Post-Construction Storm Water Control for New Developments and Redevelopment Projects Presenters: Christe Alwin, Lansing District Office Ralph Reznick, Nonpoint Source Unit
MS4 Permit References • Watershed Permit MIG610000 Part 1.A.4.b.4. (Pages 15-17) • Jurisdictional Permit MIS049000 Part 1.A.8. (Pages 10-12) City of Battle Creek—porous pavement
Core Requirement • Develop, implement, and enforce a program through an ordinance or other regulatory mechanism to address post-construction storm water runoff from: • all new and redevelopment projects that disturb ≥1 acre, and • projects that disturb <1 acre if they are part of a larger common plan of development or sale that would disturb ≥1 acre. Applies to Permittee and Non-Permittee Projects
What is Post-Construction Storm Water Runoff? The storm water that would flow from a project site to an MS4 or surface waters of the state followingconstruction. City of Battle Creek—green roof
Scope of Post-Construction Control Requirements • Watershed Permit: • Across the permittee’s jurisdiction in the regulated area (the Urbanized Area and watershed area). • Jurisdictional Permit: • Discharges to the permittee owned or operated MS4 within the Urbanized Area.
Scope of Post-Construction Control Requirements • Watershed Permit: • Across the permittee’s jurisdiction in the regulated area (the Urbanized Area and watershed area). • Jurisdictional Permit: • Discharges to the permittee-owned or operated MS4 with the Urbanized Area.
How is the post-construction program implemented and enforced? • Ordinance • Other regulatory mechanism • A combination of both
Likely Scenarios for Applicability of Ordinance and Other Regulatory Mechanisms
Permittee Expectations • Establish a post-construction control program through an ordinance or other regulatory mechanism. • May rely on others to assist with completing the necessary reviews and calculations. Compliance with permit requirements is ultimately the responsibility of the permittee.
Post-Construction Storm Water Runoff Control Program Required Components • Minimum Water Quality Treatment Volume Standard • Channel Protection Criteria • Site Plan Requirements • Operation and Maintenance Requirements • Enforcement Mechanisms • Recordkeeping Procedures City of Lansing—raingardens
Program Components • Minimum Water Quality Treatment Volume Standard Channel Protection Criteria Site Plan Requirements Operation and Maintenance Requirements Enforcement Mechanisms Recordkeeping Procedures Minimum Treatment Volumes: • 1 inch of runoff generated from the entire site (½ inch optional for the Watershed Permit if the Watershed Management Plan supports it) • The calculated site runoff from the 90 percent annual non-exceedance storm for the region or locality
Simple Method to Calculate Runoff Volume WQv = Water Quality Volume = [(P)(Rv)(A)] /12 Rv = 0.05+0.009(I) I = Impervious Cover (Percent) Minimum Rv = 0.2 P = 90% Rainfall Event Number A = site area in acres www.semcog.org/LowImpactDevelopment.aspx Minimum Treatment Volume
Treatment Design • Treatment methods shall be designed on a site-specific basis to achieve the following: • Minimum of 80 percent removal of total suspended solids (TSS), as compared with uncontrolled runoff, or • Discharge concentrations of TSS not to exceed 80 milligrams per liter (mg/l)
Treatment Mechanisms • Settling • Filtering • Absorption • Chemical Treatment • Mechanical Treatment Minimum Treatment Volume
National Pollutant Removal Performance Database, 2nd Edition Table 3.1 Median Pollutant Removal (%) of Stormwater Ponds and Wetlands
References BMP Performance data can be found in; National Pollutant Removal Performance Database, Version 3 at: www.cwp.org/Resource_Library/Center_Docs/SW/bmpwriteup_092007_v3.pdf and The International Stormwater BMP Database at: http://www.bmpdatabase.org/Docs/Performance%20Summary%20June%202008.pdf BMP design information can be found in; Low Impact Development Manual for Michigan available at: http://library.semcog.org/InmagicGenie/DocumentFolder/LIDManualWeb.pdf and the Guidebook of BMPs for Michigan Watersheds available at: http://www.michigan.gov/deq/0,1607,7-135-3313_3682_3714-118554--,00.html Minimum Treatment Volume
Minimum Treatment Volume Total Area X 1 inch or 90 percent annual non-exceedance storm and appropriate runoff model applied to the site. then Apply BMP or BMPs necessary to achieve 80% reduction of Total Suspended Solids Minimum Treatment Volume
Program Components Minimum Water Quality Treatment Volume • Channel Protection Criteria Site Plan Requirements Operation and Maintenance Requirements Enforcement Mechanisms Recordkeeping Procedures Existing levels = runoff flow volume and rate for the last land use prior to the planned new development or redevelopment. Channel Protection Criteria: • Maintain post-development site runoff volume and peak flow rate at or below existing levels • All storms up to the 2-year, 24-hour event
Stream Stability The ability of a stream, over time, to maintain its pattern, dimension and profile such that, the channel neither aggrades or degrades and is able to transport without adverse impact the flow and sediment from it’s watershed. Channel Protection Criteria
Definition: Bankfull Discharge That discharge which barely overtops the channel banks Maximumdeposition & erosioncirculation cell development Channel Protection Criteria
The most effective sediment discharge, over time, occurs at the bankfull flow rate which is between the 1 to 2 year recurrence interval. At the bankfull flow, the stream bottom picks up and moves, then redistributes its self in the same pool & riffle patterns existing prior to the bankfull discharge. Channel Protection Criteria Taken from Rosgen, 1996
Streams become unstable due to: • Changes in Climate • Changes in Hydrology • Changes in Topography • Changes in Vegetation • Changes in Geology • Alterations of the Stream Channel Channel Protection Criteria
The Channel Protection requirements in the MS4 Permit address changes in hydrology due to development and the negative channel responses that result.
