Title 210 – National Engineering Handbook

Part 650 Engineering Field Handbook National Engineering Handbook

Chapter 2 Estimating Runoff Volume and Peak Discharge

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Chapter 2 - Estimating Runoff Volume and Peak Discharge Table of Contents

650.0200 Introduction ...... 1 650.0201 Factors Affecting ...... 1 A. General ...... 1 B. Rainfall Amount ...... 2 C. Rainfall Distribution ...... 2 D. Hydrologic Soil Groups ...... 5 E. Cover Type ...... 6 F. Land Use ...... 6 G. Land Treatment ...... 6 H. Hydrologic Conditions...... 7 I. Topography ...... 7 650.0202. ...... 7 A. General ...... 7 B. Types of Hydrographs...... 7 C. Dimensionless unit peak rate factors ...... 8 650.0203 Runoff Volume ...... 9 A. General ...... 9 B. NRCS Runoff Equation ...... 9 C. Runoff Curve Numbers ...... 10 D. Estimating Runoff Volume using the NRCS CN Method ...... 10 650.0204 Time of Concentration ...... 16 A. General ...... 16 B. The NRCS Lag Equation ...... 16 (2) Flow Length ...... 17

C. Estimating Tc Using the NRCS Lag Equation ...... 18 650.0205 Peak Discharge ...... 19 A. General ...... 19 B. Estimating Peak Discharge Manually ...... 20 (1) Unit peak discharge curves ...... 20

(2) Ia/P ratio ...... 20 (3) Estimating Peak Discharge ...... 20 C. Estimating Peak Discharge Using the EFH-2 Computer Program ...... 26

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650.0206 Limitations ...... 26 650.0207 Examples ...... 27 A. Example - Estimating weighted CN ...... 27 B. Example - Estimating Time of Concentration ...... 27 C. Example – Manual Estimation of Peak Discharge ...... 27 650.0208 References ...... 30 650.0209 Acknowledgements ...... 30 650.0210 Exhibits ...... 30 A. Worksheet ...... 30 B. Time of Concentration and Peak Discharge Worksheet ...... 30

Table of Figures

Figure 2-1. Approximate Geographic Boundaries for NRCS Standard Rainfall Distributions ...... 3 Figure 2-2. Temporal Plots of Type I, IA, II, and III rainfall distributions ...... 4 Figure 2-3. NOAA Atlas 14, Volume 2 2 – Geographic Boundaries of NRCS-developed Rainfall Distributions ...... 4 Figure 2-4. Temporal plots of the NRCS-derived NOAA Atlas 14 Rainfall Distributions for the Ohio and Neighboring States ...... 5 Figure 2-5. Hydrograph plot illustrating a natural (or measured) hydrograph and a synthetic (or computed) hydrograph for the same watershed ...... 8 Figure 2-6. Dimensionless curvilinear unit hydrograph (PRF=484) and equivalent dimensionless triangular unit hydrograph ...... 9 Figure 2- 7a. Runoff Curve Numbers - (a) Cultivated Agricultural Lands 1 ...... 11 Figure 2-8. Solution to the NRCS Runoff Equation ...... 15 Figure 2-9. Runoff Depth for Selected CNs And Rainfall Amounts1 ...... 16 Figure 2-10. Natural watershed ...... 18 Figure 2-11. Watershed with Terraces ...... 18

Figure 2-12. Time of Concentration (Tc) Nomograph ...... 19

Figure 2 13. Ia Values for Runoff Curve Numbers ...... 21

Figure 2- 14. Unit Peak Discharge (qu) for NRCS Rainfall Type I Rainfall Distribution ...... 22

Figure 2-15. Unit Peak Discharge (qu) for NRCS Rainfall Type IA Rainfall Distribution ...... 23

Figure 2- 16. Unit Peak Discharge (qu) for NRCS Rainfall Type II Rainfall Distribution ...... 24

Figure 2-17. Unit Peak Discharge (qu) for NRCS Rainfall Type III Rainfall Distribution ...... 25 Figure 2-18 Solution to Example – Estimating Weighted Runoff Curve Number ...... 28 Figure 2-19. Solution to Example – Estimating Time of Concentration and Peak Discharge ...... 29

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1 Part 650 – Engineering Field Handbook

2 Chapter 2 – Estimating Runoff Volume and Peak Discharge

3 650.0200 Introduction

4 A. Surface runoff is the volume of excess that runs off a drainage area - also referred 5 to as runoff volume. Excess water is that portion of rainfall that runs off the watershed. 6 Some rainfall is retained on the watershed in the form of initial abstraction (the amount of 7 rain that falls prior to runoff starting), (water that soaks into the ground), and 8 storage (water that is trapped on vegetation and other surfaces or depressions, and does not 9 runoff). 10 B. Peak discharge is the peak rate of runoff (volume per unit time) from a drainage area for a 11 given rainfall. 12 C. This chapter presents procedures for estimating runoff volume and peak discharge from 13 small rural watersheds for use in designing soil and water conservation measures. These 14 procedures apply to drainage areas that range in size from 1 to 2,000 acres in the United 15 States, Puerto Rico, the U.S. Virgin Islands and selected Pacific Islands. 16 D. This chapter includes figures, and worksheets to estimate runoff volume and peak 17 discharge using manual methods for a range of rainfall amounts, soil types, land use, and 18 cover conditions. It also includes a short section describing the use of the EFH-2 Computer 19 program for estimating runoff volume and peak discharge. 20 E. The U.S. Department of Agriculture (USDA) Natural Resources Conservation Service 21 (NRCS) developed the EFH-2 computer program to automate the computation procedures 22 presented in this chapter. In the past, EFH-2 utilized only the manual methods discussed in 23 this chapter. In the future, EFH-2 will utilize the TR-20 computation procedures outlined in 24 the NRCS Title 210- National Engineering Handbook, Part 630, (NEH Part 630). 25 The EFH-2 User’s Manual contains full instruction for the use of the EFH-2 computer 26 program. 27 F. Use the procedures found in NEH Part 630, the WinTR–55 computer program “Small 28 Watershed Hydrology,” or the WinTR–20 computer program “Project Formulation-- 29 Hydrology,” to estimate peak discharge for conditions beyond the limits of this chapter and 30 for special situations and areas where procedures of this chapter may be considered too 31 general to provide good estimates.

