of tracer is injected at a constant rate into a channel. The concentration of the tracer in the discharge is measured at a downstream point over time. After some time period has passed, the tracer becomes completely mixed in the discharge so that the downstream concentration reaches steady-state. Discharge is calculated from the initial tracer concentration, the tracer injection rate, and the steady-state downstream concentration.

Total Recovery Tracer Dilution Studies

A discrete "slug" of tracer is injected into the channel. Near-continuous measurements of tracer concentration in the discharge are taken at a downstream point until the plume has entirely passed. Discharge is calculated from the volume and concentration of injected tracer and the total area under the concentration-time curve.

Pump Discharge Method

In some cases, the overall discharge rate for a catchment may be measured as the volume of water that is pumped out of a basin per unit time while holding the water level in the basin constant. This method can be applied at sites where discharge runs into a natural or manmade basin from several directions or as overland flow. If the pump is pre-calibrated, the number of revolutions per minute, or the electrical energy needed to pump a given volume, may be used as a surrogate for measuring the pumped volume during a stormwater runoff event. Application of this method requires considerable knowledge of the installed pump's performance. Because this setup (i.e., all of the runoff from a catchment flows into one pond or basin which can be pumped out) is rarely encountered in the field as the only available monitoring method, pumps are not discussed further in this manual.

4.2.2. Factors Influencing Equipment Selection for Measuring Discharge Rate

Various factors influence the selection of equipment for measuring discharge rate. This section provides an overview of key factors.

Site location

  • The likelihood that field personnel will be exposed to hazardous conditions while making visual observations or while installing, maintaining, or operating automated equipment.
  • The likelihood that equipment will be vulnerable to vandalism or theft if left unsecured.

Site condition

  • The presence of turbulence, foam, or mist associated with the discharge (these may affect depth of flow measurements).
  • The presence of large air or water temperature gradients (these may affect depth of flow measurements).
  • The presence of surface-active materials or organisms that can affect the accuracy of a probe (this may interfere with the functioning of the probes).
  • The expected concentrations of suspended solids, settleable solids, or debris in the discharge. (High concentrations of solids that can settle will inhibit the functioning of probes installed at the channel invert and debris may gather on or around the probes.)

Expected discharge rates

  • The minimum and maximum discharge rates expected during a storm event.
  • The likelihood of full-pipe (surcharged) flow in closed conduits (limits the use of stage-discharge relationships).

Allowable loss of capacity

  • The likelihood that the installation of a weir or flume in a channel will cause flooding upstream.

Accuracy

  • Regarding quantitative data, the accuracy achievable with a given instrument compared to that needed to meet monitoring program objectives.

Expense

  • Equipment costs plus the costs to install, operate, and maintain a piece of equipment, including training time for field personnel.

Installation requirements

  • The time required to install, and, if necessary, calibrate a given piece of equipment.
  • The potential need to retrofit a conveyance.
  • The potential need to purchase or build mounting brackets or secure housing.

Operations and maintenance requirements

  • The time required to inspect and maintain the equipment between storm events.
  • The potential for a piece of equipment to break or malfunction.
  • The possibility that repairs will be conducted in the field.
  • The degree to which electronic components are protected against water damage.
  • The type of power supply required.

Special considerations for near-highway sites

  • Catchments draining highway sites are often smaller than those draining residential, commercial, or industrial areas. Therefore, the conveyances used to transport runoff are likely to be relatively small. Methods and instruments must be evaluated with respect to their usefulness in relatively restricted spaces.

4.2.3.Equipment for Measuring Depth of Flow

A variety of instruments may be used to measure water depth. Because some techniques are relatively cumbersome, they are more useful for calibrating equipment than for routine or continuous data collection during storm events. The equipment required for each technique, and the associated advantages and disadvantages for sampling runoff at transportation sites, are described below. Table 4.2 summarizes available equipment for measuring depth of flow, major requirements for use, and typical highway use.

TABLE 4.2 EQUIPMENT FOR MEASURING DEPTH OF FLOW
Method Major Requirements for Use Typical Highway Use
Visual Observations
  • Small number of sites and events to be sampled
  • No significant health and safety concerns
Manual sampling
Float Gauge
  • Stilling well typically required
Manual or automatic sampling
Bubbler Tube
  • Open channel flow
  • No velocities greater than 5 ft/sec
Automatic sampling
Pressure Transducer
  • Better if remains submerged
Automatic sampling
Ultrasonic Depth Sensor
  • Open channel flow
  • No significant wind, loud noises, turbulence, foam, steam, or floating oil & grease
Automatic sampling
Pressure Probe
  • Open channel flow
  • No organic solvents or inorganic acids & bases
Automatic sampling
Ultrasonic Uplooking
  • No sediments or obstructions likely to cause errors in measurement
Automatic sampling
Radar/Microwave
  • Similar to Ultrasonic Depth Sensor but can see through mist and foam
Automatic sampling
3-D Point Measurement
  • Since requires highly controlled systems, typically not useful in the field
Automatic sampling

Visual Observation

The visual method of measuring depth of flow requires that personnel be present at a site and in a position to take readings throughout a sampling event. Depth of flow can be measured using a fixed or hand-held staff gauge, meter stick, or some other physical gauge. Although visual observations can be simple and inexpensive to obtain, this method is not generally recommended for programs involving large numbers of sites or sampling events. Under these circumstances, labor costs can outstrip those associated with automated equipment. Visual measurements are generally not as accurate due to difficulty in positioning to read gauges and fluctuating depths. Health and safety issues such as the potential for exposure to inadequate oxygen, toxic or explosive gases, storm waves in manhole vaults, or to hazardous traffic conditions at street level, must also be considered when evaluating manual versus automated observation techniques.

Float Gauge

A float gauge consists of a float that is free to move up and down in response to the rising and falling water surface in a channel. Prior to an actual stormwater sampling event, the site is calibrated to establish an initial reference depth. During the storm, the float rises and falls with changes in water surface elevation, and a device attached to the float records the magnitude of these changes. The changes in water surface elevation are converted to depth of flow by the float gauge. A data logger can record the depth of flow, and if capable of performing mathematical equations, can also determine the discharge rate. The data also can be used as input with compatible software to compute the discharge rate.

In some applications, use of a float gauge requires a stilling well. A stilling well is a reservoir of water connected to the side of the conveyance that isolates the float and counterweight from turbulence in the main body of the discharge. The need to retrofit an existing channel or conduit with a stilling well, a potentially expensive and time-consuming process, is the principal drawback of this technique. However, this method may be useful if sampling is conducted at a site where a float gauge and stilling well have previously been installed.

Bubbler Tube

Bubbler tubes are used by some types of automated flow meters to measure the depth of flow. Compressed air (or gas) is forced through a submerged tube attached to the channel invert (i.e., bottom of the channel). A pressure transducer measures the pressure needed to force a bubble out of the tube. This pressure, in turn, is linearly related to the depth of the overlying water:

P = ρh

where:
P = hydrostatic pressure, N/m2 (lb/ft2)
ρ = specific weight of water, N/m3 (lb/ft3)
h = depth of water, m (ft)

Bubbler tubes are commonly integrated with a flow meter, or a data logger that is capable of performing mathematical calculations. This approach allows the measurement of depth to be immediately converted to a discharge. These real-time inputs along with a program that tracks accumulated flow volumes can be used to trigger the collection of samples for flow-weighted compositing by an automated sampler.

