Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina
By Benjamin F. Pope and Gary D. Tasker
U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 99–4114
Prepared in cooperation with the North Carolina Department of Transportation
Raleigh, North Carolina 1999
U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary
U.S. GEOLOGICAL SURVEY Charles G. Groat, Director
The use of firm, trade, and brand names in this report is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey.
For additional information write to: Copies of this report can be purchased from:
District Chief U.S. Geological Survey U.S. Geological Survey Information Services 3916 Sunset Ridge Road Federal Center, Box 25286 Raleigh, NC 27607-6416 Denver, CO 80225
CONTENTS
Abstract ...... 1 Introduction...... 1 Purpose and scope...... 2 Approach...... 2 Data compilation ...... 3 Acknowledgments...... 3 Basin characteristics...... 6 Estimation of flood magnitude and frequency at gaged sites...... 7 Estimation of flood magnitude and frequency at ungaged sites...... 10 Regional regression analysis ...... 10 Region-of-influence analysis...... 13 Comparison of results ...... 14 Use of computer software ...... 15 Application of methods...... 15 Summary ...... 16 References...... 17 Appendix...... 43
FIGURES 1. Locations of gaged rural sites in North Carolina ...... 4
TABLES 1. Map identification numbers and descriptions of gaged rural sites in North Carolina with annual peak-flow record ...... 19 2. Recurrence interval discharges and basin characteristics for gaged rural sites in North Carolina ...... 31 3. Basin characteristics that were used in the North Carolina flood-frequency regionalization study ...... 6 4. Generalized skew coefficient and associated mean square error for rural North Carolina gaging sites ...... 9 5. North Carolina rural flood-frequency equations ...... 11 6. Average predictive errors and equivalent years of record associated with North Carolina rural flood-frequency equations ...... 12 7. Root mean square error for the regional regression and region-of-influence methods, presented by hydrologic area and recurrence interval ...... 14
Contents III Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina
By Benjamin F. Pope and Gary D. Tasker
ABSTRACT between flood discharge and basin characteristics for a subset of gaged sites with similar basin A statewide study was conducted to develop characteristics. This, then, can be used to estimate two methods for estimating the magnitude and flood discharges at ungaged sites. Because the frequency of floods in rural ungaged basins in computations required for this method are North Carolina. Flood-frequency estimates for somewhat complex, a computer application was gaged sites in North Carolina were computed by developed that performs the computations and fitting the annual peak flows for each site to a log- compares the predictive errors for this method. Pearson Type III distribution. As part of the The computer application also includes the option computation of flood-frequency estimates for of using the regression equations to compute gaged sites, new values for generalized skew estimated flood discharges and errors of prediction coefficients were developed. Basin characteristics specific to each ungaged site. for these gaged sites were computed by using a Root mean square errors, computed for each geographic information system and automated recurrence interval and hydrologic area, are computer algorithms. Flood-frequency estimates generally only slightly lower for the region-of- and basin characteristics for 317 gaged sites were influence method than for the regression equations combined to form the data base that was used for and do not provide sufficient basis for this analysis. recommending one method over the other. In Regional regression analysis, using addition, the region-of-influence method is a new generalized least-squares regression, was used to method that is still being improved. As a result, the develop a set of predictive equations that can be regional regression equations are considered to be used to estimate the 2-, 5-, 10-, 25-, 50-, 100-, the primary method for computing flood- 200-, and 500-year recurrence interval discharges frequency estimates at ungaged sites. for rural ungaged basins in the Blue Ridge- Piedmont, Coastal Plain, and Sand Hills hydrologic areas. The predictive equations are all INTRODUCTION functions of drainage area. Average errors of prediction for these regression equations range Reliable estimates of the magnitude and from 38 to 56 percent. frequency of floods are needed by State and local designers and managers. The design of highway and A region-of-influence method also was railroad stream crossings, delineation of flood plains developed that interactively estimates recurrence and flood-prone areas, management of water-control interval discharges for rural ungaged basins in the structures, and management of water supplies are all Blue Ridge-Piedmont and Coastal Plain activities that require estimates of the frequency hydrologic areas of North Carolina. Regression distribution of flood events. Such estimates can be techniques are used to develop a unique relation computed directly by using statistical methods at gaged
Abstract 1 sites that have at least 10 years of annual peak record; performed by a computer application that is discussed the longer the record of annual peak flows, the more later in this report. Because only gaged sites with reliable the estimate. It is not feasible, however, to similar basin characteristics are used to estimate flows collect 10 years of annual peak record for every at ungaged sites, there is less chance of extrapolation location where an estimate of the flood-frequency beyond the limits of the explanatory data. Tests of this distribution is needed, nor is it reasonable to wait approach in Texas (Tasker and Slade, 1994) and in 10 years for an estimate once a site has been identified. Arkansas (Hodge and Tasker, 1995) yielded estimates Estimates that are derived solely from gage with lower prediction errors than those produced by records do not provide sufficient spatial coverage to using traditional regional regression techniques. satisfy the need for reliable estimates of the magnitude Gunter and others (1987) contains annual and frequency of floods. Traditionally, to meet this peak-flow data collected from gages throughout 1 need, annual peak records at gaged sites have been North Carolina through the 1984 water year , whereas regionalized, or extended in space. By this process, this report contains peak-flow data collected through flood-frequency estimates at gaged sites are related to the 1996 water year. Thus, gaged sites that have measurable basin characteristics so that reliable flood- continued in operation since 1984 have as much as frequency estimates can be made at ungaged sites. In 12 additional years of peak-flow data available for response to the need to improve the accuracy of computation of flood-frequency estimates. The estimates of flood discharges for ungaged rural basins, 12 intervening years (1985–96) include several years the U.S. Geological Survey (USGS), in cooperation of pronounced drought (1985–88) as well as years in with the North Carolina Department of Transportation, which maximum peaks of record were recorded initiated an investigation in 1996 to further define the (1992–93, 1996) for North Carolina streams. In relation between flood discharges of selected addition, 64 gaged sites that were not used in Gunter recurrence intervals and selected basin characteristics and others (1987) are now available for analysis. for rural North Carolina basins. In the past, regionalization was achieved by Purpose and Scope means of regional regression analysis. Data from gaged sites were used to define a set of relations between This report describes the development, selected recurrence interval discharges and drainage application, and evaluation of two methods for area. Once defined, these relations were then used to estimating the magnitude and frequency of floods at estimate discharges at selected recurrence intervals for ungaged, unregulated, rural basins in North ungaged sites. Often the area of study was subdivided Carolina—(1) the regional regression method and into regions of similar hydrology in order to improve (2) the region-of-influence method. A comparison of the predictive ability of the equations. Gunter and these two methods, based on their predictive ability and others (1987) used this approach to develop regional ease of application, also is presented. In order to relations for estimating the magnitude and frequency of compare the two methods on an equal basis, each floods in rural North Carolina basins. method was applied to the same available data. The Recently, however, a different approach to regional regression and region-of-influence methods of regionalization has been developed. This new estimation were applied to the current data base of 317 approach, known as the region-of-influence method, sites with at least 10 years of unregulated peak-flow interactively estimates recurrence interval discharges record and evaluated. for ungaged sites based on data from gaged sites with similar basin characteristics. For each ungaged site selected, a subset of gaged sites having similar basin Approach characteristics is selected from the entire data base of A set of eight basin characteristics was computed rural gaged sites. Regression techniques are used to and compiled for each of 366 gaged rural sites in North develop a unique relation between flood discharge and Carolina that have peak-flow record. Sites that have basin characteristics for this subset of gaged sites. This relation is then used to estimate flood discharges at the ungaged site. Although computationally intensive, the 1Water year is the period October 1 through September 30 region-of-influence method is easily automated and and is identified by the year in which it ends.
2 Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina flows affected by regulation or channelization were consisting of (1) the combined Blue Ridge and identified, and where possible, records for such sites Piedmont physiographic provinces, (2) the Coastal were divided into periods of unregulated and regulated Plain Province, and (3) a subdivision of the Coastal flows. Weighted regional average skew values were Plain Province known as the Sand Hills, also were used used to compute flood-frequency estimates for 317 in this study (fig. 1). sites with at least 10 years of unregulated peak-flow An initial list of 366 rural sites with annual peak- record. Flood-frequency estimates and the computed flow record was compiled (fig. 1; table 1, p. 19–30). basin characteristics for these 317 sites were combined Records for these sites were then examined to to form the data base used in the regional analyses. determine the extent of available basin characteristic Generalized least-squares regression analysis data and to identify sites with flows affected by was used to develop predictive equations relating the channelization or regulation. The only consistently 2-, 5-, 10-, 25-, 50-, 100-, 200-, and 500-year available basin characteristics for most sites were recurrence interval flood discharges to selected basin drainage area and location. A complete evaluation of all characteristics for rural basins throughout North possible relations between flood discharges and other Carolina. In addition, a region-of-influence method characteristics of rural basins requires a more complete was developed that interactively estimates the set of basin characteristics. The computation and recurrence interval flood discharges for ungaged rural compilation of the required basin characteristics for all basins in the Blue Ridge-Piedmont and Coastal Plain of the 366 initial sites are described in the following hydrologic areas. section. Computation and compilation of basin Examination of the flow records for the 366 sites characteristics and of the selected recurrence interval revealed 19 sites with record containing only regulated/ discharges are described in the following sections. All channelized flows, 27 sites with record that could be aspects of each analysis, including the initial divided into periods of unregulated/unchannelized and exploratory multiple regression analysis using ordinary regulated/channelized flows, and 320 sites with records least-squares regression, final regional regression using unaffected by any known regulation/channelization. Of generalized least-squares regression, and the region-of- the 347 sites with at least some period of unregulated influence analysis, are described. Finally, a comparison flow record, 317 sites had the requisite 10 or more years of the results of each method is presented. of record for computation of flood-frequency estimates (table 1). Flood-frequency estimates for these sites were computed and combined with the basin Data Compilation characteristics to form the data base that was used for the regional analyses (table 2, p. 31–42). This data base The first step in the regionalization of flood- contained 222 sites in the Blue Ridge-Piedmont hydro- frequency estimates is the compilation of a list of all logic area, 80 sites in the Coastal Plain hydrologic area, gaged sites with annual peak-flow record. Such sites and 15 sites in the Sand Hills hydrologic area (table 2). are either continuous-record sites or crest-stage sites. Of the 46 sites with regulated flow records, flood- At continuous-record sites, the water-surface elevation, frequency estimates were computed for 42 sites with or stage, of the stream is recorded at fixed intervals, periods of regulated flow longer than 10 years but were typically ranging from 5 to 60 minutes. At crest-stage not included in either regional analysis. sites, only the crest, or highest, stages that occur between site visits, usually 6 to 8 weeks, are recorded. Regardless of the type of gage, measurements of Acknowledgments discharge are determined throughout the range of recorded stages, and a relation between stage and The authors gratefully acknowledge the discharge is developed for the gaged site. Using this assistance and support of Mr. Archie Hankins of the stage-discharge relation, or rating, discharges for all North Carolina Department of Transportation. The recorded stages are determined. The highest peak peak-flow data used in the analyses described herein discharge that occurs during a given year is the annual were collected throughout North Carolina at stream peak for the year, and the list of annual peaks is the gages operated in cooperation with a variety of Federal, annual peak-flow record. The three hydrologic areas State, and local agencies. The authors also would like identified and described by Gunter and others (1987), to recognize the dedicated work of the USGS field
Introduction 3 o o o o o 84 83 82 81 80 VIRGINIA
290 291 9 289 197 13 198 201 288 11 287 199 202 10 196 337 194 200 203 340 338 190 193 12 339 191 195 206 208 204 o 330 205 TENNESSEE 336 189 192 207 209 211 36 210 335 220 318 188 213 212 334 261 265 217 214 317 263 216 319 332 333 331 264 219 316 259 262 222 218 221 215 227 228 314 315 258 267 223 225 329 257 229 328 311 309 260 270 266 268 327 313 224 226 326 308 310 256 271 360 359 324 231 361 357 312 230 355 307 283 232 351 325 322 323 284 272 356 302 306 278 279 280 350 353 321 301 305 273 235 233 362 358 320 349 347 354 300 303 285 352 299 304 274 275 236 234 345 295 286 346 298 281 269 237 366 348 342 344 294 297 296 282 365 341 343 293 239 o 363 292 35 364 238 241 277 240 276 245
SOUTH CAROLINA GEORGIA o 34
Figure 1. Locations of gaged rural sites in North Carolina.
4 Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina o o o o o 80 79 78 77 76
18 VIRGINIA 13 16 19 25 26 28 29 1 17 24 15 27 48 3 14 21 2 20 23 52 6 119 22 4 5 120 70 34 35 8 122 68 71 46 30 7 123 125 121 69 72 49 51 o 126 73 53 36 127 65 36 47 32 141 64 67 74 37 42 50 75 33 130 44 140 124 128 66 76 77 43 79 41 45 142 129 131 80 38 143 134 78 54 40 55 59 132 135 144 133 39 31 136 83 98 148 84 89 95 56 150 138 149 137 85 81 96 139 86 90 60 63 97 104 151 152 99 58 146 82 61 103 57 145 91 242 153 88 100 101 106 155 102 107 243 147 92 62 156 87 108 159 93 94 105 157 175 176 112 109 113 o 158 177 35 250 154 111 160 251 172 169 110 168 170 244 162 161 178 248 114 246 253 163 116 173 179 Atlantic 164 171 181 117 Ocean 247 252 115 165 180 118 249 254
182 174 EXPLANATION 255 183 SOUTH CAROLINA 184 166 BLUE RIDGE - PIEDMONT HYDROLOGIC AREA 167 SAND HILLS HYDROLOGIC AREA (Gunter and others, 1987) 187 COASTAL PLAIN HYDROLOGIC AREA o GENERALIZED PHYSIOGRAPHIC PROVINCE LINE 34 185 183 186 STREAMGAGING STATION AND NUMBER
0 25 50 75 100 MILES
0 25 50 75 100 KILOMETERS
Introduction 5 office staff in collecting, processing, and storing the Table 3. Basin characteristics that were used in the North peak-flow data necessary for the completion of this Carolina flood-frequency regionalization study 2 report. [mi , square mile; mi, mile; ft/mi, foot per mile; ----, a dimensionless characteristic]
Unit Basin of Definition characteristic BASIN CHARACTERISTICS measure Physical characteristics The annual peak-flow data that were used in this DA mi2 Drainage area, measured area study were collected at gages in rural basins from all contained within basin areas of the State, representing the wide range of divides. physical and climatic conditions that occur in North L mi Channel length, measured from Carolina. Eight parameters that characterize the size, gage site upstream along main channel to basin divide. shape, relief, and climate of rural basins in North CSLOPE ft/mi Channel slope, computed Carolina were computed and compiled for each site between points at 10- and 85- used in the study. Physical basin characteristics include percent of the length, drainage area (DA), channel length (L), channel slope measured from the gage site. (CSLOPE), basin slope (BSLOPE), and basin shape BSLOPE ft/mi Basin slope, mean value of slope measured along several (SHAPE) (table 3). The primary climatic characteristics flow paths from basin divide relevant to flood frequency in each basin are the to channel. intensity, duration, and amount of storm rainfall, as well SHAPE ---- Shape, computed by dividing drainage area by the square as other meteorologic inputs that control evaporation 2 and transpiration. Lichty and Liscum (1978) suggested of channel length (DA/L ). Climatic characteristics the use of a regional climate factor, CFt, where t = 2-, CF2 ---- 2-year recurrence interval 25-, and 100-year recurrence intervals, that integrates climate factor long-term rainfall and pan evaporation information and CF25 ---- 25-year recurrence interval represents the effect of these climatic influences on climate factor flood frequency. In this study, a refined version of CFt, CF100 ---- 100-year recurrence interval as developed and described by Lichty and Karlinger climate factor (1990), was used to characterize climatic effects of Regional identifiers flood frequency. Climate factors, CF , for each site were BRP ---- 1, if site is in Blue Ridge- t Piedmont; 0, if not. computed by using a computer algorithm that used the CP ---- 1, if site is in Coastal Plain; maps of climate factor isolines presented in Lichty and 0, if not. Karlinger (1990) and the latitude and longitude of a site SH ---- 1, if site is in Sand Hills; to interpolate values for the three climate factors, CF2, 0, if not. CF25, and CF100. REG ---- 1, if site is in Blue Ridge- The hydrologic area for each site was determined Piedmont; 2, if site is in Coastal Plain; by examining drainage boundary maps. The appropriate 3, if site is in Sandhills. integer value for each site was then assigned to the region variable (REG) (table 3). Other than drainage area, the physical basin was used to compute the required physical basin characteristics selected for use in this study were not characteristics. readily available for most of the basins in the study. In In order to use GIS to develop basin previous studies, drainage area was the primary characteristics, a digital elevation model (DEM) was explanatory variable; thus, there was no prior need to created by combining individual data sets. These data measure or compute the other characteristics. As a sets included the U.S. Environmental Protection result, the other physical basin characteristics had to be Agency River File 3 (McKay and others, 1994), USGS computed and compiled. Because of the large number digital line graph contour lines (U.S. Geological of sites involved and the need for consistent, unbiased Survey, 1989), and the National Oceanic and methodology in making measurements and Atmospheric Administration shoreline data set computations, a geographic information system (GIS) (National Oceanic and Atmospheric Administration,
6 Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina 1999). Known drainage basin boundaries were overlain any given year. Recurrence interval is defined as the onto the DEM, and a combination of computer and number of years, on average, during which the visual interpolation techniques were used to define specified discharge is expected to be exceeded one time boundaries between the 366 gage sites and the known and is expressed as number of years. A discharge with drainage boundaries. a 10-year recurrence interval is one that, on average, Once the DEM was constructed and basin will be exceeded once every 10 years. Recurrence boundaries were delineated for all sites, a set of interval and exceedance probability are the computer algorithms was developed to automatically mathematical inverses of one another; thus, a discharge compute drainage area, L, CSLOPE, BSLOPE, and with an exceedance probability of 0.10 has a recurrence SHAPE. Although GIS-computed drainage area was interval of 1/0.10 or 10 years. Conversely, a discharge computed, the values used for DA were the drainage with a recurrence interval of 10 years has an areas compiled from site records that were hand- exceedance probability of one-tenth or 0.10. It is computed and checked when the sites were established. important to remember that recurrence intervals, The percent difference between GIS-computed regardless of length, always refer to the average drainage area and DA was automatically computed and number of occurrences over a long period of time; for used to verify the delineation of basin boundaries and example, a 10-year flood discharge is one that might the automated computations. Sites with greater than occur about 10 times in a 100-year period, rather than 10-percent difference between the computed drainage exactly once every 10 years. area and DA were flagged and re-examined. Errors in Flood-frequency estimates for gaged sites are boundary delineation were corrected by comparing computed by fitting the series of annual peak flows to USGS 7.5-minute topographic maps with the original some known statistical distribution. For the purposes of hand-delineated basin boundary and by using manual this study, estimates of flood-flow frequency are techniques to match the GIS basin boundary to the computed by fitting the logarithms (base 10) of the original. After adjusting basin boundaries, basin annual peak flows to a log-Pearson Type III characteristics were recomputed and rechecked until distribution, following the guidelines and using the satisfactory results were obtained. Several sites with computational methods described in Bulletin 17B of drainage areas less than about 1 square mile (mi2) did the Hydrology Subcommittee of the Interagency not meet the criteria of less that 10-percent difference Advisory Committee on Water Data (1982). The between computed drainage area and DA because the equation for fitting the log-Pearson Type III resolution of the GIS data and computational methods distribution to an observed series of annual peak flows were about one-tenth of a square mile. These sites were is as follows: examined manually to determine if the automated delineation of basin boundaries was consistent with the hand-drawn boundaries; if not, the boundaries were LogQt = XKS+ ,(1) adjusted accordingly and basin characteristics were recomputed. where Qt is the t-year recurrence interval discharge in cubic feet per second, ESTIMATION OF FLOOD MAGNITUDE X is the mean of the log-transformed annual peak AND FREQUENCY AT GAGED SITES flows, K is a factor dependent on recurrence interval Flood-frequency estimates for a given stream and the skew coefficient of the log- site are typically presented as a set of exceedance transformed annual peak flows, and probabilities or, alternatively, recurrence intervals S is the standard deviation of the log- along with the associated discharges. Exceedance transformed annual peak flows. probability is defined as the probability of exceeding a specified discharge in a 1-year period and is expressed Values for K for a wide range of recurrence intervals as decimal fractions less than 1.0 or as percentages less and skew coefficients are published in Appendix 3 of than 100. A discharge with an exceedance probability Bulletin 17B (Hydrology Subcommittee of the Inter- of 0.10 has a 10-percent chance of being exceeded in agency Advisory Committee on Water Data, 1982).
