Presentation on theme: "Modeling the Benefits of LID Stormwater Techniques for Developable Parcels in the Nashoba Brook Subwatershed Bob Hartzel, CLM, CPESC Senior Water Resources."— Presentation transcript:
Modeling the Benefits of LID Stormwater Techniques for Developable Parcels in the Nashoba Brook Subwatershed Bob Hartzel, CLM, CPESC Senior Water Resources Scientist
Project funded through: Provide planning to aid 5 subwatershed towns in promoting LID as part of master planning, project permitting and development of bylaws. Organization for the Assabet River Project Overview / Goals
Develop model to evaluate the suitability and priority of developable parcels in the Nashoba Brook subwatershed for LID stormwater techniques. Estimate the benefits and costs of LID stormwater management compared to “conventional” stormwater management. Vine Brook Nashoba Brook
Low Impact Development (LID) An ecosystem-based approach to land development and stormwater management. Goal: Mimic pre-development site hydrology
Conventional Development Centralized Pipe and Pond Control
LID Development Disconnected Decentralized Distributed Conservation Minimization Soil Amendments Open Drainage Infiltration BMPs Vegetative BMPs Rain Barrels Pollution Prevention Multiple Systems
LID Subdivision Cluster Only Subdivision Conventional Subdivision Pre-development Watershed LID Cluster Non-LID Cluster Non-LID Non-Cluster Undeveloped Effectiveness of Environmental Site Design Modeling: Model Scenarios Partridgeberry Place Case Study (Ipswich, MA)
Effectiveness of Environmental Site Design Development Comparison Results Compared to Pre-development Watershed: LID/Cluster Subdivision: 2 to 11 % more runoff Cluster Subdivision: 3 to 18 % more runoff Conventional Subdivision: 13 to 45 % more runoff Partridgeberry Place Case Study (Ipswich, MA)
Low Impact Development Stormwater Controls Bioretention Raingardens/barrels Porous surfacesBioretention
Raingardens A bowl-shaped garden designed to capture and absorb stormwater.
Bioretention Cell Similar to raingarden, more highly engineered: underdrain/riser pipe gravel bed engineered soils
Street Edge Alternatives (SEA) Functional Landscape Reduced Impervious Area 98% Stormwater volume reduction for 2-year storm
“SEA” Street: Maximized space for filtration, recharge and landscape elements
Watershed Communities Town % of watershed Acton43% Westford35% Littleton14% Carlisle5% Concord3%
A. “Developable Parcel” Inventory / Map (Not Retrofits) Unimproved lots > minimum lot size requirement Municipally owned property that is for sale. Parcels protected though open space preservation, deed restrictions, etc. = Not Developable Any portion of a parcel within wetlands = Not Developable Task 1: Evaluate Parcels Most Suitable for LID Inventory developed with 5 town planning departments
Developable Parcels Map 198 Parcels 11% of land in watershed Summary Statistics: Size Range: 0.3 – 150 acres 50% of land comprised by largest 8% of parcels 178 residential parcels 12 commercial parcels 8 industrial parcels
Task 1: Evaluate Parcels Most Suitable for LID B. Develop Model to Rank Site Suitability / Priority S = Soils D = Distance to Receiving Water B = Shallow Bedrock Imp = Impervious Cover Analysis PWS = Lands Critical to Public Water Supply Quality and Quantity LID Priority Ranking = 2 (S) + 1 (D) + 1 (Imp) - 1 (PWS) – 1 (B)
“…with proper design and maintenance, LID stormwater techniques can be used successfully at many sites with less than ideal initial conditions.” Soil Amendments Underdrains Steep Slope Designs (Terraces, etc.) “Not Enough Space”
A newly planted bioretention cell installed in the Dudley Pond watershed (Wayland, MA).
Lake Shirley Bioretention Cell
Lesson: Small is beautiful!
Terraced bio-infiltration system (Plymouth, MA) Lesson: Sloped areas can be put to work!