A bubbler flow meter
Bubbler Flow Meter (ISCO)

Bubbler tubes are simple to use and are not usually affected by wind, turbulence, foam, steam, or air-temperature gradients. Accuracy is not lost under dry conditions in a conveyance between runoff events. (Some other types of probes must remain submerged.) Although they are generally reliable, bubblers are susceptible to error under high velocity flow. That is, as discharge velocity increases to over 1.5-1.8 m/s (5-6 ft/s), a low-pressure zone is induced around the mouth of the bubbler tube, which is interpreted by the flow meter as a drop in discharge rate. These instruments therefore should not be used in channels where the slope of the bottom exceeds 5-7%. Also, sediments and organic material can plug bubbler tubes. Some units are periodically purged with compressed air or gas to prevent this problem, but visual inspection and periodic maintenance are recommended for any unit installed in the field. Bubblers are commonly available in integrated systems, such as those manufactured by ISCO and American Sigma, but they also are sold as independent devices.

Ultrasonic Depth Sensor

An ultrasonic depth sensor consists of a sonar-like device mounted above the surface of the water at a known distance above the bottom of the channel. A transducer emits a sound wave and measures the period of time taken for the wave to travel to the surface of the water and back to a receiver. This time period is converted to a distance and then converted to a depth of flow, based on measurements of the site configuration. As with bubbler tubes, an ultrasonic sensor can be integrated into a flow meter or interfaced with a data logger. An ultrasonic depth sensor and data logger can provide the real-time flow data necessary to trigger an automated sampler to collect a stormwater sample for flow-weighted compositing.

An ultrasonic depth sensor module
Ultrasonic-depth Sensor Module (ISCO)

Some manufacturers have built redundancy into their ultrasonic depth-measuring instruments. Redundancy helps to ensure that useful data will be collected even if some of the sensors in the array become fouled with grease, surface-active materials, or organisms. Experience has shown that this type of fouling can occur during storm events. Because an ultrasonic sensor is mounted above the predicted surface of the water, it is not exposed to contaminants in the runoff (unless the depth is greater than anticipated or installed in a pipe that reaches fully pressurized flow). However, ultrasonic signals can be adversely affected by wind conditions, loud noises, turbulence, foam, and steam, and they will require periodic inspection and maintenance. Ultrasonic signals also can be affected by changes in density associated with air temperature gradients. However, some manufacturers build a compensation routine into their instruments.

Background noise can interfere with a sensor's ability to accurately measure water depth. For example, an ultrasonic sensor was used to measure the depth of flow at an urban stormwater sampling site in Portland, Oregon located in a manhole, in which runoff from an arterial pipe splashed down into the main conveyance. To dampen the effect of the interfering signal, the ultrasonic sensor was retrofitted with a flexible noise guard.

Pressure Probe

A pressure probe consists of a transducer that measures the hydrostatic pressure of the overlying water, mounted at the bottom of the channel. This hydrostatic pressure is converted to a depth of flow. Some pressure probes have a built-in thermometer to measure the temperature of the water, allowing for temperature compensation in the depth of flow calculation. As with bubblers and ultrasonic probes, the pressure probe can be integrated into a flow meter or interfaced with a data logger to provide real-time inputs to an automated sampler. If the instrument is fitted with a thermometer, the temperature data used for compensation can possibly also be input to memory and retrieved as additional useful data.

pressure transducers
Pressure Transducers (In-Situ Inc.)

Submerged probes are not adversely affected by wind, turbulence, foam, steam, or air temperature gradients. However, because contaminants in the water may interfere with or damage the probe, periodic inspection and maintenance is recommended. Dry conditions between storms can affect the accuracy of the probe, as can sudden changes in temperature.

Ultrasonic "Uplooking"

This depth of flow sensor is mounted at or near the bottom of the channel or pipe. It uses ultrasonic signals to determine the depth of the flow. This sensor is very accurate unless interference occurs. However, according to one vendor, this equipment is not recommended for stormwater applications because the sensor is likely to become covered by sediment and debris. This then interferes with the signal and does not allow the sensor to work properly.

Radar/Microwave

A variation of the ultrasonic method is a non-water contacting instrument that emits and reprocesses electromagnetic waves in the radar/microwave spectrum. By altering the wavelength of the electromagnetic signal, problems associated with foam, mist, and rapid changes in air temperature and pressure are eliminated or significantly reduced.

A radar/microwave sensor is used in the same manner as an ultrasonic "downlooking" sensor for measuring fluid levels in tanks. Based on experience, this device does not present a significant advantage over other methods of level measurement, since foam and mist are not typically a serious concern during stormwater monitoring.

Radar/microwave sensors have not been extensively tested by manufactures for this type of application, and there is no existing literature that shows them presently being used for stormwater monitoring.

3-D Point Measurements

This instrument measures the three velocity components at a point by applying ultrasonics and the Doppler principle. This instrument can be useful for studying boundary layers in fluid systems. However, this approach is very sophisticated and requires a high degree of accuracy in a controlled system to produce reasonable results. A large amount of time is also required to make each measurement. Consequently, its use is not practical for continuously measuring the mean velocity of water in a conveyance at a highway site.

4.2.4. Equipment for Measuring Velocity

Use of the continuity equation for measuring discharge requires estimating average velocity as well as depth. The velocity of a discharge may be measured using visual methods (i.e., the float-and-stopwatch and the deflection, or drag-body methods); tracer studies; or instruments such as rotating-element current meters or pressure, acoustic, ultrasonic (Doppler) and electromagnetic sensors. Electromagnetic sensors have been found to be the most accurate. Among these methods, many are more useful for the calibration of automated equipment than for continuous data collection. Only the ultrasonic and electromagnetic methods are recommended for measuring velocity during a storm. In the following text, velocity measurement methods potentially suitable for calibration are briefly described. (More details are available in USGS, 1980). More extensive discussions, including advantages and disadvantages associated with stormwater sampling, are provided for the ultrasonic and electromagnetic sensors.

Methods Suitable for Calibration

Float-and-Stopwatch Method

In this method, the time it takes for a float to move a known distance downstream is determined. Velocity is calculated as the distance traversed divided by the travel time. The characteristics of a good float are: an object that floats such that it is partially submerged, allowing some averaging of velocity above and below the surface of the water; an object that is easily observed and tracked; an object that is not easily affected by wind; and an object that does not cause problems if not recovered. Citrus fruits such as oranges, limes, or lemons are commonly used as floats. Ping-pong balls and Styrofoam float well but are too light and are easily blown by the wind. They may also pose environmental problems if not recovered.

In a variation of this method, a vertical float with a weighted end is used. The vertical float provides a better measure of mean velocity over the depth of the water column than a float moving primarily at the surface. In addition, it can be designed to minimize bias due to wind.