Estimation of Flood Magnitude and Frequency at Gaged Sites 7 Fitting the log-Pearson Type III distribution to records may not provide an accurate estimate of the the general case of a long, well-distributed series of population skew. This is problematic because the K- annual peak flows is fairly straightforward. Often, factor in equation 1 for a given recurrence interval is however, a series of peak flows may include low or high dependent only on skew coefficient; therefore, an outliers, which are extremely low or high peak flows inaccurate skew coefficient will result in a flood- that depart significantly from the trend in the data. The frequency estimate that is not representative of the true, gage record also may frequently include information or population, value. about maximum peak flows that occurred outside of the A more accurate estimate of skew coefficient at a period of regularly collected, or systematic, record. site can be obtained by using a weighted average of the Such peak flows, known as historic peaks, are often the sample skew coefficient estimate with a generalized, or maximum peak flows known to have occurred during regional, skew coefficient. A generalized skew an extended period of time, longer than the period of coefficient is obtained by combining skew estimates collected record. The interpretation of outliers and from nearby, similar sites. A nationwide generalized historic peak information in the fitting process can skew study was conducted for the study documented in greatly affect the final flood-frequency estimate. Bulletin 17B (Hydrology Subcommittee of the Bulletin 17B (Hydrology Subcommittee of the Interagency Advisory Committee on Water Data, Interagency Advisory Committee on Water Data, 1982) 1982). Skew coefficients for long-term gage sites from provides guidelines for detecting and interpreting these all over the Nation were computed and used to produce data points and provides computational methods for a map of isolines of generalized skew. Gunter and making appropriate corrections to the distribution to others (1987) used this nationwide generalized skew in account for their presence. In some cases, high or low their flood-frequency computations. In addition, the outliers are excluded from the record, so that the USGS in North Carolina has computed other number of systematic peaks may not be equal to the unpublished flood-frequency estimates by using the number of years in the period of record. nationwide generalized skew. Statistical measures, such as mean, standard During preliminary computations of flood- deviation, or skew coefficient, can be described in frequency estimates for inclusion in the regression terms of the sample, or computed, measure and the analyses, a number of inconsistencies were noted population, or true, measure. In terms of annual peak between the computed values of sample skew flows, the period of collected record can be thought of coefficients at long-term gaging sites in North Carolina as a sample, or small portion, of the entire record, or and the values obtained from the national generalized population. Statistical measures computed from the skew study. Inconsistencies at long-term sites are of sample record are estimates of what the measure would concern because if generalized skew coefficients for a be if the entire population were known and used to region are accurate estimates of the population skew, compute the given measure. The accuracy of these then the computed values of sample skew at long-term estimates depends on the nature of the specific measure sites should approach the generalized values. Instead, it and the given sample of the population. was noted that while sample skew coefficients at long- Skew coefficient measures the symmetry of the term North Carolina sites were somewhat consistent distribution of a set of peak flows about the median of among themselves, they did not agree with the the distribution. A peak-flow distribution with the mean generalized values obtained from the nationwide equal to the median is said to have zero skew. A generalized skew study. This anecdotal evidence, when positively skewed distribution has a mean that exceeds considered along with the age and lack of resolution of the median typically as a result of one or more the national study, was deemed sufficient cause to extremely high peak flows. A negatively skewed develop new generalized skew estimates for rural distribution has a mean that is less than the median gaging sites in North Carolina. typically because of one or more extremely low peak Bulletin 17B (Hydrology Subcommittee of the flows. Interagency Advisory Committee on Water Data, 1982) The computed skew coefficient for the peak-flow describes three methods for performing generalized record of a given station is very sensitive to extreme skew studies using skew coefficients computed from events; therefore, the sample skew coefficient for short long-term gaging stations—(1) plot computed skew
8 Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina coefficients on a map and construct skew isolines, skews from each site, and a weighted average for each (2) use regression techniques to develop a skew hydrologic area was computed. Inspection of the initial prediction equation that would relate station skew results revealed no significant difference between the coefficients to some set of basin characteristics, or weighted average skew estimates for the Blue Ridge- (3) use the arithmetic mean of computed skew Piedmont hydrologic area or the Coastal Plain coefficients from long-term sites in the area. For the hydrologic area. As a result, these two areas were purposes of this report, a modification of the second combined in the computation, and two weighted method initially was decided to be the most likely regional average skew values, along with the mean method to produce satisfactory results. However, rather square error associated with each estimate, were than using ordinary least-squares regression, a weighted determined for use as generalized skew values for sites least-squares regression technique was used to in North Carolina—one for sites in the Sand Hills determine the relation between the sample skew hydrologic area and the other for sites in the remainder coefficient and selected basin characteristics. Sample of the State (table 4). skew estimates were weighted according to their respective record length; sites with long records were assigned greater weight than those with short records. Table 4. Generalized skew coefficient and associated The use of this regression technique in this study made mean square error for rural North Carolina gaging sites it possible for data from all 347 sites with unregulated Generalized Mean flows to be used in developing the estimate. Hydrologic area skew square Multiple regression analysis, using ordinary coefficient error least-squares regression, was used to determine the best Blue Ridge-Piedmont 0.195 0.038 set of basin characteristics to use as explanatory, or and Coastal Plain independent, variables in the weighted least-squares predictive model. Initial analyses were somewhat Sand Hills 0.252 0.062 disappointing; no combination of basin characteristics accounted for a significant amount of the variance in computed skew. Lacking any significant statewide As described previously, a weighted skew relationship between sample skew and basin coefficient is used in order to improve the accuracy of characteristics, three location variables—BRP, CP, and SH, one for each of the three hydrologic areas, Blue the skew coefficient used to fit peak-flow records to a Ridge-Piedmont, Coastal Plain, and Sand Hills—were log-Pearson Type III distribution. The weighted skew added to the analysis. For a given site, the location coefficient for a given site is computed as the weighted variable representing the region of the site was set at 1, average of the generalized skew coefficient and the and the other two location variables were set at 0 site’s computed skew coefficient, with weights assigned (table 3). When these variables were added to the according to the mean square error of each component multiple regression analysis, results were only skew value. Flood-frequency estimates for all sites with marginally better. None of the exploratory multiple unregulated flow records were computed by using the regression models yielded significant relations between weighted skew method. Flood-frequency estimates for sample skew and the basin characteristics. sites with regulated flow record were computed by Given the lack of satisfactory results in this fitting the recorded regulated peak flows to the log- attempt to develop predictive equations relating skew to Pearson Type III distribution. Computed sample skew some set of basin characteristics, it was decided to apply coefficients for the regulated flow record were used a modified version of the third method in Bulletin 17B (Hydrology Subcommittee of the Interagency Advisory because regulated peak-flow records typically are not Committee on Water Data, 1982). However, instead of representative of regional or generalized conditions. using an arithmetic mean of computed skews from long- Although flood-frequency estimates for regulated sites term sites as an estimate of generalized skews, weighted are presented in this report, more detailed, site-specific regional average skews were used. Weights were analyses of flood frequency at many regulated sites are assigned, according to record length, to the computed available from the U.S. Army Corps of Engineers.
Estimation of Flood Magnitude and Frequency at Gaged Sites 9 ESTIMATION OF FLOOD MAGNITUDE models included drainage area and the 25-year climate AND FREQUENCY AT UNGAGED SITES factor for the Blue Ridge-Piedmont and Sand Hills hydrologic areas; while the best two-variable models Two regional analyses were used to develop for Coastal Plain sites consisted of drainage area and methods for estimating flood discharges for ungaged channel length. rural basins in North Carolina. The first analysis, a The validity of the regionalization scheme was traditional regional regression, required the use of examined by performing additional ordinary least- generalized least-squares regression to define a set of squares regression analyses by using the two-variable predictive equations that relate peak discharges for the models determined previously and comparing the 2-, 5-, 10-, 25-, 50-, 100-, 200-, and 500-year coefficients and intercepts for each region’s model to recurrence intervals to selected basin characteristics for those for the rest of the State. In each case, the unregulated rural basins in each of three hydrologic coefficients and intercepts for each region’s model areas of North Carolina (fig. 1). The second analysis, differed from those of the model using the remaining the region-of-influence method, required the sites in the State. Additionally, a further test was development of a computer application to derive, for conducted by introducing the location variable (table 3) any given ungaged rural site in the Blue Ridge- for each region into the regression model. Each of these Piedmont or Coastal Plain hydrologic areas, unique variables was set either at 1, if the site was in a predictive relations between the 2-, 5-, 10-, 25-, 50-, particular region, or 0, if not. A five-variable ordinary 100-, 200-, and 500-year recurrence interval discharges least-squares regression model, including all available and selected basin characteristics. Just as in the sites and using (1) drainage area, (2) climate factor, traditional regional regression, generalized least- (3) location variable, (4) the product of the location squares regression is used to develop these predictive variable with drainage area, and (5) the product of relations; however, in the region-of-influence analysis, the location variable with climate factor as explanatory regression techniques are applied to only a selected variables, was constructed for each recurrence interval subset of gaged sites, rather than the entire data base of discharge in each of the three hydrologic area. For gaged sites. a given region’s model, a significant coefficient for the location variable indicates a difference in the intercept between sites in that region and sites in the Regional Regression Analysis rest of the State; a significant coefficient for either of the terms that are products of a location variable and Ordinary least-squares regression with flood another variable indicates a difference in the discharge as the dependent variable was used in coefficients of the basin characteristic in that term exploratory analyses to determine the best regression between sites in that region and the rest of the State. In models for all combinations of the eight basin this particular test, a 95-percent confidence level was characteristics that were used as explanatory variables. defined as significant. All three regional models had An additional goal of the exploratory analysis was to significant coefficients for at least one of the location determine if the subdivision of the State into three variables or location variable product terms. Given the hydrologic areas is supported by current data. results of these regression tests, the regionalization Initially, the regionalization scheme used by scheme used by Gunter and others (1987) was Gunter and others (1987), which divided the State into accepted. the Blue Ridge-Piedmont, Coastal Plain, and Sand Ordinary least-squares regression is an Hills hydrologic areas, was assumed to still be valid. appropriate and efficient regression model for use Multiple regression analysis, using Mallow’s Cp when flow estimates that are used as response variables (Stedinger and Tasker, 1985), adjusted coefficient of are independent of each other (no correlation exists determination, and hydrologic judgment as criteria, between pairs of sites) and when the reliability and resulted in one-variable and two-variable models variability of flow estimates that are used as response relating flood discharge to basin characteristics for variables are approximately equal. The flow estimates each of the three hydrologic areas. The most significant that were used in this regression were generated from one-variable models for all three regions included peak-flow records at gaging stations in all parts of drainage area only. The most significant two-variable North Carolina with periods of record ranging from
10 Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina 10 to 101 years. Records from gaging stations on the 30 years) of concurrent record. A graphical ‘best-fit’ same stream within the same basin or even in adjacent line to these points was used to define the relation basins may be highly correlated because the peak flows between cross-correlation coefficient and distance resulted from the same rainfall events, similar between sites. This relation was then used to populate antecedent conditions, and similar basin characteristics. a cross-correlation matrix for the sites contained in However, records from other sites, in basins remote each area. Variability of each peak-flow estimate is from each other, have varying degrees of correlation. In measured by the standard deviation of the peak-flow general, correlation between pairs of sites can be record used to compute that estimate. For each described as a function of distance between sites. hydrologic area, a generalized least-squares regression Additionally, the reliability of flow estimates that were of the sample standard deviations against drainage area used as response variables in this regression is, in was used to obtain estimates of the standard deviations general, a function of record length and, as such, cannot of the peak-flow records at each site. These regression be considered equal for all sites in the regression. estimates of the standard deviations were used to assign Variability of the flow estimates, characterized by the weights to flow estimates because they are independent standard deviation of the peak-flow record that was used of the sample standard deviation estimates used to to compute the flow estimate, depends in large part on characteristics of the basin and also cannot be compute the flow estimate. Finally, length of record at considered equal for all sites used in the regression. For each peak-flow site was used as a direct measure of the these reasons, ordinary least-squares regression was relative reliability of the flow estimates computed from used only as an exploratory technique in this analysis to those records. identify the best potential regression models and to Generalized least-squares regression was used to evaluate the proposed regionalization scheme. The final evaluate the 1- and 2-variable models suggested by regression equations were developed by using preliminary ordinary least-squares regression for each generalized least-squares regression techniques. of the three hydrologic areas in North Carolina. The Generalized least-squares regression, as final regression models in all of the regions relate peak described by Stedinger and Tasker (1985), is a discharge to drainage area for each recurrence interval regression technique that takes into account the (table 5). The 2-variable model for each region was correlation between, as well as differences in the tested by using generalized least-squares regression, variability and reliability of, the flow estimates used as and in each case, the addition of a second variable did dependent, or response, variables. These factors are not substantially improve the predictive ability of the accounted for in generalized least-squares regression by model. assigning different weights to each observation of the response variable used in the regression, based on its contribution to the total variance of the sample-flow Table 5. North Carolina rural flood-frequency equations statistic used as the response variable. In contrast, [DA, drainage area, in square miles. Result will be in cubic feet per second] ordinary least-squares regression assumes equal reliability and variability in flow estimates at all sites Rural Hydrologic area flood and no cross-correlation between flow records at all recur- Blue Ridge- sites, so that each flow estimate has equal variance and rence Coastal Plain Sand Hills Piedmont is assigned equal weight in the regression. interval (years) The use of generalized least-squares regression techniques to model the relations between peak 2 139 DA 0.698 61.9 DA 0.677 33.7 DA 0.711 discharges and basin characteristics of North Carolina 5 248 DA 0.672 121 DA 0.642 56.1 DA 0.700 rural basins requires estimates of the cross-correlation 10 342 DA 0.657 174 DA 0.623 73.9 DA 0.696 coefficients and standard deviation of the peak-flow 25 490 DA 0.640 261 DA 0.601 100 DA 0.692 records that were used to compute peak discharges for 50 622 DA 0.629 340 DA 0.586 122 DA 0.690 the selected recurrence intervals. For each of the three 100 774 DA 0.618 435 DA 0.573 147 DA 0.688 hydrologic areas, a scatter-plot of sample correlation 200 949 DA 0.608 548 DA 0.560 175 DA 0.686 coefficients versus distance between sites was 0.596 0.544 0.683 constructed for site pairs with long periods (at least 500 1,220 DA 727 DA 216 DA
Estimation of Flood Magnitude and Frequency at Ungaged Sites 11 Uncertainty in a flow estimate that was predicted The standard error of the model (SE(model)) can for an ungaged site by using the regression equations be converted from log (base 10) units to percent error can be measured by the standard error of prediction, Sp, by using the transformation formula, which is computed as the square root of the mean
square error of prediction, MSEp. The MSEp is the 1 --- sum of two components—the mean square error 2 2 ()2.3026()γ resulting from the model, γ2, and the sampling mean %SE()model = 100 10 – 1 .(4) square error, MSEs,i, which results from estimating model parameters from samples of the population. The Similarly, the average standard error of prediction can mean square model error, γ2, is a characteristic of the be transformed from log (base 10) units to percent error 2 γ2 model and is a constant for all sites. The mean square by substituting Sp for in equation 4. Computation of sample error, MSEs,i, for a given site, however, depends Sp,i for a given ungaged site, i, involves fairly complex on the values of the explanatory variables (DA) used to matrix algebra. Computational procedures and the develop the flow estimate at that site. The standard required matrices are provided in the Appendix. error of prediction for a site, i, is computed as: The standard errors of the model, which measure how well the regression model fits the data used to construct it, ranged from about 34 percent to just over 1 --- 50 percent. This error term is comparable to errors ()γ2 2 Spi, = + MSEsi, ,(2)often cited and referred to as ‘model error’ or ‘standard error of estimate’ in earlier studies in which ordinary and, therefore, varies from site to site. If the values of least-squares regression was used to develop predictive the explanatory variables for the gage sites used in the equations. The average standard errors of prediction, regression are assumed to be a representative sample of which provide a better overall measure of a model’s all sites in the region, then the average accuracy of pre- predictive ability, ranged from about 38 percent to diction for the regression model can be determined by about 56 percent (table 6). Another measure of computing the average standard error of prediction: predictive ability is equivalent years of record (Hardison, 1971). Equivalent years of record are the 1 number of years of peak-flow record needed to provide n --- 2 an estimate by using log-Pearson Type III techniques γ2 1 S = + --- ∑ MSE , .(3)that would be equal in accuracy to an estimate made by p n si i = 1 using regional methods (table 6).
