POST-CONSTRUCTION STORMWATER MANAGEMENT FOR EROSION CONTROL PROFESSIONALS

Transcription

1 POST-CONSTRUCTION STORMWATER MANAGEMENT FOR EROSION CONTROL PROFESSIONALS Shannon Tillack, P.E., CPESC Wright Water Engineers, Inc. Why do we care about stormwater quality? Board 1

2 Recreational Uses Slide courtesy of Jim Sipes, Sand County Studios Wildlife Habitat Slide courtesy of Jim Sipes, Sand County Studios Board 2

3 Agricultural Uses Slide courtesy of Jim Sipes, Sand County Studios History of Stormwater Regulation 1960s Cuyahoga River caught fire 2/3 of U.S. waters are polluted Houston Ship Channel most polluted 1970s USEPA created Clean Water Act restructured to regulate point source discharges through NPDES permits that impose numeric effluent limitations NRDC wins lawsuit challenging USEPA s exemption of storm water discharges 1980s Clean Water Act revised to mandate regulation of storm water Board 3

4 History of Stormwater Regulation Phase I Rules 1990s 1/3 of U.S. waters polluted Houston Ship Channel most polluted Phase II Rules promulgated 2000s Phase II Rules Major push on Total Maximum Daily Loads (TMDLs) 2010s Phase III Volume-based regulations in development by EPA, focus on Low Impact Development (LID) and Green Infrastructure (GI), mimic predevelopment hydrology Numeric limits on turbidity for construction site discharges Increased frequency of numeric limits in NPDES stormwater discharge permits Phase I and II Storm Water Regulations Phase I Eleven categories of Industrial Activity Including construction disturbing > 5 acres Large and medium Municipal Separate Storm Sewer Systems (MS4s) serving a population of at least 100,000 Other sources as designated by the Permitting Authority (PA) Phase II Regulated small MS4s Small construction activities (< 5 acres, but > 1 acre) Board 4

5 Proposed National Rulemaking to Strengthen the Stormwater Program Key proposed rulemaking actions: Develop performance standards for newly developed and redeveloped sites; Explore options for expanding the protections of the MS4 program; Evaluate options for establishing and implementing a municipal program to reduce discharges from existing development; Evaluate establishing a single set of minimum measures requirements for regulated MS4s. However, industrial requirements may only apply to regulated MS4s serving populations of 100,000 or more; Explore options for establishing specific requirements for transportation facilities; Evaluating additional provisions specific to the Chesapeake Bay watershed. Summarized from: EXPANDED PROTECTIONS OF MS4 PROGRAM Board 5

6 Engineering Approach to Green Infrastructure HYDROLOGY FOR WATER QUALITY Vegetation is Nature s BMP: It slows the flow Roots hold soil together Allows water to percolate into the ground Allows water to evaporate Filters/traps sediment Pre-Development Hydrology Board 6

7 HYDROLOGY FOR WATER QUALITY Post-Development Hydrology Paving, Concrete, Impervious Surfaces: Decrease in natural vegetation (sometimes dramatic) Higher peak discharges and greater runoff volume More frequent runoff (more runoff events per year) Concentrates flows Increases potential for erosion and pollutant transport. Why are Stormwater Management and LID Important? IMPACT OF URBANIZATION ON STREAMFLOW STREAMFLOW RATE Large Storm Higher Baseflow Higher and More Rapid Higher Peak and Discharge More Rapid Peak Discharge Small Storm More Runoff Volume More Runoff Volume Lower and Less Rapid Lower Peak and Less Rapid Peak Gradual Recession Gradual Recession Pre-development Post-development TIME Source: Schuler, T.R Controlling Urban Runoff: A Practical Manual for Planning and Designing BMPs. Washington DC: Metro Council of Governments. Board 7

8 Four Step Process Board 8

9 Runoff Reduction Directly Connected Impervious Area This is what we want to avoid! Board 9

10 Runoff Reduction Minimize Directly Connected Impervious Area Direct Runoff to Landscape Board 10

11 Runoff Reduction Reduce Impervious Area Treat the Water Quality Capture Volume Board 11

12 Number of Rainfall Events in the Denver Area Total Rainfall Depth (inches) Average Annual Number of Storm Events 0.0 to to to to to to to to >5.0 0 Total 75 Maximized Water Quality Volume Board 12

13 Grass Swale MDCIA Practices (without WQCV) WQCV Facilities Design dependent Grass Buffer Wetland Channel Pervious Pavement (no storage) Wetland Basin Bioretention (Porous Landscape Detention) EDB Sand/ Media Filter Ret. Pond Pervious Pavement (with storage) Functions Pretreatment LID/Volume Reduction Best Management Practices (BMPs), Functions & Considerations Green Roof WQCV WQCV + Detention Effectiveness for Targeted Pollutants Sediment & Solids Nutrients Total Metals Bacteria Other Considerations Land Requirements Med Med Med Low High High High High High Low Low Low Capital Costs Low Low Low High High High High High High High High High Life cycle Costs Low Low Low Med Med Med Med Med Med Med Med High Underground BMPs (various types) Maintenance Frequency/ Costs Low Low Low Med Med Med Med Med Med Med Med High Primary, Secondary and Incidental Treatment Process Provided by BMPs WQCV Evapotranspiration Adsorption/ Biological BMP Infiltration Sedimentation Filtration Straining Attenuation Absorption Uptake Grass Swale I S I S S P S S Grass Buffer I S I S S P S S Constructed Wetland I N/A P P S P S P Channel Green Roof I S P N/A P N/A I P Pervious Pavement P * P N/A N/A P N/A N/A N/A System Rain Garden P P S P P S S ** P Extended Dry Detention P I I P N/A S S I Basin Sand Filter Extended P P I P P N/A S ** N/A Detention Constructed Wetland P I P P S S P P Basin Retention Pond P I P P N/A N/A P S Underground BMPs Variable N/A N/A Variable Variable Variable Variable N/A P = Primary; S = Secondary, I = Incidental; N/A = Not Applicable * If storage is provided ** Depending on media Board 13