In most cases this method is not accurate enough to be of significant utility in stormwater monitoring studies and is particularly inaccurate for very deep systems and where there is a significant difference in velocity across the water surface (e.g., in natural channels).

Deflection (or Drag-Body) Method

In this method, the deflection or drag induced by the current on a vane or sphere is used as a measure of discharge velocity. This method is only practical for short-term, real-time measurements, such as equipment calibration, because an object of this size inserted into the flow will accumulate debris, causing it to change the hydraulic form, provide inconsistent data, and (possibly) break away.

Tracer Studies

Tracer methods have been developed to measure discharge velocity under uniform flow (USGS, 1980). As described in the flow measurement methods section, for Total Recovery Tracer Dilution studies, a discrete slug of tracer is injected into the discharge. Concentration-time curves are constructed at two downstream locations. The time for the peak concentration of the dye plume to pass the known distance between the two locations is used as an estimate of the mean velocity of the flow. This method is not practical for continuous discharge measurement, but it is useful for site calibration.

Rotating-Element Current Meters

A current meter or current meter array can be used to measure the velocity at various points throughout a flow stream. The measured point velocities can be combined to estimate a mean velocity for the flow. As with the deflection or drag-body method, if employed for longer periods, a current meter inserted into the flow will accumulate debris causing it to malfunction and possibly break away. This method should therefore only be used for short-term measurements such as during equipment calibration or to develop a rating curve. Two types of readily available instruments that meet USGS standards are the type AA Price and Pigmy current meters.

Pressure Sensors

A pressure sensor or transducer measures the dynamic pressure head at a given point in the flow. The dynamic pressure is a measure of the point velocity and can be used to estimate the mean velocity of the flow. A common example of a pressure sensor is the pitot tube used on an airplane or on some boat speedometers.

The same caution described for bubbler tubes must be applied to pressure sensors. That is, as the velocity of the discharge increases above 1.5-1.8 meter/second (5-6 feet/second), a low- pressure zone is induced across the sensor, interpreted by the flow meter as a drop in discharge rate. These instruments should not be used in channels where the slope of the bottom exceeds 5 to 7 %.

Acoustical Sensors

An acoustical sensor emits a sound wave under water across a channel and measures the time required for the signal's return. Transit time is correlated with channel width. The relative positions of the emitting and receiving sensors are used to estimate velocity. A minimum depth of flow is required. This type of sensor can only be used at sites with sufficient baseflow to provide the medium in which the sound wave travels. If there is no baseflow, the lower portions of the rising and falling limbs of the hydrograph will be lost.

Methods Most Suitable for Continuous Velocity Monitoring

Ultrasonic (Doppler) Sensors

An ultrasonic sensor applies the Doppler principle to estimate mean velocity. A sound wave, emitted into the water, reflects off particles and air bubbles in the flow. The shift in frequency of waves returning to the sensor is a measure of the velocity of the particles and bubbles in the flow stream. The instrument computes an average from the reflected frequencies, which is then converted to an estimate of the average velocity of the flow stream.

area velocity sensor modules
Area Velocity Sensors Module (ISCO)

The sensor is mounted at the bottom of the channel. However, because the ultrasonic signal bounces off suspended particles, the signal may be dampened (i.e., not able to reach portions of the flow stream) when suspended solid concentrations are high. The sensor may also be mounted on the side of the channel, slightly above the invert. Combined with the appropriate hardware and software, the sensor can filter out background signals associated with turbulence in the discharge.

Ultrasonic Doppler sensors can be used under conditions of either open channel or pressurized flow. When combined with the hardware and software required for real-time discharge measurement, data logging, and automated sampling, and when properly calibrated, this system is capable of greater accuracy than one relying on a stage-discharge (i.e., Manning equation) relationship. The ultrasonic sensor-based system may be more expensive, but the additional expense may be justified by program objectives. Without routine maintenance, the accuracy of ultrasonic sensors may decrease due to fouling by surface-active materials and organisms.

Electromagnetic Sensors

Electromagnetic sensors work under the principle stated in Faraday's Law of electromagnetic induction; that is, a conductor (water) moving through an electromagnetic field generates a voltage proportional to its velocity. This instrument, mounted at or near the channel bottom, generates the electromagnetic field and measures the voltage inducted by the flow. Although velocity is measured at only a single point, that measurement is used to estimate the average velocity of the flow stream.

Electromagnetic sensors can be pre-calibrated for many types of site configurations. The sensor is usually mounted at the channel invert but can be mounted on the side of a channel, slightly above the invert, if high solid loadings are expected. A built-in conductivity probe senses when there is no discharge in the conveyance.

These types of instruments are not sensitive to air bubbles in the water or changing particle concentrations, as is the ultrasonic sensor, but can be affected by extraneous electrical "noise." As with the ultrasonic system, when an electromagnetic sensor is combined with the hardware and software required for real-time discharge measurement, data logging, and automated sampling, and when properly calibrated, it may be capable of greater accuracy in specific circumstances than a system relying on a stage-discharge relationship. On the other hand, the electromagnetic sensor-based system may also be more expensive, but the additional expense may be justified by program objectives.

Acoustic Path

These sensors are used to determine the mean velocity of streams and rivers, and where they are applicable they have been found to be one of the most accurate flow measurement systems. The method consists of an array of sensor elements that are installed at an even elevation across the channel. The number of sensor elements used is dictated by the channel width. Larger channels require more sensors. Due to the sensor array's height above the channel bottom, its use is generally limited to larger channels that have a baseflow present. It is not practical for smaller diameter conveyances with no baseflow, as is typically found at highway sites. Additionally, stormwater conduits for highway runoff are generally small enough that a single point measurement for velocity provides a reasonable estimate for the average velocity. For these reasons, acoustic path sensors are rarely applicable to highway runoff monitoring situations.

4.3. Sampler Equipment

4.3.1. Introduction

Federal and state water quality regulations often either require or encourage the analysis of highway runoff to determine the magnitude (e.g., pollutant loads) or concentrations of pollutants present for assessing potential impacts to receiving waters. For almost all constituents, samples of stormwater runoff should be collected and taken to a laboratory for analysis. As with measurements of discharge, stormwater samples can be collected manually or automatically. Both approaches, their applicability to monitoring for various pollutants, and their advantages and disadvantages, are discussed in this section.

The American Society for Testing and Materials (ASTM) defines sampling as "obtaining a representative portion of the medium being sampled" (ASTM, 1989). A sample is representative if it possesses the same qualities or properties as the subject medium at the point and time of collection (Stenstrom and Strecker, 1993). However, one of the fundamental characteristics of water quality, whether in a stormwater conveyance or in a receiving water body, is the large inherent variability in the concentrations of constituents over time and space (USEPA, 1983; FHWA, 1989). This variability is caused in part by changes in storm intensity, the mechanism by which pollutants are mobilized from surfaces, as well as the timing of discharges from different sub-areas of a catchment. As a result, the concentration of a pollutant, even when measured at a fixed point at a single site, typically fluctuates greatly over the duration of a storm event (Woodward-Clyde Consultants, 1991a). For some pollutants, concentration can vary with depth depending on the degree of mixing. In lower velocity situations materials present in the particulate phase, those denser than water (e.g., natural sediment particles, pollutants adsorbed to sediment particles, and relatively dense pieces of debris) tend to be found in higher concentrations near the bottom of a channel. Those less dense (e.g., oil and grease, woody debris, and plastic particles) tend to float. Only completely dissolved materials, or suspended particles under turbulent flow, are likely to be well mixed over the depth of the water column.