Table 6. Average predictive errors, in percent, and equivalent years of record associated with North Carolina rural flood-frequency equations
Hydrologic area Rural flood Blue Ridge-Piedmont Coastal Plain Sand Hills recurrence Equivalent Equivalent Equivalent interval Average error Average error Average error years years years (years) of prediction of prediction of prediction of record of record of record 2 43.9 1.8 38.6 2.8 38.2 2.1 5 43.9 2.7 38.0 4.3 41.9 2.8 10 44.5 3.7 38.8 5.8 44.1 3.6 25 45.8 5.0 40.7 7.7 47.0 4.7 50 47.2 5.9 42.3 8.9 49.1 5.4 100 48.7 6.8 44.3 9.9 51.1 6.1 200 50.4 7.6 46.3 11 53.3 6.8 500 52.6 8.4 49.1 12 56.1 7.5
12 Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina Region-of-Influence Analysis characteristics and estimates of the selected recurrence interval discharge at the ungaged site computed. The region-of-influence method (Tasker and The number, p, and identity of the basin Slade, 1994) estimates flood discharges at ungaged characteristics that are used to compute dij and the basins by deriving, for a given ungaged rural site, number of gaged sites, N, that compose the region of regression relations between the flood discharges and influence are specific to a given set of flood-discharge basin characteristics of a unique subset of gaged sites. estimates and basin characteristics. In order to adapt This unique subset of gaged sites for a given ungaged the region-of-influence method to that data set, these site, first suggested by Acreman and Wiltshire (1987), parameters must be determined. In addition to these was described by Burn (1990a, b) as the region of parameters, the set of basin characteristics also must be influence for an ungaged site, hence the name of the chosen for use as explanatory variables in the method. The unique subset of gaged sites is defined as generalized least-squares regression models developed the N ‘nearest’ gages to the ungaged site, where for each region. There is a subtle but important distance between sites i and j is defined by the distinction between the two sets of basin Euclidean distance metric: characteristics—the first is used to define a region of influence; the second serves as variables in the unique 1 p --- predictive equations that are developed for that region 2 x – x 2 of influence. These two sets of characteristics need not d = ------ik jk ,(5) ij ∑ () be identical but are in some cases. In other cases, such sd Xk k = 1 as in North Carolina, the set of characteristics used as variables is a subset of the set of characteristics used to where define the region of influence. d is the distance between sites i and j in terms of ij Selection of the number of gaged sites, N, and basin characteristics, the number and identity of the basin characteristics that p is the number of basin characteristics used to will define the region of influence for North Carolina calculate d , ij was done by trial and error, using a computed root X is the kth basin characteristic, k mean square error (RMSE) as the criterion. RMSE was sd(X ) is the sample standard deviation for X , and k k computed by removing one site at a time from the data x is the value of X at the ith site. ik k base and using the remaining sites to compute an estimate of the flow characteristic. Once completed for This distance metric is directly analogous to the more familiar equation for distance, D, between two points, every site, the RMSE was computed as the square root of the arithmetic mean of the differences between the (x1, y1) and (x2, y2) in a 2-dimensional rectangular coordinate system: estimated and computed values at each site. The results of the exploratory multiple regression analyses performed as part of the traditional regional regression 1 --- analysis were used to provide some insight in selecting []()2 ()2 2 Dx= 2 – x1 + y2 – y1 ,(6)initial sets of basin characteristics. The strong evidence for using separate hydrologic areas in the traditional where the only difference is the use of sample standard regression analysis led to the decision to restrict a site’s deviation to standardize the different basin characteris- region of influence to its hydrologic area. As a result, tics and the slight notational difference of using an 15 sites in the Sand Hills region (fig. 1) were not additional subscript k rather than changing variable enough to support a valid region-of-influence analysis. symbols (x, y). For any ungaged site identified as a Sand Hills site, the same set of 15 sites would compose the region of The distances, dij’s, between a given ungaged site and all the gaged sites are computed and ranked; influence, and the unique predictive equation developed would be the same equation developed by the N gaging stations with the smallest dij compose the region of influence for that gaging station. Once using traditional regional regression techniques, as determined, generalized least-squares regression described in previous sections of this report. techniques are used to develop the unique predictive Combinations of defining variables that were relations between flood discharge and basin tested include DA and CF25; DA and REG; DA, CF25,
Region-of-Influence Analysis 13 and REG; and DA, CF25, L, and REG. Each set of region-of-influence method. However, the additional defining variables was tested by using values of 25, 30, variable, latitude and longitude of the ungaged site, is and 35 for N. For all variable combinations, N = 30 simple to determine, so that the variable requirements provided the best results; and the combination of of the methods are nearly equal. The regional variables that minimized RMSE for all recurrence regression equations are easily evaluated manually, the intervals was DA, CF25, and REG. For these initial region-of-influence method, however, is tests, DA and CF25 were used as explanatory variables computationally intensive but is made simpler by the in the unique regression relations. Subsequent testing, use of a computer application that performs the after the defining variables and N were determined, complex computations. indicated that CF25 was not significant as an The average RMSE was computed for each area explanatory variable. As a result, only DA is used as an and recurrence interval (table 7), providing a measure explanatory variable in the final version of the region- of the predictive ability of the model or method. of-influence method. Average RMSE was computed as the square root of the After determining the best combination of arithmetic mean of the differences between the flood- variables to define the region of influence and the frequency estimate determined using the log-Pearson optimal value for N, the computer application for the Type III and the flood-frequency estimate computed region of influence was completed. Equation 5 is used using either the regression equations or the region-of- to determine the region of influence for an ungaged influence method. RMSE for the region-of-influence site, given the required input variables. Unique method is slightly less than for the traditional predictive equations for the ungaged site are then regression equations in all cases except for the 200- and developed, using a generalized least-squares regression 500-year discharges in the Coastal Plain hydrologic of the sites within the region of influence, and the area. A site-specific comparison of predictive error also predicted flood-discharge estimates are computed. In is possible by using S . As discussed previously, the addition, because generalized least-squares regression p,i region-of-influence method reports the site-specific was used to develop the predictive equations, Sp,i, the site-specific standard error of prediction is computed standard error of prediction, Sp,i. The Sp,i is not for each estimated recurrence interval discharge. typically computed when evaluating the traditional regression equations manually because of the complexity of the computations involved. Automation Comparison of Results of the equations eliminates this concern, and the Sp,i is reported along with the flood-discharge estimate for Application of the regional regression equations any given site, allowing for comparison of predictive requires one less variable than application of the results on a site-by-site basis.
Table 7. Root mean square error, in percent, for the regional regression and region-of-influence methods, presented by hydrologic area and recurrence interval [n.a., not applicable]
Hydrologic area Blue Ridge-Piedmont Coastal Plain Sand Hills Recurrence interval Region Region Region Regional Regional Regional of of of regression regression regression influence influence influence 2 46.5 45.9 39.6 36.4 36.5 n.a. 5 48.1 46.5 40.8 38.3 40.8 n.a. 10 50.2 48.0 43.4 41.5 44.1 n.a. 25 53.4 50.6 47.9 46.6 48.3 n.a. 50 56.1 52.7 51.5 50.8 51.4 n.a. 100 58.8 55.0 55.3 55.0 54.4 n.a. 200 61.7 57.4 59.2 59.4 57.5 n.a. 500 65.5 60.7 64.4 65.1 61.5 n.a.
14 Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina In general, little difference was found in the ease boundaries of the ungaged site and measure the of application or in average predictive abilities between drainage area contained within those boundaries. The the regional regression equations and the region-of- corresponding regression equations (table 5) can then influence method. The region-of-influence method is a be applied to determine an estimate of the flood new technique and is still being improved. As a result, discharges for the recurrence interval of interest. the region-of-influence method is considered a Alternatively, the region-of-influence computer secondary or alternative method of determining flood- application can be initiated; it will query the user for an frequency estimates for ungaged rural sites in North output file name, an identifier for the site of interest, the Carolina. hydrologic area for the site, the drainage area of the site, and the latitude and longitude of the site. With this Use of Computer Software information, the computer application computes the climate factor, defines a region of influence, and As part of the study described by this report, a produces the desired flood-discharge estimates, along computer software package was developed that with the standard error of prediction, Sp,i, specific to the computes (1) estimates of flood-frequency discharges ungaged site. using the region-of-influence method at ungaged rural The computer application contains the sites in the Blue Ridge-Piedmont or Coastal Plain regression equations and can be used to apply either hydrologic areas of North Carolina, (2) estimates of method. Use of the computer application to evaluate flood-frequency discharges using the regional the regression equation provides an automated regression equations for ungaged rural sites in each of computation of Sp,i for the regression equations as well the three hydrologic areas of North Carolina, and as for the region-of-influence method. If evaluated (3) the associated site-specific errors of prediction, S , p,i manually, Sp,i can be computed only by using the rather for each method. The complexity of the computations complex computational procedures described required for the region-of-influence method requires previously and outlined in detail in the Appendix. the use of the software for practical application of the Although average standard errors of prediction method. The regional regression equations can be (table 6) give an idea of the relative accuracy of the evaluated manually, but the software allows for easy methods; Sp,i is the more precise measure of the evaluation of the complex computation of the Sp,i for accuracy of a specific prediction. the regional regression method. Flood-frequency estimates at gaged sites and The computer software package includes an ungaged sites on the same stream as a gaged site can be executable program file and four supporting data files. improved by combining the estimate determined by All five files are required for execution of the computer regional methods with the estimate determined by software. The software package and instructions for fitting the log-Pearson Type III distribution to the peak- down loading, installation, and execution of the flow record at the gaged site. At a gaged site, the best program currently are available at the North Carolina estimate of flood frequency can be determined by District home page on the World Wide Web at URL
Application of Methods 15 Flood estimates at an ungaged site that is on the where Qt(adjusted) is the adjusted discharge for the t- same stream as a gaged site can be determined by using year recurrence interval; Qt(HA1) and Qt(HA2) are the a combination of the regional estimate and the log- discharges computed as if the entire drainage area were Pearson Type III estimate from the nearby gaged site. within the hydrologic areas, HA1 and HA2; DA1 and In order to make the appropriate adjustment, first DA2 are portions of the total drainage area found in the compute the ratio, respective hydrologic drainage areas; and DAtotal is the total drainage area. () Qt w R = ------(),(8) Qt r SUMMARY
Accurate and reliable estimates of the magnitude for the gaged site by using Qt(w) and Qt(r) as defined in the preceding paragraph. Next, a correction factor, and frequency of floods are critical for such activities as R', is computed as follows: bridge design, flood-plain delineation and manage- ment, water-supply management, and management of water-control structures, among others. Recognizing ∆DA() R – 1 the need for accurate estimates of flood frequency at R' = R – ------,(9) 0.5DAg ungaged rural basins, the U.S. Geological Survey, in cooperation with the North Carolina Department of where ∆DA is the difference between the drainage Transportation, conducted a study to further define the relation between flood discharges of selected areas of the gaged and ungaged sites, and DAg is the ∆ recurrence intervals and selected physical and climatic drainage area of the ungaged site. If DA/DAg is less than 0.5, then the corrected discharge for the ungaged characteristics of rural North Carolina basins. This site, Q (corr), can be computed by multiplying the cor- study includes the development of two methods for t regionalizing, or extending in space, flood-frequency rection factor, R', by the regional estimate for the ∆ estimates at gaged sites. In the first method, traditional ungaged site, Qt(r). If DA/DAg is greater than 0.5, use the results of the regional methods without correction. regional regression analysis, a generalized least- squares regression analysis is used to develop a set of At times, flood-frequency estimates may be predictive equations for each of three hydrologic areas desired for an ungaged site that is between two gaged in North Carolina—the Blue Ridge-Piedmont, the sites on the same stream. In this case, select the gaged Coastal Plain, and the Sand Hills. In the second site for which ∆DA/DA is less than 0.5, compute R', g method, the region-of-influence method, flood- and apply as described above. If ∆DA/DA is less than g frequency estimates for ungaged sites are predicted 0.5 for both gaged sites, compute R' for each. If both interactively, based on data from a subset of gaged sites correction factors are greater than 1.0, use the larger R'; with basin characteristics similar to those of the if both correction factors are less than 1.0, use the ungaged site. This report documents the development smaller R'. If one correction factor is greater than 1.0 of both methods, using a data base of flood-discharge and the other smaller than 1.0, an average of the two estimates and basin characteristics for 317 rural North correction factors should be used. Carolina gaged sites. If the drainage basin for an ungaged site lies An initial set of 366 gaged sites was determined within more than one hydrologic area, the computed to have some annual peak-flow record; basin discharge should be adjusted according to the characteristics data were computed and compiled for proportion of the total drainage area that lies within all of these sites by using a GIS. While the development each hydrologic area. The adjusted discharge can be of the basin characteristics was ongoing, flow records determined by the equation: were examined to determine which sites had flows that were affected by regulation or channelization. Of the DA 366 original sites, 19 sites had only regulated record Q (adjusted) = Q ()HA1 x------1 - and 27 sites had periods of unregulated flow record t t DA total prior to regulation. After basin characteristics were DA () 2 developed and flow records were examined, + Qt HA2 x------, (10) DAtotal preliminary computations of flood-frequency estimates
16 Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina were begun. Results of these preliminary computations method being clearly superior. Both require hydrologic indicated the need for a generalized skew study for area and drainage area as input variables; the region-of- North Carolina basins to replace outdated generalized influence method additionally requires latitude and skews that were based on a nationwide study. After the longitude, but these coordinates are fairly simple to generalized skew study, flood-frequency estimates for determine. The RMSE were, in general, lower for the all sites with 10 or more years of record were region-of-influence method, but only slightly. The computed. Flood-frequency estimates were computed region-of-influence method is newly developed and for 317 rural, unregulated sites and for 42 rural, still being refined. As a result, the regional regression regulated sites. The sites with regulated record were equations are considered to be the primary method of excluded from further analysis. estimating magnitude and frequency of floods for rural Basin characteristics data and flood-frequency ungaged sites in North Carolina. The region-of- estimates for the 317 rural, unregulated sites were influence method can be considered an alternative merged to form the data base that was used to develop method. the regional regression equations and the region-of- A computer application is available that influence method. Of the 317 total sites, 222 were automates the complex computations required by the located in the Blue Ridge-Piedmont hydrologic area, region-of-influence method. This computer application 80 were located in the Coastal Plain hydrologic area, includes the option to compute flood-frequency and 15 were located in the Sand Hills hydrologic area. estimates using the predictive equations developed by Preliminary multiple regression analyses, using the traditional regional regression analysis. The ordinary least-squares regression, were conducted to computer application also computes site-specific error confirm the validity of the regionalization scheme and of prediction for each method. to identify the best combination of explanatory variables for inclusion in the generalized least-squares analysis. REFERENCES Generalized least-squares analysis was used to develop a set of equations for each region that relates Acreman, M.C., and Wiltshire, S.E., 1987, Identification of the 2-, 5-, 10-, 25-, 50-, 100-, 200-, and 500-year regions for regional flood frequency analysis [abs.], recurrence interval flood discharges to drainage area. EOS, v. 68, no. 44. Model error and error of prediction for the equations Burn, D.H., 1990a, An appraisal of the “region of influence” ranged from about 40 percent for the lower recurrence approach to flood frequency analysis: Hydrological interval equations to more than 50 percent for the 500- Sciences Journal, v. 35, no. 24, p. 149–165. year equations. ———1990b, Evaluation of regional flood frequency analy- sis with a region of influence approach: Water The region-of-influence method was adapted to Resources Research, v. 26, no. 10, p. 2257–2265. the available flood-frequency and basin characteristics Gunter, H.C., Mason, R.R., and Stamey, T.C., 1987, data for North Carolina. The drainage area, hydrologic Magnitude and frequency of floods in rural and urban area, and latitude and longitude of an ungaged site in basins of North Carolina: U.S. Geological Survey either the Blue Ridge-Piedmont or Coastal Plain Water-Resources Investigations Report 87–4096, 52 p. hydrologic areas of North Carolina are required to Hardison, C.H., 1971, Prediction error of regression predict the 2-, 5-, 10-, 25-, 50-, 100-, 200-, and 500- estimates of streamflow characteristics at ungaged sites: year recurrence interval flood discharges for a specified U.S. Geological Survey Professional Paper 750–C, p. ungaged site. The Sand Hills hydrologic area did not C228–C236. have a sufficient number of sites to apply the region-of- Hodge, S.A., and Tasker, G.D., 1995, Magnitude and influence method. Because of the complexity of the frequency of floods in Arkansas: U.S. Geological computations involved in the region-of-influence Survey Water-Resources Investigations Report 95–4224, 52 p. method, a computer application is required for the Hydrology Subcommittee of the Interagency Advisory practical use of the method. Committee on Water Data, 1982, Guidelines for A brief comparison of the regional regression determining flood frequency: U.S. Geological Survey and region-of-influence methods, based on ease of Bulletin 17B, Office of Water Data Collection, Reston, application and RMSE of prediction, resulted in neither Va., 183 p.
References 17 Lichty, R.W., and Karlinger, M.R., 1990, Climate factor for Stedinger, J.R., and Tasker, G.D., 1985, Regional hydrologic small-basin flood frequency: American Water analysis 1—Ordinary, weighted, and generalized Resources Association, Water Resources Bulletin, v. 26, least square compared: American Geophysical no. 4, p. 577–586. Union, Water-Resources Research, v. 21, no. 9, Lichty, R.W., and Liscum, F., 1978, A rainfall-runoff p. 1421–1432. modeling procedure for improving estimates of T-year Tasker, G.D., and Slade, R.M., 1994, An interactive regional annual floods for small drainage basins: U.S. regression approach to estimating flood quantiles, in Geological Survey Water-Resources Investigations Fontane, D.G., and Tuvel, H.N., eds., Proceedings of 78–7, 44 p. the Twenty-First Annual Conference, Water policy and McKay, L., Hanson, S., Horn, R., Dulaney, R., Cahoon, A., management, solving the problems, May 23–26, 1994: Olsen, M., and Dewald, T., 1994, The U.S. EPA Reach Denver, Colo., Denver Society of American Society of File Version 3.0 Alpha Release (RF3-Alpha) Technical Civil Engineers, p. 782–785. Reference: U.S. Environmental Protection Agency, Tasker, G.D., and Stedinger, J.R., 1989, An operational GLS Washington, D.C. model for hydrologic regression: Journal of Hydrology, National Oceanic and Atmospheric Administration, 1999, v. 111, p. 361–375. NOAA’s medium resolution digital vector shoreline, U.S. Geological Survey, 1989, Digital line graphs from accessed March 17, 1999, at URL http:// 1:100,000-scale maps, National Mapping Program seaserver.nos.noaa.gov/projects/shoreline.html. Technical Instructions Data Users Guide 2, 88 p.
