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Chapter 6 Estimating Runoff and Stormwater Discharge Environmental Protection Handbook 6-i CHAPTER 6: ESTIMATING RUNOFF AND STORMWATER DISCHARGE TABLE OF CONTENTS 6.1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1 6.2 ESTIMATING RUNOFF - THE SCS CURVE NUMBER METHOD . . . . . . . . . . . . . . . . . . . . . . . 6-2 6.2.1 Factors Considered in Determining Runoff Curve Numbers . . . . . . . . . . . . . . . . . . . . . . . 6-3 6.2.1.a Hydrologic Soil Groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3 6.2.1.b Cover Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-4 6.2.1.c Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-4 6.2.1.d Hydrologic Condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-4 6.2.1.e Antecedent Runoff Condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-4 6.2.1.f Urban Impervious Area Modifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-4 6.2.1.g Connected Impervious Areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-4 6.2.1.h Unconnected Impervious Areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-5 6.2.1.i Rainfall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-5 6.2.1.j Runoff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-5 6.2.2 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-5 6.2.3 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-6 6.2.3.a Example 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-6 6.2.3.b Example 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7 6.2.3.c Example 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7 6.2.3.d Example 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-8 6.3 TIME OF CONCENTRATION AND TRAVEL TIME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-8 6.3.1 Factors Affecting Time of Concentration and Travel Time . . . . . . . . . . . . . . . . . . . . . . . . 6-9 6.3.1.a Surface Roughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-9 6.3.1.b Channel Shape and Flow Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-9 6.3.1.c Slope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-9 6.3.2 Computation of Travel Time and Time of Concentration . . . . . . . . . . . . . . . . . . . . . . . . . 6-9 6.3.3 Sheet flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-10 6.3.4 Shallow Concentrated Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-10 6.3.5 Open Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-11 6.3.6 Reservoirs, Lakes or Ponds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-11 6.3.7 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-11 6.3.8 Example 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-11 6.4 GRAPHICAL PEAK DISCHARGE METHOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-12 6.4.1 Peak Discharge Computation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-13 Chapter 6 Estimating Runoff and Stormwater Discharge 6-ii Environmental Protection Handbook 6.4.2 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-13 6.4.3 Example 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-13 6.5 TR-55 WORKSHEET SOLUTIONS TO EXAMPLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-13 6.6 FIGURES AND TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-20 Figure 6.1. Solution of runoff equation for SCS curve number method . . . . . . . . . . . . . . . . . 6-20 Figure 6.2. Flow chart for selecting appropriate figure or table for determining runoff curve numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-21 Table 6.1. Runoff depth for selected curve numbers (CN's) and rainfall amounts . . . . . . . . . . 6-22 Table 6.2.a. Runoff curve numbers for urban areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-23 Table 6.2.b. Runoff curve numbers for cultivated agricultural lands . . . . . . . . . . . . . . . . . . . . 6-24 Table 6.2.c. Runoff curve numbers for other agricultural lands . . . . . . . . . . . . . . . . . . . . . . . . 6-25 Table 6.2.d. Runoff curve numbers for arid and semiarid rangelands . . . . . . . . . . . . . . . . . . . 6-26 Table 6.3. Hydrologic soil groups of the U.S. Virgin Islands . . . . . . . . . . . . . . . . . . . . . . . . . . 6-26 Figure 6.3. Composite CN with connected impervious area . . . . . . . . . . . . . . . . . . . . . . . . . . 6-27 Figure 6.4. Composite CN with unconnected impervious areas and total impervious area less than 30% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-27 Figure 6.5. 1-year, 24-hour rainfall for the U.S. Virgin Islands . . . . . . . . . . . . . . . . . . . . . . . . 6-28 Figure 6.6. 2-year, 24-hour rainfall for the U.S. Virgin Islands . . . . . . . . . . . . . . . . . . . . . . . . 6-28 Figure 6.7. 5-year, 24-hour rainfall for the U.S. Virgin Islands . . . . . . . . . . . . . . . . . . . . . . . . 6-29 Figure 6.8. 10-year, 24-hour rainfall for the U.S. Virgin Islands . . . . . . . . . . . . . . . . . . . . . . . 6-29 Figure 6.9. 25-year, 24-hour rainfall for the U.S. Virgin Islands . . . . . . . . . . . . . . . . . . . . . . . 6-30 Figure 6.10. 50-year, 24-hour rainfall for the U.S. Virgin Islands . . . . . . . . . . . . . . . . . . . . . . 6-30 Figure 6.11. 100-year, 24-hour rainfall for the U.S. Virgin Islands . . . . . . . . . . . . . . . . . . . . . 6-31 Table 6.4. Roughness coefficients (Manning's n) for sheet flow . . . . . . . . . . . . . . . . . . . . . . . 6-31 Table 6.5. Ia values for runoff curve numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-32 Figure 6.12. Average velocities for estimating travel time for shallow concentrated flow . . . . . 6-33 Figure 6.13. Unit peak discharge for SCS type-III rainfall distribution . . . . . . . . . . . . . . . . . . 6-34 Table 6.6. Adjustment factor for pond and swamp areas that are spread throughout the watershed6-35 6.7 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-35 Chapter 6 Estimating Runoff and Stormwater Discharge Environmental Protection Handbook 6-1 [Eq. 6-1] [Eq. 6-2] [Eq. 6-3] CHAPTER 6: ESTIMATING RUNOFF AND STORMWATER DISCHARGE 6.1 INTRODUCTION Runoff occurs whenever there is precipitation in excess of that which can be absorbed by pervious materials (soils, etc.). Land cover types and changes to land cover types affect the amount of runoff and stormwater discharge that occurs in conjunction with a storm event. The conversion of rural land and open spaces to urban land uses usually increases soil erosion and the discharge and volume of stormwater runoff within a watershed. These increases are due to pervious surfaces being converted to impervious surfaces, natural water flow patterns being altered, and to other changes that occur to watersheds in response to precipitation. This chapter provides information to assist the developer, planner, landowner or regulator in estimating the amount of runoff and stormwater discharge from a watershed and the amounts of runoff and stormwater discharge due to changes to a watershed. The information in this chapter is taken from Urban Hydrology for Small Watersheds, U.S. Department of Agriculture Soil Conservation Service Engineering Technical Release 55 (TR-55) (USDA-SCS, 1986). Information in this chapter was compiled by Mario A. Morales, USDA Natural Resources Conservation Service (NRCS), Resource Conservation and Development Coordinator for the Virgin Islands. For more information, contact the Virgin Islands’ USDA-NRCS Field Office or your local Cooperative Extension Service office. 