Engineering Approach to Green Infrastructure

T. Andrew Earles, Ph.D., P.E., D.WREWright Water Engineers, Inc.

Why do we care about stormwater quality?

The Regulation of Storm Water by Decade

» 1960s

» 1970s

» 1980s

» 1990s

» 2000s

» 2010s

Phase I and II Storm Water Regulations» Phase I

» Phase II

Four Step Process

Runoff Reduction

Directly Connected Impervious Area

This is what we want to avoid!

Runoff Reduction

» Minimize Directly Connected Impervious Area

Runoff Directed to Landscape

Runoff Reduction

» Reduce Impervious Area

UIA

Conceptual Model

UIA = Unconnected Impervious AreaDCIA = Directly Connected Impervious Area

RPA = Receiving Pervious AreaSPA = Separate Pervious Area

Effective Imperviousness Adjustments

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Effe

ctiv

e Im

perv

ious

ness

Total Imperviousness

Directly Connected 2-year Level 2 10-year Level 2 100-year Level 2

Based on typical development patterns & 4-cover Imperviousness model

Treat the WQv

Treat the Water Quality Volume (WQv)

Equivalent to capturing and treating roughly 90% of all runoff on an annual basis in Iowa

Source: Iowa Stormwater Management Manual

Ames, Iowa Rainfall Summary1960-2006

“Maximized” Water Quality Volume

Volumetric Runoff Coefficients (Rv)

Adjustment Factors for MDCIA

Treat the Water Quality Volume (WQv)--Example

Approximately 4,500 ft3 Wqv required per acre of impervious area.

Recharge Volume (Rev)

The Rev is a part of the WQv and can be achieved by MDCIA or a WQv practice such as bioiretention.

Stabilize Drainageways

Water Quality, Channel Protection & Flooding

Stream Channel Stability

Lane’s Stable Channel Concept

The Channel Protection Storage Volume (Cpv) is the volume required for extended detention release (24-hour) of the 1-year, 24-hour storm

Implement Source Controls

Source Control BMPs

BMP Photo Gallery

Two Key Aspects of Performance: Improving BMP Maintenance and Implementing Maintainable BMPs

When preparing landscaping plans for BMPs, remember that access is necessary.

Swales that are too flat can lead to ponding and nuisance conditions.

Runoff cannot get to BMP because curb is not cut.

Soggy bottom may be due to shallow groundwater, poor draining soils and/or overcompaction.

Hydrology and Impervious Area

Hydrologic Cycle

Source: Sustainable Sites Initiative

Hydrologic Budget Components

GAINS

» Precipitation» Runoff» Groundwater Gains

LOSSES

» Interception and Depression Storage

» Evaporation and Transpiration

» Infiltration» Groundwater Withdrawals» Diversions

Source: Kansas Biological Survey, Native Prairie Hay Meadows

Large Storm

Higher and More Rapid Peak Discharge

Small Storm

More Runoff Volume

Lower and Less Rapid Peak

Higher BaseflowGradual Recession

Pre-developmentPost-development

TIME

STR

EAM

FLO

W R

ATE

IMPACT OF URBANIZATION ON STREAMFLOW

Source: Schuler, T.R. 1987. Controlling Urban Runoff: A Practical Manual for Planning and Designing BMPs. Washington DC: Metro Council of Governments.

Large Storm Higher and More

Rapid Peak Discharge

More Runoff Volume

Lower and Less Rapid Peak

Higher Baseflow Gradual Recession

Pre-developmentPost-development

Why are Stormwater Management and LID Important?

Source: Federal Interagency Stream Restoration Working Group 1998

How do you quantify impervious area?

Source: UDFCD

Source: UDFCD

Nothing is as effective as site level drainage mapping & analysis

What Do You Need? Why Do You Need It? What Does It

Tell Us?

Typical Design Information Needs»

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Drainage Basin Mapping—Watershed Scale

Land Use and Imperviousness Mapping

Plan for future imperviousness

Regional, Sub-regional and Onsite

Regional Approach»»

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Source: Arapahoe County (Colorado) Stormwater Management Manual

Regional, Sub-regional and Onsite

Sub-regional Approach»

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Source: Arapahoe County (Colorado) Stormwater Management Manual

Regional, Sub-regional and Onsite

Onsite Approach»

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Source: Arapahoe County (Colorado) Stormwater Management Manual

Site-scale Topography and Mapping

Geotechnical Data

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Soil Classification and Characterization

Sieve Size Analysis

Soil Survey Mapping and Data

Soil Testing Parameters for Successful Revegetation

» pH»»»»

» Electrical Conductivity (EC): »»

» Free Lime»

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Soil Testing Parameters for Successful Revegetation» Nitrogen(N) - Phosphorus(P) - Potassium(K)

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» Saturation Percentage»»

»» Organic Matter Content

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Soil Testing Parameters for Successful Revegetation

» Cation Exchange Capacity (CEC)»

» Sodium Absorption Ratio (SAR) »

» Soil Texture by Hydrometer Method»

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