Module 14: Small Storm Hydrology, Continuous Simulations and Treatment Flow Rates The Integration of Water Quality and Drainage Design Objectives Robert Pitt, Ph.D., P.E., DEE Department of Civil, Construction, and Environmental Engineering University 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 late 1960s, 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 configurations used 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(Q 2 +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
Residential Sites High Density
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) Initial Rate Final Rate Range of Observed Rates 25 15 11 to 25 22 17 17 to 24 14.7 9.4 9.4 to 17 5.8 9.4 0.2 to 9.4 5.7 9.4 5.1 to 9.6 4.7 3.6 3.1 to 6.3 4.1 6.8 2.9 to 6.8 3.1 3.3 2.4 to 3.8 2.6 2.5 1.6 to 2.6 0.3 0.1 0 to 0.3 0.3 1.7 0.3 to 3.2 0.2 0 0 to 0.2 0 0.6 0 to 0.6 0 0 all 0 0 0 all 0
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 Number Average COV of tests infiltration rate (in/hr) Noncompacted sandy 36 13 0.4 soils Compacted sandy soils 39 1.4 1.3 Noncompacted and dry 18 9.8 1.5 clayey soils All other clayey soils 60 0.2 2.4 (compacted and dry, plus all wetter conditions)
Long-Term Sustainable Average Infiltration Rates (3 of 15 textures tested) Soil Compaction Dry Bulk Effects on Long-term Method Density Root Growth Average Texture (g/cc) (per NRCS) Infilt. Rate (in/hr) Sand Hand 1.451 Ideal Very high Standard 1.494 Ideal Very high Modified 1.620 May affect - 80 Silt Hand 1.508 May affect 18 Standard 1.680 May affect + 0.9 Modified 1.740 Restrict 0.08 Clay Hand 1.241 May affect 3.0 Standard n/a n/a 0 Modified n/a n/a 0
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 3.0 2.5 Predicted Runoff (in) 2.0 1.5 1.0 0.5 - - 0.5 1.0 1.5 2.0 2.5 3.0 Observed Runoff (in)
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)
3 1.5 0.5
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) 3” 8” 0.5” >8”: <0.1% of rains (2% of runoff)
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) Annual Birmingham, AL, rains (1.4 m) Annual roof compared to Seattle, WA, rains (0.84 m), and runoff volume reductions Phoenix, AZ, rains (0.24 m) Flat roofs instead of pitched roofs 13/21/25% Cistern for reuse of runoff for toilet flushing 66/67/88% and irrigation (3m D x 1.5 m H) Planted green roof 75/77/84% Disconnect roof drains to loam soils 84/87/91% Rain garden with amended soils (3m x 2m) 87/100/96%
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
Frequent high flows after urbanization WI DNR photo
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.
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