18 Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina 9 9 9 9 19 19 39 13 33 10 19 13 54 15 18 17 45 18 57 15 10 47 21 23 30 13 16 14 23 18 19 38 41 10 19 peaks Number of systematic of of Period Period record 1953–71 1953–71 1929–96 1965–73 1965–73 1940–63 1964–96 1964–73 1953–71 1916–38 1939–96 1955–71 1954–71 1954–71 1930–96 1954–73 1908–96 1958–73 1930–49 1940–49 1950–96 1954–89 1954–76 1965–96 1954–71 1965–82 1967–80 1965–73 1974–96 1954–71 1978–96 1878–49 1956–96 1940–49 1953–71 77°11'58" 80°18'11" 80°18'11" 79°11'50" 77°22'24" 77°01'36" 77°14'12" 77°09'28" 77°00'00" 77°00'00" 77°00'38" 76°47'14" 80°19'46" 80°10'28" 80°04'25" 79°59'30" 79°53'14" 79°49'35" 79°50'08" 79°46'00" 79°45'57" 79°45'57" 79°23'00" 79°18'20" 79°09'57" 79°05'48" 79°06'26" 79°01'42" 78°59'48" 79°01'14" 78°52'21" 77°38'04" 77°38'04" 77°23'03" 77°04'42" Longitude Latitude 36°11'52" 36°25'48" 36°17'54" 36°22'14" 36°14'52" 36°16'46" 36°16'48" 36°16'48" 36°16'29" 36°30'53" 36°30'53" 36°23'53" 36°27'54" 36°12'20" 36°32'05" 36°20'54" 36°24'45" 36°31'39" 36°29'00" 36°31'31" 36°31'31" 36°28'13" 36°19'29" 36°23'57" 36°22'38" 36°21'44" 36°23'09" 36°31'02" 36°31'24" 36°23'48" 36°32'26" 36°27'37" 36°27'37" 36°12'34" 35°43'51" tification and station numberfor sites having separate period of regulated n Station name Wildcat Swampnear Jackson Wildcat Cutawhiskie Creek near Woodland PotecasiCreek near Union Square Creek near Rich Ahoskie Store Mintons at Creek Ahoskie AhoskieCreek at Ahoskie AhoskieCreek at Ahoskie (channelized period) AhoskieCreek tributaryPoortown at Chinkapinnear Creek Colerain Francisco near River Dan period) (regulated Francisco near River Dan Hog Rock Creek near MooresSprings Lawsonville near Creek Snow Little Belews Creek near Kernersville Mayo River Price near CreekJacobs near Wentworth Wentworth near River Dan Leaksville near Creek Matrimony Leaksville at River Dan at Eden River Smith (regulated period) at Eden River Smith Moonnear Creek Yanceyville Hightowers near Creek Line Country South Hyco Creek near Leasburg Leasburg near tributary Creek Kilgore Double Creek near Roseville Roseville near Creek Hyco South Hyco River at McGehees Mill Hyco River below Afterbay Dam near McGehees Mill Storys Creek nearRoxboro MayoCreek near Bethel Hill at Roanoke Rapids Roanoke River at Roanoke Rapids period) (regulated Roanoke River Roanoke River near Scotland Neck Williamston near Creek tributary Smithwick Station number 02053110 02053170 02053200 02053400 02053450 02053500 02053500* 02053510 02053550 02068500 02068500* 02068610 02068660 02069030 02070500 02070810 02071000 02071410 02071500 02074000 02074000* 02075160 02075230 02077200 02077210 02077240 02077250 02077300 02077303 02077310 02077670 02080500 02080500* 02081000 02081060 , site excluded from regional analysis because flows were affected by regulation or channelization] r r r r r nc nc nc r, nc r r Map identification numbers and descriptions of gaged rural sites in North Carolina with annual peak-flow record peak-flow annual with Carolina North in sites rural gaged of descriptions and numbers identification Map 6* 9* 1 2 3 4 5 6 7 8 9 11 18* 29* 10 12 13 14 15 16 18 19 20 21 22 23 24 27 29 30 31 17 25 26 28 Map Map (fig. 1) number , flood-frequency, estimates were not computed because the site hasthan less 10 yearsofpeak-flow record; duplicate *, map ide identification identification nc Table 1. 1. Table [ or channelized flows;
Tables 19 peaks Number of of Number systematic of Period Period record 36°04'46" 76°58'36" 1953–71 14 identification and stationnumber for sites havingseparate period ofregulated Station nameStation Latitude Longitude Station Station number 0208111310 Cashie River at Secondary Road 1257 near Windsor02081935 River at Spring Tar Hope 02082506 River RiverReservoir belownear Tar Tar Rocky Mount02082585 36°02'51" River at NC97at Tar Rocky Mount02082731 76°59'07" Devils Cradle Creek nearAlert at Secondary Road1412 1988–9602082955 35°53'58" Fishing Creeknear Glenview 9 77°51'57" 1973–9602083833 36°12'03"02084160 Conetoe Creek(tributary 3) near Penny Hill02084160* 35°55'42" 78°14'19" 24 02084164 Chicod Creek at Secondary Road 1760 near Simpson Chicod Creek at Secondary Road 1760 near Simpson (channelized02084164* period) 35°57'15" 1993–96 78°08'53" Juniper BranchSecondary at Road period) 1766 nearSimpson (channelized Simpson near 1766 Road Secondary at Branch Juniper 35°33'47" 77°47'15" 1967–71 35°33'55" 4 1977–96 77°13'43" 77°14'43"02084909 5 1982–96 1979–86 20 SevenmileCreek near Efland 35°33'47" 36°08'44" 11 35°33'55" 35°46'00" 8 77°13'43" 77°50'31" 77°14'43" 77°29'26" 1976–81 1967–71 1976–78 1993–96 6 5 3 4 36°03'56" 79°08'39" 1988–96 9 r r,nc r r nc nc nc nc nc nc nc nc Map identification numbers and descriptions of gaged rural sites in North Carolina with annual peak-flow record—Continued peak-flow annual with Carolina North in sites rural gaged of descriptions and numbers identification Map 41 43 57* 3233 02081110 Windsor near Swamp Oak White 343536 0208150037 0208171038 02081747 River River near Tar Tar 39 LongCreek at40 Kittrell 02081800 River at U.S. 401 Tar at Louisburg 02082000 Cedar Creek near Louisburg42 02082500 River near Nashville Tar 44 Nashvillenear Creek Sapony 45 0208254046 02082610 Wildcat Branch near Mapleville47 0208263048 River near Rocky Tar Mount49 HartsMill Run nearTarboro 50 0208277051 02082835 02082950 SwiftCreek at Hilliardston52 Fishing Creeknear Warrenton53 02083000 Oak White near Creek Fishing Little 5455 02083090 Fishing Creeknear Enfield56 36°05'34" 02083410 02083500 Beaverdam Swamp 57 near Heathsville 36°11'41" 02083800 Deep Creek nearScotland Neck 78°17'48" River at Tarboro Tar 58 36°13'30" 36°03'14" 78°35'00" Conetoe Creeknear Bethel 1964–9659 78°27'15" 35°53'10" 78°20'24" 1940–9660 35°50'57"61 36°03'29" 1954–76 33 02084240 1935–75 77°54'40"62 77°55'51"63 57 02084500 78°08'39" Collie 1951–70 Swamp64 near Everetts 02084520 35°58'38" 1919–70 20 02084540 22 35°55'40" Herring Run nearWashington 1953–76 02084570 Creek Yeatsvillenear Goose Upper 77°45'35" 36°11'08" Durham Creek 20 at Edward 77°37'10" 36°23'00" Acre Swamp near Pinetown 36°06'42" 42 1964–73 77°52'34" 11 1953–71 36°16'49" 78°10'54" 77°55'16" 1960–96 36°09'03" 10 1954–76 77°41'48" 36°09'26" 1924–96 18 77°41'35" 37 1953–71 77°28'24" 22 33 35°46'33" 1915–96 1953–73 35°53'38" 19 77°27'45" 77°32'00" 82 21 1957–96 35°31'25" 1897–96 35°49'34" 35°34'03" 76°53'23" 40 77°12'03" 77°01'09" 95 1953–73 35°19'25" 35°35'02" 1953–76 1946–80 76°52'26" 76°50'23" 21 24 1966–92 30 1953–69 27 17 58* Map (fig. 1) number identification identification Table 1. 1. Table [nc,flood-frequency estimates werenot computed because thesitehas less than 10years of peak-flow record; *, duplicate map orchannelized siteexcluded flows;r, from regional analysisbecause flowswere affected by regulation or channelization]
20 Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina peaks Number of systematic of of Period Period record 36°04'18" 79°05'49" 1928–96 54 identification and station numberfor sites having separate period of regulated Station nameStation Latitude Longitude Station number 0208650002086849 Flat River at Dam near Bahama Ellerbe Creek nearGorman 02087183* Neuse River near Falls (regulatedperiod) 02087500* period) (regulated Clayton near River Neuse 02087570* Neuse River at Smithfield (regulatedperiod) 0208758850 Swift Creek nearMcCullars Crossroads 36°08'55" 78°49'43" 36°03'33" 1928–93 35°56'25" 78°49'58"02089000* 35°38'50"0208925200 78°34'56" 1983–94 Neuse River near Goldsboro (regulated period) 48 Store Mays at Creek Bear 78°24'22"02089500* 35°30'46" 1981–96 1981–96 Neuse River at Kinston (regulated period) 78°21'00" 8 35°41'33" 16 1981–90 16 78°41'34" 1992–96 10 35°20'14" 5 77°59'51" 1984–96 35°15'29" 77°35'09" 35°16'28" 13 1981–96 77°47'40" 1988–96 16 9 r r r r r r nc nc nc Map identification numbers and descriptions of gaged rural sites in North Carolina with annual peak-flow record—Continued peak-flow annual with Carolina North in sites rural gaged of descriptions and numbers identification Map 72 79* 81* 82* 92* 94* 66676869 02085020 0208507070 02085190 Stony Creek tributary71 nearHillsboro 0208521324 EnoRiver near Durham Rougemont near tributary River Little Fork North Little River at Secondary Road 1461near Orange Factory 02085500 02086000 Flat River at Bahama Dial Creeknear Bahama 36°08'30" 36°11'41" 78°55'10" 79°00'52" 1962–96 36°03'01" 1954–76 79°02'14" 35 1953–71 23 36°04'20" 78°54'30" 19 36°10'36" 36°10'57" 1964–96 78°51'24" 78°52'44" 33 1926–91 1926–96 47 71 65 02085000 Hillsborough River at Eno 7375 020866247677 Knap Of ReedsCreek near Butner78 0208700079 0208700780 02087030 Neuse River near Northside Little Lick Creek aboveSecondary Road 1814near Oak Grove 0208714080 02087183 Lick Creek nearDurham 81 LowerBarton Creektributary near Raleigh Neuse River near Falls82 02087240 02087500 Stirrup IronCreek 83 tributary near Nelson 35°59'11" 02087570 Neuse River near Clayton85 Neuse 78°47'58" River at Smithfield86 0208758087 1983–95 02087910 Swift Creek nearApex88 0208800089 Middle Creek nearHolly 90 Springs 02088140 36°07'40" Middle Creek nearClayton91 02088210 13 02088420 StoneCreek near NewtonGrove 92 78°48'55" 35°54'44" 02088470 Hannah Creeknear Benson 02088500 Long Branch near Selma 1983–95 36°02'54" 78°40'55" Little River near Kenly94 02089000 Little River near Princeton 35°53'06" 1954–71 78°44'59" Neuse River near Goldsboro 35°58'50" 13 02089500 78°49'37" 1928–80 78°44'19" 35°56'25" Neuse River at Kinston 18 1952–73 1954–71 35°38'50" 78°34'56" 53 35°30'46" 1945–80 20 78°24'22" 18 35°39'28" 78°21'00" 1919–80 35°43'00" 21 78°48'06" 1908–80 35°20'24" 35°34'10" 78°45'00" 53 1954–71 78°21'54" 78°35'30" 35°23'36" 48 1954–71 35°38'11" 1953–71 1940–96 78°31'48" 18 35°30'40" 35°35'20" 35°20'14" 78°15'06" 18 1953–71 19 78°09'38" 56 78°11'18" 77°59'51" 1953–71 1919–96 1965–89 19 1930–80 35°15'29" 19 77°35'09" 66 25 51 1919–80 53 74 84 93 Map Map (fig. 1) number identification identification Table 1. 1. Table or channelized flows; siter, excluded from regional analysis because flows were affected by regulation or channelization] [nc, flood-frequency estimates were not computed because the site hasthan less 10 yearsofpeak-flow record; duplicate *, map
Tables 21 peaks Number of of Number systematic of Period Period record 35°41'29" 78°06'38" 1965–76 12 identification and stationnumber for sites havingseparate period ofregulated Station nameStation Latitude Longitude Station Station number 02090380* ContentneaCreek near Lucama(regulated period)0209096970 MoccasinRun near Patetown 35°41'29" 78°06'38" 1977–96 20 35°28'46" 77°54'37"02093549 1989–96 Haw RiverAltamahaw at 8 36°10'43" 79°30'09" 1968–73 6 r nc nc Map identification numbers and descriptions of gaged rural sites in North Carolina with annual peak-flow record—Continued peak-flow annual with Carolina North in sites rural gaged of descriptions and numbers identification Map 95* 95 02090380 ContentneaCreek near Lucama 969798 0209056099 02090625 02090780 LeeSwamp tributary near Lucama Turner Swamp near Eureka 02090960 Wilsonnear tributary Swamp Whiteoak Nahunta Swampnear Pikeville 35°42'24" 35°38'21" 77°47'11" 78°01'37" 35°34'14" 1953–71 1953–71 35°30'40" 77°52'47" 77°58'56" 19 1969–87 19 1953–73 19 19 111 02092500 Trent River near Trenton 35°03'54" 77°27'24" 1928–96 45 110112113 02092290114 02092520 Rattlesnake Branch nearComfort 115 02092620116 Swamp Vine near Kinston117 02092720 UpperBroad Creek tributarynear118 Grantsboro 02092780 02093000 White Oak River at Belgrade119 02093040 Bell Swampnear Hubert 02093070 New River nearGum Branch SouthwestCreek tributarynear Jacksonville 02093290 Southwest Creek near Jacksonville Haw River nearSummerfield 35°08'06" 35°00'31" 76°56'31" 77°35'50" 1953–73 34°47'18" 35°09'29" 1953–71 34°53'30" 77°33'08" 77°33'16" 21 34°50'56" 34°43'56" 19 1954–73 77°14'02" 34°42'04" 1953–71 77°31'11" 77°32'02" 1953–73 77°14'01" 36°14'32" 19 1908–96 1953–73 19 1953–70 79°52'20" 21 1954–71 33 20 18 18 101102103 02091000104 02091430105 02091500 Nahunta Swampnear Shine106 Shepherd Run near107Snow Hill 02091700 ContentneaCreek 108 at Hookerton 02091810 02091970 Creek near Farmville Contentnea Little 109 02092000 Halfmoon Creeknear Fort Barnwell 02092020 Creeping Swamp near Vanceboro SwiftCreek near Vanceboro 02092120 PalmettoSwamp near Vanceboro Bachelor Creek near New Bern120122 35°32'40"123 02093500 35°29'20" 35°26'06" 35°25'44"124 02093800 35°17'58" 77°30'41" Haw River nearBenaja 125 77°48'22" 02094000 77°38'42"126 77°34'59" 35°23'30" Reedy Fork nearOak127 Ridge 1957–87 77°21'14" 02095000 1955–96 Horsepen Creek at Battle128 Ground 1953–71 02095500 1928–96 35°20'18" 77°13'46" 02096500 35°20'42" South 1953–75 Buffalo Creek nearGreensboro 02096660 North Buffalo Creek nearGreensboro 31 35°10'24" 77°10'16" 02096700 1972–85 Haw RiverHaw at River 42 19 77°11'45" 68 RockCreek nearWhitsett 12 Big 1953–76Alamance 77°06'14" Creeknear Elon College 1909–89 14 1953–71 24 39 19 36°08'34" 36°03'36" 36°15'06" 36°10'22" 36°07'13" 79°51'40" 79°43'33" 79°33'55" 36°02'21" 79°57'12" 79°42'30" 1926–59 1929–58 1916–71 1956–96 1929–90 79°31'29" 36°05'13" 36°04'49" 1945–80 30 29 43 79°22'02" 41 62 78°47'45" 1929–96 23 1954–71 68 17 121 100 Map (fig. 1) number identification identification Table 1. 1. Table [nc,flood-frequency estimates werenot computed because thesitehas less than 10years of peak-flow record; *, duplicate map orchannelized siteexcluded flows;r, from regional analysisbecause flowswere affected by regulation or channelization]
22 Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina peaks Number of systematic of of Period Period record 36°02'58" 79°28'35" 1954–73 19 identification and station numberfor sites having separate period of regulated Station nameStation Latitude Longitude Station number 02096846 Cane Creeknear Orange Grove0209819802098200 Haw Riverbelow B.Everett Jordan Damnear Moncure Haw Rivernear Haywood02100000 Muddy Creek nearArchdale 0210166029 35°39'11" 35°59'13" Crossroads Crutchfield near River Rocky 79°04'03" 79°12'23"02102192 1980–92 1989–9602102192* Buckhorn Creek nearCorinth Buckhorn Creek nearCorinth (regulatedperiod) 02102500* 13 8 period) (regulated Lillington at River Fear Cape 35°39'01" 79°03'59" 35°48'25" 35°52'35" 1966–72 79°31'41" 79°52'43" 35°33'34" 1988–96 1935–41 0 35°24'22" 78°58'25" 9 78°48'48" 1981–96 7 35°33'34" 1981–96 16 78°58'25" 16 1973–80 8 r r r nc nc nc nc nc Map identification numbers and descriptions of gaged rural sites in North Carolina with annual peak-flow record—Continued peak-flow annual with Carolina North in sites rural gaged of descriptions and numbers identification Map 138 152* 153* 131132133 02096850134 02096960135 02097010 Teernear Creek Cane 136 Bynumnear River Haw 137 02097314 Pittsboronear Creek Robeson 0209741955Genlee near 02097910 1100 Road Secondary at Creek Northeast New Hope Creek nearBlands 139 02098000140 Wilsonville near Creek Oak White 141 Pittsboronear River Hope New 142143 02098500 02099000144 PointHigh near 02099500 River Deep Fork West 145 PointHigh near River Deep Fork East 146 Randleman near River Deep 147 02100500148 35°52'20" 02101000 02101030 Ramseur at River Deep 149 02101480 Robbins at Creek Bear 78°54'49"150 Bennett near Creek Falls 151 Tramway near Creek Sugar 152 1983–96 02101800 02101890 Springs Vernon 02102000 Mount near Creek Tick 153 35°43'29"Goldston near Creek Bear 35°56'34" 12 35°45'48" Moncure at River Deep 35°44'47" 35°53'05"154 79°12'33"155 02102500 79°14'46" 79°08'02"156 35°44'12" 79°00'44" 78°57'58" 1954–76 36°00'15" 02102908Lillington at River Fear Cape 1960–73157 1908–96 36°02'15" 02102910 79°01'36" 1954–71158 1983–96 02102930 79°58'42" Inverness near Creek Flat 159 23 Carthage near tributary Creek Dunhams 79°56'46"160 1908–73 02103000 14 Vass near Creek 69 Crane 1924–66161 02103390 18 14 35°54'06" 1929–94 02103500 Little River at Manchester 02104000Lillington near Creek Anderson Prong South 24 02104080 Linden at River Little 42 79°51'05"Fayetteville at River Fear Cape 66 35°39'37" Fayetteville near Creek Reese 35°43'34" 1929–96 35°26'03" 35°25'28" 35°33'20" 79°24'08" 79°39'20" 79°35'39" 66 79°14'50" 79°29'56" 1959–96 1901–96 35°37'33" 1940–71 1954–73 1954–73 35°37'38" 26 79°17'54" 35°18'41" 73 35°24'22" 32 20 20 79°06'58" 35°15'31" 1952–71 79°22'53" 78°48'48" 1931–96 35°10'54" 78°55'27" 1954–71 1924–80 19 1953–71 79°10'40" 35°17'53" 66 35°11'38" 18 35°02'49" 1969–96 57 79°16'19" 19 35°04'49" 78°59'14" 78°51'36" 35°15'46" 1954–71 28 78°47'45" 1939–50 1889–76 78°46'35" 1953–71 18 1928–71 11 71 17 44 129 02096740Alamance near Branch Gun 130 Map Map (fig. 1) number identification identification Table 1. 1. Table or channelized flows; siter, excluded from regional analysis because flows were affected by regulation or channelization] [nc, flood-frequency estimates were not computed because the site hasthan less 10 yearsofpeak-flow record; duplicate *, map
