Small Storm Hydrology The Integration of Water Quality and Drainage Design Objectives

Transcription

1 Small Storm Hydrology 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 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 , the Clean Water Act, 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. Importance of Site Hydrology in the Design of Stormwater Controls Many stormwater monitoring configurations used over the years 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. 1

2 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 QP) 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). Medium Density Residential Sites Low Density Residential Sites Pitt, et al. (2000) 2

3 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 3

4 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 Ponding during very intense rain in area having sandy soils. Pitt

5 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. 5

6 WI DNR Double-Ring Infiltrometer Test Results (in/hr), Oconomowoc (mostly A and B soils) Initial Rate Final Rate Range of Observed Rates 11 to to to to to to to to to to to to 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 Compacted sandy soils Noncompacted and dry clayey soils All other clayey soils (compacted and dry, plus all wetter conditions)

7 Long-Term Sustainable Average Infiltration Rates (3 of 15 textures tested) Soil Texture Sand Silt Clay Compaction Method Hand Standard Modified Hand Standard Modified Hand Standard Modified Dry Bulk Density (g/cc) n/a n/a Effects on Root Growth (per NRCS) Ideal Ideal May affect - May affect May affect + Restrict May affect n/a n/a Long-term Average Infilt. Rate (in/hr) Very high Very high Observed vs. Predicted Runoff at Madison Maintenance Yard Outfall Natural forces and management attempts to increase infiltration in compacted soils. Nature much better at this than we are. Predicted Runoff (in) Observed Runoff (in) 7

8 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) Probability distribution of rains (by count) and runoff (by depth). Birmingham Rains: <0.5 : 65% of rains (10% of runoff) Same pattern in other parts of the country, just shifted. 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) Pitt, et al. (2000) 8

9 Design Issues (<0.5 inches) Fishing in urban waters also occurs, both for recreation and for food. 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 WI DNR photo Children frequently play in urban creeks, irrespective of their designation as water contact recreation waters 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 WI DNR photo 9

10 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. 10

11 Calculated Benefits of Various Roof Runoff Controls (compared to typical directly connected residential pitched roofs) Green roof, Portland, OR 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% Frequent high flows after urbanization Design Issues (0.5 to 1.5 inches) WI DNR photo Majority of annual runoff volume and pollutant discharges Occur approximately every two weeks Problems: Produce moderate to high flows Produce frequent high pollutant loadings 11

12 Suitable Controls for Treatment of Runoff from Intermediate- Sized Rains (0.5 to 1.5 in.) Rain Garden Designed for Complete Infiltration of Roof Runoff 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 Soil Modifications for rain gardens and other biofiltration areas can significantly increase treatment and infiltration capacity compared to native soils. Percolation areas or ponds, infiltration trenches, and French drains can be designed for larger rains due to storage capacity, or small drainage areas. (King County, Washington, test plots) 12

13 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 13

14 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. WI DNR photos 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. 14

15 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. 15

16 Combinations of Controls Needed to Meet Many Stormwater Management Objectives Smallest storms should be captured on-site for reuse, or infiltrated Design controls to treat runoff that cannot be infiltrated on site Provide controls to reduce energy of large events that would otherwise affect habitat Provide conventional flood and drainage controls Pitt, et al. (2000) 16