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Stormwater Best Management Practices in an Ultra-Urban Setting: Selection and Monitoring

Monitoring Case Study-Streetsweeping BMP Evaluation, Port Of Seattle, Washington

This case study is based on a study evaluating streetsweeping technology by Kurahashi and Associates, Inc. (1997).

The Port of Seattle owns five cargo container yards, covering a total area of approximately 162 ha (400 ac), which serve its marine terminals. A major expansion effort currently under way at two of the five yards will result in a substantial increase in the container yard area. At the present time, the only stormwater treatment BMP that is technically feasible and approved for new marine facilities is the wet vault. In May 1996, the Port of Seattle contracted with Kurahashi and Associates to evaluate the effectiveness of new high-efficiency pavement sweepers in combination with conventional sediment-trapping catch basins to determine if the combination technology provided pollutant reduction benefits that were comparable to those of wet vaults. The evaluation was prompted by the results of recent studies conducted by Kurahashi and Associates that indicated significant reductions in pollutant loadings could be achieved through the use of high-efficiency sweepers. Older studies, dating back to the Nationwide Urban Runoff Program (NURP), had indicated that streetsweeping was of limited benefit in improving the quality of urban runoff. The use of high-efficiency pavement sweepers in combination with conventional sediment-trapping catch basins would result in substantial savings for the Port of Seattle compared to the use of wet vaults (estimated life cycle costs of $2 million for high-efficiency sweepers in combination with conventional sediment-trapping catch basins versus $18 million for wet vaults).

Study Objectives

  • Calibrate a stormwater quality computer model using pollutant accumulation data from nine sites in various activity areas of a container storage yard in the Port of Seattle.

  • Use the calibrated model to evaluate the stormwater pollutant removal effectiveness of new high-efficiency pavement sweepers used in combination with conventional sediment- trapping catch basins and determine if the removal effectiveness was equivalent to that obtained through wet vaults.

Modeling Approach, Data Collection, and Calibration

The Simplified Particulate Transport Model (SIMPTM) was used in the study. SIMPTM is a continuous stormwater quality model that has been shown to accurately simulate the accumulation and washoff of sediment and associated pollutants, and the load reductions expected through the implementation of BMPs. SIMPTM accounts for sediment deposition, armoring, and resuspension processes, and models scheduled cleaning of streets, parking lots, catch basins, and maintenance hatches. The model aggregates hourly precipitation data into rainfall events and provides continuous simulation of sediment and bound pollutant transport.

Data on pollutant accumulation was obtained over a 2-month period at nine sites within three areas in the container yard that were deemed to be representative of various ongoing activities. The activity areas selected were the alleyways between stored containers, the alleyways between parked trailers, and the area beneath the trailers. One site in each activity area was sampled every week; the second and third sites were sampled every two weeks and four weeks, respectively. Samples were collected on designated days by hand sweeping and mechanical vacuuming. A mechanical grain size analysis and chemical analysis for metals and total petroleum hydrocarbons was performed on each sample.

The SIMPTM model was calibrated using data on pollutant accumulation obtained over the 2-month sampling period and rainfall data for the same period collected at a nearby airport. Calibration essentially entailed adjusting the values of washoff and accumulation parameters until the best overall match was obtained between predicted and observed sediment accumulations for each of the activity areas during the two month sampling period. The best match was determined by visually comparing line graphs of predicted and actual sediment accumulation values for different parameter combinations.

The calibrated model for each activity area was used to simulate the average annual total suspended solids (TSS) loadings using an "average year" of rainfall events, synthesized from the analysis of a 29-year precipitation record at the airport. SIMPTM simulations included copper, lead, zinc, and phosphorus. Estimates of the particulate (suspended) fraction of each pollutant were based on the mean mass-fraction of the pollutant found in the analysis of samples collected from the container yard. These estimates were that 50 percent of the copper, phosphorus, and zinc washoff at any given time was assumed to be dissolved, while only 20 percent of the lead was assumed to be dissolved.

Alternative frequencies of sweeping (daily to monthly) and alternative sizes of catch basins (normal or enlarged)) were considered in the SIMPTM simulations. Since the model does not allow alteration of basic performance characteristics of the sweeper for a given model run, two sets of results were obtained to simulate performance characteristics of dry sweeping (high pickup efficiency) and damp pavement sweeping (reduced pickup efficiency).

Wet vaults are not explicitly modeled by the SIMPTM model. Sediment and associated pollutant removals for wet vaults were computed based on a modification of Stoke's Law for determining settling velocities for various grain sizes. SIMPTM model outputs (with no sweeping assumed) were used as inputs for these computations.

Results and Conclusions

The expected range of annual pollutant load reductions for various sweeping frequencies indicated by SIMPTM are summarized in Table 41. The table also provides the expected range of pollutant load reductions for wet vaults. The following conclusions were drawn from the simulation study:

  • Pollutant removals obtained with high-efficiency sweeping at a weekly frequency in combination with normal catch basin inlets cleaned annually are comparable to removals obtained by wet vaults.

  • High-efficiency sweeping appears more effective than wet vaults in the removal of highly dissolved pollutants (copper, zinc, and phosphorus).

  • Wet vaults appear more effective than high-efficiency sweeping in the removal of TSS and sediment-bound pollutants such as lead.

  • High-efficiency sweeping carried out on a weekly basis in combination with normal catch basin inlets is a viable, cost-effective BMP, with overall pollutant removals comparable to those obtained by wet vaults.

Table 41. Comparison of pollutant load reductions from various sweeping frequencies and wet vaults (%)1
Pollutant Twice Weekly Sweeping Weekly Sweeping Biweekly Sweeping Wet Vaults
TSS 45-70 45-65 40-60 75-90
TP 35-60 30-55 20-40 35-45
Total Lead 40-60 35-60 30-50 65-80
Total Zinc 30-55 25-50 20-40 35-45
Total Copper 35-60 30-55 20-40 35-45
1 The low end of each range is obtained under the assumptions that none of the dissolved pollutants are captured by sweeping during damp pavement conditions and that parked trailers do not block the potential transport of material beneath trailers.


Kurahashi & Associates, Inc. 1997. Port of Seattle - Stormwater Treatment BMP Evaluation. Prepared for Port of Seattle, Pier 66. Prepared by Kurahashi & Associates, in association with AGI Technologies.

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Questions and feedback should be directed to Susan Jones (Susan.Jones@dot.gov, 202-493-2139) and Marcel Tchaou (Marcel.Tchaou@dot.gov, 202-366-4196).

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