Module 14: Small Storm Hydrology, Continuous Simulations

and Treatment Flow RatesThe Integration of Water Quality and

Drainage Design Objectives

Robert Pitt, Ph.D., P.E., DEEDepartment of Civil, Construction, and Environmental

EngineeringUniversity of Alabama

Tuscaloosa, AL, USA 35487

Urban Stormwater Hydrology History• Early focus of urban stormwater was on storm sewer and flood control design using the Rational Method and TR-55 (both single event, “design storm” methods).

• The Curve Number procedure was developed in the 1950s by the (then) SCS as a simple tool for estimating volumes generated by large storm events in agricultural areas, converted to urban uses in mid 1970s (TR55 in SCS 1976). Data based on many decades of observations of large storms in urban areas, at Corps of Engineers monitoring locations. Data available from the Rainfall-Runoff database report prepared by the Univ. of Florida for the EPA.

• Water quality focus results form Public Law 92-500, the Clean Water Act, 1972. Stormwater quality research started in the late1960s, with a few earlier interesting studies. Big push with Nationwide Urban Runoff Program (NURP) in late 70s and early 80s. Most still rely on earlier drainage design approaches.

Many stormwater monitoring configurationsused over the years

Importance of Site Hydrology in the Design of Stormwater Controls

• Design of stormwater management programs requires knowledge of site hydrology

• Understanding of flows (variations for different storm conditions, sources of flows from within the drainage area, and quality of those flows), are needed for effective design of source area and outfall controls.

The following equation can be used to calculate the actual NRCS curve number (CN) from observed rainfall depth (P) and runoff depth (Q), both expressed in inches:

CN = 1000/[10+5P+10Q-10(Q2+1.25QP)1/2]

The following plots use rainfall and runoff data from the EPA’s NURP projects in the early 1980s (EPA 1983), and from the EPA’s rainfall-runoff-quality data base (Huber, et al. 1982).

Low Density Residential Sites

Pitt, et al. (2000)

Medium Density Residential Sites

High Density Residential Sites

Highway Sites

Knowing the Runoff Volume is the Key to Estimating Pollutant Mass

• There is usually a simple relationship between rain depth and runoff depth.

• Changes in rain depth affect the relative contributions of runoff and pollutant mass discharges:– Directly connected impervious areas contribute

most of the flows during relatively small rains– Disturbed urban soils may dominate during

larger rains

Source Characteristics of Stormwater Pollutants

• Quality of sheetflows vary for different areas.

• Need to track pollutants from sources and examine controls that affect these sources, the transport system, and outfall.

Street dirt washoff and runoff test plot, Toronto

Pitt 1987

Runoff response curve for typical residential street, Toronto

Pitt 1987

Ponding during very intense rain in area having sandy soils.

Disturbed Urban Soils during Land Development

Road shoulder soil compaction due to parked cars along road.

Soil modifications can result in greatly enhanced infiltration in marginal soils.

Direct measurements of turf runoff for different soil conditions.

WI DNR Double-Ring Infiltrometer Test Results (in/hr), Oconomowoc (mostly A and B soils)

all 000all 000

0 to 0.60.600 to 0.2 00.20.3 to 3.21.70.30 to 0.30.10.31.6 to 2.62.52.62.4 to 3.83.33.12.9 to 6.86.84.13.1 to 6.33.64.75.1 to 9.69.45.70.2 to 9.49.45.89.4 to 179.414.717 to 24172211 to 251525

Range of Observed RatesFinal RateInitial Rate

Infiltration Rates in Disturbed Urban Soils (AL tests)

Sandy Soils Clayey Soils

Recent research has shown that the infiltration rates of urban soils are strongly influenced by compaction, probably more than by moisture saturation.

Infiltration Measurements for Noncompacted, Sandy Soils (Pitt, et al. 1999)

Infiltration Rates during Tests of Disturbed Urban Soils

2.40.260All other clayey soils (compacted and dry, plus all wetter conditions)

1.59.818Noncompacted and dry clayey soils

1.31.439Compacted sandy soils

0.41336Noncompacted sandy soils

COVAverage infiltration rate (in/hr)

Number of tests

Long-Term Sustainable Average Infiltration Rates (3 of 15 textures tested)

Very highVery high80

IdealIdealMay affect -

1.4511.4941.620

HandStandardModified

Sand

180.90.08

May affectMay affect +Restrict

1.5081.6801.740

HandStandardModified

Silt

3.000

May affectn/an/a

1.241n/an/a

HandStandardModified

Clay

Long-term Average Infilt. Rate (in/hr)

Effects on Root Growth (per NRCS)

Dry Bulk Density (g/cc)

Compaction Method

SoilTexture

Natural forces and management attempts to increase infiltration in compacted soils. Nature much better at this than we are.

