Stormwater Retrofit of Highwood Estates Detention Basins to address Water Quality Issues 2011 Low Impact Development Symposium Philadelphia, PA
Co-Authors Steven Trinkaus, PE, CPESC, CPSWQ of Trinkaus Engineering, LLC - Southbury, CT Sean Hayden, CPESC, Soil Scientist of Northwest Conservation District Torrington, CT
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
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
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)
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
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
Northfield Lake Issues Sediment deposition Unknown source Sediment deposition from Northfield Brook Known source
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
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
Sources of Non-point Source Stream Crossings Pollution Stream Bank Erosion Medium Density Residential Developments
Sources of Non-point Source Pollutants
Source of Sediment Load Significant erosion of existing stream channel clear & convincing source of sediment load reaching Northfield Lake BUT what is causing erosion?
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
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
Highwood Estates Thomaston, CT Basin Inlet Existing Stormwater Basins Basin Inlet Large Basin Small Basin
Large Stormwater Basin Inlet Outlet Sediment Delta Majority of Basin
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
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
Very fine sediments on stone bottom of natural channel
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
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
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 = 30.66 cfs (2-yr) Required Water Quality Volume (WQV) = 41,851 cu.ft.
Pollutant Loads without treatment (annual basis) Simple Method Total Suspended Solids 8,422 lbs Total Phosphorous 34.2 lbs Total Nitrogen 257.3 lbs Zinc 20.4 lbs Total Petroleum Hydrocarbons 205.6 lbs Dissolved Inorganic Nitrogen 42.1 lbs
Discharged pollutant loads after treatment (lbs/year) Total Suspended Solids 264.9 (96.9%) Total Phosphorous 10.8 (68.4%) Total Nitrogen 97.0 (62.3%) Zinc 0.8 (96.1%) Total Petroleum Hydrocarbons 137.7 (33.0%) Dissolved Inorganic Nitrogen 15.6 (63.0%)
Modified Hydrologic Conditions Staged orifice outlet will reduce 2-yr peak rate from 30.66 cfs to 5.00 cfs Forebay and basin will provide 17,996 cu.ft. (43%) of fixed storage volume for WQV.
Small Stormwater Basin Inlet swale from Road conveyance system Sediment accumulation on bottom of basin
Small Stormwater Basin -Outlet Structure - Short Circuit flow path - Sediment accumulation
Small Stormwater Basin Second inlet BUT no runoff is discharged into this swale, what is its purpose?
Small Stormwater Basin Outlet structure 12 x 12 cut opening
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
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
Basin Retrofit - Excavate bottom of basin to create 8 deep pond with aquatic bench around edge -Redirected inlet swale - Create forebay -
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 = 30.09 cfs (2-yr) Required Water Quality Volume (WQV) = 35,278 cu.ft.
Pollutant Loads without treatment (annual basis) Simple Method Total Suspended Solids 6,872 lbs Total Phosphorous 29.2 lbs Total Nitrogen 215.9 lbs Zinc 17.3 lbs Total Petroleum Hydrocarbons 163 lbs Dissolved Inorganic Nitrogen 35.3 lbs
Discharged pollutant loads after treatment (lbs/year) Total Suspended Solids 302.6 (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%)
Modified Hydrologic Conditions Staged orifice outlet will reduce 2-yr peak rate from 30.66 cfs to 28.87 cfs Forebay and basin will provide 5,278 cu.ft. (15%) of fixed storage volume for WQV.
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
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
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
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
Contact Information Steve Trinkaus, PE, CPESC, CPSWQ Trinkaus Engineering, LLC 114 Hunters Ridge Road Southbury, CT 06488 203-264-4558, Fax: 203-264-4559 Email: strinkaus@earthlink.net Website: http://www.trinkausengineering.com
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