Tables 23 peaks Number of of Number systematic of Period Period record 34°57'57" 78°55'04" 1939–54 16 identification and stationnumber for sites havingseparate period ofregulated Station nameStation Latitude Longitude Station Station number 02105500* period) (regulated Tarheel near Lock O. Huske William at River Fear Cape 34°50'05"02105769* 78°49'27" Cape Fear River at Lock1 nearKelly (regulated period) 1981–96 15 34°24'15" 78°17'38" 1981–96 16 02112000* Riverat Wilkesboro (regulated Yadkin period) 36°09'09" 81°08'45" 1962–96 35 r r r Map identification numbers and descriptions of gaged rural sites in North Carolina with annual peak-flow record—Continued peak-flow annual with Carolina North in sites rural gaged of descriptions and numbers identification Map 166* 192* 163* 162 02104500 Mills Hope near Creek Rockfish 164165166 02105570 02105630167 Browns Creek nearElizabethtown168 02105769 Turnbull Creeknear Elizabethtown169 02105900 Cape Fear River at Lock1170 nearKelly 02106000171 02106240 Hood Creek near Leland172 Creek near Roseboro Coharie Little 173 02106410 Turkey Creeknear Turkey174 02106500 02106910 Stewarts Creek tributary175near Warsaw 02107000 Black River near Tomahawk176 02107500 Big Swamp 177near Roseboro South River near Parkersburg178 02107590 Colly Creek near Kelly179 02107600 02107620 NortheastCape Fear180 River tributary nearMount Olive 02107980 NortheastCape Fear River181 near SevenSprings 02108000 Mathews Creeknear Pink Hill182 LimestoneCreek183 near Beulaville 02108500 34°36'32" NortheastCape Fear River184 near Chinquapin 34°41'32" 02108548 34°24'15" 02108610 Rockfish Creek near Wallace185 78°36'57" 02108630 Wallace at Creek Rockfish Little 78°35'02"186 02108960 78°17'38" PikeCreek near187 Burgaw 1953–73 34°57'13" Turkey Creeknear Castle Hayne188 1949–71 02109500 Buckhead Branch189 nearBolton 1970–80 02109640 35°11'06" 34°57'25" 78°29'17" 02110020 River Waccamaw at Freeland190 34°16'43" 18 02111000 Ash Swamp Wet near Ash191 19 77°57'34" 78°04'42" 35°00'11" 02111180 1924–91 Mill Branch City near192 Tabor 11 35°10'20" 78°07'34" Riverat Patterson Yadkin 02111340 34°45'17" 1954–71 Elkville at Creek Elk 1955–71193 78°11'06" 02111500 1953–96 34°48'45" 34°58'38" 77°55'56" 02112000 41 South Prong LewisFork Creek near NorthWilkesboro 78°17'21" 1953–73 Reddies River 34°49'40" at North Wilkesboro 02112120 18 1959–75 Riverat Wilkesboro 16 Yadkin 78°27'26" 78°34'07" 1928–96 34°27'48" 24 77°50'00" 35°05'49" 34°45'48" Roaring River near Roaring River 1952–86 1953–73 18 78°15'26" 17 1941–96 45 77°49'10" 77°48'15" 34°44'02" 1908–71 34°44'32" 35 20 1953–76 1953–71 56 34°23'47" 77°58'03" 36°11'23" 78°02'22" 21 34°20'52" 34°30'00" 1977–92 77°54'48" 16 81°24'40" 19 1955–81 34°05'43" 78°26'19" 77°53'58" 1953–71 1955–71 34°02'17" 16 34°10'59" 78°32'55" 27 1953–71 1953–71 35°59'29" 36°10'29" 78°30'14" 19 1940–96 78°48'08" 16 19 18 36°04'16" 81°33'30" 1953–71 81°10'09" 1953–71 36°09'09" 36°14'59" 57 1940–96 81°24'13" 1940–95 18 81°08'45" 81°02'39" 18 1940–96 56 1904–61 1916–96 55 31 48 32 163 02105500 Tarheelnear Lock O. Huske William at River Fear Cape 34°50'05" 78°49'27" 1938–80 36 Map (fig. 1) number identification identification Table 1. 1. Table [nc,flood-frequency estimates werenot computed because thesitehas less than 10years of peak-flow record; *, duplicate map orchannelized siteexcluded flows;r, from regional analysisbecause flowswere affected by regulation or channelization]
24 Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina peaks Number of systematic of of Period Period record 36°15'12" 80°51'45" 1971–80 10 identification and station numberfor sites having separate period of regulated Station nameStation Latitude Longitude Station number 02112250 River at Elkin Yadkin 02113500 River at Siloam Yadkin 02115360 River at Enon Yadkin 0211573002115740 Stanleyvillenear Creek Mill 02115810 Oldtownnear Creek Mill Little Creek nearClemmons 02116500* 36°14'30"College (regulated River at Yadkin period) Yadkin 80°50'49" 1965–95 35°16'55"02119400 80°33'46" 31 Third Creek nearStony Point02120500* 1977–87 36°07'55" Third Creek at Cleveland (regulatedperiod) 36°10'49" 35°51'23" 80°26'39" 11 80°16'19" 36°09'06" 36°02'19" 1965–96 80°23'14" 1965–72 80°19'03" 80°20'46" 1962–96 32 1965–72 1965–72 6 35 6 6 35°45'00" 80°41'00" 35°52'04" 1955–71 81°04'00" 1957–69 17 13 r r r r nc nc nc r r Map identification numbers and descriptions of gaged rural sites in North Carolina with annual peak-flow record—Continued peak-flow annual with Carolina North in sites rural gaged of descriptions and numbers identification Map 200 195 211 02115830 Kernersvillenear Creek Smith 215* 36°06'19" 80°06'19" 1954–71 18 223* 212213214 02115856215 02115860 Atwood near Creek Salem 02115900Creek Muddy near Creek Muddy 02116500 Clemmonsnear Creek Muddy Fork South College Yadkin at River Yadkin 36°00'22" 36°00'01" 80°18'07" 36°02'10" 80°20'25" 35°51'23" 1965–91 80°18'35" 1965–91 80°23'14" 1972–82 19 1916–61 19 11 33 196197198 02112360199 02112410 Road 02112500State near River Mitchell 201 Bottom near River Fisher 202 02113000 Dobson near River Fisher 203 02113850 Copeland near River Fisher 02114010205 02114450 Ararat at River Ararat 206 Mountain Pilot near Dam at River Ararat 207 Dalton at River Yadkin Little 208 02115500 02115520209 02115540 Yadkinville near Creek Forbush 210 Smithtown near Creek Logan Yadkinville near Creek Deep South 216 36°18'42"217 80°48'26"218 36°22'00" 36°26'35" 02117030219 36°23'05" 02117410220 1940–96 36°21'26" Fork near Creek Humpy 80°33'00"221 80°46'12" 02117500 Statesville near Creek McClelland 80°40'20" 02118000 80°41'10" 1938–68 1954–71 02118500 Turnersburg at Creek Rocky 223 32 36°17'56" 1922–33 02119000Mocksville near River Yadkin South 36°24'16" 1922–96 Harmony near Creek Hunting 36°08'13"224 36°08'00" 80°25'53" at Cooleemee River Yadkin South 225 16 16 02120500 80°33'43" 36°12'50"226 12 80°33'09" 1961–96 80°46'00" 74 02120780 Cleveland at Creek Third 1947–96 80°33'32" 02120820 1941–71 1954–66 02121000 Barber near Creek Second Salisbury near Branch 1954–71 Deal 36 32 Salisbury near River Yadkin 31 13 18 35°57'04" 35°50'41" 80°56'46" 35°51'17" 35°54'23" 80°39'34" 1954–76 35°48'10" 80°26'24" 36°00'00" 80°48'34" 1930–96 80°33'22" 1969–83 80°44'44" 22 1941–71 1916–65 1952–96 58 35°45'00" 15 31 35°43'05" 35°44'43" 37 80°41'00" 35°43'30" 45 80°35'45" 80°30'25" 1916–54 80°23'50" 1980–96 1954–71 1896–27 14 17 15 30 194 02112247 Elkin at River Elkin 204 222 Map Map (fig. 1) number identification identification Table 1. 1. Table or channelized flows; siter, excluded from regional analysis because flows were affected by regulation or channelization] [nc, flood-frequency estimates were not computed because the site hasthan less 10 yearsofpeak-flow record; duplicate *, map
Tables 25 peaks Number of of Number systematic of Period Period record 35°45'28" 80°19'24" 1980–90 11 identification and stationnumber for sites havingseparate period ofregulated Station nameStation Latitude Longitude Station Station number 0212250002122500* Riverat High Rock Yadkin Riverat High Rock Yadkin (regulated period) 35°35'46"02129000 80°13'59" Rockingham near River Dee Pee 1942–610213228795 35°35'46" Jordan Creeknear Silver Hill 19 80°13'59" 1916–27 8 34°56'46" 79°52'11" 1928–96 34°58'12" 79°31'35" 69 1985–93 9 r nc nc r Map identification numbers and descriptions of gaged rural sites in North Carolina with annual peak-flow record—Continued peak-flow annual with Carolina North in sites rural gaged of descriptions and numbers identification Map 230* 227 02121180 North PottsCreek at Linwood 231232233234 02122560235 02122720 02123500 Cabin Creek nearJackson 236 Hill 02123567 BeaverdamCreek 237 tributarynear Denton 02124060 Uwharrie River near Eldorado238 DutchmansCreek near239 Uwharrie 02124130 North Prong Clarke240 Creek near Huntersville 02125000 02125410 Mallard Creek241 near Charlotte 02126000 BigBear Creek near Richfield242 02127000 Chinkapin243 Creek near Monroe Rocky River nearNorwood 02127390 Brown Creek near Polkton245 02128000 02128260 Ansonvilleat Branch Palmetto 246 Star near River Little 247 02129440 Cheek Creek near Pekin249 02129530 South Fork JonesCreek 250 near Morven 02132230 35°31'57" DeePee near tributary Creek Little 251 02132320 35°25'13" Bridge Creek tributary252at Johns 02133500 35°34'57" 80°05'04"253 BigShoe Heel Creek254 nearLaurinburg 02133590 80°47'54" 35°25'47" 35°22'05" Drowning Creeknear 255 Hoffman 1954–71 80°09'12" 02133624 02133960 Beaverdam Creek256 near Aberdeen 1954–73 02134380 80°01'05" 80°01'49" Lumber 1954–71 River257 near Maxton 02134500 Raft Swamp near Red258 Springs 35°19'05" 18 35°20'02" Swamp Tenmile near Lumberton259 1928–71 1982–95 02137000 20 35°02'48" Lumber River260 at Boardman 02137727 17 80°44'16" 02138000 80°20'09" Mill Creek at Old Fort 35°08'54" 02138500 80°29'33" Catawba Rivernear PleasantGardens 32 12 35°06'03" 1954–71 35°02'10" 02138680 Catawba Rivernear Marion 1955–96 80°10'33" Linville River near Nebo 1953–71 34°53'51" 80°07'11" White Branch near Marion 80°08'42" 1908–96 18 34°55'07" 42 80°00'24" 35°23'11" 35°12'37" 1953–71 1908–71 18 34°45'01" 79°54'38" 34°42'12" 1954–71 79°49'56" 79°50'49" 67 17 36 1955–71 79°23'12" 1955–96 79°26'34" 1954–71 35°03'38" 18 35°00'42" 1987–96 1953–73 79°29'39" 11 41 18 79°26'50" 34°43'34" 34°52'16" 34°46'22" 1940–96 10 18 1953–71 35°41'09" 78°59'31" 79°10'12" 79°19'55" 34°26'32" 82°03'40" 57 1953–73 1953–71 1987–96 18 78°57'38" 35°37'59" 35°42'26" 1981–96 1901–96 18 35°47'41" 15 10 35°38'46" 82°11'14" 82°02'00" 16 81°53'25" 1940–75 1916–81 81°55'18" 67 1916–96 1955–71 15 40 74 14 228229 02121500 02121940 AbbottsCreek at Lexington Flat Swamp Creek nearLexington 35°43'59" 35°48'23" 80°06'37" 80°14'05" 1954–71 1941–95 18 23 248 230 244 Map (fig. 1) number identification identification Table 1. 1. Table [nc,flood-frequency estimates werenot computed because thesitehas less than 10years of peak-flow record; *, duplicate map orchannelized siteexcluded flows;r, from regional analysisbecause flowswere affected by regulation or channelization]
26 Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina peaks Number of systematic of of Period Period record 35°53'21" 81°44'18" 1955–71 17 identification and station numberfor sites having separate period of regulated Station nameStation Latitude Longitude Station number 02142500 Catawba River at Catawba02148500 Broad River near Chimney Rock 35°43'00" 81°03'59" 1936–62 30 35°25'29" 82°10'54" 1928–58 31 r r Map identification numbers and descriptions of gaged rural sites in North Carolina with annual peak-flow record—Continued peak-flow annual with Carolina North in sites rural gaged of descriptions and numbers identification Map 268269270 0214253830271 02142900 Troutman near Creek Norwood 272 02143000273 Creek Paw near Creek Long 274 02143040 River Henry near Fork Henry 275 02143310 02143500 at Ramsey Fork Jacob 276 02144000 Lincolntonnear Branch Inn Lithia 277 02145000Laboratory near Creek Indian City Bessemer near Creek Long 279 02146890 Lowell at River Catawba Fork South 280 02146900Waxhaw near Creek Mile Twelve Fork East 281 Waxhaw near 02149000Creek Mile Twelve 282 02150420283 Lure Lake near Creek Cove 284 02151000 Rutherfordtonnear Creek Camp 285 02151500 02152100 Cliffside at River Broad Second 286 02152420 Springs Boiling near River Broad 287 Casar 02152500 near River Broad First 288 35°40'48"Fallston near Creek Knob Big 289 02152610 Lawndale near River Broad First 290 03160610 80°56'44" 35°19'42" 03161000 Springs Boiling near Branch Sugar 291 35°41'03" Jefferson 03162110 West near Creek Field Old 292 35°27'47" 1984–96 Jefferson 03162500 near River New Fork South 80°54'35"293 34°57'46" 81°24'10" Warrensville at Creek Buffalo 294 35°17'10" 03162880 81°13'27" Crumpler at 35°25'20" River New Fork North 35°18'23"295 1966–96 03439000 80°42'40" 35°35'26" 13 1916–96 03439500 Sparta near Creek Vile 81°06'00" 1954–71 81°15'52" 81°14'05"Rosman 03440000 at River Broad French 1954–72 34°57'08" 81°34'02" 03441000Calvert at Broad French 1940–96 31 Brevard 1916–96 near 59 Creek Catheys 1954–96 1962–96 80°45'21" 14 Davidson River near Brevard 18 35°27'47" 35°25'24" 42 1949–96 45 35°14'08" 43 35°12'39" 35 81°54'29" 82°06'42" 81°45'57" 81°41'52" 1955–71 36 35°29'35" 1916–96 35°22'50" 35°29'34" 35°15'00" 1926–96 36°21'29" 1926–96 81°40'56" 36°23'35" 81°32'40" 81°32'25" 17 81°37'15" 45 81°31'46" 1960–96 81°24'26" 71 1916–80 1953–71 36°31'04" 70 1954–87 36°27'22" 1955–71 1916–96 81°23'18" 36 81°30'51" 41 18 35°08'32" 34 1878–66 17 69 1940–71 36°30'39" 82°49'28" 35°12'40" 35°08'55" 35°16'23" 39 81°06'16" 1908–96 17 82°47'00" 82°47'57" 82°42'21" 1955–71 1945–96 1916–55 62 1876–96 17 21 31 73 261 02140980 Collettsville near Creek Carroll 262263264265 02140991 02141130266 02141890 Store Arneys at River Johns 02142000 Lenoirnear Creek Fork Zacks Taylorsville near Creek Duck 02142480 Springs All Healing near River Little Lower Catawba near Creek Hagan 35°56'44" 81°14'13" 35°50'01" 35°55'32" 35°53'34" 1954–95 81°42'43" 81°31'13" 81°18'09" 1986–96 35°40'20" 1967–76 42 1954–71 81°08'12" 11 10 1954–71 18 15 278 267 Map Map (fig. 1) number identification identification Table 1. 1. Table or channelized flows; siter, excluded from regional analysis because flows were affected by regulation or channelization] [nc, flood-frequency estimates were not computed because the site hasthan less 10 yearsofpeak-flow record; duplicate *, map