6.2 ESTIMATING RUNOFF - THE SCS CURVE NUMBER METHOD The SCS Curve Number (CN) Method is described in detail in USDA-SCS (1985). The equation used by the curve number method is of the form: where Q = runoff (inches) P = rainfall (inches) S = potential maximum retention after runoff begins (inches) Ia = initial abstraction (inches) The initial abstraction term (Ia) encompasses all losses that occur before runoff begins. It includes water retained in surface depressions, water intercepted by vegetation, evapotranspiration, and infiltration. Ia is highly variable but generally is correlated with soil and cover parameters. Through studies of many small agricultural watersheds, Ia was found to be approximated by the following empirical equation: By removing Ia as an independent parameter, this approximation allows the use of a combination of S and P to produce a unique runoff amount. Substituting equation 6-2 into equation 6-1 yields: Chapter 6 Estimating Runoff and Stormwater Discharge 6-2 Environmental Protection Handbook [Eq. 6-4] S is related to the soil and cover conditions of the watershed through a Curve Number (CN). CN has a range of 0 to 100, and S is related to CN by the equation: Figure 6.1 and Table 6.1 solve equations 6-3 and 6-4 for a range of curve numbers and rainfall amounts. 6.2.1 Factors Considered in Determining Runoff Curve Numbers The primary factors that determine CN are the hydrologic soil group (HSG), cover type, hydrologic condition, and antecedent runoff condition (ARC). Another factor to consider is whether impervious areas outlet directly to the stormwater drainage system (connected) or whether the flow spreads over pervious areas before entering the drainage system (unconnected). Figure 6-2 is provided to aid in selecting the appropriate figure or table for determining curve numbers. Curve numbers in Tables 6-2 (a-d) represent average antecedent runoff conditions for urban land uses, cultivated and other agricultural land uses, and arid and semiarid rangeland uses. Tables 6-2 assume impervious areas are directly connected. The following sections explain how to determine curve numbers and how to modify them for use with non- agricultural conditions. 6.2.1.a Hydrologic Soil Groups Infiltration rates of soils vary widely and are affected by subsurface permeability as well as surface intake rates. Soils are classified into four Hydrologic Soil Groups – A, B, C, and D (see Table 6.3) – according to their minimum infiltration rate, which is obtained for bare soil after prolonged wetting. Table 6.3 defines these groups and provides the hydrologic group classification for soils in the U.S. Virgin Islands. Most urban areas are only partially covered by impervious surfaces, thus, soils remain an important factor in runoff estimates. The effect of urbanization on runoff is greater in watersheds that have soils with high infiltration rates (sands and gravels) than in watersheds with predominately silt and clay soils, which generally have low infiltration rates. Any disturbance of a soil profile can significantly change infiltration characteristics. With urbanization, native soil profiles may be mixed or removed, or fill material from other areas may be introduced. Assistance in determining HSG for altered soils is available from the Natural Resources Conservation Service Field Office on St. Croix. The soils in the watershed of interest may be identified from the revised U.S. Virgin Islands Soil Survey (USDA-NRCS, 1995), which can be obtained from the USDA Service Center in Gallows Bay, St. Croix, from the UVI Conservation Data Center, St. Thomas (on CD-ROM), or from the UVI Cooperative Extension Service, St. Thomas. The text of the Soil Survey can also be downloaded in pdf format from www.statlab.iastate.edu/soils/soildiv/surveys/virgnis.pdf. 6.2.1.b Cover Type Tables 6.2 (a-d) address most cover types found in urbanizing areas, such as vegetation, bare soil, and impervious surfaces. There are a number of methods that can be used to determine cover type. The most common are field and aerial reconnaissance, photographs, and land use maps. Chapter 6 Estimating Runoff and Stormwater Discharge Environmental Protection Handbook 6-3 6.2.1.c Treatment Treatment is a modifier for cover type (used only in Table 6.2 (b)) that describes the management of cultivated agricultural lands. It includes mechanical practices, such as contouring and terracing, and management practices, such as crop rotations and reduced or no tillage. 6.2.1.d Hydrologic Condition Hydrologic condition indicates the effects of cover type and treatment on infiltration and runoff, and is generally estimated from plant density and residue cover on sample areas. Good hydrologic condition indicates that the soil usually has a low runoff potential for that specific hydrologic soil group, cover type, and treatment. Some factors to consider in estimating the effect of cover on infiltration and runoff are: canopy or density of lawns, crops or other vegetative areas; amount of year-round cover; amount of grass; percent of residue cover; and degree of surface roughness. 6.2.1.e Antecedent Runoff Condition The index of runoff potential before a storm event is the antecedent runoff condition (ARC). ARC is an attempt to account for the variation in curve number at a site from storm to storm. The curve number for the average ARC at a site is the median value as taken from sample rainfall and runoff data. The curve numbers in Tables 6.2 are for the average ARC, which is used primarily for design applications. See USDA-SCS (1985) and Rallison and Miller (1981) for a more detailed discussion of storm-to-storm variation and a demonstration of upper and lower enveloping curves. 