Module 3: Rainfall and Hydrology for Construction Site Erosion Control

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1 Module 3: Rainfall and Hydrology for Construction Site Erosion Control Robert Pitt Department of Civil, Construction, and Environmental Engineering University of Alabama Tuscaloosa, AL Rainfall and Hydrology Factors Affecting Erosion Rates Rainfall energy (rain intensity and duration) Tractive force (shear stress) of sheet and channel flow Runoff depth and velocity (to calculate shear stress for specific site conditions)

2 Hydrology Parameters Needed for the Design of Construction Site Erosion Control Practices Mulches water velocities and water depth Ditch liners water velocities and water depth Slope down shoots peak flow rates Diversion dikes and swales peak flow rates Filter fabric fences water velocities and hydrographs Sediment ponds water volume and hydrographs Thronson (1973) presented the following equation to estimate the erosion potential for individual rains: P dur R Accumulative Percentage of Total R P = rain depth in inches and, dur = rain duration in hours Rain Depth (inches) 3 4 >4 Huntsville Birmingham Tuscaloosa Montgomery Mobile Percentage of annual rains greater than 1 inch (there are about 100 to 125 rains a year in Alabama) Percentage of annual rains greater than 5 inches (there are about 100 to 125 rains a year in Alabama)

3 Number of Large Rains per Month in Birmingham, AL, over 41.5 year period 2.00 to to to 4.00 over 4.01 total January February March April May June July August September October November December Alabama Rainfall Conditions Are there specific rains that are most important from an erosion control viewpoint? (yes, about 5 rains a year are responsible for about half of the annual erosion. These are greater than about 1.5 inches in depth) Are there seasons of the year that are less prone to erosion than other times? (not really; June, July and August, plus October and November generally have had fewer of these large rains, but they may occur during any month) Intensity-Duration-Frequency curves used for hydrology calculations. 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 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 infiltration. 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. Annual rainfall variations over Alabama (inches). The dots show the locations of the rain gages used in the analysis.

4 Birmingham Intensity - Duration - Frequency (IDF) Curve Time of Concentration (t c ) The duration must be equal to the time of concentration for the drainage area. The time of concentration (t c ) is equal to the longest flow path (by time). If the t c 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. If the t c for this same return period was 40 min, the peak rain intensity would be only 3.8 in/hr. Rainfall Frequency Rainfall frequency is commonly expressed as the average 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 20% chance of occurring in any one year. Multiple rare events may occur in any one year, but that is not very likely.

5 SCS (NRCS) Rainfall Distributions Data shown for Mobile, AL Zones of Different Rainfall Distributions Rainfall Distributions in the Southeastern U.S.

6 Probability of design storm (design return period) not being exceeded during the project life (design period). As an example, if a project life was 5 years, and a storm was not to be exceeded with a 90% probability, a 50 year design return period storm must be used. Probability of storm not being exceeded in a one year (T d ) construction period 50% 2.0 year 75% % 10 95% 20 Design storm return period (T) Different Hydrologic Calculation Methods SCS (NRCS) TR-55 Curve Number Model of Rainfall vs. Runoff Output Requirements Drainage Area Appropriate Method Peak Discharge Only Up to 20 acres Up to 2,000 acres Up to 5 square miles Up to 20 square miles Peak Discharge and Up to 2,000 acres Total Runoff Volume Up to 5 square miles Up to 20 square miles Runoff Hydrograph Up to 5 square miles Up to 20 square miles Rational Method 2 NRCS/SCS TR-20 Method 3 NRCS/SCS TR-55 Tabular Method 4 NRCS/SCS TR-55 Graphical Peak Discharge Method 5 COE HEC-1 Method (HEC- HMS)

7 Typical curve number (CN) values for urban areas. Typical CN Values for Pastures, Grasslands, and Woods Watershed Delineation Example X

8 Watershed Delineation Example Watershed Delineation Hints X The determination of a watershed s area begins with the analysis of a topographic map of the region. The most downstream point of interest (a potential dam site, a culvert location, the outlet of a stream, where a stream reaches a river, etc.) is located. The area contributing flow to that site is then identified by application of a few simple rules: Water flows downhill Water tends to flow perpendicularly across the contour lines Ridges are indicated by contour V s pointing downhill Drainages are indicated by contour V s pointing upstream. Watershed Delineation Process Information Sources USGS Topographic Maps The fundamental source of data for delineating and studying watersheds is the U.S. Geological Survey Quadrangle map. Each Quad Sheet map covers 7.5 minutes of longitude and latitude. These maps give a wealth of information including topographic contour lines, locations of cities, buildings, roads, road types, railroads, pipelines, water bodies, forested land, stream networks, and USGS stream gauging stations and benchmarks. Quad sheet maps typically have a scale of 1:24,000 (i.e. 1 inch on the map = 24,000 inches on land) Depending on the age of the map, elevation data may be in English or Metric units. Typically, in the Midwest, the contour intervals of the elevation data are 5 feet or 1.5 meter, in the south, the contour intervals may be 20 ft. For watershed delineation, quad sheet maps offer us the best starting point. Detailed site maps having 1 to 5 ft contour intervals are usually required for final analyses.