Tables 27 18 peaks Number of of Number systematic of Period Period record 1943–58 35°11'32" 82°36'49" 1963–90 28 identification and stationnumber for sites havingseparate period ofregulated Station nameStation Latitude Longitude Station Station number 0344894205 North Fork Swannanoa Rivernear Walkertown03449000* North Fork Swannanoa Rivernear BlackMountain (regulated period)03451000* period) (regulated Biltmore River at Swannanoa 35°39'11" 82°21'04" 1953–57 35°41'07"0345577330 82°19'58"03456100 ForkPigeon River West near Retreat 5 1990–96 ForkPigeon West River at Bethel 35°34'06"03458500 82°32'42" 7 Pigeon River nr Crabtree 1980–96 17 35°25'36" 82°55'12" 35°27'48" 1989–96 82°54'00" 1955–96 8 35°34'37" 41 82°57'07" 1922–30 9 r r,nc nc r,nc nc r Map identification numbers and descriptions of gaged rural sites in North Carolina with annual peak-flow record—Continued peak-flow annual with Carolina North in sites rural gaged of descriptions and numbers identification Map 311312* 03450000 Beetree Creeknear Swannanoa 35°39'11" 82°24'20" 1927–96 61 307308 03448000 03448500 French Broad River at Bent Creek Hominy Creekat Candler 35°30'07" 82°35'33" 35°32'28" 1916–86 82°40'35" 52 1940–78 35 296297298299 03441440300 03441500 03442000Mountain Cedar near Falls High above River Little 301 03443000 Penrose near River Little 302 03444000 Crab Creeknear Penrose303 French Broad River304 at Blantyre 03444500 Boylston Creeknear Horseshoe305 03446000 03446410 South Fork Mills306 River at The PinkBeds 03446500 MillsRiver near Mills River 03447000Edneyville near Branch Laurel Hendersonville near Creek Clear 03447500 Mud Creek at Naples Cane CreekFletcher at 35°21'59" 35°17'56" 35°13'23" 35°22'10" 35°14'02" 82°44'20" 82°38'07" 82°37'26" 82°33'50" 82°36'39" 1927–73 35°22'15" 1916–73 35°23'55" 1875–96 35°21'14" 1943–73 1916–65 82°24'10" 82°35'42" 82°26'40" 31 13 76 1955–70 13 1910–65 1876–96 13 35°22'52" 35°26'08" 82°29'54" 12 10 64 82°29'23" 1939–55 1901–02; 17 310 03449000312 North Fork Swannanoa313 Rivernear BlackMountain 03451000314315 03451500 Biltmore River at Swannanoa 316317 03452000 Asheville at River Broad French 318 03453000 03453500 Sandymush Creeknear319 Alexander 03453880 Ivy Creek near Marshall320 03454000 French Broad River321 at Marshall Brush CreekWalnut at 03454500 BigLaurel Creek near323 Stackhouse 35°39'11" 03455500 French Broad River at Hot324 Springs 82°21'04" ForkPigeon River West above LakeLogan 325near Hazelwood 03456500327 1926–52 03456991 East ForkPigeon 328 Rivernear Canton 03457500 Pigeon Rivernear Canton 03459000 Allen Creek near Hazelwood 27 03459500 JonathanCreek near CoveCreek 35°23'46" 35°34'06" Pigeon Rivernear Hepco 35°36'33" 82°56'17" 35°43'49" 82°32'42" 82°34'43" 1955–96 1921–79 82°40'11" 35°47'10" 1896–96 35°55'12" 35°46'10" 1940–55 35°53'23" 82°39'39" 42 51 82°45'42" 101 35°50'40" 82°37'16" 1916–96 82°49'16" 13 1935–78 35°27'42" 1876–96 82°44'30" 1796–78 54 82°52'13" 1954–71 39 42 1955–96 15 35°25'49" 35°31'19" 35°37'21" 17 83°00'30" 82°50'53" 83°00'25" 42 35°38'05" 1950–73 1810–96 1931–73 82°59'21" 24 71 43 1876–96 69 309 310* 326 322 Map (fig. 1) number identification identification Table 1. 1. Table [nc,flood-frequency estimates werenot computed because thesitehas less than 10years of peak-flow record; *, duplicate map orchannelized siteexcluded flows;r, from regional analysisbecause flowswere affected by regulation or channelization]
28 Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina peaks Number of systematic of of Period Period record 35°40'02" 83°04'22" 1935–96 52 identification and station numberfor sites having separate period of regulated Station nameStation Latitude Longitude Station number 03500500Highlands at River Cullasaja 03505500 Nantahala at River Nantahala 0350800003508000* Tuckasegee River at Tuckasegee Tuckasegee River at (regulated Tuckasegee period)03510500* period)(regulated Dillsboro at River Tuckasegee 03513000* 35°04'14" Tuckasegee River at Bryson City(regulated period) 83°13'57" 35°16'55" 1928–71 83°07'37" 35°17'55" 35°22'00" 1940–76 44 35°16'55" 83°39'21" 83°15'37" 35°25'40" 37 1943–82 1940–82 83°07'37" 83°26'51" 1840–40 39 43 1940–95 6 55 r r r nc r r Map identification numbers and descriptions of gaged rural sites in North Carolina with annual peak-flow record—Continued peak-flow annual with Carolina North in sites rural gaged of descriptions and numbers identification Map 352* 354* 357* 344345346347 03501000348 03501760 03502000Cullasaja at River Cullasaja 03503000 Coon Creek nearFranklin 350Iotla at River 03504000 Tennessee Little 351Needmore at River Tennessee Little 352 Springs Rainbow near River Nantahala 03506500 03507000353 Almond at River Nantahala 354River Little Judson at Tennessee 355 03509000356 03510500 Sylva above Creek Scott 357 03511000Dillsboro At River Tuckasegee 03512000358 Cherokee at River Oconaluftee 359 03513000 Birdtown at River Oconaluftee 360City Bryson at 03513410 River Tuckasegee 03513500 03514000 JenkinsBranch tributary at Bryson City 35°07'37" 35°20'11" 35°09'59" City Bryson near Creek Noland 35°13'59" Proctor at Creek Hazel 35°14'04" 83°37'09" 83°31'37" 83°19'25" 82°23'32" 83°20'28" 1940–96 1899–96 1908–76 1899–49 35°24'30" 35°22'32" 1957–73 57 51 52 83°33'26" 83°33'59" 17 17 35°22'00" 1897–44 1923–41 35°23'02" 35°29'04" 83°15'37" 35°27'41" 35°25'40" 48 83°12'51" 17 35°24'50" 83°18'56" 1928–40 83°21'13" 83°26'51" 1929–95 83°27'20" 1867–49 35°29'05" 1946–96 1898–40 13 1957–71 48 83°30'15" 28 48 35°28'38" 43 1936–71 13 83°42'58" 36 1943–52 10 329 03460000 Cataloochee Creek near Cataloochee 330331332333 03461910 03462000334 03463300 Newland at River Toe North 335 03463500 Altapass at River Toe North 336Celo near River Toe South 337 03463910Newdale at River Toe South 338 03464000 03464500 Burnsvillenear Creek Phipps 339 03478910Sioux near River Cane 340 03479000 Poplarat River Nolichucky 341 at Sherwood Creek Cove 342 03480540 Grove Sugar near River Watauga 03481000 Elk 03500000 Banner near Branch Peavine 03500240Park Elk near River Elk Prentiss near River Tennessee Little Franklin near Creek Cartoogechaye 36°05'01" 35°53'59" 81°55'45" 35°49'53" 35°54'22" 82°01'50" 35°54'40" 1955–73 82°11'04" 82°11'19" 1935–78 82°22'10" 36°14'18" 36°04'29" 1958–96 1916–78 36°00'52" 19 36°10'20" 36°15'50" 1957–73 81°49'22" 24 82°20'41" 35°08'59" 82°19'40" 35°09'31" 39 81°54'42" 18 81°47'03" 1916–96 1926–78 83°22'47" 1893–78 14 36°11'01" 83°23'40" 1953–72 1940–72 1899–96 57 30 1949–96 81°57'45" 38 11 18 1935–78 52 35 21 349 343 Map Map (fig. 1) number identification identification Table 1. 1. Table or channelized flows; siter, excluded from regional analysis because flows were affected by regulation or channelization] [nc, flood-frequency estimates were not computed because the site hasthan less 10 yearsofpeak-flow record; duplicate *, map
Tables 29 peaks Number of of Number systematic of Period Period record 35°26'45" 83°48'20" 1939–44 6 identification and stationnumber for sites havingseparate period ofregulated Station nameStation Latitude Longitude Station Station number 0351500003515000* Dam Fontana at River Tennessee Little period) (regulated Dam Fontana at River Tennessee Little 0354700003548500* Hiwassee River below Chatuge Dam near Hayesville Hiwassee Riverabove Murphy (regulatedperiod) 35°26'45" 83°48'20" 1945–54 35°01'45" 10 83°47'45" 35°04'49" 1943–74 84°00'10" 1941–96 32 55 r r nc r Map identification numbers and descriptions of gaged rural sites in North Carolina with annual peak-flow record—Continued peak-flow annual with Carolina North in sites rural gaged of descriptions and numbers identification Map 365* 361* 362363 03516000365 03546000 SnowbirdCreek near366 Robbinsville Hayesvillenear Creek Shooting 03548500 03550000 Hiwassee Riverabove Murphy RiverTomotla at Valley 35°18'40" 35°01'29" 83°51'35" 83°42'27" 1943–52 35°04'49" 1923–55 84°00'10" 10 35°08'20" 1897–41 13 83°58'50" 44 1898–96 86 361 364 Map (fig. 1) number identification identification Table 1. 1. Table [nc,flood-frequency estimates werenot computed because thesitehas less than 10years of peak-flow record; *, duplicate map orchannelized siteexcluded flows;r, from regional analysisbecause flowswere affected by regulation or channelization]
30 Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina REG 100 CF 25 CF 2 CF ) 2 SHAPE (DA/L (ft/mi) BSLOPE (ft/mi) CSLOPE L (mi) E, basin slope; SHAPE, basin shape; CF, climate factorrecurrence for interval basinE, slope;basin SHAPE, shape; CF, ) flow record; *, duplicate map identification numberfor sites having separate 2 channelization; n.a., data not available] DA (mi 500 Q 200 Q 100 Q 50 Q 25 Q 10 Q 5 Q n.a.n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a.n.a. n.a. n.a. n.a. n.a. 3.7 n.a. 24.0 2.65 n.a. 8.03 2.38n.a. 2.18 n.a. 4.10 n.a. .58 9.79 n.a. 2.25 .34 2.90 n.a. 2.26 n.a. 3.09 2.90 2 n.a. 3.09 n.a. 2 1150 n.a. 116.76 n.a. 20.23 154.63 n.a. .09 2.10 n.a. 2.79 191 2.97 1 30.26 6.66 108.97 .22 2.11 2.80 2.98 1 2 963 1340 1630 2070 2440 2860 3330 4040 63.3 18.55 2.34 8.23 .18385 2.26 2.90 1010 3.10 2 1700 3030 4430 6280 8690 13000 53.5 16.06 13.21 108.45 .21 2.14 2.82 3.01 1 Q 4210 7370 9940 13700 17000 20600 24600 30500 129 46.524450 51.21 211.90 8400 .06 10900 2.07 2.77 13700 2.95 15600 1 17100 18500 20100 202 32.72 6.28 108.48 .19 2.14 2.82 3.01 1 10300 15300 19000 24100 28200 32500 37000 4350022800 29400 538 35100 77.81 43900 10.76 51600 194.65 60500 .09 2.10 70800 2.79 87000 2.97 1 8386 280.12 4.03 159.83 .11 2.22 2.87 3.06 1 r r r r Recurrence interval discharges and basin characteristics for gaged rural sites in North Carolina North in sites rural gaged for characteristics basin and discharges interval Recurrence nc nc nc r, nc r r 6* 9* 123 51.6 331 1940 99.6 641 2950 143 3700 918 211 4750 1360 274 5600 1770 6510 347 2240 7490 2800 433 8900 3680 568 225 11.8 0.7 30.86 5.84 1.17 3.18 2.93 16.19 8.61 13.25 .23 .33 4.33 2.26 0.53 2.25 2.90 2.24 2.90 3.10 2.89 3.09 2 3.08 2 2 6789 816 1380 209 221 4630 1840 253 7620 427 2510 280 9990 611 3090 13400 314 3740 906 16400 338 19600 4460 1180 23200 363 5540 1490 28500 386 1860 63.3 129 18.55 2450 418 46.52 2.34 51.21 8.9 2.6 211.90 8.23 .06 .18 4.96 3.07 2.07 2.26 2.77 7.12 6.71 2.90 2.95 3.10 2.28 1 1.53 2 .25 .35 2.26 2.31 2.90 2.94 3.10 3.14 2 1 4 5 18* 29* 11 616 945 1190 1540 1820 2120 2450 2920 5.4 5.21 53.24 154.08 .20 2.08 2.77 2.95 1 1819 150002021 2430022 31500 81623 91224 1850 42000 1640 1380 3700 44.5 50700 2400 726 1960 1730 77.8 60300 5390 3630 127029 3680 70900 106 2220 8150 4780 172030 86400 5190 1070031 2610 147 77700 6150 2400 13700 105000 7560 538 3040 52000 184 7780 124000 17300 2990 78600 9700 149000 3490 77.81 10400 70.2 23100 225 3660 98500 169000 12200 10.76 148 4150 126000 188000 194.65 29.9 45.9 15100 4410 272 148000 209000 221 .09 14.66 19500 8.97 5560 7.1 172000 237000 2.10 343 13.59 20.81 198000 2.79 344 8386 118.79 56.5 117.88 4.32 2.97 235000 7.5 .22 .41 40.20 280.12 1 461 .2 14.40 8671 2.12 2.11 103.92 4.39 2.80 13.70 4.03 2.80 .35 603 311.75 .39 53.25 2.98 104.89 159.83 2.98 2.12 1 19.18 1 94.67 .27 .11 3.83 2.80 774 2.12 66.32 155.40 .39 2.22 2.98 2.80 2.12 2.87 .66 1050 .09 1 2.98 2.80 2.12 3.06 2.25 2.98 2.80 1 1 2.89 2.98 .9 1 3.09 1 2 1.17 18.78 10.29 .38 2.29 2.92 3.12 2 101213 1281415 88416 186 708017 1510 11800 884 18100 227 15600 2020 25400 1560 957 21200 283 30500 2780 1640 2120 26000 37300 328 3430 2200 2980 31300 42700 4170 3040 375 37300 3730 48200 4990 4610027 3760 53900 4580 424 6220 62000 261 4570 5550 495 1035 172 5480 14.9 7020 36.02 259 21.01 97.48 6850 .3 6.25 16.2 145.96 23.62 18.65 324 .20 1.96 148.17 12.0 7.98 87.83 393.39 2.10 .11 40.33 225.04 .37 414 2.79 9.17 2.10 137.37 2.11 2.97 .33 2.80 30.50 2.80 .26 487 1 2.08 138.34 2.97 2.97 2.11 2.78 1 .14 1 2.80 2.95 564 2.10 2.97 1 2.79 1 648 2.97 1 768 2.0 2.60 38.00 100.05 .28 2.16 2.83 3.01 1 25 28 26 Map (fig. 1) number identification identification Table 2. Table [Q, recurrence interval flood discharge for years indicated; DA, drainage area; L, channel length; CSLOPE, channel slope; BSLOP years indicated; REG,region; nc, flood-frequency estimates werecomputed not becausethe siteless hasthan 10 years of peak- periodsofregulated or channelized siteexcluded flows;r, from regional analysis because flowswereaffected by regulation or
Tables 31 REG 100 CF 25 CF 2 CF ) 2 SHAPE (DA/L (ft/mi) BSLOPE (ft/mi) CSLOPE L (mi) —Continued E, basinslope; SHAPE, climate factor basin shape; for CF, recurrence interval ) flow record; duplicate *, mapidentification number for siteshaving separate 2 channelization; n.a.,data not available] DA (mi 500 Q 200 Q 100 Q 50 Q 25 Q 10 Q 5 Q n.a. n.a. n.a.n.a. n.a. n.a. n.a. n.a. n.a. n.a.n.a. n.a. n.a. n.a. n.a.n.a. n.a. n.a. n.a. 108 n.a. n.a. n.a. 24.98 n.a. n.a.n.a. 660 2.45 n.a.n.a. n.a. n.a. 9.75 76.62n.a. n.a. n.a. .19n.a. n.a. 4.14 n.a. n.a. 2.36 90.51 n.a. n.a. n.a. 2.98 n.a. .11 n.a. n.a. 3.16 2.21 n.a. 13.4 n.a. n.a. 2 2.87 n.a. n.a. 3.05 7.79 n.a. n.a. n.a. 1 24.43 440 n.a. n.a. n.a. 81.63 n.a. 53.86 .22 n.a. n.a. n.a. 2.18 n.a. 3.65 2.85 n.a. n.a. 82.13 3.03 n.a. n.a. .16 2 11.0 45.0 n.a. 2.24 n.a. 2.89 10.82 9.76 7.5 3.08 7.5 3.64 2 2.24 4.90 12.34 4.90 3.45 9.66 .38 9.66 .13 2.32 13.23 13.23 2.26 2.94 .34 3.12 2.91 .34 2.32 2 2.32 3.10 2.94 2.94 2 3.12 3.12 2 2 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 14.1 6.11 26.95 79.86 .38 2.15 2.82 2.99 1 2 Q 7530 100008570 11200 11300 12400 12400 13100 13400 13700 13900 14100 14300 14600 14500 777 14800 95.12 925 3.811460 102.35 84.58 2260 .09 3.42 2.25 2690 80.30 2.90 .09 3.09 3110 2.25 2 3370 2.90 3.09 3570 2 3740 3920 45.0 10.82 3.64 12.34 .38 2.32 2.94 3.12 2 r Recurrence interval discharges and basin characteristics for gaged rural sites in North Carolina North in sites rural gaged for characteristics basin and discharges interval Recurrence r,nc r r nc nc nc nc nc nc nc nc 41 43 57* 32638101012901690202023802780335017.16.765.839.420.422.272.913.102 34353637 4990 35139 8160 591040909162022303150396048805930753064.819.854.8344.34.172.252.903.091 1180 10700 688 9120 1980 14300 6720 11500 993 17400 9720 15000 2620 1480 20700 11900 17800 357047 24400 14800 20800 194048 4370 2990049 17100 24000 2470 5260 19600 28700 182051 167 3100 116052 22100 3000 6260 2440 42753 2180 30.12 4090 2580054 3990 4590 3930 774055812125015702030241028103240387078.116.592.148.86.262.272.913.102 11.33 52.26 3070 7460 701 5230 211 77.62 5280 3.3 47.8 387 7.45 13900 4490 .18 9720 7020 423 87.60 6430 20300 93.14 14.33 2.17 5.41 809 5760 13000 .16 2.84 8540 24900 14.77 4.17 35.92 7690 61759672116015702180271033103980500029.09.753.4316.82.332.282.913.112 15700 2.19 1210 3.02 7240 104.99 10200 88.94 93.74 3110060 2.85 1 9090 18800 .22 93461 .09 .23 1880 12100 3.03 8960 3610062 2.18 11200 22100 2.25 2.19 1 1480063 11600 1230 2510 41300 2.84 2.90 2.85 27000 243 166 3.02 3.09 3.03 177 46900 102 1580 3280 45.0 1 1 1 526 446 397 54800 40.80 624 192 1990 4210 30.93 11.36 752 57.85 2183 5.81 1150 518 11.65 6.85 2660 5710 270 87.37 3.97 85.77 148.95 76.60 999 1610 695 .10 77.03 .36 .19 11.7 393 9.4 2.63 1360 2.24 2330 .16 2.17 2.24 843 2.89 66.04 2.24 2.84 5.95 2.89 5.22 503 1680 2970 3.08 2.89 3.02 3.08 .10 1010 15.08 4.60 2 3.08 2 2 2.26 2030 3710 631 36.16 2 10.38 1190 2.90 2410 .37 4560 .36 3.09 779 1450 2.23 2.25 2 3000 5880 2.88 2.89 1010 3.07 3.09 9.6 26.0 32.2 2 2 1.5 4.76 9.15 7.85 2.37 8.21 2.07 3.90 5.55 8.18 16.80 5.16 .43 .28 3.21 .53 2.32 2.37 .29 2.32 2.95 2.98 2.32 2.95 3.14 3.17 2.95 3.14 2 2 3.14 2 2 424445 56.2 7690 115 266 10100 170 11700 399 13800 262 497 15400 348 17000 634 18600 451 744 20800 574 862 930 772 987 104.89 1170 3.71 .3 79.30 8.6 .09 .53 2.25 98.69 5.49 2.90 39.08 10.82 3.09 .76 2 37.10 2.20 .29 2.86 2.26 3.04 2.90 2 3.09 2 33 38 50 56 57 64 46 58 58* Map (fig. 1) (fig. number identification identification years indicated; REG, region; nc,flood-frequency estimates were not computedbecause the sitehasthan less 10 yearsofpeak- periods of regulatedor channelizedsiteflows; excluded r, from regional analysisbecause flows were affected by regulation or Table 2. Table recurrence[Q, interval flood discharge for yearsindicated; drainage DA, area; L, channel length;CSLOPE, channel slope; BSLOP