32 650.0201 Factors Affecting Surface Runoff

33 A. General 34 (1) Rainfall is the primary source of water that runs off the surface of small rural 35 watersheds. The main factors affecting the volume of rainfall that runs off are soil 36 type and the type of vegetation in the watershed. Factors that affect the rate at which 37 water runs off are watershed topography and shape, along with land use and 38 conservation practices on a watershed. 39 (2) Intense rainfall usually causes the peak discharge from a small rural watershed. The 40 intensity of rainfall affects the peak discharge more than it does the volume of runoff. 41 The melting of accumulated snow in the mountains and northern plains may result in 42 a greater volume of runoff, but usually at a lesser discharge rate than that caused by

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43 rainfall. The melting of a winter’s snow accumulation over a large area may cause 44 major flooding along . Intense rainfall that produces high peak discharges on 45 small watersheds usually does not extend over a large area. Therefore, the same 46 intense rainfall that causes flooding in a small is not likely to cause major 47 flooding along a main that drains 100 or more square miles. 48 B. Rainfall Amount 49 (1) Originally, NRCS primarily used precipitation-frequency data (rainfall amount by 50 ) as published by the National Oceanic and Atmospheric Administration 51 (NOAA) National Weather Service (NWS) in Technical Paper No. 40 (TP-40) in 52 1959 and other publications for specific areas of the United States. 53 (2) In the early 2000s, the NWS began the process of updating precipitation-frequency 54 data to take advantage of a denser network of rain gages, to bring an additional 40 to 55 60 year of rainfall data into the precipitation-frequency estimates, and to use updated 56 statistical methodologies. The NWS published these data as NOAA Atlas 14, in 57 several volumes with each volume covering a portion of the United States. As of this 58 publication, the only states without updated NOAA Atlas 14 precipitation-frequency 59 estimates are Washington, Oregon, Idaho, Montana, and Wyoming. 60 (3) As the updated precipitation-frequency data became available, NRCS adopted the 61 data for use in analysis and design for NRCS projects by policy as described in the 62 NRCS National Engineering Manual (NEM) Part 530, Subpart A, 530.1. 63 (4) NRCS developed a rainfall database with rainfall values representative of individual 64 counties (or portions thereof) for each county in the nation and incorporated these 65 data into the NRCS hydrology computer programs. These data are also available in 66 NRCS State supplements to this chapter. If you need these county-based data, 67 contact the appropriate State Conservation Engineer. 68 C. Rainfall Distribution 69 (1) A rainfall distribution describes the amount of rainfall that falls in successive time 70 increments over the duration of a storm event. The intensity of rainfall varies 71 considerably during the storm period. 72 (2) To avoid the use of a separate set of rainfall intensities for each drainage area, NRCS 73 developed a set of synthetic rainfall distributions having nested rainfall intensities. 74 This set of distributions maximizes the rainfall intensities by including short-duration 75 intensities within those needed for longer duration. 76 (3) NRCS chose a storm duration of 24 hours for the synthetic distributions, based on the 77 watershed size for which NRCS typically provides assistance. The 24-hour storm, 78 while longer than that needed to determine peak discharges, is suitable for 79 determining runoff volumes. Thus, NRCS uses a single storm duration and 80 associated synthetic rainfall distribution to estimate peak discharges for a wide range 81 of watershed areas. 82 (4) In the intermountain and northern tier of the United States, the annual peak discharge 83 may occur in some years from rainfall falling on snow or from rapid snowmelt on 84 frozen or saturated soils. This chapter considers only rainfall-generated runoff and 85 not runoff generated from snowmelt. To estimate runoff from snowmelt, use special 86 procedures in NEH Part 630. 87 (5) NRCS Standard Rainfall Distributions 88 (i) Using the U.S. National Weather Service (NWS) Technical Paper No. 40 and 89 other reference data, NRCS, in the 1960s and 1970s developed four 24-hour 90 storm distributions Type I, Type IA, Type II, and Type III as typical design 91 storms, associated with four broad climatic regions across the United States.

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92 NRCS refers to them as the NRCS Standard Rainfall Distributions. Figure 2-1 93 shows the approximate geographic boundaries for the four rainfall distributions. 94 If a watershed is near a boundary, contact the NRCS State Conservation Engineer 95 for a better definition of actual location. 96 97 Figure 2-1. Approximate Geographic Boundaries for NRCS Standard Rainfall Distributions

98 99 (ii) Type IA maximum intensities are less than Type I; Type I intensities are less 100 than Type III; and Type III intensities are less than Type II intensities. 101 (iii) Types IA and I storm distributions are typical of maritime climates in the 102 Western United States where winters are wet, and summers are dry. The Type 103 IA storm distribution is characteristic of the coastal side of the Cascade and 104 Sierra Nevada Mountains in Oregon, Washington, and northern California. The 105 Type I storm distribution is the characteristic storm distribution of the coastal 106 side of the Sierra Nevada Mountains in southern California and for Hawaii and 107 Alaska. The Type III storm distribution represents the Gulf of Mexico and the 108 Atlantic coastal areas where tropical storms bring large 24-hour rainfalls. The 109 Type II storm distribution is typical of the more intense storms that occur over 110 the remainder of the United States, Puerto Rico, and the Virgin Islands. 111 (iv) Figure 2-2 shows temporal plots of the of the four rainfall distributions 112 illustrating the concepts described. 113 (v) The examples presented in section 650.0207 of this handbook chapter use the 114 TP-40 rainfall and standard distributions. 115

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116 Figure 2-2. Temporal Plots of Type I, IA, II, and III rainfall distributions

117 118 (6) NRCS-developed NOAA Atlas 14-derived Rainfall Distributions 119 (i) As the NWS updated precipitation-frequency data in NOAA Atlas 14, they 120 published these data in volumes covering specific regions of the nation. NRCS 121 then developed updated rainfall distributions using the NOAA Atlas 14 data. 122 Due to more and better-quality data availability and being able to take advantage 123 of advancements in computer technology, NRCS developed many more 124 distributions to cover the nation. 125 (ii) Figure 2-3 shows the approximate geographic boundaries for NRCS-derived 126 rainfall distributions from the NOAA Atlas 14, Volume 2, Precipitation- 127 Frequency Atlas of the United States, Ohio Basin and Surrounding States. 128 129 Figure 2-3. NOAA Atlas 14, Volume 2 2 – Geographic Boundaries of NRCS-developed 130 Rainfall Distributions

131

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132 (iii) Figure 2-4 shows temporal plots of the NRCS-derived NOAA Atlas 14 data 133 rainfall distributions for the Ohio River Basin and surrounding states. Contact 134 the State Conservation Engineer for more information about the rainfall 135 distributions used by NRCS within specific states. 136 137 Figure 2-4. Temporal plots of the NRCS-derived NOAA Atlas 14 Rainfall Distributions for 138 the Ohio Valley and Neighboring States