Observed vs. Predicted Runoff at Madison Maintenance Yard Outfall

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Observed Runoff (in)

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icte

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Design Issues Related to Storm Size

• Recognize different objectives of storm drainage systems • Recognize associated rainfall conditions affecting different

objectives• Select the appropriate tools for design• Example - 4 major rainfall categories for Milwaukee, WI:

<0.5 in (<12 mm)0.5 to 1.5 in (12 to 40 mm)1.5 to 3 in (40 to 75 mm)>3 in (>75 mm)

0.5 1.5 3

Probability distribution of rains (by count) and runoff (by depth).

Birmingham Rains:<0.5”: 65% of rains(10% of runoff)

0.5 to 3”: 30% of rains(75% of runoff)

3 to 8”: 4% of rains(13% of runoff)

>8”: <0.1% of rains(2% of runoff)

0.5” 3” 8”

Same pattern in other parts of the country, just shifted.

Pitt, et al. (2000)

Design Issues (<0.5 inches)• Most of the events (numbers of rain storms)• Little of annual runoff volume• Little of annual pollutant mass discharges• Probably few receiving water effects• Problem:

– pollutant concentrations likely exceed regulatory limits (especially for bacteria and total recoverable heavy metals) for each event

Fishing in urban waters also occurs, both for recreation and for food.

WI DNR photo

Children frequently play in urban creeks, irrespective of their designation as water contact recreation waters

WI DNR photo

Suitable Controls for Almost Complete Elimination of Runoff Associated with

Small Rains (<0.5 in.)

• Disconnect roofs and pavement from impervious drainages

• Grass swales• Porous pavement walkways• Rain barrels and cisterns

Roof drain disconnections

Grass-Lined Swales

Ponds, rain barrels and cisterns for stormwater storage for irrigation and other beneficial uses.

Rural airport and rural home near Auckland, New Zealand, examples

Simple porous paver blocks used for walkways, overflow parking, and seldom used access roads.

Green roof, Portland, OR

Calculated Benefits of Various Roof Runoff Controls (compared to typical directly connected residential pitched roofs)

87/100/96%Rain garden with amended soils (3m x 2m)

84/87/91%Disconnect roof drains to loam soils

75/77/84%Planted green roof

66/67/88%Cistern for reuse of runoff for toilet flushing and irrigation (3m D x 1.5 m H)

13/21/25%Flat roofs instead of pitched roofs

Annual roof runoff volume reductions

Annual Birmingham, AL, rains (1.4 m) compared to Seattle, WA, rains (0.84 m), andPhoenix, AZ, rains (0.24 m)

Design Issues (0.5 to 1.5 inches)

• Majority of annual runoff volume and pollutant discharges

• Occur approximately every two weeks• Problems:

• Produce moderate to high flows• Produce frequent high pollutant loadings

WI DNR photo

Frequent high flows after urbanization

Suitable Controls for Treatment of Runoff from Intermediate-

Sized Rains (0.5 to 1.5 in.)

• Initial portion will be captured/infiltrated by on-site controls or grass swales

• Remaining portion of runoff should be treated to remove particulate-bound pollutants

Rain Garden Designed for Complete Infiltration of Roof Runoff

Soil Modifications for rain gardens and other biofiltration areas can significantly increase treatment and infiltration capacity compared to native soils.

(King County, Washington, test plots)

Percolation areas or ponds, infiltration trenches, and French drains can be designed for larger rains due to storage capacity, or small drainage areas.

Bioretention and biofiltration areas having moderate capacity

Temporary parking or access roads supported by turf meshes, or paver blocks, and advanced porous paver systems designed for large capacity.

Wet detention ponds, stormwater filters, or critical source area controls needed to treat runoff that cannot be infiltrated.

Design Issues (1.5 to 3 inches)

• Larger events in category are drainage design storms• Establishes energy gradient of streams• Occurs approximately every few months (once to

twice a year)• Problems:

– Unstable streambanks– Habitat destruction from damaging flows– Sanitary sewer overflows– Nuisance flooding and drainage problems/traffic

hazards

WI DNR photos

Infrequent very high flows are channel-forming and may cause severe bank erosion and infrastructure damage.

High flows may cause separate sewer overflows (SSOs), resulting in the discharge of raw sewage.