Pollutant concentrations vary on other scales, such as along the length of a conveyance, between conveyances in a catchment, and between catchments and watersheds. However, it is primarily the spatial and temporal variability discussed above that influence the choice of method for water quality sampling. "Method" means not only the choice of manual versus automated sampling, but also the number of grab samples necessary to meet program objectives. If a single grab is not adequate, a series of grab samples collected over the course of a storm may be analyzed individually to provide discrete measures of pollutant concentrations over time.

Individual grab samples can be composited in one of several ways (USEPA, 1992):

1. Constant time - constant volume: Samples of equal volume are taken at equal increments of time and composited to make an average sample. (Note: This method is not acceptable for samples collected for compliance with USEPA's NPDES Municipal Stormwater Permit Application.)

2. Constant time - volume proportional to flow increment: Samples are taken at equal increments of time and are composited proportional to the volume of flow since the last sample was taken. This method does result in a flow-weighted sample; however, it is seldom employed, as it requires extensive measurements to determine volumes applicable for each sample.

3. Constant time - volume proportional to flow rate: Samples are taken at equal increments of time and are composited proportional to the flow rate at the time each sample was taken.

4. Constant volume - time proportional to flow volume increment: Samples of equal volume are taken at equal increments of flow volume and composited.

Each of these methods results in a sample that is more representative of average conditions during a storm than a single grab sample. However, use of the second and fourth methods described above would require continuous monitoring of the discharge. Automated samplers can be programmed to act in this manner; however, this is not a practical approach for personnel sampling by hand. The third method described above also requires some system for the measurement of the discharge rate, but this must only be monitored periodically, when each grab sample is retrieved.

As mentioned above, the series of samples can also be flow-weight composited to provide an average picture of water quality and a better estimate of the impact of the discharge on receiving water quality. The time composite methods are also sometimes used, but are only typically recommended where flow rate does not vary significantly with time. The type of chemical constituent being measured may also dictate the type of sampling method. The extent of grab sampling or selected compositing will significantly influence equipment selection.

Grab Versus Composite Samples

A grab sample can be considered representative of runoff at a single site at the precise time of collection. Depending on the degree to which pollutant concentrations vary during a storm, data derived from a grab sample may or may not present an accurate representation of pollutant concentrations and loadings over the course of a storm. Despite this potential shortcoming, grab sampling for some constituents is incorporated into most sampling programs for the following reasons:

  • A grab sample collected during the first 30 minutes or less of a storm has been used to characterize pollutants associated with the first flush (those pollutants that build up in the collection system, on paved surfaces, and in storm sewer system during the antecedent dry period).
  • Some pollutants such as temperature, pH, total residual chlorine, bacteria, and volatile organic compounds transform rapidly. The compositing techniques when used with these parameters will introduce a source of bias.
  • Some pollutants (i.e., oil and grease) adhere to surfaces so that transfer between sampling containers must be minimized. If program objectives require characterization of the average oil and grease concentration over the duration of a storm, this information should be derived from a number of grab samples that are analyzed individually.

Two approaches may be taken to obtain more representative data for those parameters that do not transform rapidly:

  • Grab samples may be taken at predetermined intervals throughout a storm and analyzed individually.
  • Grab samples taken at predetermined intervals may be mixed together in equal volumes or in volumes weighted by the discharge rate at the time of collection.

The first method provides the most detailed information about the variability of pollutant concentrations during a storm. However, the analysis of each grab sample separately increases laboratory costs and is typically only used to answer specific questions about stormwater quality.

More typically, the practice (compositing) of mixing together a series of grab samples is used. Whether the compositing procedure entails sampling at equal time intervals between grab samples or by the rate of discharge at the time each grab sample is collected, the resulting sample is considered to represent an average picture of stormwater quality over the compositing period. Time-weighting gives a representation of the average concentration while flow weighting presents an average that can be used to estimate pollutant loads. The use of composite samples provides a clear opportunity for reducing monitoring program expenses and provides a good method for obtaining pollutant loads.

Manual Versus Automated Sampling Methods

For a monitoring program that is small in scope, with relatively few sampling sites and storm events, manual methods for obtaining grab and composite samples may be preferable to those employing automated equipment. The principal advantage to manual sampling is the relatively low cost of equipment and station setup. In addition to the capital outlay required for the purchase of automated samplers, other potentially substantial costs include installation, training personnel to use the samplers correctly, field maintenance and operations (i.e., replacing batteries, interrogating data loggers, retrieving and cleaning sampling receptacles).

However, manual sampling is not recommended for monitoring programs involving large numbers of sites or sampling events where flow-weighted composites will be collected over multiple events. Under these circumstances, labor costs can far exceed those associated with automated equipment. Health and safety issues such as the potential for exposure to inadequate oxygen, toxic or explosive gases, storm waves in manhole vaults, and hazardous traffic conditions at street level, must also be considered when evaluating manual versus automated sampling techniques.

The following sections describe methods and equipment used to collect grab and composite samples.

4.3.2. Water Quality Sampling Equipment

Water quality monitoring equipment can be broken into two general categories: manual equipment and automated equipment. Each are discussed in this section. In addition, overland flow samplers and in situ water quality devices are discussed.

Manual Water Quality Sampling Equipment

Manual equipment can be used in collecting grab samples, composite samples, or both.

Manual Grab Sampling Equipment

Stenstrom and Strecker (1993) provide a more detailed review of manual sampling techniques and equipment. If site conditions allow, a grab sample can be collected by holding the laboratory sample bottle directly under the lip of an outfall or by submerging the bottle in the flow. A pole or rope may be used as an extension device if field personnel cannot safely or conveniently approach the sampling point. Alternatively, a clean, high-density polyethylene bucket may be used as a bailer and sample bottles may be filled from the bucket. Care should be taken not to stir sediments at the bottom of the channel.

As described earlier, the concentrations of suspended constituents tend to stratify within the flow stream depending on their specific gravity and the degree to which flow is mixed by turbulence. Use of a discrete-depth sampler for multiple samples should be considered when constituents lighter or heavier than water are targeted, or if the discharge is too deep and/or not mixed well enough to be sampled in its entirety (Martin et al., 1992). However, highway runoff sampling sites usually drain relatively small catchments and contain fairly shallow flows. Collection of depth-integrated samples at these sites is not typical.

Given the extremely low detection limits that laboratory analytical instruments can achieve, leaching of water quality constituents from the surface of a bailing device or sample bottle can affect water quality results. Sample bottles of the appropriate composition for each parameter are usually available from the analytical laboratory. Depending upon the pollutant to be analyzed, bailers and discrete-depth samplers should be made of stainless steel, TeflonTM-coated plastic, or high-density polyethylene. When in doubt, a laboratory analyst should recommend an appropriate material type for the collection device.

