Implementing Low Impact Development in Utah -An Engineer’s PerspectivePRESENTED BY:

CRAIG BAGLEY, P.E., CFM

BOWEN, COLLINS & ASSOCIATES

What is Low Impact Development?

A comprehensive, landscape-based approach to sustainable developmentA management strategy to maintain pre-development natural systems, hydrology and ecologyA cost-effective, flexible approach based on a set of simple techniquesA collection of practices that have been implemented nationwide

What is Low Impact Development?NOT rocket scienceNOT formulaicNOT the answer to every storm water challengeNOT yet finished evolvingNOT very common in Utah

Typical pre-development conditions: Runoff = 10% Infiltration = 50%

Typical post-development conditions:

Runoff = 55% Infiltration = 15%

LID PrinciplesWork with the landscape and natural systemsFocus on prevention (flood, erosion, pollution, etc.)Micromanage stormwater – treat near sourceKeep it simpleDesign multi-functional features (filter, treat, infiltrate)Maintain and sustain

1. Use existing natural systems as the integrating framework for site

planning•Land use planning and watershed planning

•Identify environmentally sensitive resources: wetlands, mature trees, slopes, drainageways, permeable soils, waterway buffers

•Mimic pre-development hydrology

2. Focus on prevention

•Minimize clearing and grading

•Cluster buildings and reduce building footprints

•Reduce road widths, use shared driveways, reduce parking area

•Align roads to minimize impact

•Use green rooftops

•Use permeable paving

•Create subwatersheds and “micromanage” runoff in a treatment train of small structures

•Flatten slopes, lengthen flow paths, maximize sheet flow

•Maintain natural flow paths, use open drainage

•Use LID techniques to manage frequent, low-intensity storms

3. Treat stormwater close to the source

•Open drainage systems and filter strips

•Disconnection of roof runoff

•Rain barrels

•Street sweeping

•Reduce construction disturbance

4. Emphasize simple, nonstructural, low-tech, low-cost methods

•Use stormwater management components that provide filtration, treatment, and infiltration.

•Provide open space and wildlife habitat.

•Store water for landscape use

•Reduce heat island effect

•Enhance site aesthetics

5. Create a multifunctional landscape

•Reduce use of pesticides and fertilizers.

•Use drought-resistant plants.

•Maintain rain gardens and bioretention areas.

•Provide adequate funding for public works departments

6. Maintain and sustain

Key LID StrategiesConserve natural hydrology, trees and vegetationPreserve stream and wetland buffersMinimize impervious surfaces (DCIA)Micromanage stormwaterPlant ecological landscaping

LID Advantages over Conventional Means

Reduces need for large detention facilities and flood control structuresReduces pollutant loading to receiving watersReduces stream bank erosion associated with peak flow and volumeProvides visual amenityCosts less to construct (less pipe, less pavement, smaller facilities)

Conventional LID BMPs/Integrated Management Practices

Bioretention Dry wells

Filter/buffer strips Vegetated swales

Rainwater harvesting Infiltration trenches

Reducing roads and parking areas

Permeable pavement

LID Design ProcessDevelop site planPerform hydrologic analysisEvaluate/select BMPs/Integrated management practicesDevelop erosion and sediment control measures

LID Site PlanningDefine development envelopeReduce/minimize total impervious area on siteDisconnect impervious areasModify/increase flow paths

LID Hydrologic AnalysisDelineate drainage basin and sub-basin areasDefine the design storm characteristicsSelect modeling techniquesEstimate runoff from pre-development conditions (baseline)Estimate runoff from propsed new development without BMPsEvaluate alternative BMPs to mitigate impacts of development on stormwater

Select LID BMPs/Integrated Management Practices

Define needed hydrologic controls to mitigateEvaluate site constraintsScreen potential BMPsEvaluate impacts of implementing BMPs on post-developed modelSelect BMPs to implementIncorporate additional controls if necessary

Select Erosion & Sediment Control MethodsPlan and phase constructionSelect appropriate erosion control measuresSelect sediment control measuresIdentify needed maintenance activities

LID Planning/Design ConsiderationsSpace requirementsSoil characteristicsSlopes (hillside, channel, etc.)Depth to water table or bedrockProximity to building foundations & wellsMaximum depth AND Maintenance costs

Low Impact Site Design Example

•Conservation of natural hydrology, trees, vegetation

•Stream & wetland buffers

•Minimize impervious surfaces

•Stormwater micromanagement

•Ecological landscaping

Typical Subdivision

Conservation

Low Impact Site Design ExampleOpen Space Residential Design1. Identify Conservation Areas

Low Impact Site Design ExampleOpen Space Residential Design1. Identify Conservation Areas

2. Locate House Sites

Low Impact Site Design ExampleOpen Space Residential Design1. Identify Conservation Areas

2. Locate House Sites

3. Align Roads & Trails

1. Identify Conservation Areas

2. Locate House Sites

3. Align Roads & Trails

4. Draw the Lot Lines

Open Space Residential Design

Low Impact Site Design Example

Buildings and RoadwaysLow Impact Site Design Example

•Cluster buildings within the development envelope•Design buildings with smaller footprints•Roadways should follow existing grades.•Use parking structures •Separate parking areas

Buildings and RoadwaysLow Impact Site Design Example

•Cluster buildings within the development envelope•Design buildings with smaller footprints•Roadways should follow existing grades.•Use parking structures •Separate parking areas

Stormwater ManagementLow Impact Site Design Example

•Minimize directly connected impervious area•Create multiple sub-basins •Increase time of concentration•Use a “treatment train” of LID techniques to deal with frequent, low-intensity storms.