Soils A: Sand, loamy sand or sandy loam soils. High infiltration rates! B: Silt loam or loam. Moderate infiltration when fully wetted. C: Sandy clay loam. Poor infiltration when thoroughly wetted. D: Clay loam, silty clay loam, sandy clay, silty clay or clay. Highest runoff potential, very low infiltration when fully wetted. Hydrologic Soil Groups
Impervious Cover Assessment Total Subwatershed Imperviousness = 12.1%
Impervious Cover Assessment Total Subwatershed Imperviousness = 12.1%
Range = 5.9% % Impervious Cover Assessment
Public Water Supply Protection Public Water Supply Wells Interim Wellhead Protection Areas Zone 2 Wellhead Protection Areas Water quantity Drinking water quality concerns
Next Steps: Evaluate the Relative Benefits of LID Techniques A. Create “Standardized” Development Scenarios LID Conventional Development Cluster / Open Space Development (Residential parcels only) B. Compare LID vs. Traditional Stormwater Mgt. Water Quality / Quantity Benefits Cost
Review Existing Municipal By-Laws and Regulations Review of bylaws and regulations: Identify existing conflicts/overlaps relative to stormwater mgt. Identify areas of recommended modification / revisions Review LID by-laws (and similar bylaws, regulations and policies) which have passed in MA and neighboring states.
Parcel Statistics: Choosing a “Representative” Parcel Median (50 th Percentile) Values: Size: 0.9 Acres % IMP: 8.4%
PRECIPITATION STORAGE INFILTRATION RUNOFF EVAPORATION SWMM (VOLUME) EVENT MEAN CONCENTRATION (QUALITY) LOAD = RUNOFF VOLUME X EMC Quantifying LID Benefits: Infiltration and Load Reduction
Modeling Hydrologic Budget: EPA’s SWMM INPUTS Precipitation: 57 years of Boston rainfall data Evaporation: data obtained from MassGIS Subwatersheds based on surface types: roofs, pavement, lawn, wooded, etc. Infiltration BMPs: drywells, bioinfiltration OUTPUTS Runoff totals from subwatersheds Precipitation/Runoff statistics
Event Mean Concentration: Total Suspended Solids (TSS)
Event Mean Concentration: Total Phosphorus (TP) !
Single Family Lot vs. Residential Subdivision Parcels with 1-4 buildable lots = “Single Family Lots” (138 parcels, 285 ac.) Parcels with 5+ buildable lots = “Residential Subdivisions” (40 parcels, 920 ac.)
Conceptual Single Family Lot: Conventional Development Runoff: 8.9 in/yr Infiltration: 28.1 in/yr TSS: 213 lb/ac/yr TP: 0.72 lb/ac/yr
LID Benefits: Infiltration The added infiltration from LID being applied to developable parcels may be as high as: … for the population of Acton, Littleton, and Westford (~51,500 people) 22 gal/person/day
2 extra toilet flushes (1.6 gal each) per person per day… … for the population of Acton, Littleton, and Westford (~51,500 people) 1 5-minute shower (10 gal) per person per day… 1 dishwasher load (8 gal) per person per day… 22 gal/person/day = approximately… LID Benefits: Infiltration
LID Benefits: TSS … entering Nashoba Brook The reduction in weight of TSS from LID being applied to developable parcels may be as high as: 190 tons/year
LID Benefits: TSS 140 cubic yards of sand/sediment… entering Nashoba Brook each year 190 tons = approximately…
LID Benefits: TP … entering Nashoba Brook The reduction in weight of TP from LID being applied to developable parcels may be as high as: 1190 lbs/year
1190 lbs/year of TP = approximately… ….growing per year in Nashoba Brook (and tributary ponds) *assuming 1 lb P : 1026 lb wet Phytoplankton, density Phytoplankton ~ density H ,760 cu. ft. of algae
COST: LID vs. Conventional Stormwater
Thank you for your time!
Conventional Design Low Impact Design UnitsUnit Cost Qty.Total Qty.Total LANDSCAPING Item: Tree Removal 1 ac$9, $7, $2,745 Clearing/Grubbing 1 ac$7, $6, $2,850 Grass Seed/Sod 1 sy$6 3900$23, $7,968 Native Trees/Shrubs 1,2 ea$30 0$0 5$150 Soil Amendment 2 cy$51 0$0 12$627 Raingarden/Bioretention 2,3 sf$11 0$0 770$8,470 INFRASTRUCTURE/CONVEYANCE/STORAGE Item: Standard Asphalt Driveway 2 sy$10 94$940 0$0 Porous Pavement Driveway 2,3 sy$108 0$0 94$10,152 Drywell - (including inst.) 2 ea$1,500 0$0 1$1,500 TOTAL $38,410 $34, % Notes: Construction/Material costs only. Does not include maintenance or property values. 1. From PA DOT construction cost spreadsheet (ftp://ftp.dot.state.pa.us/public/Bureaus/design/Pub2 87/Pub%20287.pdf) 2. Geosyntec cost estimate data, past projects 3. "Evaluation of LID Best Management Practices (BMPs) Opportunities," Charles River Watershed Association Single Family Home