Stormwater Retrofit of Highwood Estates Detention Basins to address Water Quality Issues Low Impact Development Symposium Philadelphia, PA

Transcription

1 Stormwater Retrofit of Highwood Estates Detention Basins to address Water Quality Issues 2011 Low Impact Development Symposium Philadelphia, PA

2 Co-Authors Steven Trinkaus, PE, CPESC, CPSWQ of Trinkaus Engineering, LLC - Southbury, CT Sean Hayden, CPESC, Soil Scientist of Northwest Conservation District Torrington, CT

3 Funding Funded in part by the Connecticut Department of Environmental Protection through a United States Environmental Protection Agency Clean Water Act Section 319 Non-point Source Grant

4 Northfield Lake Watershed Northfield Brook is Listed on CT 2008 Impaired Waters List as one designated use cannot be supported Watershed contains 3,700 acres & contains 10 miles of perennial and intermittent streams

5 Northfield Lake Watershed Deciduous Forest & Wetlands 61% (2257 ac) Agricultural 18% (667 ac) Developed 12% (444 ac) Coniferous Forest 5.5% (203.5 ac) Meadows 1.5% (55.5 ac) Utility ROW 1.5% (55.5 ac) Barren 0.5% (18.5 ac)

6 What are the problems? High bacteria levels in Northfield Lake were causing frequent closures of swimming area at Army COE Facility Bacteria samples routinely exceeded 235 CFU/100 ml. Several grab samples exceeded 1000 CFU/100 ml

7 What are the problems? A large deposition of sediment had formed in Northfield Lake which was not result of flows in Northfield Brook High phosphorous levels were observed in samples routinely 20 ug/l with some greater than 50 ug/l

8 Northfield Lake Issues Sediment deposition Unknown source Sediment deposition from Northfield Brook Known source

9 Finding the source of the problems Northwest Conservation District conducted a watershed track down survey. This involves walking all stream channels with watershed to observe and document problems

10 Track Down Surveys Developed by Center for Watershed Protection Continuous streamwalk to evaluate conditions of stream channel Identify restoration opportunities Stormwater retrofits, stream restoration, riparian management, & discharge prevention

11 Sources of Non-point Source Stream Crossings Pollution Stream Bank Erosion Medium Density Residential Developments

12 Sources of Non-point Source Pollutants

13 Source of Sediment Load Significant erosion of existing stream channel clear & convincing source of sediment load reaching Northfield Lake BUT what is causing erosion?

14 What Caused Erosion of Channel? Answer: Stormwater management system for Highwood Estates Subdivision Directly connected impervious areas: Driveways drain directly to road Roof & footing drains connected to catch basin Curb and gutter drainage collection system Catch Basin & pipe conveyance system Detention Basins

15 Highwood Estates Thomaston, CT Highwood Estates ½ ac. building lots all directly connected impervious area Large Basin Small Basin Sediment in Lake Northfield Brook Lake

16 Highwood Estates Thomaston, CT Basin Inlet Existing Stormwater Basins Basin Inlet Large Basin Small Basin

17 Large Stormwater Basin Inlet Outlet Sediment Delta Majority of Basin

18 Summary of problems with this basin No forebay sediment accumulating in bottom of basin Inlet & outlet are too close to each other, flows are short circuiting majority of constructed basin Short flow path within basin Opening of outlet control structure to large reduce peak rate of runoff for either design storm or smaller storm events No water quality benefit Design not in compliance with State Manual

19 Large Stormwater Basin Design Parameters: -Peak rate reduction only -25-yr storm event only -NO outside engineering review during approval process -Basin outlet is 12 x 24 cut opening in standard catch basin

20 Very fine sediments on stone bottom of natural channel

21 What can we do to fix this basin? Create forebay to trap sediments Redesign outlet to reduce peak rates of runoff Redesign basin to focus on water quality improvement: Design micro-pools Design high and low marsh areas Increase contact time of runoff and vegetated systems Demonstrate effectiveness of design to improve water quality

22 Basin Retrofit -Forebay providing >10% of WQV -2 deep micro-pool -High Marsh Areas -Low Marsh Areas -Low flow path -4 deep micro-pool -Modified Outlet Structure

23 Existing Hydrologic Conditions Watershed Area = 28.3 acres ½ acre building lots & 26 wide roadways Average impervious coverage on lots = 37% Peak Rate of Runoff = cfs (2-yr) Required Water Quality Volume (WQV) = 41,851 cu.ft.