Controls for Treatment of Runoff from Drainage Events (1.5 to 3 in.)• Infiltration and other on-site controls will

provide some volume and peak flow control• Treatment controls can provide additional

storage for peak flow reduction • Provide adequate stormwater drainage to

prevent street and structure flooding• Provide additional storage to reduce magnitude

and frequency of runoff energy• Capture sanitary sewage overflows for storage

and treatment

Storage at treatment works may be suitable solution in areas having SSOs that cannot be controlled by fixing leaky sanitary sewerage.

Golf courses can provide large volumes of storage.

Design Issues (> 3 inches)• Occur rarely (once every several years to

once every several decades, or less frequently)

• Produce relatively little of annual pollutant mass discharges

• Produce extremely large flows and the largest events exceed drainage system capacity

WI DNR photo

Controls for Treatment of Runoff from Very Large

Events (> 3 in.)

• Provide secondary surface drainage system to carefully route excess flood water away from structures and roadways

• Restrict development in flood-prone areas

Appropriate Combinations of Controls• No single control is adequate for all problems• Only infiltration reduces water flows, along with soluble

and particulate pollutants. Only applicable in conditions having minimal groundwater contamination potential.

• Wet detention ponds reduce particulate pollutants and may help control dry weather flows. They do not consistently reduce concentrations of soluble pollutants, nor do they generally solve regional drainage and flooding problems.

• A combination of bioretention and sedimentation practices is usually needed, at both critical source areas and at critical outfalls.

Example of design of integrated program to meet many objectives

•Smallest rains (<0.5 in.) are common, but little runoff. Exceed WQ standards, but these could be totally infiltrated.

•Medium-sized storms (0.5 to 1-1/2 in.) account for most of annual runoff and pollutant loads. Can be partially infiltrated, but larger rains will need treatment.

•Large rains (>1-1/2 in.) need energy reduction and flow attenuation for habitat protection and for flood control.

Example of monitored rain and runoff distributions during NURP. Similar plots for all locations, just shifted.

Relationship between basin development, riparian buffer width, and biological integrity in Puget Sound lowland streams. (From May, C.W. Assessment of the Cumulative Effects of Urbanization on Small Streams in the Puget Sound Lowland Ecoregion: Implications for Salmonid Resource Management. Ph.D. dissertation, University of Washington, Seattle. 1996.

EXCELLENT

GOOD

FAIR

POOR

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Riparian IntegrityBiotic Integrity

PoorFair/GoodGood/ExcellentAquatic Life Biodiversity

Highly UnstableUnstableStableChannel Stability

Damaged26–100%

Imperviousness

Impacted11– 25%

Imperviousness

Sensitive0 – 10%

Imperviousness

Urban Steam Classification

Figure and Table from Center of Watershed Protection

00.10.20.30.40.50.60.70.80.9

1

1 10 100Directly Connected Impervious Area (%)

Rv

Sandy Soil Rv Silty Soil Rv Clayey Soil Rv

GoodFair

Poor

Relationship between Directly Connected Impervious Areas, Volumetric Runoff

Coefficient, and Expected Biological Conditions

WinSLAMM v 9.2 Output Summary

12034302.11.5 (bankfullconditions)

900380901.241.01 (critical mid-bankfullconditions)

Exceedence for Ultimate Development Conditions, with ZRI Controls (hrs per 5 yrs)

Exceedence for Existing Development Conditions, with ZRI Controls (hrs per 5 yrs)

Exceedence for Predevelopment Conditions (hrs per 5 yrs)

Existing Flowrate(m3/s)

Recurrence Interval (yrs)

Hours of Exceedence of Developed Conditions with Zero Runoff Increase Controls Compared to Predevelopment Conditions (MacRae(1997)

Poor0.291221675120RESLittle

Shades Creek

Poor0.613.46136228COM ALJC 012

Poor0.307.92864133Resid. Med. Dens.

ALJC 010

Poor0.37123454102Resid. High Dens.