6.2.1.f Urban Impervious Area Modifications Several factors, such as the percentage of impervious area and the means of conveying runoff from impervious areas to the stormwater drainage system, should be considered in computing the curve number for urban areas (Rawls et. al., 1981). For example, do the impervious areas connect directly to the drainage system or do they outlet onto lawns or other pervious areas where infiltration can occur? 6.2.1.g Connected Impervious Areas An impervious area is considered connected if runoff from that area flows directly into the stormwater drainage system. It is also considered connected if runoff from the area occurs as concentrated shallow flow that runs over a pervious area and then into a drainage system. Urban curve numbers (Table 6.2(a)) were developed for typical land use relationships based on specific assumed percentages of impervious area. These curve number values were developed on the assumptions that pervious urban areas are equivalent to pasture in good hydrologic condition and impervious areas have a curve number of 98 and are directly connected to the stormwater drainage system. Some assumed percentages of impervious area are shown in Table 6.2(a). If all of the impervious area is directly connected to the drainage system, but the impervious area percentages or the pervious land use assumptions in Table 6.2(a) are not applicable, use Figure 6.3 to compute a composite curve number. For example, Table 6.2(a) gives a curve number of 70 for a ½-acre lot in hydrologic soil group B with an assumed impervious area of 25%. However, if the lot has only 20% impervious area and a pervious area curve number of 61, then the composite curve number obtained from Figure 6.3 is 68. The decrease in curve number from 70 to 68 reflects the smaller percentage of impervious area for that parcel. Chapter 6 Estimating Runoff and Stormwater Discharge 6-4 Environmental Protection Handbook 6.2.1.h Unconnected Impervious Areas Runoff from these areas is spread over a pervious area as sheet flow. To determine curve number when all or part of the impervious area is not directly connected to the drainage system, use Figure 6.4 if total impervious area is less than 30 percent or use Figure 6.3 if the total impervious area is greater than 30 percent, because the absorptive capacity of the remaining pervious areas will not significantly affect runoff. When impervious area is less than 30 percent, obtain the Composite Curve Number by entering the right half of Figure 6.4 with the percentage of total impervious area and the ratio of total unconnected impervious area to total impervious area. Then move left to the appropriate pervious curve number and read down to find the composite curve number. For example, for a ½-acre lot with 20 percent total impervious area (75 percent of which is unconnected) and pervious curve number of 61, the composite curve number from Figure 6.4 is 66. If all of the impervious area is connected, the resulting curve number (from Figure 6.3) would be 68. 6.2.1.i Rainfall The highest peak discharges from small watersheds are usually caused by intense, brief rainfall that occurs as a distinct event or as a part of a longer storm. These rainfall events have intensities that vary greatly and normally do not extend over a very large area. Because of the great variability in storm events and the need for design information, synthetic (a typical storm event as calculated by NRCS) rainfall distributions have been developed by the USDA Natural Resources Conservation Service using U.S. National Weather Service data for typical storms. The twenty-four (24) hour storm, while longer than that needed to determine peaks for small drainage areas, is appropriate for determining runoff volumes. Therefore, a single storm duration and associated synthetic rainfall distribution can be used to represent not only peak discharge but also runoff volumes. The following twenty-four hour rainfall distribution charts are presented in Figures 6.5 through 6.11: the one-year, two-year, five-year, ten-year, twenty five-year, fifty-year, and one hundred-year storms. 6.2.1.i Runoff When curve number and the amount of rainfall (P) have been determined for the watershed, runoff can be determined by using Figure 6.1, Table 6.1 or equations 6-3 and 6-4. The runoff amount calculated is usually rounded to the nearest hundredth of an inch. 6.2.2 Limitations C Curve numbers describe average conditions that are useful for design purposes. If the rainfall event used is a historical storm, the modeling accuracy decreases. C Use the runoff curve number equation with caution when recreating specific features of an actual storm. The equation does not contain an expression for time, and therefore, does not account for rainfall duration and intensity. C The user should understand the assumption reflected in the initial abstraction term (Ia) and should ascertain that the assumption applies to the situation. Ia – which consists of interception, initial infiltration, surface depression storage, evapotranspiration, and other factors – was generalized as 0.2S based on data from agricultural watersheds (S is the potential maximum retention after runoff begins). This approximation can be especially important in an urban application because the combination of impervious areas with pervious areas can imply a significant initial loss that may not actually take place. The opposite effect – a greater initial loss – can occur if the impervious areas have surface depressions that store some runoff. To use a relationship other than Ia = 0.2S, one must re-calculate equation 6-3, Chapter 6 Estimating Runoff and Stormwater Discharge Environmental Protection Handbook 6-5 The site covers eighteen (18) acres at Bordeaux Hills, on the western end of St. Thomas. The acreage is classified as a Cramer soil, which is in hydrologic soil group C (see Table 6.3). A 25-year, 24-hour storm event produces a total rainfall (P) of 6.2 inches (see Figure 6.9). Figure 6.1, Table 6.1, and Tables 6.2 by using the original rainfall-runoff data to establish new S or CN relationships for each cover and hydrologic soil group. C The curve number procedure is less accurate when runoff is less than 0.5 inches. A different procedure should be used to verify runoff accuracy in this case. C The SCS runoff procedures apply only to direct surface runoff: do not overlook large sources of subsurface flow or high groundwater levels that contribute to runoff. These conditions are often related to hydrologic soil group A soils and wooded areas that have been assigned relatively low curve numbers in Tables 6.2. Good judgement and experience based on stream gage records are needed to adjust curve numbers as conditions warrant. C When the weighted curve number is less than 40, use another procedure to determine runoff. 