9 1. Trace out the main drainage pathways upstream from the point of concern. 2. Using a different color, trace the drainage pathways draining away from the area. 3. Extend the obvious drainage paths and locate the peaks in the contours between these drainage systems 4. Starting at the bottom, connect the peaks between the drainage systems along the ridges to delineate the watershed boundary. HSG A B C D Site soil information also needed for TR-55 Soil Textures Sand, loamy sand, or sandy loam Silt, silt loam or loam Sandy clay loam Clay loam, silty clay loam, sandy clay, silty clay, or clay Group A soils have low runoff potential and high infiltration rates, even when thoroughly wetted. They consist chiefly of deep, well to excessively drained sands or gravels and have a high rate of water transmission (greater than 0.30 in/hr). Group B soils have moderate infiltration rates when thoroughly wetted and consist chiefly of moderately deep to deep, moderately well to well drained soils, with moderately fine to moderately coarser textures. These soils have a moderate rate of water transmission (0.15 to 0.30 in/hr).

10 Soil Surveys Group C soils have low infiltration rates when thoroughly wetted and consist chiefly of soils with a layer that impedes downward movement of water and soils with moderately fine to fine textures. These soils have a low rate of water transmission (0.05 to 0.15 in/hr). Group D soils have high runoff potential. They have very low infiltration rates when thoroughly wetted and consist chiefly of clay soils with a high swelling potential, soils with a permanent high water table, soils with a claypan or clay layer at or near the surface, and shallow soils over nearly imperious material. These soils have a very low rate of water transmission (0 to 0.05 in/hr). Soil surveys have been generated by the U.S. Department of Agriculture (National Resource Conservation Service formerly the Soil Conservation Service) and are typically available through the county extension office. Information typically available in a soil survey: Soil type by general area Descriptions of the various soil types Tables of information regarding the various soil types Soil classification (Hydrologic Soil Group A, B, C, and D) Soil Survey Background Information: Soil scientists observed the steepness, length, and shape of the slopes; the general pattern of drainage; the kinds of crops and native plants; and the kinds of bedrock. They dug many holes to study the soil profile, which is the sequence of natural layers, or horizons, in a soil. The profile extends from the surface down into the material in which the soil formed. The material is devoid of roots and other living organisms and has not been changed by other biological activity. Some Alabama Soil Surveys are available on-line at: (some, including Jefferson and Tuscaloosa counties, are just scans of the older reports and maps) General Soil Map for Alabama

11 Tuscaloosa, AL, soil survey (USDA), Cripple Creek Church area. Small numbers are the soil types, while the larger numbers are the quadsheet section numbers. An example of an older USDA soil map (uses symbols to describe soil groups, slopes, and existing severity of erosion)

12 Soil number (name) and depth Hydrologic Soil Group Depth to Bedrock (inches) Permeability (in/hr) Erosion Factor, k Tolerable Soil Loss, T (tons/ac/yr) Organic Matter (%) 21 (Montevallo) D Soil Building Site Development Limitations Shallow Excavations Local Streets and Roads Dwellings with Basements Lawns and Landscaping 23 (Nauvoo) B (Smithdale) B > (Montevallo) 23 (Nauvoo) 33 (Smithdale) Severe (depth to rock, slope) Slight Severe (slope) Moderate (low strength) Severe (depth to rock, slope) Slight Moderate (slope) Moderate (slope) Moderate (slope) Severe (droughty, slope, thin soil layer) Slight Moderate (slope) Time of Concentration (t c ) The duration must be equal to the time of concentration for the drainage area. The time of concentration (t c ) is equal to the longest flow path (by time). If the t c 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. If the t c for this same return period was 40 min, the peak rain intensity would be only 3.8 in/hr. Time of Concentration Estimates The TR-55 procedures estimate t c using three flow segment types: Sheetflow (maximum of 300 ft; WinTR-55 has a maximum of 150 ft now, similar to some state agency restrictions) Shallow concentrated flow (paved or unpaved surfaces Channel flow (using Manning s equation) Candidate t c pathways are drawn on the site map and the travel times for the three flow segments of each pathway are calculated. The t c for the drainage area is the longest travel time calculated.