139 140 (vi) The updated rainfall distributions provide better definition, particularly in what 141 was the Type II rainfall distribution geographic area, accounting for climatic and 142 orographic conditions. 143 (v) NRCS typically uses data from NOAA Atlas 2, Volume 9, published by NWS in 144 1973, or other local precipitation-frequency studies for those regions of the 145 country not yet covered by NOAA Atlas 14 (Wyoming, Washington, Oregon, 146 Idaho, and Montana). As NWS develops updated precipitation-frequency data, 147 NRCS will develop updated rainfall distributions. 148 (vi) In those regions where NOAA Atlas 14 precipitation-frequency data are 149 available, NRCS has incorporated them and the updated rainfall distributions into 150 NRCS computer programs for estimating peak discharge. 151 D. Hydrologic Soil Groups 152 (1) The NRCS defines four hydrologic soil groups (HSGs) that, along with land use, 153 management practices, and hydrologic conditions, determine a soil’s associated 154 runoff curve number (NEH Part 630 Chapter 9). 155 (2) In general, the definitions for the four HSGs are as follows: 156 (i) Group A—Soils in this group have low runoff potential when thoroughly wet. 157 Water transmits freely through the soil. Group A soils typically have less than 10 158 percent clay and more than 90 percent sand or gravel and have gravel or sand 159 textures. Some soils having loamy sand, sandy loam, loam, or silt loam textures 160 may be in this group if they are well aggregated, of low bulk density, or contain 161 greater than 35 percent rock fragments. 162 (ii) Group B—Soils in this group have moderately low runoff potential when 163 thoroughly wet. Water transmission through the soil is unimpeded. Group B

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164 soils typically have between 10 percent and 20 percent clay and 50 percent to 90 165 percent sand and have loamy sand or sandy loam textures. Some soils having 166 loam, silt loam, silt, or sandy clay loam textures may be in this group if they are 167 well aggregated, of low bulk density, or contain greater than 35 percent rock 168 fragments. 169 (iii) Group C—Soils in this group have moderately high runoff potential when 170 thoroughly wet. Water transmission through the soil is somewhat restricted. 171 Group C soils typically have between 20 percent and 40 percent clay and less 172 than 50 percent sand and have loam, silt loam, sandy clay loam, clay loam, and 173 silty clay loam textures. Some soils having clay, silty clay, or sandy clay textures 174 may be in this group if they are well aggregated, of low bulk density, or contain 175 greater than 35 percent rock fragments. 176 (iv) Group D—Soils in this group have high runoff potential when thoroughly wet. 177 Water movement through the soil is restricted or very restricted. Group D soils 178 typically have greater than 40 percent clay, less than 50 percent sand, and have 179 clayey textures. In some areas, they also have high shrink-swell potential. 180 (v) Certain wet soils are in group D based on the presence of a water table, when 181 adequately drained might meet the definition of another of the groups. These 182 soils are assigned a dual classification and may be listed as A/D, B/D, or C/D, 183 where the first letter applies to the drained condition and the second to the 184 undrained condition. Take special care to assess the hydrologic soil group when 185 making hydrologic analyses. 186 (3) NEH Part 630, Chapter 7, Hydrologic Soil Groups, provides a complete definition of 187 each of the soils groups including the limits on the diagnostic physical characteristics 188 of each group and assignment of soils to HSGs. 189 (4) The assigned groups can be found by consulting the NRCS Field Office Technical 190 Guide; published soil survey databases; or the NRCS Web Soil Survey at 191 https://websoilsurvey.sc.egov.usda.gov/. 192 E. Cover Type 193 Cover type affects runoff in several ways. The foliage and its litter maintain the soil’s 194 infiltration potential by preventing the impact of the raindrops from sealing the soil 195 surface. Some of the raindrops are retained on the surface of the foliage, increasing their 196 chance of evaporation back into the atmosphere. Some of the intercepted moisture takes 197 so long to drain from the plant down to the soil that it is withheld from the initial period 198 of runoff. Ground cover also allows soil moisture from previous rains to transpire, 199 leaving a greater void in the soil to be filled. Vegetation, including its ground litter, 200 forms numerous barriers along the path of the water flowing over the surface of the land. 201 This increased surface roughness causes water to flow more slowly, lengthening the time 202 of concentration (Tc) and reducing the peak discharge. 203 F. Land Use 204 Land use is the watershed cover and includes every kind of vegetation, litter and mulch, 205 fallow, and bare soils, as well as nonagricultural uses, such as water surface (lakes, 206 wetlands), impervious surfaces (roads, roofs), and urban lands. 207 G. Land Treatment 208 (1) Land treatment, or conservation practices, reduces , and thereby maintains an 209 open structure at the soil surface. This reduces the runoff, but the effect diminishes 210 rapidly with increases in storm magnitude.

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211 (2) Contouring and terracing reduce erosion and decrease the amount of runoff by 212 forming small reservoirs. Closed-end level terraces act as storage reservoirs without 213 spillways. It is possible to exclude land areas in which level terraces have been 214 constructed from the drainage area above downstream measures if the terrace system 215 has enough capacity to store the depth of the runoff commensurate with the 216 frequency of the runoff event. Gradient terraces increase the distance water must 217 travel and thereby increase the Tc. 218 H. Hydrologic Conditions 219 (1) Hydrologic condition of the site can affect the volume or runoff significantly. 220 Estimation of the hydrologic condition considers the effects of cover type and 221 treatment on infiltration and runoff based on density of plant cover and residue on the 222 ground surface. Good hydrologic condition indicates that the site usually has a lower 223 runoff potential. Crop residue tilled into the soil and the residual root system from 224 grasses that have been in crop rotations produce a good hydrologic condition. 225 (2) A grassland cover is good if the vegetation covers 75 percent or more of the ground 226 surface and is lightly grazed. A cover is poor if vegetation covers less than 50 227 percent of the ground surface or is heavily grazed. Evaluation of grass cover uses the 228 basal area of the plant, while evaluation of trees and shrubs uses the canopy cover. 229 (3) For arid and semiarid rangelands, poor conditions exist if the ground cover (grass, 230 litter, and brush canopy) is less than 30 percent. Fair conditions exist when the 231 ground cover is between 30 and 70 percent, and good conditions exist when ground 232 cover is greater than 70 percent. 233 I. Topography 234 (1) The slopes in a watershed have a major effect on the peak discharge at downstream 235 points. Slopes have little effect on how much of the rainfall will run off. As 236 watershed slope increases, velocity increases, Tc decreases, and peak discharge 237 increases. An average small watershed is fan shaped. As the watershed becomes 238 elongated or more rectangular, the flow length increases and the peak discharge 239 decreases. 240 (2) Potholes may trap a small amount of rain, thus reducing the amount of expected 241 runoff. If potholes and wetland areas make up a third or less of the total watershed 242 and do not intercept the drainage from the remaining two-thirds, they will not 243 contribute much to the peak discharge. These areas may be excluded from the 244 drainage area for estimating peak discharge. If potholes constitute more than a third 245 of the total drainage or if they intercept the drainage, use the procedures in NEH Part 246 630 to estimate the peak discharge.

247 650.0202. Hydrographs

248 A. General 249 (1) A hydrograph is a graph of flow (rate versus time) at a stream. The hydrograph 250 shape is a function of watershed conditions and all the factors previously discussed. 251 (2) For most NRCS on-farm conservation work, it is necessary to know only the peak 252 rate of runoff, or the peak of the hydrograph. However, determining the peak rate of 253 runoff requires developing a hydrograph 254 B. Types of Hydrographs 255 There are several types of useful hydrographs.