14 Stabilize Drainageways Water Quality, Channel Protection & Flooding Board 14

15 Stream Channel Stability Lane s Stable Channel Concept The Channel Protection Storage Volume (Cpv) is the volume required for extended detention release (24-hour) of the 1-year, 24-hour storm Board 15

16 Implement Source Controls Source Control BMPs Board 16

17 Overview of Common LID/GI Practices Vegetative Swales Board 17

18 Vegetative Swales Installation & Maintenance Perform fine grading, soil amendment, and seeding only after upgradient surfaces have been stabilized and utility crossings have been completed. Avoid compaction of soils to preserve infiltration capacities. Weed the area during the establishment of vegetation by hand or mowing. Mechanical weed control is preferred over chemical treatment. Protect the swale from other construction activities. When using an underdrain, ensure no filter sock is placed on the pipe. Locating swales in prominent locations on site, as a part of landscaping, will encourage upkeep and maintenance. Provide access for mowing equipment and avoid plantings that make mowing operations overly difficult. Vegetative Buffers Board 18

19 Sheet Flow: FL x SI 1 Board 19

20 Level Spreaders Source: Vegetative Buffers Installation & Maintenance Where compatible with vehicles, install the top of the buffer 1 to 3 inches below adjacent pavement so vegetative growth does not impede flow and trap sediment at edge of pavement. Amend soils as needed for deep rooting and infiltration. Restrict vehicle access to buffers using slotted curb or similar methods. Final grade is critical allow room for sod layer below surface that is contributing runoff. Perform fine grading, soil amendment, and seeding only after upgradient surfaces have been stabilized and utility crossings have been completed. Avoid compaction of soils to preserve infiltration capacities. When using sod, stagger ends of tiles to avoid preferential flow paths at joints. Use roller to eliminate air pockets between sod and soil. Provide access for mowing. Board 20

21 Bioretention Infiltration Practices Infiltration Trenches Infiltration Basins Bioretention & Raingardens Soil Quality Restoration Native Landscaping Board 21

22 Full Infiltration Section No Infiltration Section Partial Infiltration Section Bioretention Installation & Maintenance Do not put a filter sock on the underdrain. This is not necessary and can cause the BMP to clog. The best surface cover for a rain garden is full vegetation: Do not use rock mulch within the rain garden because sediment build-up on rock mulch tends to inhibit infiltration Wood mulch floats and may clog the overflow, bury plants or flow downstream. Consider all potential maintenance requirements such as mowing (if applicable) and replacement of the growing medium. Provide pre-treatment when it will reduce the extent and frequency of maintenance. Make the rain garden as shallow as possible. Board 22

23 Bioretention Installation & Maintenance Protect area from excessive sediment loading during construction. Avoid over compaction of the area to preserve infiltration rates (for partial and full infiltration sections). Provide construction observation to ensure compliance with design specifications. Improper installation, particularly related to facility dimensions and elevations and underdrain elevations, is a common problem with rain gardens. When using an impermeable liner, ensure enough slack in the liner to allow for backfill, compaction, and settling without tearing the liner. Provide necessary QA/QC when constructing an impermeable geomembrane liner system. Provide adequate construction staking to ensure that the site properly drains into the facility, particularly with respect to surface drainage away from adjacent buildings. Permeable Pavements Board 23

24 Permeable Pavements Installation Hold a pre-construction meeting to ensure that the contactor has an understanding of how the pavement is intended to function. Discuss the contractor s proposed sequence of construction and look for activities that may require protection of the permeable pavement system. Ensure that the permeable pavement is protected from construction activities following pavement construction (e.g., landscaping operations): Covering pavement alternative construction vehicle access education for all parties working onsite. Include an observation well to monitor the drain time of the pavement system over time. Call for construction fence on the plans around pervious areas where infiltration rates need to be preserved and could be reduced by compaction from construction traffic or storage of materials. Board 24

25 Permeable Pavements Installation Subgrade preparation Involve a geotechnical engineer who understands permeable pavements! Compaction may not be needed or may only be needed in soft areas, if reservoir is excavated into sub-grade. Geogrid and compacted granular fill can be used to bridge soft areas. When final subgrade is placed fill, specify hydraulic conductivity for material to be placed at least one order of magnitude higher than native material. Low ground pressure (LGP) track equipment should be used within the pavement area to limit overcompacting the subgrade. Wheel loads should not be allowed. Filter layer compaction to a relative density between 70% and 75% (ASTM D4253 and ASTM D4254) using a walk-behind vibratory roller, vibratory plate compactor or other light compaction equipment. Specify reservoir layer compaction using method specification (i.e. type of equipment & number of passes) depending on equipment and depth of layer. Questions? For additional information and assistance, please contact Shannon Wright Water Engineers, Inc West 26 th Avenue, Suite 100A Denver, Colorado stillack@wrightwater.com (303) Board 25