13 Area of watershed contributing runoff as a function of flow travel time (T t ) In this example, 13 minutes is the watershed time of concentration, but almost all of the watershed area is contributing runoff at 9 or 10 minutes. McCuen 1989 McCuen 1989 Only a rain duration equal to the T c produces the maximum peak runoff rate at the critical rain intensity. Shorter duration rains do not produce runoff from the complete area, while longer duration rains do not have any additional contributing areas. Rains having durations equal to the T c must be used in drainage designs as they produce the critical intensity for the area and the level of service (likelihood of failure in any one year). Longer duration rains have lower intensities for the same service, while shorter duration rains do not have the complete drainage area contributing flows during that time period.

14 Figure illustrating sheetflow travel time for dense grass surfaces, for varying slopes and flow lengths. nl P S T t Figure illustrating shallow concentrated flow velocities for paved and unpaved surfaces and for different slopes. Channel Flow using Manning s Equation: r V n s V = average velocity (ft/s), and r = hydraulic radius (ft) and is equal to a/pw a = cross sectional flow area (ft2) pw = wetted perimeter (ft) s = slope of hydraulic grade line (channel slope, ft/ft) n = Manning roughness coefficient (for open channel flow) NRCS Travel Time Example: A-B sheetflow (100 ft) B-C shallow concentrated flow (1,400 ft) C-D channel flow (7,300 ft)

15 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 Runoff curve number (and associated initial abstraction) Layout of Subwatersheds for NRCS Example

16 The initial abstraction values (mostly detention storage) are a direct function of the curve number. The dimensionless unit hydrograph is selected from tables in TR-55 Example Urban Watershed 1) subdivide the watershed into relatively homogeneous subareas

17 2) calculate the drainage area for each subarea: I 0.10 mi 2 II 0.08 III 0.6 IV 0.32 Total: ) calculate the time of concentration (Tc) for each subarea I 0.2 hrs II 0.1 III 0.3 IV 0.1 4) calculate the travel time (Tt) from each subarea discharge location to the location of interest (outlet of total watershed in this example): I 0.1 hrs II 0.05 III 0.05 IV 0 5) select the curve number (CN) for each subarea: I Strip commercial, all directly connected CN = 97 II Medium density residential area, grass swales CN = 46 III Medium density residential area, curbs and CN = 72 gutters IV Low density residential area, grass swales CN = 40 9) determine Ia for each subarea (assumes Ia = 0.2 S) (SCS table 5-1): 6) rainfall distribution: Type II for all areas 7) 24-hour rainfall depth for storm: 4.1 inches 8) calculate total runoff (inches) from CN and rain depth (from SCS fig. 2-1) I CN = 97 P = 4.1 in. Q = 3.8 in. II CN = 46 P = 4.1 in. Q = 0.25 III CN = 72 P = 4.1 in. Q = 1.5 IV CN = 40 P = 4.1 in. Q = 0.06 I CN = 97 Ia = in. II CN = 46 Ia = in. III CN = 72 Ia = in. IV CN = 40 Ia = in. 10) calculate the ratio of Ia to P I Ia/P = 0.062/4.1 = II Ia/P = 2.348/4.1 = 0.57 III Ia/P = 0.778/4.1 = 0.19 IV Ia/P = 3.000/4.1 = ) use worksheets SCS 5a and 5b to summarize above data and to calculate the composite hydrograph

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21 Example Applications to Construction Sites Upslope and downslope drainage areas for construction site. Subdrainages on and near construction site affecting control design. Upslope Subdrainage Area Characteristics Calculation objective Acres Cover n Flow path slope CN t c (min) U1 Direct to site Hydrograph (to be combined with U2 and U3) % U2 Diversion to site stream Peak flow rate and hydrograph (to be combined with U1 and U3) U3 Diversion to site stream Peak flow rate and hydrograph (to be combined with U1 and U2)

22 Upslope Subdrainage Area TR-55 Calculations ( 5-year storm) Calculations for areas draining to sediment pond Area Notation Location U1 Upslope direct to on site stream U2 Upslope diversion to on site stream U3 Upslope diversion to on site stream Direct Runoff, Q (inches) area-depth (AmQ), (mi 2 - inches) Peak unit area flow rate (csm/in) Peak discharge (ft 3 /sec) Etc. Acceptable Levels of Protection for Different Site Activities (1 year construction period) Example Acceptable Failure Rate Design Storm Return Period 24-hr Rain Depth Diversion 25% 6.5 years 5.5 inches channels Main site 5% channel Site slopes 10% Site filter 50% fence Sediment pond 5% and 1% 20 and and 8.4

23 Runoff Water Depth (needed for sheetflow calculations on slopes for slope protection) qn y 1.49s y is the flow depth (in feet), q is the unit width flow rate (Q/W, the total flow rate, in ft 3 /sec, divided by the slope width, in ft.) n is the sheet flow roughness coefficient, and s is the slope (as a fraction) Homework Problem: 1) Determine the watershed delineation above the cross on the attached map (hint: it goes off the map very slightly on the bottom edge). 2) Calculate the peak runoff rate for the 25 year storm (Birmingham area), assuming B soils, and good wood cover (make other reasonable assumptions as needed). 3) Conduct a similar watershed analysis for the watershed containing your construction site (subdividing the area into upstream, on-site, and downstream areas). Note: 20 ft contour intervals Watershed boundary based on topographic information. Highlighted main drainage above location of interest, and adjacent drainages.

