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Hydrology for Drainage Design

Robert PittDepartment of Civil and Environmental Engineering

University of AlabamaTuscaloosa, AL

Objectives for Urban Drainage Systems are Varied

• Ensure personal safety (minimize local• Ensure personal safety (minimize local flooding)

• Minimize economic damage (water in homes and businesses and nuisance conditions)

• Preserve environmental health (aquatic life, non-contact recreation, aesthetics)

Design ConsiderationsUse appropriate design tools for the job at hand:

• Problems arise when trying to use drainage design hydrology models for water quality analyses.

• As an example, TR-55 greatly under predicts flows from small rains: NRCS recommends that TR-55 not be used for rains less than 0.5 inch.

• Most drainage models assume that all/most flows originate from directly connected impervious areas, with very little originating from pervious areas.

• Most stormwater managers overlook the importance of small and intermediate sized rains when investigating water quality problems.

Land Development Results in Increased Peak Flow Rates and Runoff Volumes

Developed area

Similar undeveloped area

Large Rain Small Rain

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Historical concerns focused on increased flows during rains and associated flooding. However, decreased flows during dry periods are now seen to also cause receiving water problems.

Historical approach to urban drainage has been devastating to environment and recharge of groundwaters

Factors Affecting Runoff• Rainfall – The duration of the storm and the

distribution of the rainfall during the storm are the two major factors affecting the peak rate of runofftwo major factors affecting the peak rate of runoff. The rainfall amount affects the volume of runoff.

• Soil conditions – antecedent moisture conditions generally affects the infiltration rate of the rainfall falling on the ground. Soil texture and compaction (structure) usually has the greatest effect on the i fil iinfiltration.

• Surface cover – the type and condition of the soil surface cover affects the rain energy transferred to the soil surface and can affect the infiltration rate also.

Rainfall Loss Components

Rainfall Losses (remainder of rainfall occurs as runoff):- Initial abstractions: these losses must be satisfied before any runoff occurs (interception, detention storage, flash evaporation, etc.)- Long-term losses: mostly infiltration

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Initial Abstractions

Micro-scale “puddles” on rough pavement

Evaporation Losses

US Weather Bureau Class A evaporation pan

Map showing annual evaporation in SW

Infiltration Losses

Double-ring infiltromenter to measure infiltration rates in soil

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.

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“Green roof” can be used to enhance evapotranspiration losses and delay runoff (Portland, OR)

Roof downspout disconnections enhance infiltration of runoff and reduce volume and help recharge groundwater (AL, WI, and Sweden).

Rain Garden Designed for Complete Infiltration of Roof Runoff Grass Swales Designed to Infiltrate Large Fractions of Runoff (AL, WI, and OR).

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Porous paver blocks have been used in many locations to reduce runoff to combined systems, reducing overflow frequency and volumes (Germany, Sweden, and WI).

Bioretention areas can be located between buildings and parking areas to infiltrate almost all roof and paved area runoff (Portland, OR).

Other types of on-site infiltration can be built to fit in area (MD and WI). 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 compacted, probably more than by moisture saturation.

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Developed soil modifications that result in greatly enhanced infiltration in marginal soils.

Example Calculations (using SLAMM) to Predict the Benefits of Alternative Roof Runoff Control Options (%

reduction of annual roof runoff)

Phoenix, AZ (9.3

Seattle, WA (33.4 in/yr)

Birmingham, AL (52.5 (

in/yr)( y ) (

in/yr)Roof garden (1 in/hr amended soils, 60 ft2/home)

96% 100% 87%

Cistern for on-site reuse of roof runoff (375ft3/home)

88 67 66

Disconnect roof runoff for infiltration into silty soil

91 87 84

Green roof (vegetated roof surface)

84 77 75

Wet Detention Ponds to Treat Large Flows

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Dry ponds and extended detention ponds having large storage capacities to reduce runoff energy and peak flow rates.

Wetlands can be used to provide additional water quality control, enhance habitat, and increase infiltration for groundwater recharge.

Berlin, Germany

Mature Wetlands and Wet DetentionPond Facility, Malmo, Sweden

The potential for groundwater contamination associated with stormwater infiltration is often asked.

