Stormwater Retrofitting: The Art of Opportunity. Presented by the Center for Watershed Protection

Stormwater Retrofitting: The Art of Opportunity Presented by the Center for Watershed Protection

What Are Stormwater Retrofits? Retrofits are stormwater management measures inserted in an urban or ultra-urban urban landscape where little or no prior stormwater controls existed.

Prerequisites Creativity Basic knowledge of common stormwater management practices Local stormwater criteria Some hydrology and hydraulics background is beneficial but not mandatory for identifying candidate sites

What are Typical Goals of Stormwater Retrofitting?

Enhance aquatic habitat & Minimize accelerated channel erosion

Improve water quality

Reduce flood peaks and volumes

Provide groundwater recharge Source: Martin Covington, P.E.

Source: Martin Covington, P.E. Educate the public

Correct past mistakes

The Challenge is to Think Outside the Box

Classification of Retrofitting Techniques Offsite Storage Onsite Nonresidential Onsite Residential

Typical Locations for Storage Retrofits: Culverts

Typical Locations for Storage Retrofits: Storm Drain Outfalls

Typical Locations for Storage Retrofits: Large Parking Lots

Offsite Storage Locations and Subwatershed Objectives Subwatershed Objective Location Water Channel Flood Quality Recharge Protection Control Existing detention/retention facilities Above roadway culverts Below outfalls at the terminus of a drainage network Within urban conveyance system* Public streets and highway right-of-ways Large parking lots (> 5 ac) Notes always Sometimes Rarely * Practices within urban conveyance can be off-line or on-line. In off-line scenarios, some portion of wet weather flow is diverted to the facility. On-line scenarios may rarely occur in highly altered systems (e.g., channelized, lined, perennial streams) and would manage a full range of wet weather events. Photo Copyright 1999, Center for Watershed Protection

Typical Locations for Nonresidential Retrofits: Roofs/Plazas/Courtyards

Typical Locations for Nonresidential Retrofits: Roofs

Typical Locations for Nonresidential Retrofits: Parking Lots

Typical Locations for Nonresidential Retrofits: Streets

Onsite Nonresidential Locations and Subwatershed Objectives Subwatershed Objective Location Water Channel Flood Quality Recharge Protection Control Parking lots (2-5 ac) Rooftops Private streets Notes always Sometimes Rarely Photo Copyright 1999, Center for Watershed Protection

Yards Drives Roofs Typical Locations for Residential Retrofits:

Onsite Residential Locations and Subwatershed Objectives Subwatershed Objective Location Water Channel Flood Quality Recharge Protection Control Rooftops Yard Driveway Notes always Sometimes Rarely Photo Copyright 1999, Center for Watershed Protection

Subwatershed Factors to Consider in Retrofitting Subwatershed IC (current and future) Land use and ownership Use designation of receiving water Presence of existing stormwater management Pollutants of concern Drinking water supply (volume and quality) Small watershed org./hoa presence (onsite) Channel erosion and stream restoration goals Physiographic features (e.g., steep slopes) Climate considerations CSOs

Basic Elements of a Stormwater Retrofitting Implementation Strategy Step Element Purpose 1. 2. 3. 4. Preliminary Watershed Retrofit Inventory Field Assessment of Potential Retrofit Sites Prioritize Sites for Implementation Public Involvement Process Identify potential retrofit sites Verify that sites are feasible and appropriate, produce concept designs. Set up a priority for implementing future sites Solicit comments and input from the public and adjacent residents on potential sites 5. Retrofit Design Prepare construction drawings for specific facilities 6. Permitting 7. Construction Inspections Obtain the necessary approvals and permits for specific facilities Ensure that facilities are constructed properly in accordance with the design plans 8. Maintenance Plan Ensure that facilities are adequately maintained Note: Steps 5-7 will be limited for onsite residential and Step 6 will be limited for onsite nonresidential

Step 1: Watershed Retrofit Inventory Define initial guidelines - existing facilities receiving runoff from at least X acres and outfalls of at least X inches in diameter,, WQv and Cpv targets. Identify feasible and appropriate sites as an office exercise, using available mapping When available, use data from USA and USSR Delineate and quantify drainage area to each site Compute a Target Volume and measure the site s available area

Useful Materials Aerial Photographs Drainage Maps Gas, Water, and Sewer Maps Zoning/Tax Assessor Maps Topographic Maps

Large Parking Lot

Highway Right-of of-way

Upstream of Existing Road Culvert

Sizing Rules of Thumb Water Quality Where: V t = P/12 * IC/100 * DA V t = Target storage volume (acre feet) P = Target rainfall depth (inches) IC = Impervious Cover (%) DA = Drainage Area (acres) 12 = conversion factor (inches to feet)

Sizing Rules of Thumb Channel Protection Where: V t = P/12 * IC/100 * DA * 0.6 V t = Target storage volume (acre feet) P = 1-year 24-hour storm depth (in) IC = Impervious Cover (%) DA = Drainage Area (acres) 12 = Conversion factor (inches to feet) 0.6 = Pond routing factor

Sizing Rules of Thumb Surface Area Table 2. Typical Surface Areas for Stormwater Management Practices Stormwater Management Practice Ponds and Wetlands Bioretention Filters Infiltration Swales Common Surface Area Rules of Thumb ~1% of Total Drainage Area ~5-9% of Impervious Area draining to facility ~1-2% of Impervious Area draining to facility ~1-2% of Impervious Area draining to facility ~6-15% of Impervious Area draining to facility

Step 2: Field Verification of Candidate Sites Complete a ground-truth to determine if the contemplated retrofit will work Prepare a conceptual sketch and take photographs Use inventory forms Look for pollution prevention opportunities in the field

Typical Reasons to Reject a Candidate Retrofit Site The parking lot identified from aerial photographs is really a roof Land use has changed since aerial photographs were taken Open space has an incompatible use Site elevations make it difficult to convey stormwater Soils are inappropriate for the proposed practice

Step 3: Prioritize Sites for Implementation Pollutant removal capability (storage provided and type of BMP) Stream channel protection capability (ability to control subbankful flow events) Cost of facility (design, construction and maintenance costs) Ability to implement the project (land ownership, construction access, permits) Potential for public benefit (education, location within a priority watershed, visible amenity, supports other pubic involvement initiatives)

Keys to Successful Retrofitting Follow a systematic and straightforward approach Take an approach geared toward innovation Practice and experience