32 Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina REG 100 CF 25 CF 2 CF ) 2 SHAPE (DA/L (ft/mi) BSLOPE (ft/mi) CSLOPE L (mi) —Continued E, basin slope; SHAPE, basin shape; CF, climate factorrecurrence for interval basinE, slope;basin SHAPE, shape; CF, ) flow record; *, duplicate map identification numberfor sites having separate 2 channelization; n.a., data not available] DA (mi 2692 203.19 2.10 63.32 .07 2.33 2.94 3.11 2 500 Q 200 Q 100 Q 50 Q 25 Q 10 Q 5 Q n.a. n.a. n.a. n.a. n.a. n.a.n.a. n.a. n.a. n.a. n.a. 21.9 n.a. 12.15 14.29 n.a.n.a. 84.86 .15 n.a. n.a. 2.17 2.84 n.a. n.a. 3.01 1 n.a. n.a. 35.8 n.a. 11.55 13.54 n.a. 89.20 n.a. .27 2.24 n.a. 2.87 3.04 1 .0 15.33 4.61 19.67 .25 2.32 2.93 3.10 2 2 Q 6650 10200 12400 15000 16800 184004820 19900 59007090 21700 6620 10000 1687630 12300 7540 10300 15600 29.13 8230 11700 11.83 18300 13000 8930 82.17 21300 13700 .20 9640 24600 2.17 14300 10600 29600 2.84 14700 3.01 1150 772 1 15200 94.85 64.87 1206 4.90 7.08 108.53 87.62 87.30 4.37 .13 .18 86.24 2.25 2.19 2.88 2.85 .10 3.05 3.03 2.26 1 1 2.88 3.05 1 10200 1570010800 19700 15700 25100 18900 29500 22800 34000 25500 38900 28200 45700 30900 2399 34200 169.34 2.78 68.54 .08 2.31 2.93 3.09 2 r r r r r Recurrence interval discharges and basin characteristics for gaged rural sites in North Carolina North in sites rural gaged for characteristics basin and discharges interval Recurrence r nc nc nc 72 79* 92* 94* 81* 82* 7576777879 8530 13300 864 696 16800 1330 108 7020 827 21900 1670 9710 195 26100 908 11600 2160 267 30600 14100 1010 256085 35500 1600086 379 1080 42600 2980 180008788 478 1150 20100 3440 535 53689 141090 23000 590 121091 4110 980 2620 580 47.55 14092 772 1300 1120 3660 1360 483 9.18 718 10.1 163093 297 2290 83.44 1020 64.87 1610 529094 1950 2620 13.8 12700 5.01 914 3490 .24 447 7.08 1540 18400 6750 2380 2480 16.67 3390 2.18 7.04 4380 87.30 13500 2.84 22400 91.21 2410 701 8440 3100 .7 3090 4510 15.49 .18 19800 3.02 .40 5620 27900 100.58 2.19 10400 1 3230 3930 5440 2.18 3780 24400 1.06 944 2.85 .28 32200 6630 13400 2.84 3.03 98.62 4240 30600 2.18 4910 6460 4850 1240 3.02 36800 1 100.04 7700 2.84 35500 83.5 1 5450 7570 6450 41600 1600 .60 3.02 8860 8.2 40800 22.16 2.18 1 7430 48400 9220 2180 10500 27.9 2.85 46400 9.08 6.53 2399 3.02 191 81.46 54300 232 23.77 7.6 9.77 2.6 1 169.34 .17 82.99 11.67 2692 39.18 49.28 2.25 5.36 3.45 .19 2.78 46.15 2.88 5.87 203.19 21.29 2.23 34.25 5.32 .29 68.54 3.04 56.06 2.87 45.76 80.30 50.49 2.26 2.10 1 .08 .12 3.03 .27 2.88 .23 .09 2.31 63.32 2.25 1 2.25 3.05 2.26 2.26 2.93 2.89 .07 2.89 2 2.88 2.89 3.09 3.06 2.33 3.06 3.04 3.06 2 2 2.94 2 2 2 3.11 2 6566676869 276070 4340 87.0 476071 157 165 8450 3440 555073 1150074 6770 274 6160 233 7270 10800 338 16200 8460 371 2210 8690 13800 20300 340 606 12000 3970 10200 18200 25000 517 15100 438 833 11900 21900 30300 5470 18600 64480 14400 1180 25900 38400 55181 7760 22600 30200 787 1490 14182 66.0 28800 9790 682 36600 1840 49.5 9720 15.62 949 12100 33.5383 78.2 888 13200 14984 11.78 85.2 2240 8870 14700 11.00 1200 15700 18.92 81.54 11500 92.00 18800 115 24.50 2850 0.27 1380 15.26 .8 18800 .13 13200 1.0 2.17 12.08 2.17 2100 76.96 159 43.0 21300 2.84 15400 79.98 4.8 1.11 2.84 .22 1.34 3.01 2640 126.53 23800 14.25 3.01 .25 17100 197 2.17 1 39.06 5.13 135.53 1 2.17 17.05 26400 2.84 3380 18700 63.54 31.73 2.84 .72 239 3.01 85.63 30000 112.77 .52 3.01 20400 2.17 3980 1 .21 2.17 .18 1 2.84 1150 287 22800 2.17 4630 2.84 2.17 3.01 2.84 3.01 2.84 1206 1 94.85 359 3.02 5330 1 3.01 1 4.90 108.53 1 6330 87.62 .3 4.37 .13 19.5 86.24 2.25 .73 .10 2.88 7.18 75.75 2.26 3.05 21.38 111.31 2.88 1 .48 89.93 3.05 2.18 .37 1 2.84 2.23 3.02 2.87 1 3.03 1 Map (fig. 1) number identification identification Table 2. Table [Q, recurrence interval flood discharge for years indicated; DA, drainage area; L, channel length; CSLOPE, channel slope; BSLOP years indicated; REG,region; nc, flood-frequency estimates werecomputed not becausethe siteless hasthan 10 years of peak- periodsofregulated or channelized siteexcluded flows;r, from regional analysis because flowswereaffected by regulation or
Tables 33 REG 100 CF 25 CF 2 CF ) 2 SHAPE (DA/L (ft/mi) BSLOPE (ft/mi) CSLOPE L (mi) —Continued E, basinslope; SHAPE, climate factor basin shape; for CF, recurrence interval ) flow record; duplicate *, mapidentification number for siteshaving separate 2 channelization; n.a.,data not available] DA (mi 500 Q 200 Q 100 Q 50 Q 25 Q 10 Q 5 Q n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 1.9 3.15n.a. 13.58 n.a. 10.80 .23 n.a. 2.30 2.92 n.a. 3.10 2 n.a. n.a. n.a. n.a. 188 42.64 5.17 90.27 .10 2.12 2.81 2.98 1 2 Q 2160 3250 3900 4620 5090 5520 5900 6350 161 28.02 6.14 55.52 .20 2.25 2.90 3.09 2 r Recurrence interval discharges and basin characteristics for gaged rural sites in North Carolina North in sites rural gaged for characteristics basin and discharges interval Recurrence nc nc 95* 95 1770 2870 3730 4980 6030 7180 8450 10300 161 28.02 6.14 55.52 0.20 2.25 2.90 3.09 2 96979899 170 121 126 293 252 388 258 394 658 375 381 544 876 580 586 674 1200 774 779 820 1480 1010 1010 1790 984 1290 1290 2130 1230 1740 1730 2650 2.8 2.1 2.6 18.6 3.74 2.12 10.51 2.51 11.87 15.71 17.67 7.48 30.38 20.07 38.55 .21 8.17 .43 2.28 .16 2.29 .44 2.91 2.30 2.92 2.29 3.09 2.92 3.10 2.92 2 3.09 2 3.10 2 2 Map 111 1770 2910 3820 5130 6250 7480 8840 10900 168 32.46 2.04 17.33 .16 2.34 2.94 3.11 2 110112113 219114115 22911615802760375052406530800096501220074.515.744.0724.27.332.352.953.112 391 143117118 434 613 537 368 123119 1310 614 760 108 264 617 772 1980 898 214 467 957 1090 1570 401 3120 1160 757 1180 309 1580 2320 633 4210 1450 1440 2230 3550 464 984 5550 857 1800 1830 4700 3070 1310 7160 607 1130 2340 6080 4550 1590 9810 2.5 1460 776 7720 6.3 1890 2010 3.23 3.3 53.3 975 10400 3.77 2230 5.47 15.10 3.60 1290 11.66 4.9 26.9 11.12 2720 2.22 5.93 .44 7.75 10.28 2.55 1.0 2.34 5.60 .46 2.05 26.3 10.28 6.16 2.94 2.33 .26 .28 1.21 15.94 12.57 3.11 22.31 2.94 2.36 2.39 31.68 2 .71 3.11 10.60 .25 2.96 2.99 2.40 22.23 2 2.40 3.13 85.79 3.17 2.99 2.97 .38 2 2 .17 3.15 3.11 2.35 2.11 2 2 2.95 2.80 3.11 2.97 2 1 1011100175022502970357042204940599080.422.824.0328.29.152.332.953.132 1021031041430215026903430403046805370636093.318.493.0817.46.282.322.933.112 1053397101060165022102880370050104.93.717.9021.89.352.332.943.112 106 70.0 3960107 124108 6380109 502 8280 192 1910 1000 11000 516 3090 309 877 13300 1140 1460 4000 1600 15800 423 1760 2200 5320 18500 2210 564 2840 2890 22600 6420 3160 3880 3710 737120 7630 733 4010121 5170 4680 1020122 8950 71.80123 4980 6760 1690 6230 10900124 2.84 1.5 6090125 3030 9390 888 35.09 182126 27.0 669127 2.11 7800 4160 .14 1620 1670128 24.0 27.24 43.56 1130 8.46 2.32 2120 11400 5910 2910 33.6 2260 2.93 39.01 2.11 3610 6.29 5.17 18200 1510 1250 3.11 7440 .34 3930 3710 3240 8.56 11.26 23400 2 9.57 2320 3.48 4820 2.32 2070 5330 .41 9190 5470 .25 30900 2.93 16.11 2.56 4110 3250 6610 2.33 2550 2.33 3.10 11200 .57 6490 37200 6810 6.86 2.94 5110 2.94 2 4700 2.33 8160 14300 3090 3.12 44000 .51 3.12 8050 2.94 8320 6250 2 2.34 5990 2 9890 51500 168 3.12 3690 10000 2.95 9280 2 8020 62400 11800 7480 3.12 12600 4590 10600 32.93 14700 2 606 9200 20.6 11900 6.66 33.6 15.9 11900 37.1 13900 89.48 52.96 9.17 13.47 7.48 .16 12.83 19.56 14.4 6.84 12.21 116 2.12 21.30 12.47 88.69 85.86 74.59 2.80 81.00 6.27 73.20 20.29 .25 2.98 .22 .19 .29 21.48 .23 1 2.11 2.14 2.12 12.27 2.12 2.12 86.92 2.80 2.81 2.80 88.54 2.80 2.80 2.98 2.97 2.98 .37 .28 2.98 2.98 1 1 1 2.17 2.13 1 1 2.84 2.81 3.02 2.98 1 1 100 (fig. 1) (fig. number identification identification years indicated; REG, region; nc,flood-frequency estimates were not computedbecause the sitehasthan less 10 yearsofpeak- periods of regulatedor channelizedsiteflows; excluded r, from regional analysisbecause flows were affected by regulation or [Q, recurrence[Q, interval flood discharge for yearsindicated; drainage DA, area; L, channel length;CSLOPE, channel slope; BSLOP Table 2. Table
34 Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina REG 100 CF 25 CF 2 CF ) 2 SHAPE (DA/L (ft/mi) BSLOPE (ft/mi) CSLOPE L (mi) —Continued E, basin slope; SHAPE, basin shape; CF, climate factorrecurrence for interval basinE, slope;basin SHAPE, shape; CF, ) flow record; *, duplicate map identification numberfor sites having separate 2 channelization; n.a., data not available] DA (mi 500 Q 200 Q 100 Q 50 Q 25 Q 10 Q 5 Q n.a. n.a. n.a. n.a.n.a. n.a. n.a. n.a.n.a. n.a. n.a. n.a. n.a. n.a. n.a.n.a. n.a. 7.5 n.a. n.a. n.a. 4.66n.a. n.a. n.a. 25.70 n.a. n.a. 94.32 n.a. n.a. .35 n.a. n.a. 2.18 1689 n.a. n.a. 2.83 2.99 n.a. 91.01 n.a. n.a. 1 6.68 16.7 n.a. n.a. 87.29 9.61 n.a. .20 n.a. 19.55 .00 79.42 n.a. 7.4 .22 .18 .86 6.09 2.17 n.a. 1 2.82 22.99 76.3 2.99 69.92 1 15.61 .21 2.18 9.64 2.83 103.60 3.00 .31 1 2.23 2.87 3.03 1 2 746 1470 2170 3380 4570 6070 7930 11100 76.3 15.61 9.64 103.60 .31 2.23 2.87 3.03 1 Q 14800 17000 17600 18000 18200 18200 18300 18300 1689 91.01 6.6828700 87.29 36300 .20 41100 2.22 46700 2.86 50800 3.02 1 54700 58600 63600 3464 119.72 6.22 93.60 .24 2.25 2.87 3.04 1 r r Recurrence interval discharges and basin characteristics for gaged rural sites in North Carolina North in sites rural gaged for characteristics basin and discharges interval Recurrence r nc nc nc nc nc Map 138 152* 153* 129 222 566 944 1650 2390 3360 4610 6800 5.0 4.16 22.72 65.39 0.22 2.13 2.81 2.98 1 131132133134 1810 25000135136 2860 36600 175137 2320 45000 3660 1310 312 4030 56500 828140 4790 2260 3860141 65600 429 5450 1250142 5610 5730 3050 75300 7570 1560 607 1590 6860144 6760 4240 85600 1640145 9410 2600 4750 1980 100000 8570146 763 2760 7870 5260 11500147 7280 3390 1275 12100 2330 9930 3650 9490 6420 13800 942 17400 6470 9180149 4540 11400 2700 82.36 11800 17300 520150 21200 4970 7720 1140 11900 33.7 141151 12900 5500 16300 6.20 3100 26300 860 14000 6100 75.9 9700 1460 15000 1090 265 9.75 85.28 23400 6570 30400 2920 16400 3670 1130 22000 21.38153 7350 2040 .19 18.51 285 29600 21.1 34600 4480 18900 373 1.1 29700 7740 18.18 2.21 93.90154 1520 23.6 8740 36800 2880 39100 2.85 22500155 93.48 35000 34.81 8.79 5650 .35 42400 9480 1.93 542156 3.01 10800 45100 1860 11.69 .17 45400 4190 2.18 11.44 57400 11.61 42000 125 40.95 1 7290157 2.21 16.49 2.83 57800 91.08 32.1 145 78.16 2230158 695 67700 47300 14.8 349 5380 95.44 2.85 2.99 90.08159 8630 .23 91.8 23.34 .27 3.01 137 856 81200 .41160 52800 1 229 2640 9.71 .19 2.22 6750 871 2.21 168 6.54 10.90 10100 1 161 2770 45.45 2.19 1480 2.86 2.22 91600 58500 15.85 2.85 83.89 121 18.72 3250 18.60 2.83 11600 3700 294 8340 1080 3600 3.02 2.86 8.79 102000 3.02 235 66.01 46100 66300 3.00 2010 .23 79.92 17.28 1 3.02 13900 1 113000 199 10800 94.27 5560 63500 .34 4340 1 2.17 1390 175 387 1434 .34 1 3.0 90.65 128000 341 2810 .17 2.16 2.82 75500 2.12 7070 43.2 .40 5170 262 15.5 2.18 2.82 2.99 319 3464 115.73 2.80 465 2.38 3510 2.20 91400 436 2.83 1 2.99 .9 16.66 9220 2.98 5810 36.12 5.50 2.84 8.06 119.72 355 3.00 1 104000 442 4300 1 550 11000 9.64 3.01 1.60 546 70.12 91.10 1 24.42 116000 6480 6.22 1 61.78 434 13000 5200 .11 .48 68.37 631 85.61 130000 644 93.60 7160 675 .15 2.23 2.19 91.52 148000 15100 .24 6590 .24 523 2.19 2.86 2.84 799 2.19 .32 782 8110 4395 2.25 18200 2.83 877 3.02 3.00 2.83 2.24 2.87 3.00 32.4 1 623 1 991 3.00 348 2.87 156.35 460 3.04 1 7.6 2.2 1 3.03 11.10 1 1210 772 5.15 1 40.74 57.53 17.11 5.87 2.10 89.44 1550 86.34 7.27 42.55 5.33 63.92 7.6 .18 .27 84.77 86.78 80.68 88.21 2.26 2.24 7.9 4.37 .21 .22 .14 2.88 .48 2.87 2.25 2.25 2.26 3.04 17.00 2.21 3.03 4.81 2.87 2.87 2.88 2.84 2 77.76 3 3.03 3.03 3.04 3.01 7.27 .39 3 3 3 3 23.11 2.25 .43 2.87 2.26 3.04 2.88 3 3.04 2 130 143 148 152 139 (fig. 1) number identification identification Table 2. Table [Q, recurrence interval flood discharge for years indicated; DA, drainage area; L, channel length; CSLOPE, channel slope; BSLOP years indicated; REG,region; nc, flood-frequency estimates werecomputed not becausethe siteless hasthan 10 years of peak- periodsofregulated or channelized siteexcluded flows;r, from regional analysis because flowswereaffected by regulation or
Tables 35 REG 100 CF 25 CF 2 CF ) 2 SHAPE (DA/L (ft/mi) BSLOPE (ft/mi) CSLOPE L (mi) —Continued E, basinslope; SHAPE, climate factor basin shape; for CF, recurrence interval ) flow record; duplicate *, mapidentification number for siteshaving separate 2 channelization; n.a.,data not available] DA (mi 500 Q 200 Q 100 Q 50 Q 25 Q 10 Q 5 Q 2 Q 7340 9530 11000 12700 14100 15400 16700 18500 504 48.21 21.29 243.78 .22 2.13 2.79 2.96 1 28500 33500 3620025100 39100 33900 41000 39800 42700 47400 44200 53100 46100 58800 4852 64700 176.67 72600 4.67 5255 86.19 229.76 .16 3.55 2.30 2.90 81.22 3.05 .10 2 2.38 2.95 3.09 2 r r r Recurrence interval discharges and basin characteristics for gaged rural sites in North Carolina North in sites rural gaged for characteristics basin and discharges interval Recurrence Map 163* 166* 192* 193 6200 12000 17100 25300 32800 41600 51800 67900 128 20.23 33.91 254.90 .31 2.09 2.77 2.95 1 162163164165 2090 36500166 3600 45500 148167 51300 4870 494168 3250016939276110901620211026803350441015.75.786.9919.12.452.332.923.072 386 58500 1010 6790 47400170 63800 652171 651 58200 1490 8470 862172 69100 1200173 72900 10400 1150 2280 1480174 74400 56.8 1670 4150 12600 84500 1680175 3020 1990 81500 561176 6870 15900 96.8 1970 96900 2390177 2380 4852 3900 2750 1120 488 11000017810802110304045305900752094201240049.715.464.8424.90.222.382.953.092 2990 129 9040 284 3040179 3280 128000 4960 176.67 3400 30.8 1630 732 12200 3750 939180 178 3790 5255 39.33 140181 4880 68.4 4.67 6670 14900 4140 2460 1590 4800182 912 4830 4640 5.14 229.76 219183 86.19 311 106 17900 14.1 4960 3230 2110 1510184 5660 7620 60.1 1160 66.05 .16 5970 3.55 21200 279 0.18185 266 480 2600 170 2.30 6210 4140 2880 7.56 19.22 6580 127 9760 136018641579511301670216027303400444016.05.024.36.72.662.352.933.082 81.22 2.29 26100 2.90 330187 345 12.15 21.6 3490 2.89 12800 318 3.19 .10 5220 774 410 3550188 232 7570 330 3.05 1580 92.8 24.86 676 3.05189 3850 2.38 734 11.61 2 15300 4830 387 7.51 1060 700 6950 3 4290 .30 8990 24.69 1810 396 2.95190 310 554 6080 .16 18100 149 1140 52.96 2.31191 5980 8.09 3.09 1400 2.32 1420 4.25 439 5110 379192 2140 936 2.90 32.3 4080 7800 2 21100 405 978 12.90 2.90 2.15 322 1830 2750 22.86 3.05 7280 .5 1850 6350 3.05 10300 103 8160 11.64 1290 .38 25400 478 459 31.48 65.20 2 .16 1430 3610 2520 562 2 12800 3970 8730 2.39 490 2580 .24 12300 2.29 11900 7.19 .86 1590 2.72 599 770 47.5 5980 2.95 29.39 21000 516 2010 2.33 10900 2.89 3360 14500 5940 23.35 18000 25.51 3.09 776 8.83 2.91 1930 3.04 10.83 27400 1.79 .6 1020 7870 8.6 2770 47.76 .25 2 .09 16900 554 4400 41.15 23700 3.05 2 69.3 7760 1050 36900 2.28 2.32 5.65 2300 6.03 10600 2 1390 .58 2.47 1.41 20400 4100 4.84 30400 6130 2.89 13.50 2.90 605 .12 18.88 9910 44800 2.33 1390 13000 31.94 26.63 2870 19.02 3.04 3.05 38400 1710 2.38 680 2.92 .41 5.64 12400 53500 10.32 .27 2 2 36.42 15500 1.1 10.2 2.95 1800 3.07 2.32 51200 7.8 26.50 15.3 2.34 2060 .33 3.09 16500 .36 63200 2 38.60 2.93 18400 .37 2470 2.93 1.27 2.32 4.40 2 2.33 3.09 5.35 77600 2450 4.35 2.37 48.1 3.09 22700 2.93 2.93 .87 28.8 2 3.71 6.96 2.94 2 3.09 3.10 4.79 5.03 3.8 20.64 3030 504 3.08 2 7.67 15.93 2 89.2 7.38 12.77 .81 2 48.76 6.70 .47 3.57 89.91 .49 .29 .70 314.89 48.21 11.0 20.61 .75 2.33 323.81 2.40 2.38 2.39 9.32 .12 21.29 2.32 32.65 2.92 2.96 2.94 2.95 .12 6.66 243.78 2.14 2.91 17.28 265.44 3.07 3.10 3.08 3.10 2.15 386.26 3.06 2.78 2 .30 .22 .20 2 2 2 2.78 325.50 2 2.95 2.32 2.13 2.13 2.94 .26 1 2.91 2.79 2.79 1 2.11 3.06 2.96 2.96 2.77 1 1 1 2.95 1 (fig. 1) (fig. number identification identification Table 2. Table years indicated; REG, region; nc,flood-frequency estimates were not computedbecause the sitehasthan less 10 yearsofpeak- periods of regulatedor channelizedsiteflows; excluded r, from regional analysisbecause flows were affected by regulation or [Q, recurrence[Q, interval flood discharge for yearsindicated; drainage DA, area; L, channel length;CSLOPE, channel slope; BSLOP
