low impact development training design examples presented by: the low impact development center,...

Low Impact Development TrainingDesign Examples

Presented by:

The Low Impact Development Center, Inc. A non-profit water resources and sustainable design organizationwww.lowimpactdevelopment.org

Copyright 2009 Low Impact Development Center, Inc.

Objectives

• Planning and site design approach

• Site Analysis tools and information

• Site Analysis techniques (Hydrology and Hydraulics)

• Demonstration of Process (NRCS, SWMM, EISA, other)

The HydrologIic Goals and Design Approach



Hydromodification

• Alteration of flow characteristics through a landscape which has the capacity to result in degradation of water resources


1. Conservation

2. Minimization

3. Strategic timing

4. Best management practices retain / detain / filter / recharge / evaporate / use

5. Pollution prevention

LID Design Approach


1. Conservation

2. Minimization

3. Strategic timing

4. IMP’s

5. Pollution Prevention

LID Design Approach

Perform Site Evaluation

Develop LID Strategies

Prepare LID Concept Design

Target O & M Strategy

Critical Elements

Iterative Process

LID Planning Steps


1. Conservation Plans/Regulations

Local watershed and conservation plans• Forest (contiguous and interior habitat)• Streams (corridors)• Wetlands• Habitats• Step slopes • Buffers• Critical areas• Parks• Scenic areas• Trails • Shorelines• Difficult soils• Ag lands• Minerals


2. Minimize Impacts

• Minimize clearing • Minimize grading• Save A and B

soils • Limit lot

disturbance • Reforestation• Reduce

impervious surfaces

replace


Q

T

Developed condition, with reduced or disconnected impervious areas

Pre-developed

Reduced runoff volume

Developed - no controls

Reduced peak flow

Reduce the Runoff Generated by Source Areas


3. Strategic Timing

• Maintain natural flow paths • Maximize sheet flow• Reduce pipes, curb & gutter• Open drainage• Use green space• Flatten slopes • Disperse drainage • Save headwater areas• Vegetative swales


4. Storage, Detention & Filtration “LID BMP’s”

Uniform distribution at the source • Open drainage swales• Rain gardens / bioretention• Permeable pavement• Infiltration trenches• Green roofs• Blue roofs• Rainwater reuse• Soil amendments


5. Pollution Prevention

• Maintenance• Proper use, handling, and disposal

– Individuals• vehicles / hazardous wastes / reporting /

recycling– Navy facilities

• Good house keeping / proper disposal / reuse / spills

Joint Service Pollution Prevention and Sustainability Technical Library A Website Supported by the Joint Services, the Defense Logistics Agency, and the U.S. Coast Guard

http://205.153.241.230/p2_documents/navy.html

Regional Municipal and Joint Services Resources

LID Analysis Tools


Design Storm Approaches

Single Event vs. Continuous Simulation- peak flows / - volume / hydromodification

flooding - water quality


Hydrologic Analysis

• National Resources Conservation Service (NRCS) methods– curve numbers (0-100)– storm type (I, IA, II, III)– time of concentration

Curve Numbers for Hydrologic Soil Group

Cover Type A B C DWoods (Good Condition) 30 48 65 73Paved Areas 98 98 98 98

• Computed process simulations– ex. Hydrologic simulation program – FORTRAN (HSPF)


Models and Tools

Stormwater Models• EPA Stormwater Management Model (EPA

SWMM)• Source Loading and Management Model

(SLAMM)

LID Sizing Tools• National LID manual technique• 438 Guidance


EPA SWMM

Developer US EPA; Oregon State U.; Camp, Dresser and McKee (CDM)

Rainfall Modeled Single Event and Continuous

Watershed Size 1 to 100+ Acre Drainage Areas

Primary Use Flooding, Quantity, and Quality

Land Use & Source Area

User defined land uses and source areas

Application to LID Can be adapted to simulate LID controls, models storage and infiltration processes


SLAMM

Developer Dr. Robert Pitt, U of Alabama; John Voorhees

Rainfall Continuous

Watershed Size 10 to 100+ acre Drainage Areas

Land Uses Residential, Commercial, Industrial, Highway, Institutional, and other Urban

Source Areas Roofs, Sidewalks, Parking, Landscaped, Streets, Driveways, Alleys, etc.

Primary Use Runoff Quantity and Quality

Application to LID Infiltration, Wet Ponds, Porous Pavement, Street Sweeping, Biofiltration, Vegetated Swales, Other Urban Control Device


National LID Manual Technique

Developer US EPA; Prince George’s County

Rainfall Single Event

Watershed Size

Small Sites

Primary Use Estimates retention and detention requirement to meet quantity and peak flow goals

Application to LID

Applies to any BMP with retention storage: bioretention, infiltration, porous pavement, swales, and planters


Case Studies Used to Demonstrate Models

• Hypothetical DoD Housing Design– National method (PG County, MD

method)

– 438 Norfolk


National LID Manual Techniques

• Based on NRCS methods

• Uses peak storm event

• Nomographs that reflect graphical peak discharge method

Example New Development: Hypothetical DoD New Housing Design (Site A)

From LID UFC 2004 Version• 6.5 acres of

woods• Avg. slopes of

2%• Soils are Type B

– moderate infiltration

– typically silt loam or loam

• Drains to small channel & wetland



Proposed Conventional Design

• Clustered housing, classified as “Townhouse Residential District”

• Stormwater system is curb and gutter and closed drainage to stormwater pond

Proposed LID Site



Conventional


LID


Conventional


LID


Conventional


LID


Site A Project Objectives

• Integrate LID practices into a medium density residential housing area

• Hydrology goals:– Maintain the 2-year 24-hour and 10-year

24-hour peak runoff rate

– Provide water quality treatment

Calculate a Composite Curve Number (CN) for Pre-Developed

and Developed Conditions

Weighted CN = Sum of Products ÷ Drainage Area



CN Pre-Developed, Proposed, & LID Composite Conditions

Condition 10yr Runoff (in)Existing (CN = 63) 1.5

Proposed (CN = 80) 2.9

LID Composite CN (CN=73) 2.3


Condition CN Tc

Peak Discharge (CFS)Runoff depth

(in.)

