By Curtis Hinman
Pullman, WA (November 1, 2009)- Washington State University and project partners implemented a flow monitoring project on a 3.35-hectare (8.27-acre) pilot project in western Washington (Meadow on the Hylebos) that incorporates low impact development (LID) stormwater management practices.
LID practices used in the project design include bioretention swales, permeable concrete, compost amended soils, and surface flow dispersion. The primary goals of the monitoring effort are to evaluate the performance of individual LID practices and evaluate the effectiveness of integrating these practices into a stormwater management system.
Continuous simulation modeling (Western Washington Hydrology Model) was used to assess peak flow and flow durations compared to stated flow control goals of the project. Flow rates for individual storms were assessed and water budgets developed that include surface and subsurface flow, infiltration and evapotranspiration in relation to precipitation inputs for bioretention swales and the project as a whole. Infiltration rates over time for the permeable concrete were also measured.
The first year of monitoring was conducted October 2006 through May 2007; however, project construction was not complete until the spring of 2007. Accordingly, evaluation for sections of the project that were complete by the 2006-07 wet season are presented, as well as overall project performance for comparison to October 2007 through May 2008 flow statistics. Specifically, the following performance evaluations are included for 2006 through 2007: a) Summary statistics for overall project performance b) Surface and subsurface flow characteristics of specific bioretention swales c) Permeable concrete infiltration rates over time.
Overall, the project performed poorly for the 2006-07 wet season and significantly exceeded flow control targets. Total precipitation volume retained on site was approximately 62 percent. Initial flow control was, on the other hand, impressive. The site received approximately 104 mm (4.09 in.) of precipitation before any flow discharged from the site essentially eliminating the seasonal first flush of pollutants. Several factors may be contributing to the overall poor performance during the 2006-07 wet season. First, the largest bioretention area with the highest infiltration capacity was lined with an impervious membrane and effectively eliminated for flow control due to concerns of infiltration to sewer lines.
The loss of this central infiltration feature likely degraded project performance significantly. Second, surface and subsurface flow monitoring of individual bioretention swales indicate that storm flow infiltrates rapidly through the bioretention soil mix and to the under-drains. As a result, detention time is not adequate for storm flows to infiltrate into the underlying native soil. Finally, measurements for the overall project were taken at the POC not accounting for the final flow control and water quality treatment practice (Dispersion Slope). The Dispersion Slope and associated monitoring station was completed and monitored for the 2007-08 wet season.
Construction of the project was complete for the second year of monitoring conducted October 2007 through May 2008. The project performed well and actually exceeded flow control objectives for 2007-2008. At the final outfall of the project (Dispersion Slope), peak flow and durations met the DOE standard for forested pre-development condition. At the POC, peak flow and durations did not meet flow control goals for pasture pre-development condition. The flow control goal for the project was to match 50 percent of the two-year and up to the full fifty-year peak flows and durations for pre-developed pasture condition. However, the POC met the DOE standard for pre-development forested condition when the infiltration capacity of the largest bioretention area that was lined with an impermeable membrane was factored back into the system in the modeling analysis.
Surface and sub-surface flows were monitored for a sub-basin including seven homes and four bioretention areas 0.32 hectares (0.8 acres). During the wet season of 2006-2007 stormwater flows infiltrated rapidly through the soil mix and to the under-drains. The under-drains were plugged for the wet season 2007-2008. Once plugged, essentially all storm flows were infiltrated in the bioretention areas and the sub-basin met the DOE standard for pre-development forested condition.
Total precipitation volume retained on site for the 2007-2008 wet season was approximately 69 percent at the POC with the lined bioretention area (no infiltration) and 99 percent when the bioretention area was included without the liner in the modeled drainage system. Total precipitation volume retained at the final outfall (Dispersion Slope) was 96 percent and for the sub-basin monitored at Station 6 the volume retained was 99 percent.
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