WEFTEC.06. ** City of Caldwell, Idaho

Transcription

1 COST-BENEFIT ANALYSIS OF URBAN STORMWATER RETROFITS AND STREAM DAYLIGHTING USING LOW IMPACT DEVELOPMENT TECHNOLOGIES Sherrill Doran*, Dennis Cannon** * CH2M HILL, 322 East Front Street, Suite 200 Boise, Idaho ** City of Caldwell, Idaho ABSTRACT This project incorporates Low Impact Development (LID) concepts into the City of Caldwell (Idaho) Indian Creek daylighting and downtown redevelopment project, which is a prime candidate for a showcase project producing benefits in urban runoff within the lower Boise River watershed. The primary goal of this project was to incorporate LID concepts into the Indian Creek daylighting project. Indian Creek is an excellent focus stream because it is a high priority sub-watershed to the lower Boise River, both of which are 303(d) listed for sediments, bacteria, and phosphorus. The project recognized that the existing collection systems could be upgraded to mitigate the discharge of untreated stormwater into a highly-visible, restored stream. The LIFE model was used to determine a cost-benefit ratio. LIFE is a physically-based, continuous simulation tool that represents the stormwater and runoff processes that occur within bioretention facilities, vegetated swales, green roofs, infiltration devices, and other LID controls. On a site-specific basis, LIFE predicted increased improvements due to the LID stormwater design as compared to tradition upstream technologies. From a cost-benefit perspective, traditional stormwater controls have a cost of $8,500, with a removal efficiency of 5% (the resulting cost-benefit ratio is $1,700 per % of load removal). In contrast, the LID technologies cost more ($20,648), but the removal efficiency is much higher (32%) for a resulting cost-benefit ratio of $645 per % of load removal. On a cost-benefit basis, the improvements in LID certainly suggest a better return on investment. KEYWORDS urban stormwater, low impact development, retrofit, cost-benefit, daylighting THE PROBLEM This project incorporates Low Impact Development (LID) concepts into the City of Caldwell (Idaho) Indian Creek daylighting and downtown redevelopment project, which is a prime candidate for a showcase project producing benefits in urban runoff within the lower Boise River watershed. Indian Creek meanders through farmland, residential, and industrial areas before it enters downtown Caldwell, Idaho, where it joins the lower Boise River. As Caldwell grew, it buried Indian Creek beneath asphalt and concrete. The daylighting project provides important habitat improvement along Indian Creek and is one component of a larger effort aimed at creating an attractive core area for the revitalization of downtown Caldwell. This work builds on existing efforts in the Indian Creek watershed, including a 2002 redevelopment design charette and an existing Urban Ecology Design Manual for the Lower Boise River. 3833

2 GOALS AND OBJECTIVES The primary goal of this project was to incorporate LID concepts into the Indian Creek daylighting project. Indian Creek is an excellent focus stream because it is a high priority subwatershed to the lower Boise River, both of which are 303(d) listed for sediments, bacteria, and phosphorus. The project recognized that the existing collection systems could be upgraded to mitigate the discharge of untreated stormwater into a highly-visible, restored stream. A secondary goal of the project was to develop an LID handbook that can be distributed to developers looking to build in the redeveloping downtown area. PROJECT ISSUES Prior to the demonstration daylighting project, the initial phase of work focused on developing the LID handbook specific to Idaho s arid climate. In addition, priority sites within the downtown area were identified and screened against three initial criteria: Dovetails with larger re-alignment of Indian Creek by Corps of Engineers Applicable to identifiable downtown redevelopment projects Retrofits existing traditional urban stormwater system. Final consideration for selected sites was a favorable cost-benefit ratio as predicted by the LIFE model. LIFE is a physically-based, continuous simulation tool that represents the stormwater and runoff processes that occur within bioretention facilities, vegetated swales, green roofs, infiltration devices, and other LID controls. LIFE also accounts for runoff generated from all categories of land cover, including roadways, landscaping, and buildings over a variety of land uses and soil types, for new development and re-development. The model was used to estimate improvements in stormwater runoff quality resulting from application of LID technologies versus traditional stormwater control methods. Basins that drain to critical pinch points in the existing creek were delineated based on historic storm sewer mapping, ground-truthed with recent field visits. In addition, 70.4 acres in downtown core were delineated by land use. Results of this land use characterization are as follows: Impervious 75% Sidewalk 26% Parking 22% Roof 16% Road 11% Pervious 25% This information, as well as localized data on rainfall, runoff coefficients, and soil properties, was input into the model. Results of the modeling predicted the following runoff improvements: If all sidewalks were converted to permeable pavers, annual stormwater flows would be reduced by 13.2%. If half of all parking areas were retrofitted to include bioretention cells, annual stormwater flows would be reduced by 31.6%. 3834

3 If all roofs were retrofitted to discharge rainwater into underground swales, annual stormwater runoff would be reduced by 16.3%. If all roofs were also converted to green roofs, then an additional reduction of 0.7% of annual stormwater flows could be achieved. These reductions in flows would correspond to a reduction in sediment discharge of 6,110 lbs year (34% reduction versus traditional stormwater controls). These reductions in flows would correspond to a reduction in phosphorus discharge of 34 lbs year (33% reduction versus traditional stormwater controls). THE SOLUTION As the LIFE model was being developed for the larger downtown core, the Indian Creek daylighting project emerged as an excellent opportunity for a demonstration project. The demonstration site met all three criteria identified previously. LID components that were integrated into the design included: Permeable pavers along sidewalks, walking paths, and viewing platforms Bioretention swale for stormwater management Shoreline treatment wetlands Native plant vegetation for converted permeable areas On a site-specific basis, LIFE predicted increased improvements due to the LID stormwater design as compared to tradition upstream technologies (Exhibit 1). EXHIBIT 1. Summary of Load Removal Predictions at Demonstration Site Sediment Phosphorus Traditional Upstream Technologies 586 lbs/year 1.9 lbs/year 5% 5% LID Technologies 2,400 lbs/year 14 lbs/year 32% 30% From a cost-benefit perspective, traditional stormwater controls have a cost of $8,500, with a removal efficiency of 5% (the resulting cost-benefit ratio is $1,700 per % of load removal). In contrast, the LID technologies cost more ($20,648), but the removal efficiency is much higher (32%) for a resulting cost-benefit ratio of $645 per % of load removal. On a cost-benefit basis, the improvements in LID certainly suggest a better return on investment. Construction on the project is complete. Before and after photographs are presented in Exhibits 2 and

4 EXHIBIT 2. Pre-Construction Baseline Conditions. 3836

5 EXHIBIT 3. Post-Construction Restored Conditions. 3837