2-yr (3” depth)

10-yr (5” depth)

2-yr 10-yr

Existing Condition

63 0.24 2 10 0.4 1.5

Proposed Condition – conventional CN

80 0.22 9 23 1.3 2.9

Proposed Condition using LID site design

73 0.24 6 17 0.9 2.3

Summary of Graphical Peak Discharge Results

Conventional vs. LID


Comparision of CN and Peak Discharge

0

5

10

15

20

25

30

Peak Discharge (CFS) 2-yearStorm 3” Depth

Peak Discharge (CFS) 10-yearStorm 5” Depth

Pea

k D

isch

arg

e (C

FS

) Conventional CN ()

LID Design

Existing Condition



Design Storm

Conventional Site Design

LID Site Design

Depth, in

Volume, ac-ft

Depth, in

Volume, ac-ft

2-yr 24-hr 0.52 0.28 0.28 0.15

10-yr 24-hr 0.85 0.46 0.53 0.29

Post-Development Storage Volumes




Post-Development Storage Volumes

– Use National Method to determine volume of retention and detention needed on site

• Assumes– Landuse is homogenous– Practices are evenly distributed

Flowchart similar to TR-55 Method



VS/VR


Runoff Equation Solution


Existing CN: 63

Proposed CN: 73

Required Retention Storage Volume

= (0.30in)(1ft/12in) (6.5 ac)

= 4.5 ac-ft

Maintaining Pre-Development Runoff Volume


Existing CN: 63

Proposed CN: 73

Required Retention Storage Volume =(0.50in)(1ft/12in) (6.5 ac)

= 0.27 ac-ft

Maintaining Pre-Development Runoff Volume


LID Techniques and Objectives

Choosing LID Practice techniques


Base Storage Capacity

Base & subbase materialsNo. 57 crushed stone base or similar 1.5 - 1/8 in. (38 – 3 mm)No. 2 crushed stone subbase or similar 2½ in. – ¾ in. (65 mm – 20 mm)

~ 30% to 40% void space

Quarry or lab provides % of voids - ASTM C 29 3 in. (75 mm) base stores ~1 in. (25 mm) water Design for 24 - 48 hour storage

Base

Subbase

Base

Subbase

Infiltration

Infiltration, detention& filtering

Impermeable Liner Option for filtering

Permeable Paver Water Management Options



Soil Infiltration

• Establish suitability • Soil maps• NRCS soil classification (ABCD)• USCS soil classification• Bracket infiltration, CBR (R-value) range• Conduct on-site infiltration tests• Use lowest (conservative) values for

preliminary “desk top” design


Subgrade Infiltration

Determining soil infiltration ratesDig holes on the siteApprox. top-of-subgrade depth Double ring infiltrometer testUse lowest infiltration rate

Test areaMultiple test holes


Soil Design Strength

Design assumptions• Subgrade strength for vehicular traffic:

– Min. 96-hour soaked CBR = 5% – (Min. R-value = 11)

What if < 5%CBR? • Capping layer of geotextile and aggregate base

– Stabilize soil with cement – PCA guidelines– Use no exfiltration design


Base Sizing Steps

1 – Assess soil infiltration conditions, design storm depth, determine exfiltration option

2 – Compute increased runoff depth from area contributing to the PICP (if any), add rainfall depth falling directly on PICP

3 – Compute the depth of the base for storage: rule-of-thumb…1 in. (25 mm) rainfall storage requires 3 in. (75 mm) of base/subbase depth

4 – Compute the maximum base depth for drainage in 24 – 48 hours given conservative soil infiltration rate (safety factor = 2)

5 – If needed, revise base depth to accommodate storage and site area limitations

6 – Design perforated pipes at base to drain non-infiltrated water

7 – Design overflow for rainfalls exceeding the design storm


Base Sizing Steps (cont’d)

8 – Determine the base/subbase thickness for traffic using Figure 18 (see next slide)

9 – Compare required traffic base/subbase thickness water storage thickness: always use thicker one

10 – Check clearance from bottom of subbase to seasonal high water table (> 2 ft)

11 – Check geotextile filter criteria


Step 8 – Check the structural base thickness

Fig. 18 - Base thickness guidance


ICPI Permeable Design Pro Software


EISA Section 438 Guidance

• 95th Percentile Storm Event

• NRCS*• Continuous

Simulation (SWMM/HSPF)

NRCS/MDE Modeling Infiltration w/TR-20 -1983


Section 438Case Study

Land Use Acres Site Coverage Percent Building 0.90 56.3 Parking 0.35 21.9

Streets/Sidewalks 0.20 12.5 Undeveloped 0.15 9.3

Total 1.60 100%

95th Percentile Storm Event Method

Conditions


Parameters and Results

• Bioretention and Permeable Pavements only options due to operational requirements. Considers high water table and poor soils

• 30% of site available for “permeability”

• 2,000 sf of bioretention and 7,500 sf of permeable pavement

• 75% storm event managed

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