Objectives for Urban for Drainage Design Systems are Varied • Ensure personal safety (minimize local flooding) • Minimize economic damage (water in Robert Pitt homes and businesses and nuisance Department of Civil and Environmental Engineering conditions) University of Alabama • Preserve environmental health (aquatic life, Tuscaloosa, AL non-contact recreation, aesthetics)

Design Considerations Land Development Results in Increased Use appropriate design tools for the job at hand: Peak Flow Rates and Runoff Volumes • Problems arise when trying to use drainage design hydrology models for water quality analyses. • As an example, TR-55 greatly under predicts flows from Developed area small : 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 Similar undeveloped area small and intermediate sized rains when investigating water quality problems. Large Small Rain

1 Historical concerns focused on increased flows during rains and Historical approach to urban drainage has been devastating to associated flooding. However, decreased flows during dry environment and recharge of periods are now seen to also cause receiving water problems.

Factors Affecting Runoff Rainfall Loss Components • 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 runoff . The rainfall amount affects the volume of runoff. • Soil conditions – antecedent moisture conditions generally affects the rate of the rainfall falling on the ground. Soil texture and compaction (structure) usually has the greatest effect on the ifilinfiltrati on. • Surface cover – the type and condition of the soil Rainfall Losses (remainder of rainfall occurs as runoff): surface cover affects the rain energy transferred to - Initial abstractions: these losses must be satisfied before any runoff the soil surface and can affect the infiltration rate occurs (interception, detention storage, flash evaporation, etc.) also. - Long-term losses: mostly infiltration

2 Initial Abstractions Evaporation Losses

US Weather Bureau Class A evaporation pan

Map showing annual Micro-scale “puddles” on rough pavement evaporation in SW

Example of design of integrated program to meet many objectives Infiltration Losses •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 Example of monitored rain and runoff protection and for flood Double-ring infiltromenter to measure infiltration rates in soil distributions during NURP. Similar plots for control. all locations, just shifted.

3 “Green roof” can be used to enhance Roof downspout disconnections enhance infiltration of runoff and reduce losses and delay runoff (Portland, OR) volume and help recharge (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).

4 Porous paver blocks have been used in many locations to reduce runoff Bioretention areas can be located between buildings and parking areas to combined systems, reducing overflow frequency and volumes to infiltrate almost all roof and paved area runoff (Portland, OR). (Germany, Sweden, and WI).

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.

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

Example Calculations (using SLAMM) to Predict the Wet Detention Ponds to Treat Benefits of Alternative Roof Runoff Control Options (% Large Flows reduction of annual roof runoff)

Phoenix, Seattle, WA Birmingham, AZ (9.3 ((y)33.4 in/yr) AL (52.5 in/yr) in/yr) Roof garden (1 in/hr amended 96% 100% 87% soils, 60 ft2/home)

Cistern for on-site reuse of roof 88 67 66 runoff (375ft3/home)

Disconnect roof runoff for 91 87 84 infiltration into silty soil

Green roof (vegetated roof 84 77 75 surface)

6 Dry ponds and extended detention ponds having large storage Wetlands can be used to provide additional water quality control, capacities to reduce runoff energy and peak flow rates. enhance habitat, and increase infiltration for .

Berlin, Germany

Mature Wetlands and Wet Detention The potential for groundwater contamination associated with stormwater infiltration is often asked. Pond Facility, Malmo, Sweden

Roadcut showing direct recharge of Edwards , Austin, TX

7 Screening Model Developed to Evaluate Groundwater Contamination Potential: Knowing the Runoff Volume is the Key to Estimating Pollutant Mass Contamination potential is the lowest rating of the influencing factors: • There is usually a simple relationship • Surface infiltration with no pretreatment (grass swales or bidhdffdhbetween rain depth and runoff depth. roof disconnections) – Mobility and abundance most critical • Changes in rain depth affect the relative • Surface infiltration with sedimentation pretreatment contributions of runoff and pollutant mass (treatment train: percolation pond after wet detention pond) discharges: – Mobility, abundance, and treatability all important – Directly connected impervious areas contribute • Subsurface injection with minimal pretreatment most of the flows during relatively small rains (infiltration trench in parking lot or dry ) – Abundance most critical – Disturbed urban soils may dominate during larger rains

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

8 Example Intensity - Duration - Frequency (IDF) Curve Rainfall Frequency

• Rainfall frequency is commonly expressed as the averaggpe return period of the event. • 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 220%0% cchancehance ooff occuoccurringrring in aanyny oonene yeayear.r. • Multiple rare events may occur in any one year, but that is not very likely.

Developed by S. Rocky Durrans

9 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 ppjroject life (desi gn period).

As an example, if a project life was 5 years, and a storm was nottt to b e exceeded with a 90% probability, a 50 year design return period storm must be used.

10 SCS (NRCS) TR-55 Curve Number Model of Rainfall vs. Runoff Methods Used to Calculate Runoff in Urban Areas for Drainage Design • 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 Mo de l) (M et calf & Edd y, etlt al., 1971; CDM 2003) • Many currently available proprietary models use these methods.

Typical Typical CN Values for Pastures, Grasslands, and Woods curve number (CN) values for urban areas.

11 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]

Use Models for Intended Use and Time of Concentration (tc) within Acceptable Range • The duration must be equal to the time of • Drainage design methods only suitable for concentration for the drainage area. relatively large rains (typically larger than 2 • The time of concentration (tc) is equal to the longest or 3 inches of rainfall) flow path (by time). • If the tc is 5 min for a storm having a return period of • Cannot use these methods for water quality 25 years, the associated peak intensity (which has a investigations (which require procedures dudurationration ooff 5 minmin)) wouwouldld be about 8.6 inin/hr./hr. that are suitable for smaller rains) • If the tc for this same return period was 40 min, the peak rain intensity would be “only” 3.8 in/hr.

12 Correct Time of Concentration Estimates Figure illustrating are Crucial for Drainage Design sheetflow travel time • The TR-55 procedures estimate tc using three for dense grass flow segment types: surfaces, for varying – Sheetflow (maximum of 300 ft) sldfllopes and flow – Shallow concentrated flow (paved or unpaved lengths. 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.

NRCS Travel Time Example: Figure illustrating A-B sheetflow (100 ft) shallow concentrated B-C shallow concentrated flow (1,400 ft) flow velocities for paved C-D channel flow (7,300 ft) and unpaved surfaces and for different slopes.

13 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 volume , and the hydrograph. • Information needed includes: – Drainage area (square miles) – Time of concentration (hours) – Travel time through downstream segments (hours) – 24-hr rainfall total for design storm – Rainfall distribution type – (and associated initial abstraction)

Layout of Subwatersheds for NRCS Example

14 The dimensionless unit hydrograph is selected from tables in TR-55 The initial abstraction values (mostly detention storage) are a direct function of the curve number.

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