Maintaining Predevelopment Hydrology • Time of Concentration (Tc) • The time it takes water to get from the farthestpoint in a watershed to a point of collection. • Volume • The quantity of water that runs off a site during a storm event. • Peak Flow Rate • The highest flow rate of a storm event. Channel Protection Criteria
CN CN Channel Protection Criteria
Calculates rate and volume of storm runoff using the unit hydrograph method for runoff distribution and the curve number method of land use runoff response. www.michigan.gov/deqstormwater (under the “Municipal Program/MS4 Compliance Assistance” link, then the “Guidance for Calculating Runoff Volume and Peak Flow Rate” link)
24 HourPrecipitationRainfall Frequency Atlas of the Midwest, Bulletin 71, Midwestern Climate Center, 1992
Hydrograph for a farm on sandy soil or woods on loamy soil. Qp=23 cfs V=5 acre-ft. Dave Fongers - DNRE Channel Protection Criteria
Loss of infiltration due to development increases total runoff volume and peak flows. Qp=65 cfs V=11 acre-ft. Qp=23 cfs V=5 acre-ft. Dave Fongers - DNRE Channel Protection Criteria
More rapid runoff further increases peak flows. Qp=90 cfs V=11 acre-ft. Qp=65 cfs V=11 acre-ft. Qp=23 cfs V=5 acre-ft. Dave Fongers - DNRE Channel Protection Criteria
Controlling Time of Concentration • Maintain Natural Drainage Paths • Use open swales • Increase surface roughness • Detain flows • Minimize disturbance • Flatten grades in disturbed areas • Disconnect imperviousness • Connect pervious and vegetated areas Channel Protection Criteria
CN CN Channel Protection Criteria
Runoff Curve Numbers For Urban Areas Channel Protection Criteria
1 Acre Site on C Soils Runoff Volume to Remove 6310 ft3 Building Meadow CN = 71 Paved Parking Paved Commercial Site 100% Impervious CN = 98 Can be accomplished with rain gardens on approximately 20% of site 2 year runoff volume = 1640 ft3 2 year runoff volume = 7950 ft3 Channel Protection Criteria
Reduce Runoff Volume • Infiltrate • Evaporate • Reuse Channel Protection Criteria
1 Acre Site on C Soils Runoff Volume Reduction 1331 ft3 Open Space CN = 79 Woods CN = 70 2 year runoff volume = 2847 ft3 2 year runoff volume = 1516 ft3 Channel Protection Criteria
Channel Protection Determine Existing Runoff Volume Using CN Method then Determine Proposed Runoff Volume and Subtract Existing to determine Runoff Volume Increase then Apply BMPs to Remove the Runoff Volume Increase Channel Protection Criteria
Channel Protection • Meeting the Channel Protection Criteria for Volume Control will also meet the Peak Rate Control requirement in most cases. • For NEW development, meeting the Channel Protection Criteria for Volume Control will also meet the Minimimum Water Quality Treatment Volume criteria in almost all cases. Channel Protection Criteria
Redevelopment Transportation Tight Soils Brown Fields Permittees are encouraged to discuss special conditions with District Staff. Special Conditions Channel Protection Criteria
Program Components Minimum Water Quality Treatment Volume Channel Protection Criteria • Site Plan Requirements Operation and Maintenance Requirements Enforcement Mechanisms Recordkeeping Procedures Permittees shall include a requirement that the developer prepare and implement site plans for each project subject to the post-construction control requirements.