Innovative Stormwater Master Planning for the Town of Cary

Transcription

1 Innovative Stormwater Master Planning for the Town of Cary Charles Brown, CPESC, CFM Stormwater Program Analyst, Town of Cary Scott Brookhart, PE, CFM Michael Baker Engineering, Inc. Todd St. John, PE, CFM Kimley-Horn & Associates, NC NCAPWA Stormwater Conference September 16-17, 17, 2013

2 Introduction and Background Location SW Wake/NE Chatham County Population 148,000 +/-

3 Stormwater Master Plan (SWMP) Objectives Summarize the Town s s existing stormwater management program Investigate key components of stormwater as it relates to the Town Identify future stormwater management opportunities Establish and define tools that advance the stormwater program Provide a foundation for the future

4 Components of Stormwater Master Plan Introduction & Background Define Existing Stormwater Program Water Quantity Water Quality Toolbox Findings & Opportunities Economic Analysis & Funding

5 Flood Risk Assessment Three Non-FEMA streams assessed - Coles Branch, Swift Creek Trib,, and Walnut Creek

6 LIDAR Acquisition/FFE Determination Mobile LiDAR acquired on 700 Structures in town center area around floodplains

7 Cary Risk Assessment Floodprone Structures Townwide 743 buildings touching or within 500-year boundary 178 buildings identified within the Town Center study area Information collected from GIS on structures: Tax parcel data Ground LiDAR used to determine LAG and HAG, FFE from Mobile LiDAR,, Photo Land Classification, building value

8 Cary Risk Assessment Sample Score A single-family residential house with typical property improvements meets the following characteristics: experiences moderate finished floor flooding starting in the 25-yr storm event, but never more than 2 feet in any of the larger defined storm events (i.e. 50- and 100-yr events) is completely surrounded by the mapped 100-yr floodplain experiences moderate flooding at the lowest portion of the building starting at the 10-yr storm event, and starts to experience major (i.e. => 3 ) flooding at the building in the 100-yr event is not a critical facility is not located in a mapped floodway has drainage requests associated with flooding Label Condition Base Points Min Qualifying Storm Event Score A Finished Floor (FF) Flooding - Moderate % (25-yr) 140 B Finished Floor (FF) Flooding - Major 2500 N/A 0 C Flooding Surrounds Building % (10-yr) 250 D Flooding at Building - Moderate % (10-yr) 50 E Flooding at Building - Major % (100-yr) 10 Raw Score 450 Label Condition Min Qualifying Storm Event Score F1 Critical Facility or High-Occupancy Facility N 0% F2 Located in Floodway N 0% F3 Has Previous Documented Flooding Y 3% Significant Property Improvements F4 Flooding N 0% Adjustment% 3% Adjusted Score 464

9 Cary Risk Assessment - Results Risk Score No. of Primary Buildings Percent of Total (by No. of Buildings) > % % % % % % % %

10 Cary Risk Assessment - Advantages Moderately Cost Effective Covers areas outside FEMA SFHA s Refined analysis over the in or out Provides a basic ranking tool that can be repeated easily Flexible, other pieces can be added to risk factors Stays simple and allows for individual property assessment Sets the stage for outreach and mitigation

11 Stormwater Management BMP Toolbox Why create a Town-specific Toolbox? Complex regulatory environment (multiple watersheds) Provides Staff in all departments with decision making resource for selecting BMPs during the planning process Assists Staff in educating the community

12 Stormwater Wetland DESCRIPTION Stormwater Wetlands are engineered and constructed wetlands that have similar functions and processes of natural wetlands for treating stormwater. Stormwater Wetlands use physical, chemical, and biological processes to provide pollutant and nutrient removal to stormwater runoff. TYPICAL USERS POLLUTANT REMOVAL STORMWATER BENEFITS Total Suspended Solids 85% Peak Runoff Attenuation Yes Metals (Cadmium, Copper, Zinc) Medium Runoff Volume Reduction Yes Pathogens (Coliform, Streptococci) Medium Rainwater Harvesting No Oil, Grease, and Hydrocarbons Medium Infiltration No NEUSE NUTRIENT PERCENT REMOVAL SECONDARY BENEFITS Total Nitrogen (TN) 40% Aesthetic Potential High Total Phosphorus (TP) 35% Dual Use Low JORDAN LAKE NUTRIENT EFFLUENT CONC. Natural Habitat Function High Volume Reduction Percent 120% Natural Appearance High TN Effluent Concentration (mg/l) 1.08 Groundwater Recharge Medium TP Effluent Concentration (mg/l) 0.12 Temperature Reduction Medium WATERSHED IMPLEMENTATION BENEFITS IMPLEMENTATION CONSIDERATIONS Neuse River Basin Land Required High Pollutant Removal Nutrient Removal Water Quantity Reduction Size of Treatable Drainage Area Large Relative Cost Medium High High High Maintenance Burden Medium Swift Creek Watershed (Neuse Sub-watershed) Constructability Issues Medium Pollutant Nutrient Water Quantity Public Acceptance Medium Removal Removal Reduction Public Safety Concerns Medium High High High Groundwater Proximity Required High Jordan Lake Watershed Groundwater Avoidance Required Low Pollutant Nutrient Water Quantity Retrofit Opportunity Low Removal Removal Reduction Residential/Subdivision Use High High High High Urban/High Density Development Low BMP Toolbox Descriptions Typical users Pollutant / nutrient removal Watershed implementation benefits Primary and secondary benefits Implementation considerations

13 Typical Uses for the BMP in the Town of Cary

14 Findings and Opportunities Based assessments on current regulations and what is being promulgated Improvement of Water Quality and Quantity in face of development and redevelopment Infrastructure existing and future Provide as much background and information for informed decision making on LOS.

15 Watershed Regulations and Requirements

16 Stormwater Infrastructure

17 Summary of LOS options

18 Conclusions Meets the needs of the Town Both Technical and Non-Technical Provides guidance for next years Policy Initiation Document