3. MONITORING EQUIPMENT SELECTION
This section of the guidance provides a list of issues that are important to consider before and during equipment selection and monitoring plan development.
3.1. Monitoring Location
As discussed in Section 2.5, stormwater monitoring in the near-highway environment can be aimed at attaining one or more goals. This section discusses in detail the relationship between monitoring location and equipment selection, independent of the specific goal of the program. More specifically, this section addresses how three main factors affect equipment selection.
- Watershed type
- Specific site characteristics
- Site location within the watershed
3.1.1. Watershed Type Considerations
As mentioned in Section 2.3, the near-highway environment often presents a variety of challenges when it comes to selection of monitoring equipment. One of the most important factors to assess during site selection is whether or not a particular watershed presents complexities that unduly constrain the type of equipment that can be placed in the field.
Typically, watersheds in the near-highway environment are relatively small in size (<50 acres and often <10 acres or even smaller than 1 acre). For example, a one-mile long section of four-lane highway including median and shoulder may cover approximately 11 acres. Single inlets often drain less than 0.5 acres. In addition, these small watersheds are highly impervious and moderately sloped. These characteristics have significant implications for monitoring of quantity and quality. With small areas, moderate to steep slopes, and highly impervious surfaces, flow rates from highways vary dramatically and quickly. Typical times of concentration from highway sections can be less than five minutes. Flow monitoring in these conditions can be difficult. Some commonly used primary devices are not well suited to monitoring rapidly changing highly varied flow conditions.
Monitoring difficulties inherent to the near-highway environment can be exacerbated through poor site selection. Choosing a watershed type that is compatible with monitoring program goals is the first step in site selection.
Often, selecting a watershed type will significantly limit potential sites, expediting site selection. For example, depending on monitoring goals, it may be easier to monitor fill sections because they may have outfalls close to the inlet, which means that verification of the drainage system is not difficult and there is little chance that backup of the system potentially caused by the monitoring equipment will cause upstream flooding and public safety problems. In this case, equipment that may impede flow such as nozzles, weirs, and flumes may be appropriate, whereas in cut sections, pipe slopes may be shallow and backwater conditions may result. Consequently, some types of equipment should not be used in these situations.
3.1.2. Specific Site Characteristics Considerations
The specific location also dictates what type of equipment to use. A variety of considerations must be taken into account when selecting equipment for monitoring runoff and rainfall at a specific site including:
- proximity to mobilization location (How often will visits to the site be required based on the equipment used?);
- proximity to telemetry connections (Is there access to nearby cell towers or land-lines?);
- proximity to utilities (Are there nearby accessible electrical utilities?);
- accessibility for installation of equipment (Large or cumbersome devices such as large flumes can be difficult to transport to some locations.);
- personnel safety during installation and monitoring. (Does installation or use require personnel to take significant risks during installation?);
- potential for vandalism (High crime areas may require more covert and/or vandal-proof equipment and enclosures.);
- potential public safety risk (Is there a potential for increased flooding, upstream or downstream if flood flows occur?); and
- specific watershed hydrology (The monitoring equipment should be tailored to the watershed size, slope, and other characteristics.).
3.1.3. Considerations Related to Location Within a Watershed
In addition to selecting a monitoring site, the location within the site for conducting monitoring must be selected. There are four primary locations in the near-highway environment that are used to monitor runoff:
- on the surface (gutter flow, typically grab sample);
- at inlets (typically grab sample);
- mid-conveyance (manhole, in-pipe or open channel); and
Each of these locations has both operational and programmatic advantages and disadvantages that are program and site specific. As monitoring is conducted further downstream from the highway, flow monitoring results tend to be less variable; however, effects due to specific sources and practices may be more difficult to observe. The monitoring location is often directly related to the goals of the monitoring program. BMP effectiveness studies may benefit from upstream isolation of the source area affected by the BMP, where loading estimates may be most appropriate at outfalls to receiving waters.
Operationally, the specific location in the monitored watershed can affect accessibility of the monitoring equipment for maintenance and installation of equipment. Risk to monitoring personnel should be a high priority criterion when selecting a monitoring location.
3.2. Monitoring Frequency
Monitoring frequency directly impacts the selection of monitoring equipment. Typically, the larger the number of storms and the shorter the period between events that need to be monitored, the greater the benefit of using automated equipment. Prior to setting up the monitoring plan and selecting equipment, a detailed analysis of the number of events required to meet program goals should be conducted. This analysis may be limited to regulatory requirements or may involve a detailed statistical analysis to determine the number of samples required to achieve a selected level of confidence in water quality sampling results. This provides a basis for cost effective selection of a level of automation. Similar results from manual sampling will require a crew of two to remain at each station during an event. The cost of manpower should be compared to equipment/installation costs.
3.3. Range of Flows to be Monitored
The range of flows encountered in the near-highway environment can be quite large. Measuring low flow rates accurately is important where a significant portion of flow volume is the result of either base flow or low intensity events due to climate. Climatic regions that have significant annual rainfall depths but do not have very intense events on a regular basis and/or small drainage areas are good examples of locations where low flow measurement is important.
The smaller the watershed the larger the relative difference between significant low flows and peak flow (area normalized flows). This is due to the time of concentration of the watershed being on the same order as the duration of very brief and intense rainfall periods (< five minutes). Many primary devices lose considerable accuracy or their capacity is exceeded when flows range more than three orders of magnitude. Low flow measurements may not be accurate where the monitoring installation is designed to measure infrequent larger magnitude storm events. For example, the use of pressure transducers or area velocity sensors in small watersheds could result in a significant amount of flow not being measured as the depth of flow is on the order of the sensor size.
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Questions and feedback should be directed to Deirdre Remley (firstname.lastname@example.org, 202-366-0524).