MONITORING RESULTS: GREEN SOLUTION BENEFITS TO THE COMBINED SEWER SYSTEM
Author: Kurtz, Tim
Source: Proceedings of the Water Environment Federation, Collection Systems 2007 , pp. 311-313(3)
Publisher: Water Environment Federation
The City of Portland, Bureau of Environmental Services (BES) has become increasingly committed to using Green Solutions, also known as Low Impact Development (LID), to manage urban stormwater runoff. BES currently requires Green Solutions to control runoff for new development and redevelopment projects, but they've also been increasingly applied as retrofits in highly urban areas of existing development.
Most Green Solution facilities reduce both peak flows and flow volume, making them attractive options to manage stormwater issues such as combined sewer overflows and basement sewer backups in the combined sewer system. BES seeks to quantify runoff control benefits and identify issues that may improve performance. Better understanding of facility performance will inform more accurate analysis tools used to manage regulatory and policy decisions.
The primary facility types evaluated are: 1) ecoroofs (also called green roofs), 2) vegetated infiltration basins (also called bioretention facilities or Rain Gardens), and 3) stormwater planters. Water quantity data was collected through a combination of continuous flow monitoring and flow simulation tests. Performance is evaluated for varying storm sizes and intensities, as well as for seasonal variability.
An ecoroof was installed on the Hamilton Apartments when they were constructed in 1999, and rainfall and runoff volume have been monitored since 2002. The drainage area monitored is nearly 3,700 ft2 of which the ecoroof covers roughly 2,600 ft2. The average peak flow reduction has been consistently over 90% for the most intense storm events (those with peak rainfall intensities ≥ 1.0 inches per hour for a 5-minute period). Annual runoff retention averaged 56% during the four years between 2002 and 2005, with retention increasing each year to a high of 63% in 2005 (Figure 1). Seasonal retentions average 47% during the “winter” (defined as Nov – Apr) and 87% during the “summer” (defined as May – Oct).
Vegetated Infiltration Basins
The Glencoe Rain Garden is a vegetated infiltration basin constructed in 2003 to protect a residential area suffering from severe and frequent basement sewer backups. The 2,000 ft2 facility manages roughly 35,000 ft2 of impervious drainage (70% street, 30% parking lot). Monitoring of the combined sewer system began 10 months before construction, and inflow and outflow from the facility has been monitored since January 2004. In the 10 months prior to construction, peak flows in the combined sewer exceeded 0.5 cfs – the maximum flow allowed to protect downstream properties – was exceeded during 4 events. In the 36 months after construction, no events have exceeded 0.5 cfs and only one has come close – this despite the occurrence of storms of greater intensity since the facility was completed (Figure 2). Two flow tests were performed to simulate a basement sewer backup event and the peak outflow was 0.6 cfs – slightly above the benchmark but within the error tolerance. Flow volume reduction has been impressive, with an average annual retention of 88% over the first three years (Figure 3). The reduction appears consistent for storms similar to the combined sewer overflow design standards.
Flow-through test planters were constructed in 2004 adjacent to the BES Water Pollution Control Lab parking lot. The flow-through planter is a vegetated, lined facility that collects infiltrated water in an underdrain for discharge to the surface or sewer system. Long recognized as a water quality filter, information on their ability to reduce peak flows and retain flow volume was desired. The four planters are each 120 ft2, enough to handle 2,000 ft2 of drainage area. They are configured to assess the impact of several design variables: 1) different soil mixtures; 2) the effect of shape (long and skinny versus short and wide); and 3) the use of filter fabric or a pea gravel diaphragm to separate soil from the perforated pipe underdrain system. Inflow is controlled through use of a flow meter connected to a nearby irrigation system, and outflow is measured using v-notch flumes. Monitoring began in Summer 2005, with the simulation of basement sewer backup and combined sewer overflow events. Peak flow reduction has been excellent, exceeding 85%, and is generally consistent across all planter configurations. Flow volume reduction has varied between 25 and 50% depending upon configuration and antecedent conditions. While it is too soon to identify which configuration works best, all appear to provide significant benefit.
LID facilities appear to be very useful options for managing runoff peak flow and volume. Monitoring will continue to improve the accuracy of the results and to track changes in results over time.
Document Type: Research Article
Publication date: January 1, 2007
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