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256 (i) Natural hydrographs, or measured hydrographs, obtained directly from the flow 257 records of a gaged stream resulting from an actual rainfall event. 258 (ii) Synthetic (or modeled) hydrographs obtained using watershed parameters and 259 storm characteristics to simulate a natural hydrograph. Figure 2-5 illustrates a 260 measured (or natural) hydrograph and a synthetic (or modeled or computed) 261 hydrograph for a small watershed. 262 (iii) Unit hydrographs represent 1 inch of direct runoff distributed uniformly over a 263 watershed resulting from a rainfall of a specified duration. In the case of typical 264 NRCS work, that storm duration is 24-hours. 265 (iv) Dimensionless unit hydrograph (DUH), developed to represent several unit 266 hydrographs plotted using the ratio of the basic units of time to peak and peak 267 rate of discharge. DUHs basically describe the response rate or the rate at which 268 water flows off the watershed to the point of interest. 269 270 Figure 2-5. Hydrograph plot illustrating a natural (or measured) hydrograph and a synthetic (or 271 computed) hydrograph for the same watershed

272 273 C. Dimensionless unit hydrograph peak rate factors 274 (1) DUHs are frequently described in terms of a peak rate factor, or PRF. A PRF 275 describes the percentage of water in the rising limb of the hydrograph versus the 276 receding limb of the hydrograph. DUHs with high PRFs indicate areas where peak 277 rates of runoff will be extremely high, while DUHs with low PRFs indicate areas 278 where peak rates of runoff will be extremely low. 279 (2) NRCS typically uses a standard dimensionless unit hydrograph (DUH) with a PRF 280 equal to 484, often referred to as a 484 DUH. The 484 DUH has 37.5% of the runoff 281 volume in the rising limb of the hydrograph and 62.5% of the runoff volume in the 282 receding limb of the hydrograph. The 484 DUH represents the type of runoff 283 response seen throughout most of the United States and is typical of locations where 284 the landscape is rolling and not very flat. Figure 2-6 illustrates the 484 DUH.

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285 Figure 2-6. Dimensionless curvilinear unit hydrograph (PRF=484) and equivalent dimensionless 286 triangular unit hydrograph

287 288 (3) Another frequently used DUH within NRCS work is the 286 DUH, derived through 289 observations of runoff data on the Delaware-Maryland-Virginia peninsula and often 290 referred to as the DelMarVa unit hydrograph. The DelMarVa peninsula is in the 291 Piedmont region along the East Coast, where land slopes are generally flat and peak 292 rates of runoff are not as high as elsewhere in the country. 293 (4) NRCS also developed several other DUHs with other PRFs. For NRCS on-farm 294 conservation practices use the 484 DUH for evaluation and design of on-farm 295 conservation practices, unless otherwise directed the State Conservation Engineer.

296 650.0203 Runoff Volume

297 A. General 298 (1) As stated previously, surface runoff, sometimes referred to as runoff volume, is the 299 volume of excess water that runs off a drainage area. 300 (2) On a hydrograph plot, runoff volume represents the area under the hydrograph. 301 B. NRCS Runoff Equation 302 (1) The NRCS runoff equation, sometimes referred to as the CN Method, is a tool used 303 to estimate the volume of runoff resulting from a storm event. 304 (2) The NRCS runoff equation is: (푃−퐼 )2 305 푄 = 푎 (eq. 2–1) (푃−퐼푎)+푆 306 where: 307 Q = runoff, in 308 P = rainfall, in 309 Ia = initial abstraction, in

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310 S = potential maximum retention after runoff begins, in 311 (3) Initial abstraction (Ia) includes all losses before runoff begins. It includes water 312 retained in surface depressions, water intercepted by vegetation, and water lost to 313 evaporation and infiltration. Ia is highly variable but is generally correlated with soil 314 and cover parameters. Through studies of many small agricultural watersheds, 315 researchers found Ia to be approximated by:

316 퐼푎 = 0.2푆 (eq. 2–2)

317 (4) Removing Ia as an independent parameter allows use of a combination of S and P to 318 produce unique runoff volumes. Substituting equation 2–2 into equation 2–1 gives: (푃−0.2푆)2 319 푄 = (eq. 2–3) (푃+0.8푆) 320 C. Runoff Curve Numbers 321 (1) The potential maximum retention, S, can range from zero on a smooth, impervious 322 surface to infinity in deep gravel. Converting S values to runoff curve numbers 323 (CNs) using the following transformation provides for greater convenience: 1,000 324 퐶푁 = (eq. 2–4) 10+푆 325 (2) According to equation 2-4, the CN is 100 when S is zero, and approaches zero as S 326 approaches infinity. Runoff curve numbers can be any value from zero to 100, but 327 for practical applications fall within a limited range of about 40 to 98 328 (3) Researchers developed the runoff curve numbers in the tables in figure 2-7a through 329 2-7d by examining rainfall runoff data from small agricultural watersheds. The 330 runoff curve number for a given soil-cover type is not a constant but varies from 331 storm to storm. The index of runoff potential for a given storm is the antecedent 332 runoff condition (ARC). ARC is an attempt to account for the variation in CN at a 333 site from storm to storm. The runoff curve numbers in the tables in figure 2-7a 334 through 2-7d are for an average antecedent condition (ARC=II) and represent CN 335 values used for design. 336 (4) The Runoff Curve Number worksheet (Exhibit A in section 650.0210) is available 337 for use in estimating a representative curve number for a watershed as illustrated in 338 the Example – Estimating Weighted CN (section 650.0207). 339 D. Estimating Runoff Volume using the NRCS CN Method 340 (1) Use equation 2-1 to estimate runoff volume for a storm event using the watershed CN 341 value and the rainfall amount, P, for the storm. 342 (2) Alternatively, use figure 2-8 to estimate storm runoff volume by entering the figure 343 from the bottom axis, Rainfall, P; reading up to the appropriate CN value; and then 344 reading across to the left to the Direct Runoff, Q (or runoff volume). 345 (3) Another alternative is to use a table as shown in figure 2-9 that summarizes runoff 346 volume by curve number for various rainfall amounts. 347

