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-runoffquality 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) Initial Rate 25 22 14.7 5.8 5.7 4.7 4.1 3.1 2.6 0.3 0.3 0.2 0 0 0 Final Rate 15 17 9.4 9.4 9.4 3.6 6.8 3.3 2.5 0.1 1.7 0 0.6 0 0 Range of Observed Rates 11 to 25 17 to 24 9.4 to 17 0.2 to 9.4 5.1 to 9.6 3.1 to 6.3 2.9 to 6.8 2.4 to 3.8 1.6 to 2.6 0 to 0.3 0.3 to 3.2 0 to 0.2 0 to 0.6 all 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 of tests Average infiltration rate (in/hr) COV Noncompacted sandy soils 36 13 0.4 Compacted sandy soils 39 1.4 1.3 Noncompacted and dry clayey soils 18 9.8 1.5 All other clayey soils (compacted and dry, plus all wetter conditions) 60 0.2 2.4
Long-Term Sustainable Average Infiltration Rates (3 of 15 textures tested) Soil Texture Compaction Method Dry Bulk Density (g/cc) Effects on Root Growth (per NRCS) Long-term Average 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)
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) Annual Birmingham, AL, rains (1.4 m) compared to Seattle, WA, rains (0.84 m), and Phoenix, AZ, rains (0.24 m) Flat roofs instead of pitched roofs Cistern for reuse of runoff for toilet flushing and irrigation (3m D x 1.5 m H) Planted green roof Disconnect roof drains to loam soils Rain garden with amended soils (3m x 2m) Annual roof runoff volume reductions 13/21/25% 66/67/88% 75/77/84% 84/87/91% 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.
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
Infrequent very high flows are channel-forming and may cause severe bank erosion and infrastructure damage. WI DNR photos
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. Example of monitored rain and runoff distributions during NURP. Similar plots for all locations, just shifted. Large rains (>1-1/2 in.) need energy reduction and flow attenuation for habitat protection and for flood control.
100 90 80 70 60 50 40 30 20 10 EXCELLENT GOOD FAIR POOR Riparian Integrity Biotic Integrity 45 40 35 30 25 20 15 10 5 Benthic Index of Biotic Integrity (B-IBI) 0 0 0 10 20 30 40 50 60 70 80 90 100 Watershed Urbanization (%TIA) 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.
Figure and Table from Center of Watershed Protection Urban Steam Classification Sensitive 0 10% Imperviousness Impacted 11 25% Imperviousness Damaged 26 100% Imperviousness Channel Stability Stable Unstable Highly Unstable Aquatic Life Biodiversity Good/Excellent Fair/Good Poor
Relationship between Directly Connected Impervious Areas, Volumetric Runoff Coefficient, and Expected Biological Conditions Rv 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Poor Good Fair 1 10 100 Directly Connected Impervious Area (%) Sandy Soil Rv Silty Soil Rv Clayey Soil Rv
WinSLAMM v 9.2 Output Summary
Hours of Exceedence of Developed Conditions with Zero Runoff Increase Controls Compared to Predevelopment Conditions (MacRae (1997) Recurrence Interval (yrs) Existing Flowrate (m 3 /s) Exceedence for Predevelopment Conditions (hrs per 5 yrs) Exceedence for Existing Development Conditions, with ZRI Controls (hrs per 5 yrs) Exceedence for Ultimate Development Conditions, with ZRI Controls (hrs per 5 yrs) 1.01 (critical mid-bankfull conditions) 1.24 90 380 900 1.5 (bankfull conditions) 2.1 30 34 120
Poor 0.29 12 21 67 5120 RES Little Shades Creek Poor 0.61 3.4 61 36 228 COM ALJC 012 Poor 0.30 7.9 28 64 133 Resid. Med. Dens. ALJC 010 Poor 0.37 12 34 54 102 Resid. High Dens. ALJC 009 Poor 0.51 7.3 53 40 721 IND ALJC 002 Poor 0.67 2.8 72 25 341 IND ALJC 001 Expected Biological Conditions of Receiving Waters Vol. Runoff Coeff. (Rv) Disconnected Impervious Areas (%) Directly Connected Impervious Areas (%) Pervious Areas (%) Area (ac) Major Land Use Watershed ID
Flow-Duration Curves for Different Stormwater Conservation Design Practices 140 Flow Duration Curves are Ranked in Order of Peak Flows 120 100 Top Set: No Controls Swales Discharge (cfs) 80 60 Middle Set: Pond Pond and Swales 40 20 Bottom Set: Biorentention Swales and Bioretention Pond and Bioretention Pond, Swales and Bioretention 0 0.1 1 10 100 % Greater than Discharge Rate
Cost Effectiveness of Stormwater Control Practices for Runoff Volume Reductions 80 70 Pond 60 $/1000 cu. Ft Reduced 50 40 30 20 Pond and Swale Swale Pond and Bioretention Bioretention Pond, Swales and Bioretention Swales and Bioretention 10 0 0 20 40 60 80 Max % Runoff Reduced
Example of Stormwater Control Implementation No controls Pond Only Swales Only Bioretention Only Pond, Swales and Bioretention Annualized Total Costs ($/year/ac) 0 118 404 1974 2456 Runoff Coefficient (Rv) 0.61 0.60 0.54 0.26 0.20 % Reduction of Total Runoff Volume Discharges n/a 1.4% 10% 58% 67% Unit Removal Costs for Runoff Volume ($/ft 3 ) n/a 0.07 0.03 0.03 0.03 Expected biological conditions in receiving waters (based on Rv) poor poor poor poor fair 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.
60 Flow Rate (gpm per acre pavement) 50 40 30 20 10 0 0 20 40 60 80 100 Percent of Annual Flow Less than Flow Rate (Seattle 1991) Flow rates for Seattle, WA
100 Percent of Annual Flow Treated (Seattle 1991) 90 80 70 60 50 40 30 20 10 0 10 100 Treatment Flow Rate (gpm per acre of pavement) Treatment flow rates needed for Seattle, WA
450 Flow Rate (gpm per acre pavement) 400 350 300 250 200 150 100 50 0 0 20 40 60 80 100 Percent of Annual Flow Less than Flow Rate (Atlanta 1999) Flow rates for Atlanta, GA
100 Percent of Annual Flow Treated (Atlanta 1999) 90 80 70 60 50 40 30 20 10 0 10 100 1000 Treatment Flow Rate (gpm per acre of pavement) Treatment flow rates needed for Atlanta, GA
Annual Flow Rate Distributaries (gpm/acre pavement) Flow Rate Needed for Different Levels of Annual Flow Treatment (gpm/acre pavement) Location 50 th Percentile 70 th Percentile 90 th Percentile 50% 70% 90% Seattle, WA 16 28 44 10 18 30 Portland, ME 31 52 80 18 30 53 Milwaukee, WI 35 60 83 20 35 65 Phoenix, AZ 38 60 150 20 35 90 Atlanta, GA 45 65 160 25 40 100
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.