Manual Composite Sampling Equipment

If grab samples will be composited based on discharge rate (i.e., grab samples collected during high flow contribute more to the composited sample than those collected during low flow), some receptacle for storing the individual grab samples prior to compositing will be required. The use of polyethylene jugs, or the polyethylene cubes with screw-on caps manufactured for shipping chemicals, is recommended. These can be shaken to remix the sample prior to pouring out the required volume. The volume required from each receptacle can be measured in a graduated cylinder and poured into a bucket for compositing. Both the cylinder and the bucket should be made from a TeflonTM-coated plastic or high-density polyethylene and should be cleaned prior to use.

Automated Water Quality Sampling Equipment

An automated sampler is a programmable mechanical and electrical instrument capable of drawing a single grab sample, a series of grab samples, or a composited sample, in situ. The basic components of an automated sampler are a programming unit capable of controlling sampling functions, a sample intake port and intake line, a peristaltic or vacuum/compression pump, a rotating controllable arm capable of delivering samples into sample containers, containers, and a housing capable of withstanding moisture and some degree of shock. Commonly used brands include: ISCO, Lincoln, Nebraska; American Sigma, Medina, New York; Manning, Round Rock, Texas; and Epic/Stevens, Beaverton, Oregon.

An automated sampler can be programmed to collect a sample at a specific time, at a specific time interval, or on receipt of a signal from a flow meter or other signal, (e.g. depth of flow, moisture, temperature). The sampler distributes individual samples into either a single bottle or into separatebottles that can be analyzed individually or composited. Some automated samplers offer multiple bottle configurations that can be tailored to program objectives.

An automatic sampler
Automatic Sampler (American Sigma Inc.)

Important features of automated samplers include:

  • portability;
  • refrigeration;
  • volatile organic compound (VOC) sample collection; and
  • alternative power supplies.

Portable samplers are smaller than those designed for fixed-site use, facilitating installation in manholes and other confined spaces. If a suitable confined space is not available or undesirable (e.g., because of safety issues), the sampler can be housed in a secure shelter at the sampling site. Portable samplers can use a 12V DC battery power supply, solar battery, or AC power.

Although none of the portable samplers currently available are refrigerated, ice may be added to the housing of some units to preserve collected samples at a temperature as close to 4°C as possible. The objective of this cooling is to inhibit pollutant transformation before the sample can be analyzed. Refrigerated samplers hold samples at a constant temperature of 4°C. However, their large size and requirement for a 120V AC power prohibit most field installations.

An automated sampler designed for VOCs is currently available from ISCO.

A VOC Sampler
VOC Sampler (Isco)

The bladder pump used by this instrument minimizes physical disturbance of the samples (as opposed to the physical disturbances imparted by peristaltic vacuum pumps), reducing the loss of volatile compounds. The VOC sampler distributes the sample into sealed 40-ml sample bottles, as required by USEPA protocol. However, at present, the caps for the sample bottles are not compatible with automated laboratory equipment, requiring more handling in the laboratory.

In typical installations for highway sampling, an intake line is bracketed to the channel bottom. The intake tubing should be mounted as unobtrusively as possible, to minimize disturbance of the site hydraulics. Generally, the optimum position for the intake is at the channel bottom. However, if high solid loadings are expected and potential deposition could occur, the intake can be mounted slightly higher on one side of the channel wall. Typically, a strainer is attached to the intake to prevent large particles and debris from entering the tubing. The strainer is usually installed so that it faces upstream, into the flow. This configuration minimizes the development of local turbulence that could affect representative sampling of constituents in the particulate phase.

Two types of pumps are incorporated into automated samplers for typical water quality sampling (i.e., not VOC sampling): peristaltic and vacuum/ compressor. A peristaltic pump creates a vacuum by compressing a flexible tube with a rotating roller, drawing a sample to the pump that is then pushed out of the pump. Field experience has shown that the reliability of peristaltic pumps in drawing a consistent sample volume is greatly reduced as the static suction head (i.e., distance between the flowstream surface and the sampler) increases. It may be possible to increase the efficiency of these samplers by placing the pump closer to the sample source, reducing the suction head. In general, the sampler itself should be installed no more than 6 meters (20 feet), and preferably less, above the channel bottom. If the sampler is to be installed at greater than 20 feet above the channel invert, it may be necessary to use a remote pump that is placed closer to the flowstream to ensure reliable sample collection.

The degree to which sampler lift affects the concentration of total suspended solids (and other pollutant parameters) is not well known, especially the effect on coarser material. That is, the mean transport velocity achieved by the peristaltic pump is sufficient to draw suspended solids; however, the pulsed nature of the flow may allow suspended solids to settle back down through the pump tubing during transport. In work performed with the USGS for this study, it was found that suspended solid concentrations did not vary with pumping height (0 to 24 feet). However, sample volumes delivered to sample bottles did vary from sample to sample at high lift heights for some of the older sampler models.

Another concern with peristaltic pumps is their incompatibility with TeflonTM-lined tubing in the pump assembly. Compression of the intake tubing by the rollers tends to create stress cracks and small recesses in the lining where particles can accumulate. Under these circumstances, some pollutant concentrations could be underestimated and the cross-contamination of samples can occur. Although TeflonTM-lined tubing is preferable because it reduces the potential loss of pollutants through surface interactions, this advantage cannot be accommodated with a peristaltic pump.

A vacuum/compressor pump draws a sample by creating a vacuum. This type of pump can create a higher transport velocity in the intake tube and provide a more steady and uniform discharge than a peristaltic pump. However, the higher intake velocity can scour sediments in the channel near the sampler intake, resulting in disproportionately high concentrations of suspended solids.

After a sampler is installed, it must be programmed to collect the desired sample size. Calibration of peristaltic pumps is achieved by one of two methods: automatic or timed. In automatic calibration, the actual volume of the sample drawn is measured using a fluid sensor located at the pump and the known pump speed. In timed calibration, the volume is determined from the number of revolutions of the peristaltic pump and the time taken for the sample to travel from its source to the sample container. Calibration by this latter method is site specific, incorporating the pump speed, the head (vertical distance above the sample source), and the length and diameter of the intake tubing. The Manning and Epic samplers, which employ vacuum pumps, permit adjustment for specific sample volumes via a fluid level device in a chamber. Because it cannot be flushed as the tubing can, this chamber can cause sample cross-contamination.

Remote Communications with Automatic Equipment

The ability to remotely access the memory and programming functions of an automated sampler is a highly desirable feature for large stormwater sampling networks. Although this feature increases the capital cost for a system, it can greatly reduce the expertise and training necessary for field crews, since many of the technical aspects of equipment set-up and shutdown can be conducted by a system supervisor remotely.

Currently, modem communication is an available option to most commercially produced automated samplers. However, there are several common drawbacks that may be encountered with the communication systems currently offered by manufacturers:

  • Full access to all sampler programming features is currently not available. This means that trained field crews may still be necessary to ensure sampler programming is correct.
  • For multiple instrument systems (i.e., separate flow meter and automated sampler) communication and complete operation of both components through one modem system is generally not available.