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348 Figure 2- 7a. Runoff Curve Numbers - (a) Cultivated Agricultural Lands 1 Cover description Curve numbers for hydrologic soil group Hydrologic Cover type Treatment 2/ A B C D condition 3/ Fallow Bare soil ----- 77 86 91 94 Crop residue cover (CR) Poor 76 85 90 93 Good 74 83 88 90 Row crops Straight row Poor 72 81 88 91 Good 67 78 85 89 Straight row + CR Poor 71 80 87 90 Good 64 75 82 85 Contoured (C) Poor 70 79 84 88 Good 65 75 82 86 Contoured + CR Poor 69 78 83 87 Good 64 74 81 85 Contoured & terraced (C&T) Poor 66 74 80 82 Good 62 71 78 81 C&T + CR Poor 65 73 79 81 Good 61 70 77 80 Small grain Straight row Poor 65 76 84 88 Good 63 75 83 87 Straight row + CR Poor 64 75 83 86 Good 60 72 80 84 Contoured (C) Poor 63 74 82 85 Good 61 73 81 84 Contoured + CR Poor 62 73 81 84 Good 60 72 80 83 Contoured & terraced (C&T) Poor 61 72 79 82 Good 59 70 78 81 C&T + CR Poor 60 71 78 81 Good 58 69 77 80 Close-seeded Straight row Poor 66 77 85 89 or broadcast Good 58 72 81 85 legumes or Contoured (C) Poor 64 75 83 85 rotation Good 55 69 78 83 meadow Contoured & terraced (C&T) Poor 63 73 80 83 Good 51 67 76 80

349 1 Average runoff condition and Ia=0.2S 350 2 Crop residue cover (CR) applies only if residue is on at least 5% of the surface throughout the year 351 3 Hydrologic condition is based on combination of factors that affect infiltration and runoff, 352 including (a) density and canopy of vegetative areas, (b) amount of year-round cover, (c) amount of grass 353 or close-seeded legumes in rotations, (d) percent of residue cover on the land surface (good > 20%), and (e) 354 degree of surface roughness 355 Poor: Factors impair infiltration and tend to increase runoff 356 Good: Factors encourage average and better than average infiltration and tend to decrease runoff 357 For conservation tillage, poor hydrologic condition, 5 to 20 percent of the surface is covered with residue 358 (less than 750 lb/acre for small grain 359 For conservation tillage, good hydrologic condition, more than 20 percent of the surface is covered with 360 residue (greater than 750 lb/acre for row crops or 300 lb/acre for small grains) 361

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362 Figure 2- 7b. Runoff Curve Numbers - (b) Other Agricultural Lands 1 Cover description Curve numbers for hydrologic soil group Hydrologic Cover type A B C D condition Poor 68 79 86 89 Pasture, grassland, or range—continuous Fair 49 69 79 84 forage for grazing 2/ Good 39 61 74 80 Meadow—continuous grass, protected from ----- 30 58 71 78 grazing and generally mowed for hay Poor 48 67 77 83 Brush—brush-forbs-grass mixture with Fair 35 56 70 77 brush the major element 3/ Good 30 4/ 48 65 73 Poor 57 73 82 86 Woods-grass combination (orchard or tree Fair 43 65 76 82 farm) 5/ Good 32 58 72 79 Poor 45 66 77 83 Woods 6/ Fair 36 60 73 79 Good 30 4/ 55 70 77 Farmsteads—buildings, lanes, driveways, ----- 59 74 82 86 and surrounding lots

363 1 Average runoff condition and Ia=0.2S 364 2 Poor: < 50% ground cover or heavily grazed with no mulch 365 Fair: 50% to 75% ground cover and not heavily grazed 366 Good: >75% ground cover and lightly or only occasionally grazed 367 3 Poor: <50% ground cover 368 Fair: 50 to 75% ground cover 369 Good: >75% ground cover 370 4 Actual curve number is less than 30; use CN = 30 for runoff computations 371 5 CNs shown were computed for areas with 50% woods and 50% grass (pasture) cover. Other 372 combinations of conditions may be computed from the CNs for woods and pasture 373 6 Poor: Forest litter, small trees, and brush have been destroyed by heavy grazing or regular 374 burning 375 Fair: Woods are grazed but not burned, and some forest litter covers the soil 376 Good: Woods are protected from grazing, and litter and brush adequately cover the soil. 377 378

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379 Figure 2- 7c. Runoff Curve Numbers - (c) Arid and Semiarid Rangelands 1 Cover description Curve numbers for hydrologic soil group Cover type Hydrologic A 3/ B C D condition 2/ Herbaceous – mixture of grass, weeds, and Poor 80 87 93 low-growing brush, with brush the minor Fair 71 81 89 element Good 62 74 85 Oak-aspen – mountain brush mixture of oak Poor 66 74 79 brush, aspen, mountain mahogany, bitter Fair 48 57 63 brush, maple, and other brush Good 30 41 48 Pinyon-juniper – pinyon, juniper, or both; Poor 75 85 89 grass understory Fair 58 73 80 Good 41 61 71 Sagebrush with grass understory Poor 67 80 85 Fair 51 63 70 Good 35 47 55 Desert shrub – major plants, include Poor 63 77 85 88 saltbush, greasewood, creosotebush, Fair 55 72 81 86 blackbrush, bursage, palo verde, mesquite, Good 49 68 79 84 and cactus

380 1 Average runoff condition and Ia=0.2S. or rangelands in humid regions, use table 2–1(b). 381 2 Poor: <30% ground cover (litter, grass, and brush overstory) 382 Fair: 30% to 70% ground cover 383 Good: >70% ground cover 384 3 Curve numbers for group A have been developed only for desert shrub. 385

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386 Figure 2- 7d. Runoff Curve Numbers - (d) Urban Areas 1 Cover description Curve numbers for hydrologic soil group Cover type and hydrologic condition Average A B C D percent impervious area 2/ Fully developed urban areas (vegetation established): Open space (lawns, parks, golf courses, cemeteries, etc.): 3/ Poor condition (grass cover < 50%) 68 79 86 89 Fair condition (grass cover 50% to 75%) 49 69 79 84 Good condition (grass cover > 75%) 39 61 74 80 Impervious areas: Paved parking lots, roofs, driveways, etc. (excluding right-of- 98 98 98 98 way) Streets and roads: Paved; curbs and storm sewers 98 98 98 98 Paved; open ditches (including right-of-way) 83 89 92 93 Gravel (including right-of-way) 76 85 89 91 Dirt (including right-of-way) 72 82 87 89 Western desert urban areas: Natural desert landscaping (pervious areas only) 4/ 63 77 85 88 Artificial desert landscaping (impervious weed barrier, 96 96 96 96 desert shrub with 1- to 2- inch sand or gravel mulch and basin borders) Urban district: Commercial and business 85 89 92 94 95 Industrial 72 81 88 91 93 Residential districts by average lot size: 1/8 acre or less (town houses) 65 77 85 90 92 1/4 acre 38 61 75 83 87 1/3 acre 30 57 72 81 86 1/2 acre 25 54 70 80 85 1 acre 20 51 68 79 84 2 acres 12 46 65 77 82 Developing urban areas: Newly graded areas (pervious areas only, no vegetation) 5/ 77 86 91 94

