State of Stormwater Management Practice in Florida Inter-American Development Bank User Charge Based Funding Mechanisms for Stormwater Management James D.
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State of Stormwater Management Practice in Florida Inter-American Development Bank User Charge Based Funding Mechanisms for Stormwater Management James D. Hunt, P.E. City of Orlando, Florida Division Manager, Streets & Stormwater Services Division The Stormwater Balancing Act cost of stormwater improvements probability of damage occurring Stormwater Design Hierarchy 1. 2. 3. 4. 5. 6. 7. Road cross-section: Catch basin capacity: Storm sewer design: Minimum road elev: Pond design: Building elevs: Landlocked ponds: 3-yr storm 5-yr storm 10-yr, 6-hr storm 25-yr, 24-hr storm 25-yr, 24-hr storm 100-yr, 72-hr storm 25-yr, 96-hr storm Evolution of Stormwater Management • • • • • • • Flood protection (rate controlled) Flood protection (volume controlled) Pollution abatement Wetland preservation & enhancement Wetland creation Groundwater issues Re-use What is a Best Management Practice (BMP)? A cultural or engineering technique, or management strategy, that has been determined and accepted to be an effective and practical means of preventing or reducing non-point source pollution in a local area. Rainfall Probability Histogram % STORMS WITH VOLUME WITHIN STATED LIMITS 90% 80% 76.50% 70% 60% 50% 40% 30% 20% 12.90% 10% 5.50% 2.20% 1.10% 0.90% 0.40% 0.20% 2 2.5 3 3.5 4 0% 0.5 1 1.5 RAINFALL VOLUME (INCHES /EVENT) Origins of Pollution Abatement Criteria • 90% of all rainfall events are less than one inch • 80% of rainfall volume is received from events less than one inch in depth • assumes each event completely removes pollutants from drainage catchment and delivers them for treatment Stormwater Treatment Train Low impact development Roadside swales Inlet filters Baffle boxes End-of-pipe treatment Dry pre-treatment ponds Wet detention ponds Master (regional) facility Wetland filter Close to source Regional system Low Impact Development • • • • • • Control occurs closer to source Reduce maintenance obligations Still need educational outreach Still need enforcement Contrary to compact development? Cheaper to prevent pollution from entering receiving waters than to remove pollutants from receiving waters LID Design Features • • • • • • Vegetative cover Green roofs Rain gardens Roadside swales Disconnected impervious area Stormwater reuse Retention Ponds • Typically dry (may be wet) • Designed to store a specific amount of runoff (usually from the first inch of rainfall in the drainage area) • Will usually return to dry state or designed water elevation through percolation and evaporation in 72 hours Dry Retention Pond Dry Retention Pond Detention Ponds • Designed to fill up with runoff and then allow the water to pass through the pond at a slow, controlled rate by way of an outfall structure to the receiving water body Dry Detention Pond Dry Detention Pond Underdrains • Perforated pipe surrounded with filter fabric material and a specific medium (sand or gravel) for percolation • Transports groundwater and percolated stormwater into perforated pipe that discharges into a control structure • Typically installed in poor draining soils and wet areas • Designed to keep ponds dry or at a certain elevation Underdrains Underdrains Wet Detention Pond Design • • • • • • Greater treatment volume Slow volume recovery Littoral shelf with wetland plants Limited open water Mean depth three to ten feet Separation of inlets and outlets for biological contact Wet Detention Pond Wet Detention – How Not to Do It Wet Detention – Site Constraints Considered Control Box/Structure Exfiltration • Temporarily hold water in perforated pipes • Allow water to percolate and filter through surrounding soils • Typically installed in parking lots, under garages and other buildings (access for maintenance) • Not appropriate for poorly drained soils or high groundwater conditions • Heavy reliance in a concentrated area can alter groundwater conditions and cause damage Exfiltration System Exfiltration Why Retrofit? • It may be the only way to achieve further water quality improvements • It may be the only cost-effective way to recover diminished capacity in existing stormwater conveyance systems • It’s