Stormwater Bio-Infiltration Pilot Project Concept Proposal Submitted by

Transcription

1 Concept Proposal Submitted by Public Works Department 1947 Center Street, Fourth Floor Berkeley, CA December 17, 2009

2 Executive Summary The s Public Works Department submits this concept proposal to design, implement, and maintain its Stormwater Bio-Infiltration Pilot Program (SBIPP) to the California Department of Transportation, District 4 (Caltrans). The goal of this pilot program is to improve the quality of stormwater discharged to local waterways and San Francisco Bay. This will be achieved by implementing, within the City right-of-way, a suite of Low Impact Development (LID) techniques designed to meet full trash capture performance goals, treat stormwater run-off, and reduce run-off volumes by using landscape-based improvements to optimize infiltration and evapotranspiration. Many new development projects are incorporating elements of LID and Green Infrastructure into their design standards and guidelines as required by NPDES permits issued to Municipal Separate Storm Sewer Systems (MS4); however, a comprehensive application of LID to an existing urban setting has not been done. The SBIPP is a comprehensive LID retrofit of a City right-of-way in a typical urban neighborhood featuring residential and collector street traffic loads, designated bicycle routes, bus routes, and existing underground utilities. The proposed project covers 18 city blocks with a drainage area of about 90 acres, as shown in Figure 1. The project is designed to be scalable in two senses: 1) the individual LID components can be selected and implemented in buffet-style combinations as existing conditions prescribe to reduce peak run-off volume, peak flows, and urban runoff pollutant loads, and 2) depending on funding available, the project area may be divvied into thirds and implemented in six block increments. LID techniques planned include: Replacing concrete sidewalks with permeable materials, Installing bio-swales within the existing planter-strip area of sidewalks,

3 Reducing impermeable pavement by bumping out the curb-line for rain gardens, Converting medians and traffic circles to vegetated bio-filtration areas, Replacing impermeable asphalt streets with permeable surfacing, Installing underground stormwater retention cisterns, Using open-graded gravels and amended soils as subsurface media for storage and treatment, and Metering outflow of subdrains and underground stormwater storage areas to reduce peak discharges to downstream areas. The SBIPP will achieve water quality improvements by processing all the runoff from the project area through full trash capture measures with a goal to bioinfiltrate up to the 10-Year storm. Additionally, the SBIPP will demonstrate the extent to which an existing urban area can be retrofitted to meet the goals of Municipal Regional Permit s (MRP) Hydromodification Management (HM) requirements. The cost estimate for the design and implementation of the SBIPP ranges from approximately $15.8 million to $22.8 million, with long-term maintenance costs yet to be determined. The upper end of the cost estimate range assumes the need to relocate underground utilities to enhance water quality and HM performance. If this conceptual proposal is received favorably, Public Works staff will formally begin the City Council approval and public outreach processes. Problem Statement The successful implementation of the Federal Clean Water Act promulgated in 1972 significantly curtailed point source water pollution through regulation of industrial discharges. However, the contribution of contaminants from urban runoff remains a challenge to protecting water quality. According to the 2006 Clean Water Act 303(d) List of Water Quality Limited Segments, sediments polluted by mercury and polycyclic aromatic hydrocarbons impair the Central San Francisco Bay. Further, according to the San Francisco Regional Water Quality Control

4 Board s 2009 Staff Report, Evaluation of Water Quality Conditions for the San Francisco Bay Region the Central San Francisco Bay shoreline and two creeks in Berkeley by are deemed impaired by trash. The City understands that Caltrans is party to a Memorandum of Understanding (MOU) with the San Francisco Bay Regional Water Quality Control Board (Water Board) and with the Association of Bay Area Governments (ABAG) that credits Caltrans with satisfying its Stormwater Permit requirements by funding the construction and maintenance of off-site alternative stormwater treatment by third parties (signed August 2007). This off-site alternative compliance option is acceptable in instances where Caltrans is unable to implement these best management practices (BMPs) within its own right-of-way. The SBIPP is designed to meet the goals of the MOU, where Caltrans funds design, implementation, and maintenance, while the City provides the land. Through this cooperative arrangement, the SBIPP will pioneer and serve as a model for the next generation of comprehensive stormwater management in the Bay Area. Goals and Objectives The goals and objectives of the Stormwater Bio-Infiltration Pilot Project are to: 1. Provide stormwater treatment outside the Caltrans right-of-way that can be counted towards Caltrans Stormwater Permit requirements with the State Water Quality Control Board. 2. Implement a pilot stormwater pollutant reduction project that tests LID techniques in a highly urban area and can be a model for other agencies in the Bay Area and State. 3. Reduce peak and total stormwater run-off volumes through infiltration, evapotranspiration, and temporary storage. 4. Examine the real world buildability issues associated with implementing LID options in existing urban settings.