6.2.3 Examples Four examples illustrate the procedure for computing runoff curve numbers (CN) and runoff (Q) in inches. A blank copy of TR-55 Worksheet 2 (used to compute CN and Q) is provided at the end of this chapter. Two of the four examples are based on an imaginary watershed on St. Thomas and the other two are based on a different imaginary watershed on St. Croix. Both examples are based on a 25-year, 24-hour storm event. The examples on St. Thomas calculate runoff based on acreage. The examples on St. Croix calculate runoff based on the percentage of acreage of soil types that comprise the watershed. Section 6.5 contains the completed TR-55 Worksheets for examples 1 through 4. 6.2.3.a Example 1 Problem: Determine the curve number (CN) and volume of runoff (Q) for this site for the present cover condition of brush in fair hydrologic condition. Solution: Refer to Table 6.2(c) to find a curve number of 70 for brush in fair hydrologic condition. Because this location has only one hydrologic soil group and cover type, Worksheet 2 is not needed. However, should a location have more than one hydrologic soil group or cover type, TR-55 Worksheet 2 should be used to calculate a weighted curve number. To determine direct runoff (Q), refer to Figure 6.9, which depicts the rainfall distribution for a 25-year, 24-hour storm event. For the western end of St. Thomas, the rainfall (P) is 6.2 inches. Next, refer to Figure 6.1 and locate 6.2 inches on the P axis (horizontal), and follow that line up the chart to the line that represents curve number 70. Trace the curve number 70 line left towards the Q axis (vertical) to find the runoff volume of 2.95 inches. (See completed Worksheet for this example in section 6.5.) Chapter 6 Estimating Runoff and Stormwater Discharge 6-6 Environmental Protection Handbook The site covers twenty-five (25) acres at Villa La Reine Estates, mid-island St. Croix. Fifty-eight percent (58%) of the site has Arawak soils, hydrologic soil group B (see Table 6.3), and forty-two percent (42%) of the site is classified as a Cramer soil, which is in hydrologic soil group C (see Table 6.3). A 25-year, 24-hour storm event produces a total rainfall (P) of 8 inches (see Figure 6.9). 6.2.3.b Example 2 Problem: Determine the weighted curve number (CN) and volume of runoff (Q) for the same site but with a different cover type. The 18-acre site has been cleared for a housing development. Four (4) acres will remain in open space in good hydrologic condition. The remaining fourteen (14) acres will be developed into quarter-acre lots with thirty-eight percent (38%) impervious area. Solution: First, refer to Table 6.2(a) to determine the curve number for each cover type. For open space with good hydrologic condition, the corresponding curve number is 74. The curve number for quarter-acre lots with 38% impervious area is 83. Each of these curve numbers is multiplied by its corresponding area and then averaged to obtain a weighted curve number: The rainfall value (P) is the same as in Example 1 – 6.2 inches. Next, refer to Figure 6.1, locate 6.2 inches on the horizontal (P) axis, follow the line up to the line representing curve number 80, and then move horizontally to the left to determine runoff from the vertical (Q) axis. This example yields a direct runoff (Q) of four (4.1) inches. (See worksheet for this example in section 6.5.) 6.2.3.c Example 3 Problem: Determine the weighted curve number (CN) and runoff volume (Q) from this site for a fair cover condition in pasture on the Arawak soil. Solution: Referring to Table 6.2(c), the curve number for fair pasture condition in hydrologic soil group B is 69. The Cramer soil has good pasture condition. The curve number of good pasture condition in hydrologic soil group C is 74. The weighted curve number for this site is obtained by multiplying each above curve number by its corresponding percentage of acreage as follows: The rainfall value (P) is determined to be 8.0 inches for the central area of St. Croix by referring to Figure 6.9. Next, refer to Figure 6.1 and locate 8 on the horizontal axis (P), follow the line up to the line representing curve number 70, then move horizontally to the vertical axis (Q) to determine a runoff volume of 4.4 inches. (See Worksheet for this example in section 6.5.) Chapter 6 Estimating Runoff and Stormwater Discharge Environmental Protection Handbook 6-7 6.2.3.d Example 4 Problem: Using the information in Example 3 (above), determine the weighted curve number and runoff volume for the Villa La Reine Estates site for a post-development scenario of forty-seven percent (47%) of the Arawak soil having quarter-acre lots with thirty-eight percent (38%) impervious area, eleven percent (11%) of the Arawak soil in open space with fair condition, thirty-six percent (36%) of the Cramer soil in quarter-acre lots with thirty-eight percent (38%) impervious area, and six percent (6%) of the Cramer soil in open space with good condition. Solution: Referring to Table 6.2(a), we find the following curve numbers: Soil, Group, Cover & Condition Curve Number Percent of Total Area Arawak (B), 1/4-acre, 38% imperv. 75 47% Arawak (B), open space, fair 69 11% Cramer (C), 1/4-acre, 38% imperv. 83 36% Cramer (C), open space, good 74 6% The weighted curve number for this site (refer to example above) is: Using the rainfall value of 8.0, refer to Figure 6.1, locate 8.0 along the horizontal axis (P), follow this line up to approximately 77 curve number, follow that left to the vertical axis (Q) to determine the direct runoff volume of 5.2 inches. (See worksheet for this example in Section 6.5.) 6.3 TIME OF CONCENTRATION AND TRAVEL TIME Travel time (Tt) is the time it takes stormwater to travel from one location to another in a watershed. Tt is a component of time of concentration (Tc), which is the time it takes for runoff to travel from the hydraulically-most-distant point of the watershed to a point of interest within the watershed. Tc is computed by summing all the travel time for consecutive components of the stormwater drainage system. Tc influences the shape and peak of the runoff hydrograph (def). Urbanization usually increases Tc, thereby increasing the peak discharge (def). Tc can also be increased as a result of ponding behind small or inadequate drainage systems, including storm drain inlets and road culverts, or reduction of land slope through grading. 