Conventional Stormwater Management vs. LID

Carmen Agouridis, Ph.D., P.E.Assistant Professor

Biosystems and Agricultural EngineeringUniversity of Kentucky

January 5, 2012

Overview

Philosophy of conventional stormwater management

Philosophy of LID

LID Tools

Benefits and limitations of each

Conventional (Gray) Stormwater Management

Collect

Convey

Concrete

Site Drainage

Quickly drain stormwater from site using curbs, gutters and pipes

Typically does not address water quality – high pollutant transport capacity

Off‐site Flood Control

Alteration to natural hydrology increases off‐site flooding potential

Primarily use detention basins to control peak flows for given storm size (e.g. 10 year) “End of pipe” control 

approach

Hydrograph 1: Pre-development

Hydrograph 2: Post-development w/ no stormwater BMPs

Hydrograph 3: Post-development w/ conventional stormwater BMPs

Impact of Stormwater on Streams

LID Stormwater Management 

Runoff volume control Increase in imperviousness = increase in runoff volume for a 

given size storm

Peak runoff rate control Time of concentration

Time it takes flow to reach basin                                                                   outlet from most hydraulically                                                                remote point in watershed

Water quality control Treatment of at least first ½ inch 

of runoff via retention

LID Tools: Reduce Impervious Areas

Alternate roadway layout

Change road design  Width

Permeable pavement

Rooftops Vertical vs. horizontal construction

Density

Driveways Sharing, limiting size, minimizing building setbacks, permeable 

pavement

LID Tools: Minimize Connected Impervious Areas

Direct roof drain and driveway flows to vegetated areas

Encouraging sheet flow through vegetated areas

Break‐up flow directions from large paved areas

LID Tools: Increase Drainage Flow Paths

Factors affecting time of concentration Travel distance

Slope

Surface roughness

Maximize overland sheet flow

Use vegetated open swales

Remember …

Spread it

Slow it

Soak it

Store it

LID Techniques to Reduce Runoff Volume (Reduce Curve Number)

Limit use of s

idew

alks

Red

uce road le

ngth and w

idth

Red

uce drivew

ayleng

th and w

idth

Cons

erve natural re

source

sarea

s

Minim

izedisturba

nce

Prese

rve high

er in

filtrationso

ils

Prese

rve na

tural d

epression area

s

Use veg

etated swales

Prese

rve ve

getation

Land cover type X X X X

% Imperviousness X X X

Hydrologic soil group

X X

Natural hydrologic condition

X X X X X

Disconnectivity ofimpervious area

X X X

LID Techniques to Maintain Pre‐development Time of Concentration

LID Technique

LID Objective

On‐lot b

ioretention

Wider

and fla

tter 

swales

Maintain sh

eet flow

Clusters of trees and 

shrubs in flow path

Prov

ide tree

cons

erva

tion/tran

sitio

n zo

nes

Minim

ize storm drain 

pipe

s

Disco

nnec

t impe

rvious 

area

s

Save tree

s

Preserve existing 

topo

grap

hy

Minimize disturbance

X X X X X X X X

Flatten grades X X XX

maybe

Reduce height of slopes

XX

maybe

Increase flow path(divert and redirect)

X XX

X X X

Increase roughness X X X X X X X

Hydrologic Parameter Conventional LID

Impervious cover Increased Minimized

Vegetation/naturalcover

Decreased Maximized

Time of concentration ShortenedMaximized; 

predevelopmentconditions

Runoff volume IncreasedMinimized; 

predevelopmentconditions

Peak dischargeControlled to design 

storm

Minimized; predevelopment

conditions

Rainfall abstractions (interception, 

infiltration, depression storage)

DecreasedMinimized; 

predevelopmentconditions

Groundwater rechargeDecreased Minimized; 

predevelopmentconditions

LID Considerations

Zoning and regulatory statutes Minimum street width and school buses, ambulances, and 

garbage trucks

Safety concerns regarding ice formation

Mosquito concerns

Economics Building and maintenance costs

Who pays?

LID Considerations

Ownership of captured pollutants Heavy metal accumulation

Harvest vegetation

Periodic removal of mulch and media

Urban sprawl

Aesthetics