Remote communication for both samplers and flow meters is a rapidly advancing technology, and companies like American Sigma and ISCO currently are developing systems that address the problems described above.

Overland Flow Sampler

An overland flow sampler is a non-automated sampler that can be used to take discrete grab samples or a continuous sample over some duration. This type of sampler may be useful for collecting stormwater samples at the highway shoulder. One manufacturer's (Vortox, Claremont, California) unit within this class of samplers consists of an upper ball valve, a lower ball valve (through which runoff enters), and a sample container. The upper valve can be adjusted to control the rate of intake, allowing continuous sampling of storm events of different durations provided depth of flow is not highly variable. The lower ball valve seats and closes the intake when the water level reaches the top of the container.

Overland flow samplers (manufactured by Vortex) are available in two sizes: 3 liters (0.8 gallon) and 21 liters (5.5 gallons). They can be set into existing sumps or in the ground, but they must be installed with the top of the sampler flush with the ground surface.

This instrument is inexpensive and simple to operate. Since the overland flow is not concentrated there are few other methods for collecting this flow. However, this sampler is not capable of taking flow or time-weighted composites or of sampling the entire discharge during a large storm event. In fact, there is no way of knowing what part of the storm was actually sampled, especially where flow depths are variable.

Other manufacturers of samplers that can be used for overland flow include GKY's "FirstFlush" sampler and D-Tec Corporation's "Environmental Liquid Sampler" (ELS).

Recently, the USGS developed and began testing an automated overland flow sampler that may be capable of time-weighted composite sampling.

In Situ Water Quality Devices, Existing Technology

As described in the sampler section, the concentration of most pollutants in stormwater runoff is likely to vary significantly over the course of a given storm event. Some of this variability can be captured through the collection of multiple samples. The ideal data set would contain not just multiple samples, but a continuous record of constituent concentrations throughout a storm, capturing both the timing and magnitude of the variations in concentration. Given the availability of other continuous data, this approach might allow better correlation with potential causative factors. Unfortunately, the laboratory costs for even a near-continuous data set would be prohibitive. This study for FHWA determined that between 12 and 16 individual samples resulted in a mean that was within 10 to 20% of the actual event mean concentration. In situ monitoring devices offer a possible solution to obtaining a continuous record of water quality; however, currently, they are only practical for a limited set of parameters.

In situ water quality probes have been adapted from equipment developed for the manufacturing and water supply/wastewater industries. In situ water quality monitors attempt to provide the desirable near-continuous data set described above at a relatively low cost, eliminating (or reducing) the need for analysis of samples in the laboratory.

In general, water quality monitors are electronic devices that measure the magnitude or concentration of certain specific target constituents through various types of sensors. Discrete measurements can be made at intervals of one minute or less. Most monitors use probes that provide a controlled environment in which a physical and/or electrochemical reaction can take place. The rate of this reaction is typically driven by the concentration of the target constituent in the discharge. The rate of reaction, in turn, controls the magnitude of the electrical signal sent to the display or a data-logging device.

Probes to detect and measure the following physical and chemical parameters are currently available for practical use in the field:

Physical parameters

  • Temperature
  • Turbidity

Chemical parameters

  • pH
  • Oxidation-reduction potential (redox)
  • Conductivity
  • Dissolved oxygen
  • Salinity
  • Nitrate
  • Ammonia
  • Resistivity
  • Specific conductance
  • Ammonium

There are some potential probes for heavy metals, but given the complexities associated with highly variable solid concentrations and other factors, this study found that they are not practical for field application. Instruments can be configured to measure the concentrations of several of these parameters simultaneously (i.e., multi-parameter probes) and provide data logging and PC compatibility. Manufacturers of this type of instrument include YSI, Inc., Yellow Springs, Ohio; ELE International, England; Hydrolab, Austin, Texas; Solomat, Norwalk, Connecticut; and Stevens, Beaverton, Oregon.

In many cases, the electrochemical reaction that drives a probe's response is sensitive to changes in temperature, pH, or atmospheric pressure. Where appropriate, monitors are designed to simultaneously measure these associated properties. Data on the target constituent are then corrected through a mathematical routine built into the probe's microprocessor (e.g., dissolved oxygen probes are compensated for temperature and atmospheric pressure, pH probes for temperature, and ammonia probes for pH), or are adjusted in a spreadsheet after being downloaded to a personal computer.

Despite the advantage of these instruments for measuring near-continuous data, they require frequent inspection and maintenance in the field to prevent loss of accuracy due to fouling by oil and grease, adhesive organics, and bacterial and algal films. Therefore, these instruments should always be cleaned and calibrated before use. Because water quality probes are designed to operate while submerged in water, exposure of the electrochemically active probe surface to air should be minimized.

In Situ Water Quality Devices, Future Technologies

There are several in situ water quality devices that are used by industry, but they are not currently applicable to stormwater monitoring. However, as the technology advances they may become applicable and therefore are discussed in this section.

Ion-Selective Electrodes

An ion-selective electrode places a selectively permeable membrane between the discharge and an internal solution of known ionic strength. The voltage differential across the membrane is proportional to the difference in ionic strength between the two solutions. Ion-selective probes are currently available for the ionic forms of a number of parameters, including ammonia, ammonium, copper, lead, nitrate, and nitrite.

An ion-selective electrode is specific to the targeted ion and will not measure other ions or other complex forms. For example, depending on the target parameter, a nitrate-selective electrode will not measure the concentration of nitrite in the discharge. However, these instruments are sensitive to interference from other ions, volatile amines, acetates, surfactants, and various weak acids. At present, the degree of interference can be judged only by comparing the performance of the probe to that of one in a reference solution, a procedure likely to prove unwieldy in the field. Consequently, this type of probe is not typically used for stormwater monitoring.

On-Line Water Quality Analyzers

On-line water quality analyzers are spectrometers, similar to those used in analytical laboratories. A light source that generates a known intensity of light over a range of wavelengths (i.e., ultraviolet or infrared) is transmitted through a sample introduced into a flow cell. The instrument collects light absorbency information at multiple wavelengths and produces a light absorbency signature (manufacturer's specifications, Biotronics Technologies, Inc., Waukesha, Wisconsin, and Tytronics, Inc., Waltham, Massachusetts). The instrument is calibrated using 30 or more randomly varied mixtures of standards; the ultraviolet (UV) light-absorbency characteristics of a sample are then compared to a baseline calibration file of known "UV signatures."

On-line analyses are used in the water treatment and wastewater industries. Until recently, on-line spectrometric analyzers were impractical for stormwater field use. The state of technology of these systems was comparable to that of computers 20 years ago-highly trained specialists operated large machines in a controlled laboratory environment. However, an increased demand for portability, the increased power and decreased cost of microprocessor technology, the development of new statistical and mathematical analysis software, and the availability of standardized control systems (i.e., communication interfaces, actuators, and programmable controllers) have fostered the emergence of a new generation of instruments.