Roadcut showing direct recharge of Edwards Aquifer, Austin, TX

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Contamination potential is the lowest rating of the influencing factors:

Screening Model Developed to Evaluate Groundwater Contamination Potential:

• Surface infiltration with no pretreatment (grass swales or roof disconnections)– Mobility and abundance most critical

• Surface infiltration with sedimentation pretreatment (treatment train: percolation pond after wet detention pond)– Mobility, abundance, and treatability all important

• Subsurface injection with minimal pretreatment (infiltration trench in parking lot or dry well)– Abundance most critical

Knowing the Runoff Volume is the Key to Estimating Pollutant Mass

• There is usually a simple relationship b i d h d ff d hbetween 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

Where is this stuff coming from and where should we locate controls?

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Rainfall Frequency• Rainfall frequency is commonly expressed as the

average return period of the event.g p• The value should be expressed as the probability of

that event occurring in any one year.• As an example, a 100-yr storm, has a 1% chance of

occurring in any one year, while a 5-yr storm has a 20% chance of occurring in any one year. 20% chance of occurring in any one year.

• Multiple rare events may occur in any one year, but that is not very likely.

Example Intensity - Duration - Frequency (IDF) Curve

Developed by S. Rocky Durrans

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SCS (NRCS) Rainfall Distributions Zones of Different Rainfall Distributions

Rainfall Distributions in the Southeastern U.S. Probability of design storm (design return period) not being exceeded during the project life (design p j ( gperiod).

As an example, if a project life was 5 years, and a storm

t t bwas not to be exceeded with a 90% probability, a 50 year design return period storm must be used.

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Methods Used to Calculate Runoff in Urban Areas for Drainage Design

• Rational Method (Mulvaney 1851 in• Rational Method (Mulvaney, 1851, in Ireland; Kuichling, 1889, in the US)

• NRCS TR-20 and TR-55 (SCS 1975; 1982; 1986)

• US EPA SWMM (Stormwater Management M d l) (M t lf & Edd t l 1971 CDMModel) (Metcalf & Eddy, et al., 1971; CDM 2003)

• Many currently available proprietary models use these methods.

SCS (NRCS) TR-55 Curve Number Model of Rainfall vs. Runoff

Typical curve number (CN) values for

burban areas.

Typical CN Values for Pastures, Grasslands, and Woods

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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:

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

Use Models for Intended Use and within Acceptable Range

• Drainage design methods only suitable for• Drainage design methods only suitable for relatively large rains (typically larger than 2 or 3 inches of rainfall)

• Cannot use these methods for water quality investigations (which require procedures that are suitable for smaller rains)

Time of Concentration (tc)• The duration must be equal to the time of

concentration for the drainage area.g• The time of concentration (tc) is equal to the longest

flow path (by time).• If the tc is 5 min for a storm having a return period of

25 years, the associated peak intensity (which has a duration of 5 min) would be about 8.6 in/hr.duration of 5 min) would be about 8.6 in/hr.

• If the tc for this same return period was 40 min, the peak rain intensity would be “only” 3.8 in/hr.

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Correct Time of Concentration Estimates are Crucial for Drainage Design

• The TR-55 procedures estimate tc using three flow segment types:– Sheetflow (maximum of 300 ft)– Shallow concentrated flow (paved or unpaved

surfaces– Channel flow (using Manning’s equation)

• Candidate tc pathways are drawn on the site map and the travel times for the three flow segments are calculated.

• The tc for the drainage area is the longest travel time calculated.

Figure illustrating sheetflow travel time for dense grass surfaces, for varying l d flslopes and flow

lengths.

Figure illustrating shallow concentrated flow velocities for paved and unpaved surfaces and for different slopes.

NRCS Travel Time Example:A-B sheetflow (100 ft)B-C shallow concentrated flow (1,400 ft)C-D channel flow (7,300 ft)

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Tabular Hydrograph Method• The NRCS TR-55 Tabular Hydrograph Method uses

watershed information and a single design storm to predict the peak flow rate the total runoff volumepredict the peak flow rate, the total runoff volume, and the hydrograph.

• Information needed includes:– Drainage area (square miles)– Time of concentration (hours)

Travel time through downstream segments (hours)– Travel time through downstream segments (hours)– 24-hr rainfall total for design storm– Rainfall distribution type– Runoff curve number (and associated initial abstraction)

Layout of Subwatersheds for NRCS Example

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The initial abstraction values (mostly detention storage) are a direct function of the curve

bnumber.

The dimensionless unit hydrograph is selected from tables in TR-55

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