6.3.1 Factors Affecting Time of Concentration and Travel Time 6.3.1.a Surface Roughness One of the most significant effects of urban development on stormwater velocity is the decrease in flow retardance. In undeveloped areas, stormwater runoff moves slowly as shallow overland flow through vegetation and over rough surfaces. When these areas are modified by urban development, stormwater moves much more rapidly over smooth, impervious paved areas and through gutters and storm drains. This leads to a significant decrease in the travel time of stormwater runoff through the watershed. Chapter 6 Estimating Runoff and Stormwater Discharge 6-8 Environmental Protection Handbook [Eq. 6-5] [Eq. 6-6] 6.3.1.b Channel Shape and Flow Patterns In small, non-urban watersheds, much of the total travel time results from overland flow in upstream areas. Typically, urbanization reduces overland flow distances by conveying storm runoff into a channel as soon as possible. Since channel designs have efficient hydraulic characteristics, runoff flow velocity increases and travel time decreases. 6.3.1.c Slope Slopes may be increased or decreased by urbanization, depending upon the amount of site grading or the extent to which storm drains and ditches are used in the design of the stormwater management system. Slopes generally tend to increase when channels are straightened and decrease when overland flow is directed through storm drains, street gutters and swales, and diversions. 6.3.2 Computation of Travel Time and Time of Concentration Stormwater moves through a watershed as sheet flow, shallow concentrated flow, open channel flow, or some combination of these. The type of flow that occurs is a function of the type of conveyance system and is best determined by field inspection. Travel time (Tt) is the ratio of flow length to flow velocity: where Tt = travel time (hours) L = flow length (feet) V = average velocity (feet/second) 3600 is a conversion factor from seconds to hours Time of concentration (Tc) is the sum of Tt values for the various consecutive flow segments: where Tc = time of concentration (hours) m = number of flow segments 6.3.3 Sheet flow Sheet flow is flow over level surfaces. It usually occurs in the headwater of streams. With sheet flow, the friction value (Manning’s n) is an effective roughness coefficient that includes the effect of raindrop impact; drag over the plane surface; obstacles such as litter, crop ridges, and rocks; and erosion and transportation of sediment. These “n” values are for very shallow flow depths of about one tenth (0.1) of a foot (1.2 inches). Table 6.4 gives Manning’s n values for sheet flow for various surface conditions. For sheet flow of less than 300 feet, use Manning’s kinematic solution (Overton and Meadows, 1976) to compute Tt: Chapter 6 Estimating Runoff and Stormwater Discharge Environmental Protection Handbook 6-9 [Eq. 6-7] [Eq. 6-8] [Eq. 6-9] where Tt = travel time (hours) n = Manning’s roughness coefficient (see Table 6.4) L = flow length (feet) P2 = 2-year, 24-hour rainfall (inches) s = slope of hydraulic grade line (land slope, feet/feet) This simplified form of the Manning’s kinematic solution is based on the following assumptions: shallow steady uniform flow; constant intensity of rainfall excess (that part of a rain available for runoff); rainfall duration of 24 hours; and minor effect of infiltration on travel time. 6.3.4 Shallow Concentrated Flow After traveling a maximum distance of 300 feet, sheet flow usually becomes shallow concentrated flow. The average velocity for this flow can be determined from Figure 6.12, in which average velocity is a function of watercourse slope and type of channel. For slopes less than 0.005 ft/ft, compute velocity using the following equations: where V = average velocity (ft/s) s = slope of hydraulic grade line (watercourse slope, ft/ft) After determining average velocity in Figure 6.12, use equation 6-5 to estimate travel time for the shallow concentrated flow segment. 6.3.5 Open Channels Open channels are assumed to begin where surveyed cross section information has been obtained, where channels are visible on aerial photographs, or where blue lines (indicating permanent or intermittent streams) appear on United States Geological Survey (USGS) quadrangle sheets. Manning’s equation or water surface profile information can be used to estimate average flow velocity. Average flow velocity is usually determined for bank-full elevation. Manning’s equation is: where V = average velocity (ft/s) r = hydraulic radius (ft) and is equal to a/pw pw = wetted perimeter (ft) a = cross sectional flow area (ft2) s = slope of the hydraulic grade line (channel slope, ft/ft) Chapter 6 Estimating Runoff and Stormwater Discharge 6-10 Environmental Protection Handbook St. Croix watershed for Example 5. n = Manning’s roughness coefficient for open channel flow Manning’s n values for open channel flow can be obtained from standard textbooks such as Chow (1959) or Linsley et al. (1982). After average velocity is computed using equation 6-9, Tt for the channel segment can be estimated using equation 6-5. 6.3.6 Reservoirs, Lakes or Ponds Sometimes it is necessary to estimate the velocity of flow through a reservoir, lake or pond at the outlet of a watershed. This travel time is normally very small and can be assumed to be zero. 6.3.7 Limitations C Manning’s kinematic solution should not be used for sheet flow longer than 300 feet. Equation 6-7 was developed for use with the four standard rainfall intensity-duration relationships. C In a watershed with storm drains, carefully identify the appropriate hydraulic flow path to estimate Tc. Storm drains generally handle only a small portion of a large event. The rest of the peak flow travels over streets, lawns, etc. to the outlet. Consult a standard hydraulics textbook to determine average velocity in pipes for either pressure or non- pressure flow. C The minimum Tc used in TR-55 is 0.1 hour. C A culvert or bridge can act as a reservoir outlet if there is significant storage behind it. The procedures in TR-55 can be used to determine the peak flow upstream of the culvert. Detailed storage routing procedures should be used to determine the outflow through the culvert. 