387 1 Average runoff condition and Ia=0.2S 388 2 The average percent impervious area shown was used to develop the composite CNs. Other assumptions are 389 as follows: impervious areas are directly connected to the drainage system, impervious areas have a CN of 98, and 390 pervious areas are considered equivalent to open space in good hydrologic condition. 391 3 CNs shown are equivalent to those of pasture. Composite CNs may be computed for other combinations of 392 open space cover type. 393 4 Composite CNs for natural desert landscaping should be computed based on the impervious area (CN=98) 394 and the pervious area CN. The pervious area CNs are assumed equivalent to desert shrub in poor hydrologic condition. 395 5 Composite CNs to use for the design of temporary measures during grading and construction should be 396 computed using the degree of development (impervious area percentage) and the CNs for the newly graded pervious 397 areas. 398

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399 Figure 2-8. Solution to the NRCS Runoff Equation

400

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401 Figure 2-9. Runoff Depth for Selected CNs And Rainfall Amounts1 Rainfall ------Runoff (Q) in inches for curve number of ------(in) 40 45 50 55 60 65 70 75 80 85 90 95 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 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 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 1.6 0.00 0.00 0.00 0.00 0.01 0.05 0.11 0.20 0.34 0.52 0.76 1.11 1.8 0.00 0.00 0.00 0.00 0.03 0.09 0.17 0.29 0.44 0.65 0.93 1.29 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 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 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 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 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 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 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 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 7.0 0.84 1.24 1.67 2.12 2.60 3.10 3.62 4.15 4.69 5.25 5.82 6.41 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 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 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 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 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 13.0 4.00 4.89 5.76 6.61 7.42 8.21 8.98 9.71 10.42 11.10 11.76 12.39 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 15.0 5.33 6.36 7.35 8.29 9.19 10.04 10.85 11.63 12.37 13.07 13.74 14.39 402 Interpolate the values shown to obtain runoff depths for CNs or rainfall amounts not shown. 403

404 650.0204 Time of Concentration

405 A. General

406 (1) Time of concentration, Tc, is the time it takes for runoff to travel from the 407 hydraulically most distant point of the watershed to the outlet. Tc influences the peak 408 discharge. For the same size watershed, the shorter the Tc, the larger the peak 409 discharge. This means that peak discharge has an inverse relationship with Tc. 410 (2) On a hydrograph, time of concentration is the time period from the end of the excess 411 rainfall (when the storm event ends) to the point of inflection on the receding limb of 412 the hydrograph. Figure 2-6 shows this relationship between the end of the excess 413 rainfall and time of concentration. 414 (3) Estimating peak discharge requires estimating the runoff curve number, average 415 watershed slope, and flow length. 416 (4) An estimate of the time of concentration is necessary for developing a hydrograph to 417 estimate the peak discharge from a watershed. 418 B. The NRCS Lag Equation

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419 Estimate Tc for small rural watersheds using the following empirical relationship 420 referred to as the lag equation:

1,000 0.7 푙0.8[( )−9] 421 푇 = 퐶푁 (eq. 2–5) 푐 1140푌0.5 422 where: 423 Tc = time of concentration, hr 424 l = flow of length, ft 425 CN = runoff curve number 426 Y = average watershed slope, % 427 (1) Average Watershed Slope 428 (i) The average watershed slope (Y) is the slope of the land and not the watercourse. 429 It can be determined from soil survey data or topographic maps. Measure 430 hillside slopes with a hand level, Locke level, or clinometer in the direction of 431 overland flow. Average watershed slope is an average of individual land slope 432 measurements. 433 (ii) The average watershed slope can be determined using the following relationship: 100퐶퐼 434 푌 = (eq. 2–6) 퐴 435 where: 436 Y = average watershed slope, % 437 C = total contour length, ft 438 I = contour interval, ft 439 A = drainage area, ft2 440 (2) Flow Length 441 (i) Flow length (l) is the longest flow path in the watershed from the watershed 442 divide to the outlet. It is the total path water travels overland and in small 443 channels on the way to the outlet. The flow length can be determined using a 444 topographic map. 445 (ii) Figures 2-10 and 2-11 illustrate some typical examples of how flow lengths can 446 differ between a natural watershed and a terraced watershed. 447 (iii) Natural watershed—In this case, water flows from the watershed divide to a 448 small , down the small channel to the main stream and from there to the 449 watershed outlet. 450 (iv) Watershed with Terraces—In this case, water flows from the watershed divide 451 to the terrace, along the terrace to its outlet or main stream and then along the 452 main stream to the watershed outlet.

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453 Figure 2-10. Natural watershed Figure 2-11. Watershed with Terraces

454 C. Estimating Tc Using the NRCS Lag Equation 455 (1) Figure 2-12 is a nomograph for solving equation 2–5. The worksheet in 650.0210 456 (Exhibit B) is available for use to compute Tc. Example – Estimating Time of 457 Concentration and Peak Discharge (section 650.0207) demonstrates this procedure. 458 (2) For watersheds where hydraulic conditions where it is necessary to use velocities of 459 water flow in different Tc flow path segments, such as urban areas, then estimate Tc 460 using the velocity method described in NEH Part 630 Chapter 15 Time of 461 Concentration.

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462 Figure 2-12. Time of Concentration (Tc) Nomograph

463

464 650.0205 Peak Discharge

465 A. General 466 (1) For most NRCS on-farm conservation practice analysis and design, the modeler 467 needs only to know peak discharge. However, to compute the peak discharge we do 468 need to also know the runoff volume.

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469 (2) Estimating peak discharge can be done manually provided unit peak discharge curves 470 are available or using the NRCS EFH-2 computer program. 471 B. Estimating Peak Discharge Manually 472 (1) Unit peak discharge curves 473 (i) Unit peak discharge curves represent a combination of rainfall distribution and 474 dimensionless unit hydrograph for use in specific locations. NRCS developed 475 these curves in order to develop a generalized and simplified method of 476 computing peak discharge since long-hand computations of peak discharge 477 involved development of full hydrographs manually which is a long and tedious 478 process. Unit peak discharge curves are sometimes referred to as Rainfall Type 479 curves but are actually a combination of a rainfall distribution and a 480 dimensionless unit hydrograph. 481 (ii) For locations where unit peak discharge (qu) curves are available, peak discharge 482 (qp) can be estimated manually as the product of the unit peak discharge (qu), 483 drainage area (A), and runoff volume (Q):