24 Pollutant Loads without treatment (annual basis) Simple Method Total Suspended Solids 8,422 lbs Total Phosphorous 34.2 lbs Total Nitrogen lbs Zinc 20.4 lbs Total Petroleum Hydrocarbons lbs Dissolved Inorganic Nitrogen 42.1 lbs

25 Discharged pollutant loads after treatment (lbs/year) Total Suspended Solids (96.9%) Total Phosphorous 10.8 (68.4%) Total Nitrogen 97.0 (62.3%) Zinc 0.8 (96.1%) Total Petroleum Hydrocarbons (33.0%) Dissolved Inorganic Nitrogen 15.6 (63.0%)

26 Modified Hydrologic Conditions Staged orifice outlet will reduce 2-yr peak rate from cfs to 5.00 cfs Forebay and basin will provide 17,996 cu.ft. (43%) of fixed storage volume for WQV.

27 Small Stormwater Basin Inlet swale from Road conveyance system Sediment accumulation on bottom of basin

28 Small Stormwater Basin -Outlet Structure - Short Circuit flow path - Sediment accumulation

29 Small Stormwater Basin Second inlet BUT no runoff is discharged into this swale, what is its purpose?

30 Small Stormwater Basin Outlet structure 12 x 12 cut opening

31 Summary of problems with this basin No forebay sediment accumulating in bottom of basin Inlet & outlet are too close to each other, flows are short circuiting majority of constructed basin Short flow path within basin Opening of outlet control structure to large reduce peak rate of runoff for either design storm or smaller storm events No water quality benefit Design not in compliance with State Manual

32 What can we do to fix this basin? Options are limited as space is at a premium: Redesign outlet to reduce peak rates of runoff, if possible Redesign basin to improve pollutant removal: Create forebay to trap sediments, Move inlet to create longest feasible flow path, Create deep pond environment

33 Basin Retrofit - Excavate bottom of basin to create 8 deep pond with aquatic bench around edge -Redirected inlet swale - Create forebay -

34 Existing Hydrologic Conditions Watershed Area = 24.5 acres ½ acre building lots, 26 wide roadways & ballfield Average impervious coverage on lots = 37% Peak Rate of Runoff = cfs (2-yr) Required Water Quality Volume (WQV) = 35,278 cu.ft.

35 Pollutant Loads without treatment (annual basis) Simple Method Total Suspended Solids 6,872 lbs Total Phosphorous 29.2 lbs Total Nitrogen lbs Zinc 17.3 lbs Total Petroleum Hydrocarbons 163 lbs Dissolved Inorganic Nitrogen 35.3 lbs

36 Discharged pollutant loads after treatment (lbs/year) Total Suspended Solids (95.6%) Total Phosphorous 17.2 (41.1%) Total Nitrogen 57.6 (73.3%) Zinc 0.2 (98.8%) Total Petroleum Hydrocarbons 18.6 (88.6%) Dissolved Inorganic Nitrogen 16.2 (54.1%)

37 Modified Hydrologic Conditions Staged orifice outlet will reduce 2-yr peak rate from cfs to cfs Forebay and basin will provide 5,278 cu.ft. (15%) of fixed storage volume for WQV.

38 Discussion Large Basin: It was very important to significantly reduce the peak rate of discharge for the 2-yr event for this basin to minimize on-going erosion of the stream Small Basin: Area restrictions along with budgetary issues constrained the ability to enlarge the smaller basin to enhance the water quality benefits, and peak rate control

39 Things that could be implemented to reduce runoff volumes Roof drains to Bioretention Systems Grading driveways to direct runoff to swales or bioretention systems prior to reaching road edge Use of grass swale along road in lieu of catch basins and pipe Use of narrower roads

40 Conclusions The stormwater retrofits will provide a significant improvement in quality of the runoff being discharged to Northfield Lake The reductions of peak rate for smaller storm events will prevent the erosion of the existing channel from getting worse

41 Conclusions Site limitations can hinder the ability to address all stormwater issues in a retrofit situation Implementing Low Impact Development strategies on existing lots would provide additional water quality improvements as well as providing a reduction of runoff volume

42 Contact Information Steve Trinkaus, PE, CPESC, CPSWQ Trinkaus Engineering, LLC 114 Hunters Ridge Road Southbury, CT , Fax: Website:

43 QUESTIONS??