5 These goals and objectives will be accomplished by designing and implementing, within the 90-acre drainage area, a suite of LID approaches within the City rightof-way (streets and sidewalks) sized to infiltrate stormwater generated by up to the 10-year storm. As a pilot project, the City will develop the specifications and details for each LID measure as a stand-alone component. This modular approach will allow the overall project designs to be expandable as needed to achieve the desired water quality improvements. Site Description The Stormwater Bio-Infiltration Pilot Project will be located within an 18-block, 90- acre drainage area bounded by Allston Street to the north, Dwight Way to the south, Grant Street to the east, and Sacramento Avenue to the west, see Figure 1. The proposed project area is isolated from stormwater run-on by the arrangement of stormdrain pipes and drainage patterns surrounding it. This is essential both for: 1) sizing the proposed LID retrofits for maximum stormwater treatment, as well as 2) developing monitoring programs to quantify project success. The proposed project area is part of the 3.7 square-mile Potter Watershed (the largest in the City), drained almost entirely by stormdrain infrastructure, which primarily discharges to the San Francisco Bay. Runoff from the proposed project area can discharge into the Aquatic Park lagoons (located at the southwestern side of the City) during combinations of major rainfall events and high tides. Thus water quality improvements would benefit both the San Francisco Bay and local water bodies. Within the project area, the SBIPP will apply LID retrofits to two high traffic volume streets (Dwight and Sacramento) in addition to the eight residential streets (Grant, Roosevelt, McGee, Jefferson, California, Spaulding, Bancroft, and Channing), see Figure 2. If funding amounts limit the scope of the SBIPP, the initial project area can be reduced to the six block area bounded by Allston Street

6 to the north, Dwight Way to the south, California Street to the east, and Sacramento Avenue to the west. This smaller project area would still include two high volume collector streets, designated bicycle routes, bus routes, and underground utilities. Project Description The SBIPP will design, install, and maintain an array of LID applications (see Conceptual schematic, Figure 3) within the City right-of way (back of sidewalk to back of sidewalk) to treat and reduce stormwater run-off. The pilot program will apply a modular approach to determine which combinations of LID components are necessary (and feasible) within each segment of a typical existing city right-of way to meet overall urban runoff quality and quantity requirements applicable to new developments of similar density. LID techniques planned for design and implementation for the SBIPP include: Replacing impermeable concrete sidewalks with permeable materials, Installing bio-swales within the existing planter-strip area of sidewalks, Reducing impermeable pavement with rain gardens along curb bumpouts, Converting medians and traffic circles to vegetated infiltration basins, Installing underground stormwater cisterns, and Partially or totally replacing asphalt streets with permeable surfacing. Vegetated swales, basins, and rain gardens will promote both increased infiltration and evapotranspiration. Underneath the proposed permeable surface layers, the City will install open graded gravels, structural soils, and amended soils to provide treatment and temporary storage of infiltrated stormwater. Berkeley s notoriously clayey native soil types will most likely require sub-drains plumbed to existing downslope stormdrain infrastructure to ensure proper drainage. The above LID techniques will facilitate the following results:

7 Full Trash Capture: With the proposed permeable surfacing of some or all of the sidewalks, curbs & gutters, and streets all trash and litter mobilized by stormwater run-off will be left at the surface as stormwater infiltrates through the various media. The trash and coarse debris would then be readily collected by existing street sweeping and litter operations. Stormwater Treatment: Proposed bio-infiltration swales, rain gardens, and basins will use amended soils, structural soils, and mulches in various combinations as infiltration media. Bio-Infiltration facilities have been found to remove 80 percent or more of the total suspended solids from stormwater and therefore a high percentage of the metals, organochlorines, nutrients, pesticides, bacteria, oil and gas, and other contaminants that are attached to sediment. Underneath permeable surfaces (i.e., sidewalks, curbs & gutters and roadways), open graded gravels will be installed to capture and detain stormwater, capture suspended solids, and maximize infiltration opportunities. Preliminary analysis shows, on a per block basis, the bio-infiltration area needed to treat 0.5 cubic feet per second (cfs) of run-off flow from the.2 inch/hour intensity design storm is approximately 6,100 square feet, see Table 1. This area is available within the City s right-of-way. Peak Flow Reductions: The SBIPP will test the feasibility of LID approaches to reduce peak flow or unwind the hydrograph impacts of existing development. The goal of the test will be to apply new development hydromodification management (HM) goals to the project s existing urban setting. The Bay Area Hydrology Model (BAHM) was used to calculate the needed storage to achieve the HM goals. Our preliminary analyses show that retrofitting just 20% of the existing right-of-way can reduce the runoff from the 2-year storm event from 2.0 cfs to 0.8 cfs, which is a 60% reduction, see Figure 5. Retrofitting 50% of the right-of-way can achieve full HM goals. Thus, significant peak flow reductions should be achievable without fully retrofitting an entire existing neighborhood.

8 Evaluation The City will work with the Alameda Countywide Clean Water Program s Monitoring and Special Studies Program to develop and seek funding for a monitoring program to assess reductions in runoff rate and volume as well as water quality improvements resulting from the pilot project. The water quality monitoring will focus on particular pollutants found in, or expected to be associated with, runoff from this catchment. Potential parameters to be assessed include copper, lead, zinc, mercury, and trash. Balance Hydrologics will assist in the development of a flow-monitoring program to measure the infiltration rates achieved by the LID applications. This proposed project site is particularly well suited for quantifying results, as it is protected from stormwater-run-on from outside areas by the existing upland stormdrain infrastructure. Additionally, similar sized and land-use catchments adjacent to the project site can serve as control sites. Water quality and flow data on stormwater entering stormdrain pipe inlets on Sacramento and Grant will be collected prior to and post project implementation to quantify the level of treatment achieved. Estimated Costs Caltrans funding will be used for costs associated with SBIPP design, construction, and long-term maintenance activities only. The City will use its own resources and seek additional funding from other sources to develop and implement the monitoring plan. The estimated cost to fully design and implement the SBIPP over the18 block, 90 acre study area (approximately 540 single family equivalent units) ranges from approximately $15.8 million to $22.8 million. These costs are summarized in Table 2. The SBIPP will demonstrate that an existing urban area can fully comply with Provision C3 requirements for new development. The range in costs reflects potential utility relocations to facilitate and expand opportunities for subsurface run-off treatment, storage, and infiltration measures. The need for utility relocation will remain a project unknown until the design phase has begun. Maintenance costs are yet to be

9 determined. Tasks and Potential Timeline In preparing this proposal, the City worked with Balance Hydrologics to examine the feasibility and buildability of the SBIPP, and prepare cost estimates for implementing the SBIPP. This initial examination shows that the SBIPP concept is feasible. If funded, Balance Hydrologics will continue to work on this pilot project to provide additional hydraulic and hydrologic modeling, development of construction documents and specifications, and development of a pre- and postproject monitoring program. We anticipate this project will follow the following time line: Year One Perform Geotechnical Study Perform Hydraulic Study Refine Conceptual Plan Conduct Public Outreach Coordinate Utilities, Traffic, etc Year Two Continue Public Outreach Develop Construction Docs & Specs Develop Monitoring Plan Develop Maintenance Plan Conduct Pre-Project monitoring Bid Project Year Three Issue Notice to Proceed to Contractor(s) Construct project (in phases as necessary) Year Four Continued Implementation (if needed). Initiate Post-Project Monitoring Plan Initiate Maintenance Plan Year Five Continued Monitoring Continued Maintenance

10 Submit Final Reports Year Six and Beyond Continued Maintenance Continued Monitoring (as needed and as funding is available)