Three types of spectrometers are currently available or under development for environmental applications:

  • Ultraviolet-Array Spectroscopy (UVAS) employs a broad-spectrum light generated by a Xenon lamp and delivered to the sample through fiber optic cables. Light is transmitted through the sample in specially designed optical probes. The light transmitted through the sample is collected and returned to the analyzer where it is dispersed into wavelengths and projected onto a photodiode detector array. Current applications are the detection of multiple contaminants (metals, nitrates, organics, and aromatic hydrocarbons) in groundwater, the detection of metals (chromium, zinc, and mercury) in industrial wastewater, and water treatment quality parameters (copper, iron, molybdate, triazole, phosphorate) in industrial process and cooling waters.
  • Liquid Atomic Emission Spectrometry (LAES) employs a photodiode detector array, similar to that used in UVAS. A high-energy arc is discharged directly into the liquid as the source of excitation and the resulting atomic light emission is analyzed by special pattern recognition techniques. Qualitative analysis is derived from the detection of emission lines whereas quantitative analysis is a function of intensity. Use of LAES has been demonstrated for the analysis of metals, hydrogen, and sulfur.
  • Like UVAS, Near Infrared (NIR) analysis employs the transmission of light through a liquid. This technology has been used extensively in the food processing industry and is under evaluation for application elsewhere.

To date, portable on-line analyzers have not been tested extensively for use in stormwater monitoring. The "ChemScan" analyzer, manufactured by Biotronics Technologies, Inc., reportedly adjusts automatically for changes in the turbidity of the discharge and fouling of the optical windows, features that suggest applicability to stormwater situations. According to the manufacturer, routine maintenance is limited to a periodic baseline correction and occasional chemical cleaning of the flow cell.

Particle Size Analyzers

There is a particle size analyzer available that can be installed in situ. It employs laser diffraction to determine the particle size distribution. However, the unit costs approximately $30,000, is 3 feet long and 5 inches in diameter, and must be submerged. Currently it is not applicable for stormwater monitoring.

Research has been ongoing for many years on applying ultrasonics for particle size analysis. However, it is presently not available for field stormwater application.

In Situ Filtration and Extraction System

Axys Environmental Systems, Ltd., British Columbia, Canada manufactures an in situ filtration and extraction system for monitoring trace organics, metals, and radionuclides in stormwater. These systems retain the target pollutant on a resin filter as a portion of the discharge passes through it. After the storm event, the filter is taken to the laboratory and the pollutant is removed through solid phase extraction. The filtration system is comprised of a microprocessor, a pump, a flow meter, and a DC power supply. A pre-filter for suspended solids can be attached if levels high enough to clog the resin filter are anticipated. Pollutants trapped in the pre-filter can also be extracted and analyzed.

These systems can be programmed so that samples of the discharge pass through the filter at equal time intervals, or so that signals from an external flow meter trigger flow- or time-weighted composite sampling. As with other types of automated samplers, the sampling history is stored in internal memory.

Filtration and extraction systems reduce the potential for contamination of a sample during handling in the field and eliminate the need to transport large volumes of water to an analytical laboratory. The detection limit of the samples depends on the amount of water flowing through. Because large volumes of water can be passed through the system, even very small concentrations of pollutants can be detected. On the other hand, where suspended sediment concentrations are high, the pre-filter may become clogged as a large volume of water passes through it. Metals can be lost from the filter if the pH drops below 6.0 and resin filters are available for only a limited number of pollutants. Due to the potential for clogging, this methodology may not be useful for highway sites.

4.3.3. Sampling Equipment Summary

In general, manual sampling is not practical for accurate flow-weighted composite sampling of a large number of sites or sampling events over multiple years. Automated flow-measured systems are ideally suited for this role. An automated sampler combined with a reliable and accurate flow meter will provide a user with a useful system for monitoring highway stormwater runoff. There is currently a trend by some sampler manufacturers to integrate flow meters and samplers as one unit. This greatly facilitates the site installation and the role of field personnel during a storm event. It may also allow for better remote communications with both units.

The ability to interact with samplers via remote communications is a desirable advance in sampler technology. Additional improvements could include:

  • The ability to make changes in programming by entering the sampler program at a specific step, rather than having to step through the entire program. This would be useful in general as well as in regard to the particular task of calibrating the sample volume.
  • The ability to program the sampler to distribute a grab sample into one bottle and to composite a sample into the others (e.g., in a 4- or 8-bottle configuration).
  • The ability to draw a sample at a velocity equal to or greater than the mean velocity of the discharge. This would ensure the collection of a representative sample of the discharge, including suspended particles. Because the discharge rate is likely to vary over the duration of a storm, this feature would require a variable-speed pump as well as additional programming capabilities.

The development of robust in situ instruments has changed the notion of appropriate technology for water quality monitoring. It is clear from the discussion above that these instruments are still evolving and that entirely new products are likely to become available in the next few years.

Typically, in situ filtration and extraction systems and overland flow samplers currently are not well suited to monitoring highway stormwater runoff.

4.3.3.1. Flow Measurement Equipment

A variety of methods and instruments are available for measuring flows within stormwater conveyances. The most useful technologies for the continuous measurement of discharge during a storm event have the following capabilities and characteristics:

  • tolerant of site hydraulics and environmental conditions;
  • accurate and capable of maintaining calibration over the range of hydraulic and environmental conditions expected during a sampling event;
  • employ probes that are streamlined and non-intrusive or otherwise do not accumulate debris;
  • retrievable internal memory or capable of integration with an external data logging device;
  • capable of integration with a rain gauge, automated sampler, and other stormwater monitoring devices;
  • capable of receiving programming instructions from a remote workstation by telemetry;
  • capable of transmitting monitoring data to a remote work station by telemetry; and
  • minimize the changes to hydraulics such that flooding is not increased.

Nearly all of these characteristics are currently available in flow metering equipment. However, not all of these characteristics are built into each instrument, nor are they available for all site conditions. Each site must be evaluated, including a site inspection, before a discharge measurement method and instrumentation are chosen.

In general, discharge measurement devices that are likely to collect a great deal of debris (i.e., drag-bodies and rotating-element current meters) are not useful for continuous monitoring. Some instruments (i.e., pressure transducers and ultrasonic and electromagnetic probes) are designed with a more streamlined profile but, in practice, pieces of flexible debris can become wrapped around these instruments as well. Adaptations that aid in the shedding of debris would be extremely useful and would widen the application of these instruments. It is also important that the hydraulic capacity of weirs or flumes not be reduced.

4.4. Precipitation Gauging Equipment

Precipitation gauges are devices used to detect and measure rainfall and snowfall. The use of such data includes investigating the relationship between rainfall and runoff. In addition, rainfall can be used to initiate sampling or flow measurement. The most common gauges measure precipitation by either volume or weight. New technology uses an infrared beam to measure precipitation via the frequency and via the frequency and blockage of a light beam.

The purpose of a precipitation gauge is to make a point estimate of rainfall and snowfall that is used as an index to approximate the volume of water falling over an area (the amount of liquid produced when snow is melted is known as the liquid equivalent). Precipitation amounts that fall at a particular gauge are equal to the amount of rainfall plus the amount of snowfall liquid equivalent.