6.3.8 Example 5 The sketch on the right shows a hypothetical watershed on St. Croix. The problem is to compute Tc at the outlet of the watershed (point D). The 2-year, 24-hour rainfall depth is 3.6 inches. All three types of flow occur from the hydraulically most distant point (A) to the point of interest (D). To compute Tc, first determine Tt for each segment from the following information: Segment AB: Sheet flow, dense grass cover, slope (s) = 0.01 ft/ft, and length (L) = 100 feet. Segment BC: Shallow concentrated flow, unpaved, s = 0.01 ft/ft, and L = 1400 feet. Segment CD: Channel flow, Manning’s n = 0.05, flow area (a) = 27 ft2, wetted perimeter (pw = 28.2 feet, s = 0.005 ft/ft, and L = 7300 feet. TR-55 Worksheet 3 is used to compute Tc. TR-55 Worksheet 3 is completed to solve Example 5 in section 6.5. A blank copy of TR-55 Worksheet 3 is provided for the user at the end of this Chapter. Chapter 6 Estimating Runoff and Stormwater Discharge Environmental Protection Handbook 6-11 [Eq. 6-10] 6.4 GRAPHICAL PEAK DISCHARGE METHOD This section presents the Graphical Peak Discharge method for computing peak discharge from rural and urban areas. The graphical method was developed from hydrograph analyses using TR-20 “Computer Program for Project Formulation - Hydrology” (SCS, 1983). The peak discharge equation used is: where qp = peak discharge (cfs) qu = unit peak discharge (csm/in) Q = runoff (in) Am = drainage area (mi2) Fp = pond and swamp adjustment factor The input requirements for the Graphical method are the Tc (hr); the drainage area (mi2); the appropriate rainfall distribution (Type III for the Virgin Islands); the 24-hour rainfall (in); and the curve number. If pond and/or swamp areas are spread throughout the watershed and are not considered in the Tc computation, then an adjustment for pond and swamp areas is also required. 6.4.1 Peak Discharge Computation For a selected rainfall frequency, the 24-hour rainfall (P) is obtained from Figures 6.5 to 6.11 or more detailed local precipitation maps. Curve number (CN) and direct runoff (Q) for the watershed are computed according to the methods outlined in Section 6.2. The curve number is used to determine the initial abstraction (Ia) from Table 6.5, and the Ia/P is then computed. Peak discharge per square mile per inch of runoff (qu) is obtained from Figure 6.13 by using Tc (see Section 6.3), rainfall distribution type, and Ia/P ratio. The pond and swamp adjustment factor is obtained from Table 6.6 (rounded to the nearest Table value). Use TR-55 Worksheet 4 provided at the end of this chapter to aid in computing the peak discharge using the Graphical method. 6.4.2 Limitations The Graphical method provides a determination of peak discharge only. If a hydrograph is needed or watershed subdivision is required, use the Tabular Hydrograph method described in Chapter 5 of TR-55 (SCS, 1986). Use TR-20 if the watershed is very complex or if a higher degree of accuracy is required. C The watershed must be hydrologically homogenous, that is, describable by one curve number. Land use, soils, and cover are distributed uniformly throughout the watershed. C The watershed may have only one main drainage channel (stream or gut) or, if more than one, the branches must have nearly equal Tc’s. C This method cannot perform valley or reservoir routing. C The Fp factor can be applied only for ponds or swamps that are not in the Tc flow path. Chapter 6 Estimating Runoff and Stormwater Discharge 6-12 Environmental Protection Handbook C Accuracy of peak discharge estimated by this method will be reduced if Ia/P values are used that are outside the range given in Figure 6.15. The limiting Ia/P values are recommended for use in this case. C This method should be used only if the weighted curve number is greater than 40. C When this method is used to develop estimates of peak discharge for both present and developed conditions of a watershed, use the same procedure for estimating Tc. C Tc values with this method may range from 0.1 to 10 hours. 6.4.3 Example 6 Compute the 25-year peak discharge for the 25-acre watershed described in examples 3 and 5. Worksheet 4 in section 6.5 shows how qp is computed for this example. 6.5 TR-55 WORKSHEET SOLUTIONS TO EXAMPLES The following worksheets detail the computation of CN and Q for examples 1 through 4, the computation of Tc for example 5, and the computation of qp for example 6. Chapter 6 Estimating Runoff and Stormwater Discharge Environmental Protection Handbook 6-13 Solution to Example 1. Chapter 6 Estimating Runoff and Stormwater Discharge 6-14 Environmental Protection Handbook Solution to Example 2. Chapter 6 Estimating Runoff and Stormwater Discharge Environmental Protection Handbook 6-15 Solution to Example 3. Chapter 6 Estimating Runoff and Stormwater Discharge 6-16 Environmental Protection Handbook Solution to Example 4. Chapter 6 Estimating Runoff and Stormwater Discharge Environmental Protection Handbook 6-17 Solution to Example 5. Chapter 6 Estimating Runoff and Stormwater Discharge 6-18 Environmental Protection Handbook Solution to Example 6. Chapter 6 Estimating Runoff and Stormwater Discharge Environmental Protection Handbook 6-19 Figure 6.1. Solution of runoff equation for SCS curve number method (USDA-SCS, 1986). 6.6 FIGURES AND TABLES Chapter 6 Estimating Runoff and Stormwater Discharge 6-20 Environmental Protection Handbook Figure 6.2. Flow chart for selecting the appropriate figure or table for determining runoff curve numbers (USDA-SCS, 1986). Chapter 6 Estimating Runoff and Stormwater Discharge Environmental Protection Handbook 6-21 Runoff depth for curve number of -- Rain- fall 40 45 50 55 60 65 70 75 80 85 90 95 98 ---------------------------------------------------------------inches---------------------------------------------------------------- 1.0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.03 0.08 0.17 0.32 0.56 0.79 1.2 0.00 0.00 0.00 0.00 0.00 0.00 0.03 0.07 0.15 0.27 0.46 0.74 0.99 1.4 0.00 0.00 0.00 0.00 0.00 0.02 0.06 0.13 0.24 0.39 0.61 0.92 1.18 1.6 0.00 0.00 0.00 0.00 0.01 0.05 0.11 0.20 0.34 0.52 0.76 1.11 1.38 1.8 0.00 0.00 0.00 0.00 0.03 0.09 0.17 0.29 0.44 0.65 0.93 1.29 1.58 2.0 0.00 0.00 0.00 0.02 0.06 0.14 0.24 0.38 0.56 0.80 1.09 1.48 1.77 2.5 0.00 0.00 0.02 0.08 0.17 0.30 0.46 0.65 0.89 1.18 1.53 1.96 2.27 3.0 0.00 0.02 0.09 0.19 0.33 0.51 0.71 0.96 1.25 1.59 1.98 2.45 2.77 3.5 0.02 0.08 0.20 0.35 0.53 0.75 1.01 1.30 1.64 2.02 2.45 2.94 3.27 4.0 0.06 0.18 0.33 0.53 0.76 1.03 1.33 1.67 2.04 2.46 2.92 3.43 3.77 4.5 0.14 0.30 0.50 0.74 1.02 1.33 1.67 2.05 2.46 2.91 3.40 3.92 4.26 5.0 0.24 0.44 0.69 0.98 1.30 1.65 2.04 2.45 2.89 3.37 3.88 4.42 4.76 6.0 0.50 0.80 1.14 1.52 1.92 2.35 2.81 3.28 3.78 4.30 4.85 5.41 5.76 7.0 0.84 1.24 1.68 2.12 2.60 3.10 3.62 4.15 4.69 5.25 5.82 6.41 6.76 8.0 1.25 1.74 2.25 2.78 3.33 3.89 4.46 5.04 5.63 6.21 6.81 7.40 7.76 9.0 1.71 2.29 2.88 3.49 4.10 4.72 5.33 5.95 6.57 7.18 7.79 8.40 8.76 10.0 2.23 2.89 3.56 4.23 4.90 5.56 6.22 6.88 7.52 8.16 8.78 9.40 9.76 11.0 2.78 3.52 4.26 5.00 5.72 6.43 7.13 7.81 8.48 