Modeling the Benefits of LID Stormwater Techniques for Developable Parcels in the Nashoba Brook...

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Modeling the Benefits of LID Stormwater Techniques for Developable Parcels in the Nashoba Brook Subwatershed

Bob Hartzel, CLM, CPESCSenior 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 Centralized Pipe and Pond Controland Pond Control

LID Development

Disconnected Disconnected Decentralized Decentralized DistributedDistributed

Conservation Conservation Minimization Minimization Soil Amendments Soil Amendments Open Drainage Open Drainage Infiltration BMPsInfiltration BMPsVegetative BMPs Vegetative BMPs Rain Barrels Rain Barrels Pollution PreventionPollution Prevention

Multiple SystemsMultiple Systems

Residential LID Site

LID Stormwater Controls

Rain Garden Treatment Train Approach

Raingarden Cell

Storm Drain System

Raingarden Cell

Flow Path

Grass SwaleGrass Filter

Low Impact Development Stormwater Controls

Bioretention Raingardens/barrels

Porous surfaces Bioretention

RaingardensA 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

Acton 43%

Westford 35%

Littleton 14%

Carlisle 5%

Concord 3%

Subwatershed Delineation

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

Lake Shirley Bioretention Cell Lesson: Small is beautiful!

Terraced bio-infiltration system (Plymouth, MA)

Lesson:

Sloped areas can be put to work!

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

Shallow Bedrock

Impervious Cover Assessment

Total Subwatershed Imperviousness = 12.1%

Range = 5.9% - 20.2%

Public Water Supply Protection

Public Water Supply Wells

Interim Wellhead Protection Areas

Zone 2 Wellhead Protection Areas

Water quantity

Drinking water quality concerns

Parcel Ranking

Parcel Ranking…Wetlands

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

Parcel Statistics: Choosing a “Representative” Parcel

Median (50th Percentile) Values: Size: 0.9 Acres % IMP: 8.4%

Examples: “Representative” Residential Parcels

Size: ~0.9 acres Imperviousness: ~8%

Examples: “Representative” Commercial/Industrial Parcels

Size: ~1.3 acres Imperviousness: ~30%

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

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

Conceptual Single Family Lot: Low Impact Development

Runoff: -7.0 in/yr

Infiltration: +7.1 in/yr

TSS: -145 lb/ac/yr (-68%)

TP: -0.44 lb/ac/yr (-63%)

Conceptual Residential Subdivision: Conventional Development

Conceptual Residential Subdivision: Low Impact Development

Runoff: 0.7 in/yr Infiltration: 36.5 in/yr TSS: 38.4 lb/ac/yr TP: 0.06 lb/ac/yr

Runoff: -11.1 in/yr

TSS: -292 lb/ac/yr (-88%)

TP: -0.86 lb/ac/yr (-93%)

Conceptual Commercial/Industrial: Conventional Development

Conceptual Residential Subdivision: Low Impact Development

Runoff: -15.2 in/yr

TSS: -391 lb/ac/yr (-94%)

TP: -1.66 lb/ac/yr (-96%)

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

0 5 10 15 20

Single Family Lot

Residential Subdivision

Commercial/Industrial

gal/person/day

Added Infiltration from LID

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

0 50 100 150

Single Family Lot

tons/yr

Reduction in TSS from LID

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

0 400 800

Single Family Lot

lbs/yr

Reduction in TP from LID

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 H20

14,760 cu. ft.

of algae

COST: LID vs. Conventional Stormwater

Thank you for your time!

Conventional Design Low Impact Design

Units Unit Cost Qty. Total Qty. Total

LANDSCAPING

Tree Removal 1 ac $9,150 0.8 $7,320 0.3 $2,745

Clearing/Grubbing 1 ac $7,500 0.9 $6,750 0.38 $2,850

Grass Seed/Sod 1 sy $6 3900 $23,400 1328 $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

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 $34,462

-10.3%

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/Pub287/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

Residential Subdivision Conventional Design Low Impact Design Units Unit Cost Qty. Total Qty. TotalLANDSCAPING Item:

Tree Removal 1 ac $9,150 0.8 $7,320 3.6 $32,940

Clearing/Grubbing 1 ac $7,500 0.9 $6,750 4.6 $34,500

Grass Seed/Sod 1 sy $6 46800 $280,800 15936 $95,616

Native Trees/Shrubs (for bioretention areas) 1,2 ea $30 0 $0 200 $6,000

Soil Amendment 2 cy $51 0 $0 148 $7,525

Raingarden/Bioretention 2,3 sf $11 0 $0 19100 $210,100 INFRASTRUCTURE/CONVEYANCE/STORAGE Item:

Standard Asphalt Driveway 2 sy $10 1130 $11,300 0 $0

Standard Asphalt Roadway 2 sy $20 4670 $93,400 3090 $61,800

Porous Pavement Driveway 2,3 sy $108 0 $0 1130 $122,040

Drywell - (including inst.) 2 ea $1,500 0 $0 12 $18,000

Catch Basins 1,2 ea $3,000 7 $21,000 0 $0

CPP Storm Drain Pipe 1 lf $75 1710 $128,250 180 $13,500

Vegetated Swale 2 sf $2 0 $0 1840 $3,680

Detention Pond 2,4 cy $15 2450 $36,750 0 $0

Outlet Structure 2 ea $1,500 1 $1,500 0 $0 TOTAL $587,070 $605,701 3.2%

Commercial / Industrial Conventional Design Low Impact Design

UnitsUnit Cost Qty. Total Qty. Total

LANDSCAPING Item: Tree Removal 1 ac $9,150 1.1 $10,065 0.8 $7,320 Clearing/Grubbing 1 ac $7,500 1.1 $8,250 0.8 $6,000 Grass Seed/Sod 1 sy $6 3390 $20,340 1940 $11,640 Native Trees/Shrubs (for bioretention areas) 1,2 ea $30 0 $0 24 $720 Soil Amendment 2 cy $51 0 $0 18 $916 Raingarden/Bioretention 2,3 sf $11 0 $0 1800 $19,800 INFRASTRUCTURE/CONVEYANCE/STORAGE Item: Standard Asphalt Parking 2 sy $10 1475 $14,750 1100 $11,000 Porous Pavement Parking 2,3 sy $108 0 $0 370 $39,960 Drywell - (including inst.) 2 ea $1,500 0 $0 1 $1,500 Catch Basins 1,2 ea $3,000 3 $9,000 0 $0 CPP Storm Drain Pipe 1 lf $75 325 $24,375 0 $0 Vegetated Swale 2 sf $2 0 $0 470 $940 Detention Pond 2,4 cy $15 340 $5,100 0 $0 Outlet Structure 2 ea $1,500 1 $1,500 0 $0 TOTAL $93,380 $99,796 6.9%

Costs: Literature Review Comparison

Modeling the Benefits of LID Stormwater Techniques for Developable Parcels in the Nashoba Brook...

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