24 Subdivided into 3 subwatersheds for more appropriate hydrologic analyses. Need area, CN, and Tc for each surbarea, then can use WinTR-55 to calculate the hydrographs from each and the combined hydrograph. Need to evaluate alternative flow drainage pathways to identify the likely Tc. In this case, four candidate paths are examined. Subwatershed A area: 96 acres CN (B soils and good woods cover): 55 Tc: 0.7 hrs alternative flow paths need to be evaluated (there is no specific channel, so only sheetflow and shallow concentrated flow) flowpath A1 (woods, light underbrush): sheetflow: 150 ft at 4.7% 19 min shallow conc flow: 1800 ft at 4.7 % 3.9 ft/sec and 1800 ft = 460 sec = 8 min 950 ft at 0.5% 1.1 ft/sec and 950 ft = 860 sec = 14 min total travel time: 41 min = 0.7 hrs flowpath A3 (woods, light underbrush): sheetflow: 150 ft at 3.1% 22 min shallow conc flow: 1030 ft at 6.8% 3.9 ft/sec and 1030 ft = 260 sec = 4 min 1420 ft at 2.1% 2.0 ft/sec and 1420 ft = 710 sec = 12 min total travel time: 38 min = 0.6 hrs flowpath A4 (woods, light underbrush): sheetflow: 150 ft at 1.9% 26 min shallow conc flow: 370 ft at 1.9% 2.2 ft/sec and 370 ft = 170 sec = 3 min 2540 ft at 3.2% 2.8 ft/sec and 2540 ft = 910 sec = 15min total travel time: 44 min = 0.7 hrs flowpath A2 (woods, light underbrush): sheetflow: 150 ft at 7.8% 18 min shallow conc flow: 860 ft at 5.8% 3.3 ft/sec and 860 ft = 260 sec = 4 min 950 ft at 0.5% 1.1 ft/sec and 950 ft = 860 sec = 14 min total travel time: 36 min = 0.6 hrs

25 Subwatershed B area: 139 acres CN (B soils and good woods cover): 55 Tc: 1.2 hrs alternative flow paths need to be evaluated (there is no specific channel, so only sheetflow and shallow concentrated flow) flowpath B1 (woods, light underbrush): sheetflow: 150 ft at 1.4% 31 min shallow conc flow: 740 ft at 1.4 % 1.5 ft/sec and 740 ft = 490 sec = 8 min 3100 ft at 1.3% 1.6 ft/sec and 3100 ft = 1940 sec = 32 min total travel time: 71 min = 1.2 hrs flowpath B2 (woods, light underbrush): sheetflow: 150 ft at 1.4% 31 min shallow conc flow: 820 ft at 8.5% 5.0 ft/sec and 820 ft = 160 sec = 3 min 2110 ft at 1.4% 2.0 ft/sec and 2110 ft = 1060 sec = 18 min total travel time: 52 min = 0.9 hrs flowpath B3 (woods, light underbrush): sheetflow: 150 ft at 1.0% 35 min shallow conc flow: 240 ft at 3.0% 2.8 ft/sec and 240 ft = 85 sec = 1 min 3820 ft at 1.3% 2.0 ft/sec and 3820 ft = 1910 sec = 32 min total travel time: 68 min = 1.1 hrs

26 Subwatershed C area: 134 acres CN (B soils and good woods cover): 55 Tc: 0.7 hrs flowpath C1 (woods, light underbrush): sheetflow: 150 ft at 2.4% 9 min shallow conc flow: 2530 ft at 3.6 % 2.8 ft/sec and 2530 ft = 900 sec = 15 min 1550 ft at 0.7% 1.4 ft/sec and 1550 ft = 1110 sec = 18 min total travel time: 42 min = 0.7 hrs flowpath C2 (woods, light underbrush): sheetflow: 150 ft at 9.1% 14 min shallow conc flow: 830 ft at 8.3% 3.8 ft/sec and 830 ft = 220 sec = 4 min 1550 ft at 0.7% 1.4 ft/sec and 1550 ft = 1110 sec = 18 min total travel time: 36 min = 0.6 hrs

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