484 푞푝 = 푞푢퐴푄 (eq. 2–7) 485 where: 486 qp = peak discharge (cfs) 487 qu = unit peak discharge (cfs/ac/in) 488 A = watershed drainage area (ac) 489 Q = runoff volume (in) 490 (ii) Tc and Ia /P values are needed to obtain a value for qu from the unit peak 491 discharge curves (figures 2-10 through 2-13 or from appropriate NRCS State 492 supplement to this chapter). 493 (2) Ia/P ratio 494 (i) The Ia/P ratio is a parameter that indicates how much of the total rainfall is needed 495 to satisfy the initial abstraction. The larger the Ia/P ratio, the lower the qu for a 496 given Tc. This indicates that if Ia is a high portion of rainfall, the peak discharge 497 will be lower. Thus, the Ia/P ratio is greater for small storms. 498 (ii) Use either equations 2-4 and 2-2, or the table in figure 2-13 with the watershed 499 CN to determine the initial abstraction (Ia). 500 (ii) If the computed Ia/P ratio is outside the range shown (0.1 to 0.50) in figures 2-10 501 through 2-13, then the limiting values should be used; i.e., use 0.1 if less than 0.1 502 and use 0.5 if greater than 0.5. If the ratio falls between the limiting values, use 503 linear interpolation. 504 (3) Estimating Peak Discharge 505 (i) For the NRCS standard rainfall distributions (old distributions), obtain the unit 506 peak discharge (qp) using the unit peak discharge curves illustrated in figures 2- 507 14 through 2-17 for the appropriate rainfall type. Figure 2-1 shows the 508 approximate geographic boundaries for the four rainfall distributions. 509 (ii) For states with updated NOAA Atlas 14 rainfall data and distributions (new 510 distributions developed by NRCS using the NOAA Atlas 14 rainfall data), obtain 511 the unit peak discharge (qu) from the unit peak discharge curves available in the 512 appropriate State supplement to this chapter. Contact the appropriate State 513 Conservation Engineer for this information if needed. 514 (iii) The worksheet in 650.0210 (Exhibit B) is available for use in determining qp as 515 illustrated in the example in 650.0207 B. 516

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517 Figure 2 13. Ia Values for Runoff Curve Numbers

Curve Ia (in) Curve Ia (in) Curve Ia (in) number number number 40 3.000 60 1.333 80 0.500 41 2.878 61 1.279 81 0.469 42 2.762 62 1.226 82 0.439 43 2.651 63 1.175 83 0.410 44 2.545 64 1.125 84 0.381 45 2.444 65 1.077 85 0.353 46 2.348 66 1.030 86 0.326 47 2.255 67 0.985 87 0.299 48 2.167 68 0.941 88 0.273 49 2.082 69 0.899 89 0.247 50 2.000 70 0.857 90 0.222 51 1.922 71 0.817 91 0.198 52 1.846 72 0.778 92 0.174 53 1.774 73 0.740 94 0.128 54 1.704 74 0.703 95 0.105 55 1.636 75 0.667 56 1.571 76 0.632 57 1.509 77 0.597 58 1.448 78 0.564 59 1.390 79 0.532 518

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519 Figure 2- 14. Unit Peak Discharge (qu) for NRCS Rainfall Type I Rainfall Distribution

520 521

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522 Figure 2-15. Unit Peak Discharge (qu) for NRCS Rainfall Type IA Rainfall Distribution

523 524 525 526

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527 Figure 2- 16. Unit Peak Discharge (qu) for NRCS Rainfall Type II Rainfall Distribution

528 529 530 531

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532 Figure 2-17. Unit Peak Discharge (qu) for NRCS Rainfall Type III Rainfall Distribution

533 534

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535 C. Estimating Peak Discharge Using the EFH-2 Computer Program 536 (1) The EFH-2 Computer Program Automates estimation of runoff volume and peak 537 discharge. 538 (2) EFH-2 uses the procedures described in NEH Part 630, Hydrology, Chapter 16, 539 Hydrographs to develop full hydrographs for the location of interest and provide an 540 estimate of peak discharge. (2) To use the EFH-2 computer program, the user must 541 select the appropriate rainfall distribution and an appropriate dimensionless unit 542 hydrograph as separate inputs. The manual method for estimating peak discharge 543 uses the unit peak discharge curves, often described as rainfall-type curves that 544 represent a combination of the rainfall distribution and peak discharge. 545 (3) The EFH-2 User’s Manual details usage of the computer program for estimating 546 runoff curve number, time of concentration, and peak discharge.

547 650.0206 Limitations

548 A. The watershed drainage area must be greater than 1.0 acre and less than 2,000 acres. Use 549 another procedure to estimate peak discharge if the drainage area is outside these limits. 550 B. The watershed should have only one main stream. If more than one exists, the branches 551 must have nearly equal Tcs. 552 C. The watershed must be hydrologically similar; i.e., able to be represented by a single 553 weighted CN. Land use, soils, and cover are distributed uniformly throughout the watershed. 554 The land use must be primarily rural. If urban conditions are present and not uniformly 555 distributed throughout the watershed, or if they represent more than 10 percent of the 556 watershed, use another procedure. 557 D. If using the unit peak discharge curves, the accuracy of peak discharge estimated by the 558 method described in this chapter is reduced if Ia /P ratio used is outside the range of 0.1 to 0.5 559 as shown in figures 2-10 through 2-13. 560 E. When the average watershed slope is less than 0.5 percent, a different unit hydrograph 561 shape can be used. Contact the State Conservation Engineer for necessary information.

562 F. If the computed Tc is less than 0.1 hour, use 0.1 hour. If the computed Tc is greater than 563 10 hours, estimate peak discharge using the procedures documented in NEH Part 630. 564 WinTR-55 and WinTR-20 computer programs automate these procedures. 565 G. When the flow length is less than 200 feet or greater than 26,000 feet, use another 566 procedure to estimate Tc. 567 H. Do not estimate runoff and peak discharge from snowmelt or rain on frozen ground using 568 these procedures. A procedure for estimating peak discharge in these situations is in NEH 569 Part 630, Chapter 11, Snowmelt. 570 I. If potholes constitute more than a third of the total drainage area or if they intercept the 571 drainage, use the procedures in NEH Part 630. 572 J. When the average watershed slope is greater than 64 percent or less than 0.5 percent, use 573 another procedure to estimate Tc. WinTR-55 presents alternative procedures for estimating 574 Tc and peak discharge. 575 K. When the weighted CN is less than 40 or more than 98, use another procedure to estimate 576 peak discharge.