9.13 9.77 10.39 10.76 12.0 3.38 4.19 5.00 5.79 6.56 7.32 8.05 8.76 9.45 10.11 10.76 11.39 11.76 13.0 4.00 4.89 5.76 6.61 7.42 8.21 8.89 9.71 10.42 11.10 11.76 12.39 12.76 14.0 4.65 5.62 6.55 7.44 8.30 9.12 9.91 10.67 11.39 12.08 12.75 13.39 13.76 15.0 5.33 6.36 7.35 8.29 9.19 10.04 10.85 11.63 12.37 13.07 13.74 14.39 14.76 1 Interpolate the values shown to obtain runoff depths for CN's or rainfall amounts not shown. Table 6.1. Runoff depth for selected CN's and rainfall amounts1 (USDA-SCS, 1986). Chapter 6 Estimating Runoff and Stormwater Discharge 6-22 Environmental Protection Handbook Cover description Curve numbers for hydrologic soil group -- Cover type and hydrologic condition Average percent impervious area2 A B C D Fully developed urban areas (vegetation established) Open space (lawns, parks, golf courses, cemeteries, etc.)3: Poor condition (grass cover < 50%) Fair condition (grass cover 50% to 75%) Good condition (grass cover > 75%) 68 49 39 79 69 61 86 79 74 89 84 80 Impervious areas: Paved parking lots, roofs, driveways, etc. (excluding right-of-way) Streets and roads: Paved; curbs and storm drains (excluding right-of- way) Paved; open ditches (including right-of-way) Gravel (including right-of-way) Dirt (including right-of-way) 98 98 83 76 72 98 98 89 85 82 98 98 92 89 87 98 98 93 91 89 Western desert urban areas: Natural desert landscaping (pervious areas only)4 Artificial desert landscaping (impervious weed barrier, desert shrub with 1- to 2-inch sand or gravel mulch and basin borders) 63 96 77 96 85 96 88 96 Urban districts: Commercial and business Industrial 85 72 89 81 92 88 94 91 95 93 Residential districts by average lot size: 1/8 acre or less (town houses) 1/4 acre 1/3 acre 1/2 acre 1 acre 2 acres 65 38 30 25 20 12 77 61 57 54 51 46 85 75 72 70 68 65 90 83 81 80 79 77 92 87 86 85 84 82 Developing urban areas Newly graded areas (pervious areas only, no vegetation)5 77 86 91 94 Idle lands (CN's are determined using cover types similar to those in Table 2.2(c)) 1 Average runoff condition, and Ia = 0.2S. 2 The average percent impervious area shown was used to develop the composite CN's. Other assumptions are as follows: impervious areas are directly connected to the drainage system, impervious areas have a CN of 98, and pervious areas are considered equivalent to open space in good hydrologic condition. CN's for other combinations of conditions may be computed using Figures 6.3 or 6.4. 3 CN's shown are equivalent to those of pasture. Composite CN's may be computed for other combinations of open space cover type. 4 Composite CN's for natural desert landscaping should be computed using Figures 6.3 or 6.4 based on the impervious area percentage (CN = 98) and the pervious area CN. The pervious area CN's are assumed equivalent to desert shrub in poor hydrologic condition. 5 Composite CN's to use for the design of temporary measures during grading and construction should be computed using Figures 6.3 or 6.4 based on the degree of development (impervious area percentage) and the CN's for the newly graded pervious areas. Table 6.2.a. Runoff curve numbers for urban areas1 (USDA-SCS, 1986). Chapter 6 Estimating Runoff and Stormwater Discharge Environmental Protection Handbook 6-23 Cover description Curve numbers for hydrologic soil group -- Cover type Treatment2 Hydrologic Condition3 A B C D Fallow Bare soil Crop residue cover (CR) -- Poor Good 77 76 74 86 85 83 91 90 88 94 93 90 Row crops Straight row (SR) SR + CR Contoured (C) C + CR Contoured & terraced (C&T) C&T + CR Poor Good Poor Good Poor Good Poor Good Poor Good Poor Good 72 67 71 64 70 65 69 64 66 62 65 61 81 78 80 75 79 75 78 74 74 71 73 70 88 85 87 82 84 82 83 81 80 78 79 77 91 89 90 85 88 86 87 86 82 81 81 80 Small Grain SR SR + CR C C + CR C&T C&T + CR Poor Good Poor Good Poor Good Poor Good Poor Good Poor Good 65 63 64 60 63 61 62 60 61 59 60 58 76 75 75 72 74 73 73 72 72 70 71 69 84 83 83 80 82 81 81 80 79 78 78 77 88 87 86 84 85 84 84 83 82 81 81 80 Close-seeded or broadcast legumes or rotation meadow SR C C&T Poor Good Poor Good Poor Good 66 58 64 55 63 51 77 72 75 69 73 67 85 81 83 78 80 76 89 85 85 83 83 80 1 Average runoff condition, and Ia = 0.2S. 2 Crop residue cover applies only if residue is on at least 5% of the surface throughout the year. 3 Hydrologic condition is based on combination of factors that affect infiltration and runoff, including (a) density and canopy of vegetative areas, (b) amount of year-round cover, (c) amount of grass or close-seeded legumes in rotations, (d) percent of reside cover on the land surface (good > 20%), and (e) degree of surface roughness. Poor: Factors impair infiltration and tend to increase runoff. Good: Factors encourage average and better than average infiltration and tend to decrease runoff. Table 6.2.b. Runoff curve numbers for cultivated agricultural lands1 (USDA-SCS, 1986). Chapter 6 Estimating Runoff and Stormwater Discharge 6-24 Environmental Protection Handbook Cover description Curve numbers for hydrologic soil group -- Cover type Hydrologic Condition A B C D Pasture, grassland or range -- continuous forage for grazing2 Poor Fair Good 68 49 39 79 69 61 86 79 74 89 84 80 Meadow -- continuous grass, protected from grazing and generally mowed for hay. -- 30 58 71 80 Brush -- brush-weed-grass mixture with brush the major element3 Poor Fair Good 48 35 304 67 56 48 77 70 65 83 77 73 Woods -- grass combination (orchard or tree farm)5 Poor Fair Good 57 43 32 73 65 58 82 76 72 86 82 79 Woods6 Poor Fair Good 45 36 304 66 60 55 77 73 70 83 79 77 Farmsteads -- buildings, lanes, driveways, and surrounding lots -- 59 74 82 86 1 Average runoff condition, and Ia = 0.2S. 2 Poor: <50% ground cover or heavily grazed with no mulch. Fair: 50% to 75% ground cover and not heavily grazed. Good: >75% ground cover and lightly or only occasionally grazed. 3 Poor: <50% ground cover. Fair: 50% to 75% ground cover. Good: >75% ground cover. 4 Actual curve number is less than 30; use CN = 30 for runoff computations. 5 CN's shown were computed for areas with 50% woods and 50% grass (pasture) cover. Other combinations of conditions may be computed from the CN's for woods and pasture. 