the right thing to do! Rainfall Probability Histogram % STORMS WITH VOLUME WITHIN STATED LIMITS 90% 80% 76.50% 70% 60% 50% 40% 30% 20% 12.90% 10% 5.50% 2.20% 1.10% 0.90% 0.40% 0.20% 2 2.5 3 3.5 4 0% 0.5 1 1.5 RAINFALL VOLUME (INCHES /EVENT) Example #1 • treat runoff from 1 inch of rainfall over project area (100 acres) • 1,000 lbs of pollutants generated • 894 lbs of pollutants are removed • 106 lbs are not removed • an adjacent identical 100-acre tract generates an additional 1,000 lbs of pollutants, for a total of 1,106 lbs Example #2 • • • • treat half the runoff over twice the area 2,000 lbs of pollutants generated 1,530 lbs of pollutants are removed 470 lbs are not removed Technology Transfer Lake Rowena Screening Facility Leu Gardens Corrine Driv e • Basin 1 area – 539 acres Colonial Driv e Concord Street Bumby Ave Virginia Driv e Hampton Ave Mills Ave Lake Rowena Sub Basin 1 • Pipe Length – 3.45 miles • 75% of pollution to lake is from this one area Colonial Plaza Mall Fest i val Par k Periphyton Water Garden Regional Stormwater Management Systems When a Regional Approach is Best • • • • Multiple use objectives Multi-use can mean multi-funding Economies of scale Improve the chance of success Southeast Lakes Basin Characteristics • • • • • • • Intensely developed, 3.5 sq mile area Lowest elevation 58 feet Much of the basin 65-75 feet Lowest point on basin rim 103 feet 18 lakes Over 60 drainage wells No outfall to any surface receiving water Southeast Lakes Basin Strategies to Solve Flooding • Gravity diversion of stormwater away from basin rim • Creation of additional impoundments • Increased storage through lowered regulation schedules • Transfer of surface water from lakes that flood to those requiring augmentation Greenwood Urban Wetland • • • • Over 75 homes flooded for 3 weeks in 1960 2-acre lake expanded to 13 acres New parkland created Stormwater re-use employed for irrigating park as well as adjacent cemetery Greenwood Urban Wetland Karst Geology in the Southeast Lakes Basin Lake Lancaster Augmentation Well • Cannot be used if lake is above 68 feet above sea level • Must be turned off once lake reaches 69 feet above sea level • Limited to 114,000,000 gallons per year • Use is prohibited during water emergencies • Has more nutrients than surface water Lake Lurna-Lake Lancaster Interconnection • • • • • Protects Lake Lurna from flooding Moves excess water to Lake Lancaster for augmentation Is a cleaner source than groundwater Mimics natural processes in a landlocked basin Postpones or eliminates the need for more drainage wells or surface outfall from a large, intensely developed landlocked basin • No guarantee that there will always be enough water for augmentation! Use of Re-Use Water (Highly Treated Effluent) • Regulatory concerns about impact to surface waters • Other, greater demands for a new resource that cannot meet all demands • The good news: we will be drinking our own wastewater in the future. • The bad news: there won’t be enough to go around! Lake Fran Area, 1890 Lake Fran Area, 1965 100-Year Flood Plain, Prior to Lake Fran 100-Flood Plain, After Construction Lake Fran Flood Control Project • Nearly 2,000 homes removed from 100-year flood plain • 47 acres of wetland preservation • 38 acres of wetland creation • Another 200 acres of wetland mitigation for nearby road project • New school site • Future environmental center • Hub of new park and trail system Flood Control Impoundments • Potential for catastrophic failure • Higher level of protection – Greater return frequency – Longer storm duration • Freeboard (factor of safety) • Plan for controlled failure – Berm elevations set to direct flows that exceed design – Erodible plugs Regional Stormwater Management Lessons Learned • • • • • • Science and engineering – not politics must lead design development Proper location (mimic nature) Apply sufficient resources (land, money, manpower) Ease of maintenance increases chance of success Still the most cost-effective way to recover capacity in an over-capacity system Can be an engine of redevelopment and revitalization Don’t forget the little things – like pedestrian circulation Future Challenges • Numeric Nutrient Criteria • State-wide Stormwater Rule • Water Body Classification & Use Determination Lake Baldwin Outfall Canal Questions?