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577 650.0207 Examples

578 A. Example - Estimating weighted CN 579 (1) Given a 90-acre watershed in the Type II storm distribution area, determine the 580 weighted curve number for the drainage area above a proposed waterway. The 581 available soils map shows that the major soils are Dover, Berks, and Easton in field 582 #2 of A.B. Smith’s farm in Adams County, Maryland. By soil survey, the HSGs 583 were determined to be B for Dover, C for Berks, and D for Easton. By soil, the cover 584 description breaks down as 25 acres of pasture in good condition on Dover, 55 acres 585 of row crop in straight rows in good condition on Berks, and 10 acres of woods in 586 poor condition on Easton. 587 (2) Solution: Figure 2-14 shows the completed computations for estimating the 588 weighted curve number for this example. Exhibit A in 650.0210 at the end of this 589 document is a blank worksheet for estimating weighted curve number. 590 B. Example - Estimating Time of Concentration 591 (1) For the watershed in the Example – Estimating Weighted Runoff Curve Number, 592 determine the time of concentration. The average watershed slope is 1 percent, and 593 the flow length is 3,400 feet. 594 (2) Figure 2-15 shows the completed computation for estimating the time of 595 concentration. Note that this examples uses the runoff curve number estimated in the 596 previous example in the Tc computation. Exhibit B in 650.0210 at the end of this 597 document is a blank worksheet for estimating time of concentration and peak 598 discharge. 599 C. Example – Manual Estimation of Peak Discharge 600 (1) For the watershed in the examples in 650.0207 A and B, determine the peak 601 discharges for the 2-, 5-, and 10-year events. The 2-year, 24-hour precipitation is 3.4 602 inches; the 5-year, 24-hour precipitation is 4.5 inches; and the 10-year, 24-hour 603 precipitation is 5.5 inches. 604 (2) Figure 2-15 shows the completed computations for the peak discharge for this 605 example. Exhibit B in 650.0210 at the end of this document is a blank worksheet for 606 estimating time of concentration and peak discharge. 607 (3) Please note that this example for the State of Maryland is based on using TP-40 608 rainfall and the NRCS Standard Type II Rainfall Distribution. The State of Maryland 609 has updated NOAA Atlas-14 rainfall data and distributions available and is using 610 them, however at the time these examples were prepared that information was not 611 available. Results obtained using the Maryland updated rainfall data and 612 distributions will be different than those illustrated in this example. If you need 613 updated rainfall data, distributions, unit peak curves, for Maryland, or other states, 614 reach out to the State Conservation Engineer for the appropriate State supplement to 615 this chapter. 616 D. Example – Using EFH-2 to Estimate Peak Discharge 617 See the EFH-2 computer program User’s Manual for examples of estimating peak 618 discharge using the computer program. 619

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620 Figure 2-18 Solution to Example – Estimating Weighted Runoff Curve Number

621 622 623

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624 Figure 2-19. Solution to Example – Estimating Time of Concentration and Peak Discharge

625 626

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627 650.0208 References

628 A. United States Department of Agriculture, Natural Resources Conservation Service 629 (NRCS). Title 210-National Engineering Handbook (NEH), Part 630, Hydrology. (multiple 630 chapters), (multiple publication dates). 631 B. United States Department of Commerce, National Oceanic and Atmospheric 632 Administration, National Weather Service (NOAA-NWS). NOAA Atlas 14, Precipitation- 633 Frequency Atlas of the United States (multiple volumes), (multiple publication dates). 634 https://www.nws.noaa.gov/oh/hdsc/relevant_publications.html 635 C. United States Department of Commerce, National Oceanic and Atmospheric 636 Administration, National Weather Service (NOAA-NWS). NOAA Atlas 2, Precipitation- 637 Frequency Atlas of the Western United States (multiple volumes)4, Precipitation-Frequency 638 Atlas of the United States, volumes 1 - XI, 1973. 639 https://www.nws.noaa.gov/oh/hdsc/relevant_publications.html

640 650.0209 Acknowledgements

641 A. Kenneth M. Kent (retired) and Wendell A. Styner (retired) originally prepared Chapter 642 2, Estimating Runoff, which appeared in the Soil Conservation Service (SCS, now Natural 643 Resources Conservation Service, NRCS) in the SCS Engineering Field Handbook, in 1971. 644 B. Donald E. Woodward (retired) directed updates to Chapter 2 in 1989 and 1990 and 645 retitled the chapter Estimating Runoff and Peak Discharge. SCS published the 1990 version 646 as Chapter 2 of the Engineering Field Handbook 647 C. This edition incorporates the following revisions to the 1990 edition: 648 (1) Updates references from the Soil Conservation Service to the Natural Resources 649 Conservation Service. 650 (2) Retitles the chapter Estimating Runoff Volume and Peak Discharge and publishes it 651 as Chapter 2 in the NRCS NEH Part 650, Engineering Field Handbook. 652 (3) Brings the chapter into NRCS National Engineering Handbook format. 653 (4) Adds information the use of updated precipitation-frequency data and updated 654 rainfall distributions. 655 (5) Adds a discussion of dual classification of hydrologic soil groups. 656 (6) Adds a section on hydrographs. 657 (7) Adds a discussion on the use of the EFH-2 computer program. 658 C. Claudia Hoeft, National Hydraulic Engineer, Conservation Engineering Division, NRCS, 659 Washington, D.C. prepared the majority of the revisions with reviews and recommendations 660 from William Merkel (retired), Helen Fox Moody (deceased), Quan Quan, Hydraulic 661 Engineer, West National Technology Support Center, Beltsville, Maryland, and Geoff 662 Cerrelli (retired). 663 D. Lynn Owens (retired), Suzy Self (retired), and Wendy Pierce, illustrator, National 664 Geospatial Management Center, NRCS, Fort Worth, Texas assisted with initial editing and 665 provided illustrations.

666 650.0210 Exhibits

667 A. Runoff Curve Number Worksheet 668 B. Time of Concentration and Peak Discharge Worksheet

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Runoff Curve Number (CN)

Client: ______By: ______Date: ______County: ______State: _____ Checked: ______Date: ______Practice: ______

Cover description Soil name and CN Area Product of (cover type, treatment, and hydrologic soil group (table 2-3) (acres or %) CN x Area hydrologic condition)

TOTALS:

Product of CN × Area CN (weighted)= = =_____ USE CN = Total Area

Time of Concentration and Peak Discharge

Client: ______By: ______Date: ______County: ______State: _____ Checked: ______Date: ______Practice: ______

Estimating time of concentration: 1. Data: Rainfall distribution type ...... = ______From NRCS standard distribution type (I, IA, II, or III) OR Updated NRCS rainfall distribution type (from State Supplement to NEH Part 650 Chapter 2) Drainage area ...... A = ______acres Runoff curve number ...... CN = ______From Runoff Curve Number worksheet. Watershed slope ...... Y = ______%

Flow length ...... l = ______ft

2. Tc using l, Y, CN, and figure 2-6 ...Tc = ______hrs OR using equation 2-5: 0.7 0.7 0.8 1,000 1,000 ( - 9) (_____)0.8 ( - 9) l CN __ Tc= = = _____ hours 1,140Y0.5 1,140(_____)0.5

Estimating peak discharge:

Storm #1 Storm #2 Storm #3 1. Frequency ...... yr 2. Rainfall, P (24-hour) in 3. Initial abstraction, I ...... a in (use CN with figure 2-4)

4. Compute Ia/P ratio ......

5. Unit peak discharge, qu* . . . . . cfs/ac/in 6. Runoff, Q ...... in 7. Peak discharge, q ...... p cfs (where qp = qu A Q)

*(use Tc and Ia/P with figures 2-10 through 2-13 OR with appropriate unit peak discharge figures for updated NRCS rainfall distributions from State supplements to NEH Part 650 Chapter 2)