6 Poor: Forest litter, small trees, and brush are destroyed by heavy grazing or regular burning. Fair: Woods are grazed but not burned, and some forest litter covers the soil. Good: Woods are protected from grazing, and litter and brush adequately cover the soil. Table 6.2.c. Runoff curve numbers for other agricultural lands1 (USDA-SCS, 1986). Chapter 6 Estimating Runoff and Stormwater Discharge Environmental Protection Handbook 6-25 Cover description Curve numbers for hydrologic soil group -- Cover type Hydrologic Condition2 A3 B C D Herbaceous -- mixture of grass, weeds, and low- growing brush, with brush the minor element. Poor Fair Good 80 71 62 87 81 74 93 89 85 Oak-aspen -- mountain brush mixture of oak brush, aspen, mountain mahogany, bitter brush, maple, and other brush. Poor Fair Good 66 48 30 74 57 41 79 63 48 Pinyon-juniper -- pinyon, juniper, or both; grass understory. Poor Fair Good 75 58 41 85 73 61 89 80 71 Sagebrush with grass understory. Poor Fair Good 67 51 35 80 63 47 85 70 55 Desert shrub -- major plants include salt bush, greasewood, creosote bush, black brush, bursage, palo verde, mesquite, and cactus. Poor Fair Good 63 55 49 77 72 68 85 81 79 88 86 84 1 Average runoff condition, and Ia = 0.2S. For range in humid regions, use Table 6.2.c. 2 Poor: <30% ground cover (litter, grass, and brush overstory). Fair: 30% to 70% ground cover. Good: >70% ground cover. 3 Curve numbers for group A have been developed only for desert shrub. Table 6.2.d. Runoff curve numbers for arid and semi-arid rangelands1 (USDA-SCS, 1986). Group A B C D Jaucas Lameshur Redhook Arawak Cinnamon Bay Parasol Sion Victory Cramer Dorothea Glynn Jealousy Annaberg Aquents Beaches Carib Fredriksdal Hesselburg Hogensborg Maho Bay Rock Outcrop Salt Flats Sandy Point Solitude Southgate Sugar Beach Susannaberg Table 6.3. Hydrologic soil groups of the U.S. Virgin Islands (USDA-NRCS, 1995). Chapter 6 Estimating Runoff and Stormwater Discharge 6-26 Environmental Protection Handbook Figure 6.3. Composite CN with connected impervious area (USDA-SCS, 1986). Figure 6.4. Composite CN with unconnected impervious areas and total impervious area less than 30% (USDA-SCS, 1986). Chapter 6 Estimating Runoff and Stormwater Discharge Environmental Protection Handbook 6-27 Figure 6.5. 1-year, 24-hour rainfall for the U.S. Virgin Islands (in.) (USDA-SCS, 1986). Chapter 6 Estimating Runoff and Stormwater Discharge 6-28 Environmental Protection Handbook Figure 6.6. 2-year, 24-hour rainfall for the U.S. Virgin Islands (in.) (USDA-SCS, 1986). Chapter 6 Estimating Runoff and Stormwater Discharge Environmental Protection Handbook 6-29 Figure 6.7. 5-year, 24-hour rainfall for the U.S. Virgin Islands (in.) (USDA-SCS, 1986). Chapter 6 Estimating Runoff and Stormwater Discharge 6-30 Environmental Protection Handbook Figure 6.8. 10-year, 24-hour rainfall for the U.S. Virgin Islands (in.) (USDA-SCS, 1986). Chapter 6 Estimating Runoff and Stormwater Discharge Environmental Protection Handbook 6-31 Figure 6.9. 25-year, 24-hour rainfall for the U.S. Virgin Islands (in.) (USDA-SCS, 1986). Chapter 6 Estimating Runoff and Stormwater Discharge 6-32 Environmental Protection Handbook Figure 6.10. 50-year, 24-hour rainfall for the U.S. Virgin Islands (in.) (USDA-SCS, 1986). Chapter 6 Estimating Runoff and Stormwater Discharge Environmental Protection Handbook 6-33 Figure 6.11. 100-year, 24-hour rainfall for the U.S. Virgin Islands (in.) (USDA-SCS, 1986). Chapter 6 Estimating Runoff and Stormwater Discharge 6-34 Environmental Protection Handbook Curve number Ia Curve number Ia 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 3.000 2.878 2.762 2.651 2.545 2.444 2.348 2.255 2.167 2.082 2.000 1.922 1.846 1.774 1.704 1.636 1.571 1.509 1.448 1.390 1.333 1.279 1.226 1.175 1.125 1.077 1.030 0.985 0.941 0.899 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 0.857 0.817 0.778 0.740 0.703 0.667 0.632 0.597 0.564 0.532 0.500 0.469 0.439 0.410 0.381 0.353 0.326 0.299 0.273 0.247 0.222 0.198 0.174 0.151 0.128 0.105 0.083 0.062 0.041 Table 6.4. Ia values for runoff curve numbers (USDA-SCS, 1986). Surface description n1 Smooth surfaces (concrete, asphalt, gravel, or bare soil) 0.011 Fallow (no residue) 0.05 Cultivated soils: Residue cover < 20% Residue cover $ 20% 0.06 0.17 Grass: Short grass Dense grasses2 Bermuda grass 0.15 0.24 0.41 Range (natural) 0.13 Woods:3 Light underbrush Dense underbrush 0.40 0.80 1 The n values are a composite of information compiled by Engman (1986). 2 Includes species such as weeping lovegrass, bluegrass, buffalo grass, blue grama grass, and native gra 3 When selecting n, consider cover to a height of about 0.1 ft. This is the only part of the plant cover Table 6.5. Roughness coefficients (Manning's n) for sheet flow (USDA-SCS, 1986). Chapter 6 Estimating Runoff and Stormwater Discharge Environmental Protection Handbook 6-35 Figure 6.12. Average velocities for estimating travel time for shallow concentrated flow (USDA-SCS, 1986). Chapter 6 Estimating Runoff and Stormwater Discharge 6-36 Environmental Protection Handbook Figure 6.13. Unit peak discharge (qu) for SCS Type-III rainfall distribution (the type that occurs in the U.S. Virgin Islands) (USDA-SCS, 1986). Chapter 6 Estimating Runoff and Stormwater Discharge Environmental Protection Handbook 6-37 Percentage of pond and swamp areas Fp 0.0 1.00 0.2 0.97 1.0 0.87 3.0 0.75 5.0 0.72 Table 6.6. Adjustment factor (Fp) for pond and swamp areas that are spread throughout the watershed (USDA-SCS, 1986). 6.7 REFERENCES Chow, V.T. 1959. Open Channel Hydraulics. McGraw-Hill Book Company, Inc, New York, NY. Engman, E.T. 1986. “Roughness Coefficients for Routing Surface Runoff,” Journal of Irrigation and Drainage Engineering, 112(1):39-53, American Society of Civil Engineers. Linsley, R.K., M.A. Kohler, and J.L.H. Paulhus. 1982. Hydrology for Engineers, Third Ed., McGraw-Hill Book Company, Inc, New York, NY. Overton, D.E. and M.E. Meadows. 1976. Stormwater Modeling, Academic Press, New York, NY. Rallison, R.E. and N. Miller. 1981. “Past, Present and Future SCS Runoff Procedure,” IN: V.P. Singh (ed.) Rainfall-Runoff Relationships: Proceedings, International Symposium on Rainfall-Runoff Modeling, Mississippi State University. p. 353-364. Rawls, W.J., A. Shalaby, and R.H. McCuen. 1981. “Evaluation of Methods for Determining Urban Runoff Curve Numbers,” Transactions of the American Society of Agricultural Engineers, 24(6):1562-1566. USDA-NRCS. 1995. Soil Survey of the U.S. Virgin Islands. U.S. Department of Agriculture, Natural Resources Conservation Service, Caribbean Field Office, St. Croix, U.S. Virgin Islands. USDA-SCS. 1986. Urban Hydrology for Small Watersheds. Technical Release 55, U.S. Department of Agriculture, Soil Conservation Service, Washington, DC. USDA-SCS. 1985. National Engineering Handbook, Section 4 - Hydrology, U.S. Department of Agriculture, Soil Conservation Service, Washington, DC. USDA-SCS. 1983. Computer Program for Project Formulation - Hydrology. Technical Release 20. U.S. Department of Agriculture, Soil Conservation Service, Washington, DC.