Stormwater ManagementLow Impact Site Design Example

•Minimize directly connected impervious area•Create multiple sub-basins •Increase time of concentration•Use a “treatment train” of LID techniques to deal with frequent, low-intensity storms.

Low Impact Site Design Examples

Roadways and Parking AreasRoad Profile

• Narrower roadways (18-24 feet)

• Permeable parking lanes

• Open section roadways

• Alternative curb designs

Roadways and Parking AreasAlternative Turnarounds

• Smaller cul-de-sacs• Bioretention islands• One-way-loops• Hammerhead

turnarounds

Roadways and Parking AreasParking Lots

• Create multiple small lots

• Allow shared parking• Reduce requirements

near transit• Require compact

spaces • Set parking

maximums

Roadways and Parking AreasParking Lots

•Create multiple small lots

•Allow shared parking

•Reduce requirements near transit

•Require compact spaces

•Set parking maximums

Permeable Paving

• Grass pavers• Paving stones• Porous asphalt• Pervious concrete

Runoff reduction

Permeable Paving

• Parking stalls• Overflow parking• Driveways• Walkways and plazas

Applications

Bioretention

• Excavation filled with engineered soil mix

• Herbaceous perennials, shrubs, trees

• Ponded water infiltrates within 72 hours

• Overflow outlet and optional underdrain

Treatment, retention, infiltration, landscaping

Bioretention

• Parking lot islands• Median strips• Rooftop runoff

Applications

BioretentionApplications

• Urban retrofits• High-density areas

Vegetated Swales

• Roadside swales “country drainage”

• Parking lots• Low-angle slopes

only• Opportunity for snow

storage

Conveyance, treatment, infiltration

Vegetated SwalesConveyance, treatment, infiltration

Grassed Filter StripsPretreatment and Attenuation

• Low-angle vegetated slopes

• Adjacent to parking lots and roadways

• Opportunity for snow storage

Grassed Filter Strips

Infiltration Trenches / Dry WellsInfiltration and Volume Reduction

• Runoff stored in void space; slowly percolates into the ground

• Excellent for rooftop runoff

• Pretreatment is critical for surface runoff

Infiltration and Volume ReductionInfiltration Trenches / Dry Wells

Runoff Reduction and Water Conservation

Rain Barrels and Cisterns

• Downspouts directed to tanks or barrels

• UT – up to 2500 gal/lot• Excess diverted to

drywell or rain garden• Landscaping, car

washing, other nonpotable uses

Runoff Reduction and Water Conservation

Rain Barrels and Cisterns

Runoff Reduction, Treatment, Attenuation Stormwater Planters

• “Bioretention in a Box”

• Vegetative uptake of stormwater pollutants

• Pretreatment for suspended solids

• Aesthetically pleasing

• Reduction of peak discharge rate

Stormwater Planters

LID BENEFITS

Lower Peak Discharge Rates Reduced Runoff Volume

Rainfall / Runoff LID vs Conventional

0

0.2

0.4

0.6

0.8

Hours

Flo

w

Conventional

LID

Rainfall

0 4 8 12 16

LID BENEFITS

Improved Water QualityIncreased Aquifer Recharge

  Conventional  Low Impact Grading/Roads $569,698 $426,575

Storm Drains $225,721 $132,558

SWM Pond/Fees $260,858 $ 10,530

Bioretention/Micro — $175,000

Total $1,086,277 $744,663Unit Cost $14,679 $9,193Lot Yield 74 81

Lower Construction Costs Higher Lot Yield

Important Design Considerations•Selection of pretreatment prior to

infiltration •Wellhead protection•Emergency access•Snow removal issues•Overflow/bypass controls•Mosquito issues and concerns

Important Design Considerations•Select appropriate cold climate techniques

•Ensure adequate emergency access•Prioritize pedestrian safety•Define ownership: public, private,

mix?•Develop monitoring and inspection

plan•Assign maintenance responsibilities

QUESTIONS?

LID IMPLEMENTATION

Long-Term Maintenance

•All stormwater systems require maintenance

•LID maintenance often simple, low cost

•Ensure adequate funding for PW Depts

•Homeowner/landscaper education•Consider requiring permanent

sureties

LID IMPLEMENTATION

Review of Local Codes

•Zoning Bylaw and Site Plan Review•Subdivision Rules and Regulations•Board of Health Regulations •Wetland Regulations•Building Codes

LID IMPLEMENTATION

Stormwater/LID Bylaw•Replace the “patchwork” of stormwater codes

with a single set of local standards•Establish Stormwater Permitting Authority to

review all projects over a certain size•Specify performance standards •Utilize expert review when necessary •Permit/promote the use of LID techniques•Require a maintenance plan

LID IMPLEMENTATION

Collaboration is Critical!•Local boards•Public works officials•Fire department and emergency

response•Developers and builders•Consultants and engineers•Business interests and property

owners•Environmental advocates