ALJC 009

Poor0.517.35340721INDALJC 002

Poor0.672.87225341INDALJC 001

Expected Biological

Conditions of Receiving

Waters

Vol. Runoff Coeff. (Rv)

DisconnectedImperviousAreas (%)

DirectlyConnectedImperviousAreas (%)

PerviousAreas(%)

Area(ac)

MajorLand Use

WatershedID

Flow-Duration Curves for Different Stormwater Conservation Design Practices

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% Greater than Discharge Rate

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Top Set:No ControlsSwales

Bottom Set:BiorententionSwales and BioretentionPond and Bioretention Pond, Swales and Bioretention

Flow Duration Curves are Ranked in Order of Peak Flows

Middle Set:PondPond and Swales

Cost Effectiveness of Stormwater Control Practices for Runoff Volume Reductions

Swales andBioretention

Pond and Bioretention

Bioretention

Pond, Swales and Bioretention

Pond

Pond and Swale

Swale

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Max % Runoff Reduced

$/10

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Example of Stormwater Control Implementation

fairpoorpoorpoorpoorExpected biological conditions in receiving waters(based on Rv)

0.030.030.030.07n/aUnit Removal Costs for Runoff Volume ($/ft3)

67%58%10%1.4%n/a% Reduction of Total Runoff Volume Discharges

0.200.260.540.600.61Runoff Coefficient (Rv)

245619744041180Annualized Total Costs ($/year/ac)

Pond, Swales and

Bioretention

Bioretention Only

SwalesOnly

PondOnly

No controls

• Site ALJC 012

• Area 228 acres = 92.3 ha

• Bioretention devices give the greatest reduction in runoff volume discharged

• The biological conditions improved from “poor” to “fair” due to stormwater controls

• These graphs illustrate the relationships between the directly connected impervious area percentages and the calculated volumetric runoff coefficients (Rv) for each land use category (using the average land use characteristics), based on 43 years of local rain data.

• Rv is relatively constant until the 10 to 15% directly connected impervious cover values are reached (at Rv values of about 0.07 for sandy soil areas and 0.16 for clayey soil areas), the point where receiving water degradation typically is observed to start.

• The 25 to 30% directly connected impervious levels (where significant degradation is observed), is associated with Rv values of about 0.14 for sandy soil areas and 0.25 for clayey soil areas, and is where the curves start to greatly increase in slope.

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Percent of Annual Flow Less than Flow Rate (Seattle 1991)

Flow

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Flow rates for Seattle, WA

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Treatment Flow Rate (gpm per acre of pavement)

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cent

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91)

Treatment flow rates needed for Seattle, WA

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Percent of Annual Flow Less than Flow Rate (Atlanta 1999)

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Flow rates for Atlanta, GA

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Treatment Flow Rate (gpm per acre of pavement)

Perc

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low

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ated

(Atla

nta

1999

)

Treatment flow rates needed for Atlanta, GA

10040251606545Atlanta, GA

9035201506038Phoenix, AZ

653520836035Milwaukee, WI

533018805231Portland, ME

301810442816Seattle, WA

90%70%50%90th

Percentile 70th

Percentile 50th

Percentile Location

Flow Rate Needed for Different Levels of Annual Flow

Treatment (gpm/acre pavement)

Annual Flow Rate Distributaries (gpm/acre pavement)

Creating Flow-Duration Probability Plots in WinSLAMM

• Export 6-minute flow increment data (select this as an output option; was created to allow WinSLAMM to interface with hydraulic and drainage models, such as SWMM)

• Import this *.csv file into Excel (Office 2003 version limits the spreadsheet to about 65,000 rows, allowing only about 9 months of observations, suitable for a typical rain period in a northern area after selecting a typical rain year; Office 2007 allows 1,000,000 rows, allowing about 11 years of observations).

• WinSLAMM has a rain utility that assists in selecting the typical rain period. This utility sorts the rain years in a large mulit-year rain file by total annual rain totals, and calculates the residuals from the long-term average value. It also shows the monthly totals (depths and numbers of events) and compares those values to the long-term averages.

• Sort the flow column in descending order and remove all zero values (most of the flow increments will be zero, allowing possible appending new data sets if using older version of Excel to extend the analysis period).

• If a treatment flow rate is desired, then a candidate treatment flow rate (such as 25 gpm) is subtracted from each increment value (after unit conversions!).

• All negative results are removed (corresponding to when the treatment flow rates are larger than the actual flow, and all is treated).

• These excess values (flows that bypass the treatment device) are then summed for the whole analysis period and compared to the total flow that occurred during the period.

• These calculated percentages for each treatment flow rate are then plotted.

• If coarser flow-increment data is all that is needed, then the direct model output for the flow-duration option can be directly used, without using the higher resolution flow data and Excel.

Summary

• WinSLAMM output options and many of the built-in utilities enable a stormwater manager to investigate flow-duration conditions in many ways

• Continuous simulations, especially considering the effects of stormwater controls, over many decades are a very powerful tool.