Precipitation gauges provide adequate measures of precipitation amounts at a point but are less proficient at estimating precipitation amounts over large areas. How accurately the precipitation amounts are measured at such points is less important than how consistently the point measurements estimate the total precipitation amounts over an area represented by an individual gauge.

A typical precipitation gauge is 6 to 12 inches in diameter, and the area covered by this size gauge is approximately 55 to 110 square inches, or 14 to 28 billionths of a square mile. Most precipitation gauge networks have gauge densities on the order of one gauge (point) per 10, 20, or 50 square miles. Networks routinely sample the precipitation at an area-ratio of parts per trillion.

The number of precipitation gauges installed in a precipitation gauge system directly affects the quality of precipitation data. Generally, the higher the number of precipitation gauges, the better the estimate of incoming precipitation amounts. Locating a gauge at each monitoring site for small catchments is imperative because local variations in total rainfall and rainfall intensity can have significant effects on runoff when the watershed is minimal in size. Nearby locations may not be useful in estimating rainfall at the actual site.

Tipping Bucket Precipitation Gauges

The most widely used type of precipitation gauge is the tipping bucket rain gauge. This type of gauge measures precipitation amounts in various increments, usually 0.01 inch or 0.1 millimeter, depending on the model.

Tipping bucket gauges operate by funneling precipitation into a bucket mechanism that tips when filled to a calibrated level. A magnet attached to the tipping mechanism triggers a switch as the bucket tips. The momentary switch closure is counted by pulse counting circuitry of data loggers. The tipping of a bucket also brings a second bucket into position under the funnel. The second bucket is then ready for filling. After measurement, water is directed into drain tubes that allow it to exit out holes in the base of a gauge. Screens cover the exit holes to prevent insect entry. Heaters can be installed on these gauges so that snow and other freezing precipitation will melt on contact with the collection orifices, and the liquid equivalent of the frozen precipitation can be measured.

Tipping bucket rain gauges
Tipping Bucket Rain Gauges (American Sigma Inc.)

These gauges have been found to be quite reliable and require little manpower to operate. Most data loggers can be used with a tipping bucket precipitation gauge. However, since it takes 0.01 inches of precipitation to tip the bucket, it is possible that smaller precipitation amounts will not register due to evaporation between precipitation events. This can cause some error in precipitation amount data. The cost of these gauges ranges from $500 to $900, (including the price of heaters and mounting equipment). Overall, most tipping bucket gauges have a resolution of 0.01 inches and an average percentage error of ± 2.25% over an average range of 0 to 6 inches of precipitation per hour. The percent error rises significantly for higher precipitation intensity (greater than 6 inches per hour) and under windy conditions.

Optical Precipitation

Optical Precipitation Gauges are one of the more recent precipitation gauge technologies. This type of gauge has two sensors that face each other at either end of a "U" shaped bracket. The precipitation is measured by detecting the optical irregularities induced by drops falling through an infrared optical beam. These irregularities, known as scintillation, have characteristic patterns that are detected by the sensor and converted to precipitation rates. The higher cost models are equipped with a heating device for the sensors making it possible for the gauge to measure frozen precipitation.

Optical gauges are exclusively designed for remote data collection and require little to no manpower to operate, have no moving parts, are not affected by wind, and are able to measure very low intensity rainfall that may get held upon the side wall of a tipping bucket rain gauge. However, these gauges can only interface with a data logger that has RS232 input capability. Optical range gauges have an average percent error of ±5% over almost all precipitation intensities. The major drawback to this type of gauge is the cost ranging from $2,000 to $3,000.

Siphoning Precipitation Gauges

The siphoning precipitation gauge was the cutting edge of technologies for precipitation gauges until optical precipitation gauges emerged.

This type of gauge measures water levels in a tube called the measuring chamber. Each 0.04 inch of captured precipitation produces a 0.197 inch rise of the sample column in the measuring chamber giving added resolution to the measuring circuitry. A full sample column of 9.842 inches represents 1.969 inches of collected precipitation. Additional precipitation starts a self-siphoning process that empties the measuring tube in approximately 30 seconds. During this time, the gauge is unable to measure precipitation, and this could lead to inaccurate measurements during heavy precipitation events. There are heaters on this gauge that melt frozen precipitation so that the siphoning precipitation gauge can measure liquid equivalents for snowfall and other frozen precipitation.

Siphoning precipitation gauges are some of the most complex precipitation gauges used today, and have the advantage of no moving parts to wear or break. This type of rain gauge may have difficulty interfacing with some data loggers. Siphoning precipitation gauges cost about $1,100. Resolution of this type of rain gauge is 0.004 inches with a margin of error of ± 0.04 inches.

Manual Precipitation Gauges

The manual precipitation gauge consists of a funnel, an inner measuring tube, an outer over flow cylinder, and a dipstick. The inner tube has a capacity of two inches, and the over flow cylinder can hold up to 18 inches. Precipitation is measured by inserting the dipstick into the inner tube and reading the measurement shown on the stick at the water line. In cases where there is water in the overflow cylinder, the water from the cylinder is emptied into the inner tube and measured in the same manner. In the case of frozen precipitation, the funnel is removed allowing frozen precipitation to accumulate in both the inner tube and overflow cylinder. The frozen precipitation can be melted down and measured in the same manner as liquid precipitation.

Manual precipitation gauges do not have electronic data gathering capabilities. It is therefore necessary to measure the precipitation by hand for every precipitation event. This type of gauge has a resolution of 0.01 inches and can handle up to 20 inches for each precipitation event. The manual precipitation gauge is the least expensive of all the gauges discussed, ranging in price from about $250 to $400. This price includes the cost of a mounting stand. However, since data loggers cannot interface with this type of gauge the labor cost of taking readings for every precipitation event must be taken into consideration.

4.4.1. Precipitation Equipment Summary

Because of the additional labor costs associated with manual gauges, and the difficulty of siphoning gauges to interface with some data loggers, the choice of a precipitation gauge to be used as part of a water quality monitoring system of highway runoff is typically reduced to a decision between a tipping bucket type and an optical type precipitation gauge. The determining factor is the required accuracy. Some accuracy is lost when using tipping bucket gauges (several hundredths of an inch). However, the cost of the increased accuracy associated with optical precipitation gauges is high. If both types of gauges are equipped with a heater to permit measurement of snowfall, the costs of a tipping bucket gauge and the optical gauge are approximately $1,000 and $3,500, respectively. In addition, the data feed for tipping bucket gauges is not continuous, while the data feed for optical gauges is continuous. Finally, optical gauges are less susceptible to errors associated with windy conditions.

For the purpose of a water quality monitoring program, a tipping bucket type gauge is considered adequate. It provides a measurement accuracy to the few hundredths of an inch and is less expensive than an optical type gauge. However, if wind is an issue, the optical gauge should be considered. This may be especially important for highway sites, which often have windy conditions.

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Questions and feedback should be directed to Deirdre Remley (deirdre.remley@dot.gov, 202-366-0524).

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