36 Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina REG 100 CF 25 CF 2 CF ) 2 SHAPE (DA/L (ft/mi) BSLOPE (ft/mi) CSLOPE L (mi) —Continued E, basin slope; SHAPE, basin shape; CF, climate factorrecurrence for interval basinE, slope;basin SHAPE, shape; CF, ) flow record; *, duplicate map identification numberfor sites having separate 2 channelization; n.a., data not available] DA (mi 500 Q 200 Q 100 Q 50 Q 25 Q 10 Q 5 Q n.a.n.a. n.a.n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 10.2 n.a. 27.8 6.32 6.8 10.14 28.92 19.34 6.67 66.74 69.36 31.49 .28 .27 2.10 60.00 2.10 2.79 .16 2.79 2.97 2.12 2.97 1 2.80 1 2.98 1 2 63.2 68.3 70.9 73.6 75.3 76.9 78.2 79.9 4.8 4.56 29.75 52.42 .23 2.15 2.80 2.97 1 Q 1490 1830 2050 2310 2490 2680 2860 3100 87.4 33.20 11.75 91.45 .08 2.16 2.80 2.98 1 16200 21900 25000 2830028000 35300 30400 39200 3220039200 43300 33800 53100 45800 35700 59000 48100 869 63800 50100 66100 70.50 52500 67700 12.10 1226 218.10 68800 .17 69800 93.1332700 2.08 46400 1694 2.77 6.98 208.67 2.95 54600 110.42 1 .14 64100 5.25 2.20 70700 193.78 2.82 2.99 .14 76700 1 2.10 82400 2.78 89400 2.96 1 2280 149.50 4.38 169.42 .10 2.15 2.80 2.98 1 r r Recurrence interval discharges and basin characteristics for gaged rural sites in North Carolina North in sites rural gaged for characteristics basin and discharges interval Recurrence r r nc nc nc r r Map 195 200 223* 211 210215* 400 568 835 1080 1360 1690 2210 2.2 1.69 47.86 66.44 .73 2.12 2.80 2.98 1 224225226 2770 557 53000 4040 79300 1030 4970 98800 1450 6240 126000 2090 147000 7240 170000 2670 195000 8310 3340 230000 9440 3450 4100 11000 170.84 5300 118 3.90 3.9 170.74 16.59 .12 12.07 3.17 2.16 96.08 28.98 2.81 .43 99.43 2.98 2.16 .36 1 2.81 2.16 2.98 2.81 1 2.98 1 194 3250 4640 5620 6950 7990 9080 10200 11800 35.5 14.74 32.24 168.32 0.16 2.08 2.77 2.95 1 223 1430 2080 2550 3190 3700 4240 4810 5620 87.4 33.20 11.75 91.45 .08 2.16 2.80 2.98 1 196197198199 3220 1800201 5280 4710202 2650203 5490 6530 6900 3260 9100 6690205 7810 9260 7640 11200206 12000 4100 3220 13100207 11200 9500 14900 16200 5390 4770 17700 1150 13400 10800 20200 19800 215 24400 7130 1700 5480 15800 1570 12200 24800 23800 30200212 356 19400 2900 9690 13600 2100 6240 29900213 28200 36700 11900 15600214 4040 35600 469 2660 34800 7310 78.8215 44100 2650 14300 44000 109 5820 4200 22.18 3100 128 635 55100216 3280 44.7 17000217 1240 7030 31.52 231 30400 7410 27.61 3570 21000 15.33 287 3680 281.93 30.68218 775 1940 43700 9310 14.56219 9240 27.18 28.32 .16 13.23 4070220 202.20 4180 53300 93.0 2480 12700 42.8 31.47 257.85 930 198.38 2.08 225221 19.16 11300 .15 4790 180 2640 66300 17.57 2.77 .21 15500 4550 173.69 .13 13.55 3240 1100 4020 2.08 14600 315 165.49 2.95 2.08 2.08 4230 18700 5040 .29 76500 22.17 2.77 4910 258 22.1 1 6590 2.77 3860 6990 .29 2.77 1360 2.08 145.31 2.95 22200 7600 87300 379 19.5 5470 2.95 2.08 10200 2.95 2.77 5280 11.48 1 .25 8620 4540 383 27500 1 2.77 98700 1 2.95 9490 12500 2.09 8.29 7240 21.49 463 .9 2.95 11600 5760 115000 1 5280 105.01 2.78 186 12100 38.11 498 15600 1 8710 14000 2.95 2280 134.61 529 1.13 .17 6350 14200 65.6 18100 1 25.82 10300 2.09 16800 632 .26 85.26 149.50 16500 20800 16.39 10.45 2.78 597 2.14 12100 42.9 88.49 19800 2.96 90.93 13.47 4.38 789 2.80 18900 23600 .64 14700 24300 14.12 1 169.42 669 2.97 .27 86.79 2.09 22400 27600 1040 15.63 2.13 1 .10 2.78 .24 101 306 768 2.80 2.15 92.00 155 2.95 2.12 569 2.98 2.80 1.0 1 2.80 .21 34.64 48.80 1 2.98 2.98 2.12 30.06 1.6 56.83 18.90 11.36 1.60 1 1 2.80 13.21 138.79 10.23 120.89 2.98 2.80 56.18 148.05 125.33 .08 .13 1 100.24 78.01 .17 .17 2.15 2.15 132.30 .40 2.15 2.16 2.80 2.80 2.15 .19 2.80 2.80 2.97 2.98 2.80 2.15 2.97 2.98 1 1 2.98 2.80 1 1 1 2.97 1 208 209 210 204 222 (fig. 1) number identification identification Table 2. Table [Q, recurrence interval flood discharge for years indicated; DA, drainage area; L, channel length; CSLOPE, channel slope; BSLOP years indicated; REG,region; nc, flood-frequency estimates werecomputed not becausethe siteless hasthan 10 years of peak- periodsofregulated or channelized siteexcluded flows;r, from regional analysis because flowswereaffected by regulation or
Tables 37 REG 100 CF 25 CF 2 CF ) 2 SHAPE (DA/L (ft/mi) BSLOPE (ft/mi) CSLOPE L (mi) —Continued E, basinslope; SHAPE, climate factor basin shape; for CF, recurrence interval ) flow record; duplicate *, mapidentification number for siteshaving separate 2 0.5 1.07 526.60 243.48 .48 2.18 2.78 2.95 1 channelization; n.a.,data not available] DA (mi 500 Q 200 Q 100 Q 50 Q 25 Q 10 Q 5 Q n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 4000n.a. 186.48 n.a. 3.43 160.97 n.a. .11 2.18 n.a. 2.83 3.00 n.a. 1 n.a. n.a. n.a. .1 .37 119.76 58.22 1.45 2.25 2.86 3.03 3 2 Q 41300 51800 58000 65200 70200 74900 79400 85000 400073500 106000 186.48 132000 3.43 170000 160.97 202000 .11 237000 2.18 276000 2.83 335000 3.00 1 6863 255.33 4.59 134.56 .11 2.25 2.86 3.02 1 r Recurrence interval discharges and basin characteristics for gaged rural sites in North Carolina North in sites rural gaged for characteristics basin and discharges interval Recurrence nc nc r Map 230* 227228229230 565 4640231 450232 933 6530233234 685 1230 7880235 850 400 1650 96602361680242029603680426048705510641020.77.0229.6993.64.422.182.812.991 7740 861237 955 11100 10600 426238139023203050413050506070720088808.55.2627.5955.44.282.222.833.001 2020 717 1110 573239 1020 12500 12500240 720 2420 1080 987 4580 1310 14100 1090241 15000 32800 2860242 7000 1520 958 16200 1400 1520 17000243 1140 47100 2220 3520 8810 2220 1310 174 1760 19000245 57300 4140 1750 1190 175 4460 11300246 21100 71000 2840 1610 2180 1030 5810 9.6 29.85 1240247 2090 283 6340 13400 81800 24000248 1760 3570 1950 2650 9.77 845 8440 1300 5.98 15600 7670 93200 368 97.43 6.6 342 2350 1220 10800 17.12 4420 16.9 3380 2330 105000 18000 107 .20 13.7 9460 96.67 490 122000 13500 28.8 6.14 1480 3220 21400 5740 2.17 51.53 2900 0.27 195 10900 1372 2.9 16700 2.82 6.59 16.73 2.15 592 1850 6.63 3980 38.5 2.99 12300 55.6 2.81 21.96 21700 3.6 272 89.50 131.78 3.4 2.80 81.80 1 2.98 2130 86.01 13900 4820 704 52.9 14.09 .17 .13 45.36 110 1 3.55 5.70 .31 391 4.41 2.17 2.19 16000 21.06 113.22 2430 5760 65.2 2.19 84.63 35.95 828 2.82 2.83 72.33 26.68 .37 90.60 2.83 499 .21 106 2750 95.22 173.68 3.00 3.00 7180 2.19 .28 1010 79.1 3.00 7.72 2.23 1 1 .28 2.83 .18 2.20 1 3200 624 2.85 83.54 29.55 2.18 3.00 2.20 15.4 94.5 2.83 3.01 .16 2.81 1 .9 12.14 2.83 3.00 1 769 16.7 118 2.24 2.98 108.15 9.24 3.00 1 2.85 1.06 1 1 .12 47.53 5.91 996 3.01 89.92 142.49 2.20 .1 1 33.41 2.84 65.70 .18 75.65 6.2 3.00 1.03 .73 2.20 .47 114.65 1 2.24 2.84 5.46 2.24 2.85 86.40 3.01 2.86 11.97 3.01 .14 1 3.02 1 23.56 2.25 1 2.86 .19 3.02 2.26 1 2.87 3.03 3 249250251252 361253 1380254255 527 2450 76.4 1440256 466 3370 116257 649 2190 230258 4850259 647 4800 2760 145 816260 344 1130 7590 6250 6080 3560 775 1820 7160 186 950 9700 429 9430 4620 12600 7550 4220 2360 946 12700 1090 11800 57.1 8780 220 17100 547 9240 4940 15200 3150 1080 15100 12500 93.3 1250 23900 257 11900 643 5710 18000 17700 18300 1220 122 3800 29900 1460 183 21000 20600 296 6830 23600 746 36700 1370 4530 163 83.3 25400 23600 29800 365 26.23 44300 353 1570 858 5320 20.22 1228 28000 198 37000 10.26 56000 64.79 1020 6490 7.30 39.8 76.68 48200 130.44 4.7 127 236 172 40.58 4.11 .27 14.30 16.1 2.09 20.7 4.32 2.23 .20 63.07 66.7 20.20 279 23.12 7.65 27.34 2.29 2.85 33.83 .09 6.85 67.00 34.93 8.95 2.89 3.02 24.83 52.06 .07 2.29 66.99 341 299.40 268.53 3.04 3 86.54 299.74 3.83 2.89 2.31 .21 .24 407.99 .31 3 285.37 3.04 2.28 2.90 .32 7.32 2.25 2.18 .26 3 2.88 .05 3.05 2.18 2.86 .36 2.78 2.19 3.04 2.17 3 2.78 3.03 2.30 2.94 2.78 3 2.78 2.94 3 2.89 1 2.94 2.94 1 3.05 1 1 3 244 (fig. 1) (fig. number identification identification years indicated; REG, region; nc,flood-frequency estimates were not computedbecause the sitehasthan less 10 yearsofpeak- periods of regulatedor channelizedsiteflows; excluded r, from regional analysisbecause flows were affected by regulation or Table 2. Table recurrence[Q, interval flood discharge for yearsindicated; drainage DA, area; L, channel length;CSLOPE, channel slope; BSLOP
38 Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina REG 100 CF 25 CF 2 CF ) 2 SHAPE (DA/L (ft/mi) BSLOPE (ft/mi) CSLOPE L (mi) —Continued E, basin slope; SHAPE, basin shape; CF, climate factorrecurrence for interval basinE, slope;basin SHAPE, shape; CF, ) flow record; *, duplicate map identification numberfor sites having separate 2 channelization; n.a., data not available] DA (mi 500 Q 200 Q 100 Q 50 Q 25 Q 10 Q 5 Q 2 Q 3020 7990 13100 22100 30900 41500 54300 74900 97.0 21.78 96.11 394.11 .20 2.19 2.78 2.94 1 20500 39400 58400 92700 128000 174000 233000 338000 1535 99.51 5.45 174.39 .16 2.16 2.80 2.97 1 Recurrence interval discharges and basin characteristics for gaged rural sites in North Carolina North in sites rural gaged for characteristics basin and discharges interval Recurrence r r Map 279280281282 3000283 593284 4740285 4840 16900 964286 6070 7610 25800 3050287 1070 1260288 4880 32600 7970 7050 9730289 1680 10300290 42100 1680 344 12700 6310 9540 104 2150291 12600 5150 49800 15200 2030 11300292 599 8370 1170293 15800 2800 148 8910 58100 5960 17900 13100294 10100 2420 1930 18300 810295 12000 67000 9660 3350 169 20800 15800 179 4150 12000 2850 2530 21000 4690 16700 80000 12600 1130 25100 3930 276 6150 14000 696 79.0 221 23800 3480 7180 3420 20800 2740 16800 875 1400 220 17000 4570 7620 1250 362 27800 19.99 25400 4290 20300 9050 4170 253 13.0 1720 5500 64.54 22.73 9630 42.35 1710 30500 60.5 24200 200 11700 5490 486 295.91 5000 27.19 288 2070 8.05 15.04 11200 38400 28500 20.27 2430 13800 16.4 .20 223.48 212.62 7170 40.97 5920 40.64 590 13000 2600 25.87 2.19 34900 .21 16100 323 205 280.73 .12 3050 13.36 7.85 305.26 8570 2.78 2.23 14800 7280 2.23 212.84 18600 706 .20 277 50.61 2.95 .15 2.81 3770 374 10100 66.19 1.4 2.81 17400 2.19 .12 1 22200 2.19 91.82 2.97 21.8 2.97 10.53 2.79 833 2.21 11700 41.20 4580 2.79 1 .25 2.29 1 314.07 2.95 67.9 2.4 2.80 103 2.96 19.42 14100 2.19 8.58 1020 72.02 1 5830 .05 2.97 386.04 1 2.79 14.21 100.64 74.20 1.98 2.09 1 19.57 2.96 .16 40.4 430.02 159.21 67.39 .30 2.76 3.5 11.7 1 79.36 2.09 286.39 407.60 .29 2.22 2.93 13.36 373.79 2.75 2.09 2.81 .58 .34 4.85 2.32 131.60 1 .27 2.92 2.75 205.76 219.54 2.10 502.95 2.97 2.27 2.26 1 446.05 266.66 2.92 2.76 1 2.80 .22 2.80 1 2.93 2.95 .49 .38 2.25 2.95 1 1 2.26 2.07 2.80 1 2.80 2.77 2.95 2.95 2.94 1 1 1 261262263264265 229 11900266 21200 448 420 947 29000 1510 776 1560 584 40800 2660 855 1050 2040 51200 1430 3620 839 1450 63000 2750 1900 5080 1070 76400 1800 2580 3340 6350 1330 96900 2200 3160 4000 7780 1630 20.0 2640 3810 4730 9410 2090 34.09 3320 4530 5810 11900 20.52 2.4 5600 277.87 9.1 18.4 28.2 .17 3.09 2.17 7.8 6.71 572.66 9.83 8.08 2.78 225.03 31.80 44.78 47.94 4.30 2.95 0.23 227.46 189.99 173.16 1 54.76 2.16 .19 .19 .43 2.78 63.08 2.16 2.15 2.15 2.94 .44 2.78 2.79 2.79 1 2.17 2.95 2.97 2.97 2.80 1 1 1 2.97 1 268269270271 566272 1330273 4990 1010274 2040275 2280 8530 1380 199 2580276 3550 2110 11400277 1410 1950 413 3340 3600 9470 15700 4510 2360 14300 2380 2450 19400 3960 4830 614 5870 3130 3130 17900 3210 23600 3020 4620 4720 6650 7000 22900 950 4250 3770 28200 3670 5340 5910 8220 26900 8210 1270 35200 5210 4510 4660 31200 7550 6380 9980 9520 1650 6280 5070 83.2 35900 12000 8880 11400 16.4 7.2 2110 7470 42500 5640 14900 30.41 10300 7.16 2850 25.7 35.01 9250 4.34 11800 630 6240 69.2 165.65 19.53 31.23 14000 8.74 7050 .09 31.8 80.95 139.86 21.98 76.32 1.0 112.47 2.17 .31 .35 15.60 76.5 223.96 10.79 9.82 2.80 41.8 1.49 2.18 2.16 72.28 .33 20.61 13.19 88.95 2.97 2.81 81.90 2.80 2.18 .14 9.79 1 79.15 .11 2.98 2.98 9.34 43.94 2.18 2.79 2.18 12.37 .26 1 1 75.36 .46 2.80 2.95 2.81 2.19 65.58 2.18 2.98 .44 1 2.98 2.81 .43 2.80 1 2.23 1 2.98 2.23 2.98 2.83 1 2.83 2.99 1 2.99 1 1 267 278 (fig. 1) number identification identification Table 2. Table [Q, recurrence interval flood discharge for years indicated; DA, drainage area; L, channel length; CSLOPE, channel slope; BSLOP years indicated; REG,region; nc, flood-frequency estimates werecomputed not becausethe siteless hasthan 10 years of peak- periodsofregulated or channelized siteexcluded flows;r, from regional analysis because flowswereaffected by regulation or
Tables 39 REG 100 CF 25 CF 2 CF ) 2 SHAPE (DA/L (ft/mi) BSLOPE (ft/mi) CSLOPE L (mi) —Continued E, basinslope; SHAPE, climate factor basin shape; for CF, recurrence interval ) flow record; duplicate *, mapidentification number for siteshaving separate 2 channelization; n.a.,data not available] DA (mi 500 Q 200 Q 100 Q 50 Q 25 Q 10 Q 5 Q n.a. n.a.n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a.n.a. n.a. n.a. n.a. n.a. n.a. n.a. 14.5n.a. n.a. n.a. n.a. 4.76 23.8 503.19 641.91 7.84 n.a. n.a. 282.05 .66 600.92 2.19 n.a. n.a. .39 2.78 2.19 2.94 n.a. n.a. 2.78 1 2.94 n.a. n.a. 1 33.5 n.a. 12.63 213.40 n.a. 568.89 243 .21 2.26 35.84 2.79 79.37 2.95 478.74 1 .19 2.21 2.77 2.92 1 2 Q 2900 4940 6670 9370 11800 146004400 17800 6590 22900 8210 130 10500 12300 23.03 21.11 14200 426.04 16300 .24 19300 2.21 2.78 58.4 2.94 1 17.24 172.62 565.25 .19 2.25 2.79 2.95 1 r Recurrence interval discharges and basin characteristics for gaged rural sites in North Carolina North in sites rural gaged for characteristics basin and discharges interval Recurrence r,nc nc r,nc nc r Map 311312* 246 427 575 799 992 1210 1450 1820 5.5 3.89 574.31 591.79 .36 2.19 2.78 2.94 1 313314315316 15000317 22000318 2060 4100 27200319 19500 3080320 6680 30200 34200 624321 3350 3840 38300 39900 23500 8720323 940 5410 4890 37200 4090 49600 45900 11700324 1180325 7020 47700 5820 58900 52300 5740 14200326 4310 62800 1500 9350 68900 61400 7060 16900 6650 7740 6990 75200 11300 79700 764 11900 1770 19900 945 8720 7620 9090 88800 13400 95300 15000 1110 24400 10000 2050 12100 104000 9010 72.49 15800 1332 19300 1370 11400 158 125000 2350 14700 2.59 19200 22900 79.5 12800 1567 90.72 1710 320.73 17500 2780 24.08 26700 126 14900 19.44 .18 4.09 20500 106.96 47.85 1990 30800 2.20 337.83 34.12 440.28 8.0 25100 28.76 6.30 27.6 2.78 403.22 2280 36800 .16 .27 358.29 2.94 72.67 .21 2.21 5.34 10.24 2.21 51.5 2590 508.25 1 .14 130 2.21 142.20 2.77 259.95 2.77 2.20 .15 400.62 2.77 2.92 565.57 19.40 2.92 3020 2.77 2.20 24.00 2.92 1 155.96 .27 1 .26 125.34 2.92 2.76 569.36 1 2.20 2.26 14.4 533.76 1 2.92 2.77 .14 2.80 1 .22 2.92 2.25 2.95 5.16 2.22 1 2.79 556.54 1 2.77 589.19 2.95 2.93 1 .54 1 2.26 2.80 2.95 1 296297298299300 1510 1670301 2430 561302 2530 7050303 3140 10900 482304 938 3180305 663 13900 4170 2500 717 1240306 4080 1120307 18100 3850 80.0 1460 5030308 1690 890 3270 4810 21600 109309 1500 2420 4870 5970 1820 6380 2070 10700 1130 25300 5590 2050 3190 129 2940 6300 2060 15300 7000 9170 29400 2490 1320 6430 2530 3630 18700 4310 3810 7470 155 13700 35300 8520 2960 1520 7640 23200 3060 4950 5260 17800 5070 8730 296 175 26.8 3660 1740 26800 22700 3650 10100 41.4 6950 6320 6130 30500 11.33 36.20 2050 195 28400 12100 10.9 13.96 4540 8710 7490 7290 37.09 34400 3.88 37400 44.43 239.70 217 10700 14.8 4.91 343.47 66.7 9230 40000 242.70 8570 10.0 0.21 338.78 109 13000 .23 11.44 2.24 .22 27.47 285.36 10400 246 676 2.24 4.78 42.2 2.80 2.24 16400 50.40 .44 50.49 15.41 179.84 2.80 2.96 263.91 2.80 13.01 440.37 63.1 2.24 387.36 61.46 2.95 10.25 1 2.96 .6 79.8 .11 2.80 .09 24.24 1 187.68 .45 15.97 1 2.71 2.23 2.96 226.31 2.23 2.25 13.83 .46 300.03 28.17 2.79 .88 1 2.78 .25 2.79 2.23 57.53 307.71 436.52 2.95 .18 2.94 2.24 2.95 415.17 2.79 199.55 1 2.23 .25 1 2.80 1 2.95 .42 2.78 2.23 .72 2.96 1 2.22 2.94 2.79 2.24 1 2.78 1 2.95 2.80 2.93 1 2.96 1 1 310312 1800 3120 3090 4210 5270 5840 7060 7250 9730 8850 12000 10600 14600 13400 17500327 21900 23.8328 130 7.84 1930 282.05 11100 600.92 23.03 2710 16900 .39 21.11 21200 3250 426.04 2.19 2.78 27200 .24 3980 2.94 2.21 32000 1 2.78 4550 37200 2.94 5140 1 42800 50800 5760 350 6630 42.50 65.3 51.32 17.58 482.01 74.41 .19 510.37 2.21 .21 2.77 2.21 2.92 2.77 1 2.92 1 310* 322 (fig. 1) (fig. number identification identification years indicated; REG, region; nc,flood-frequency estimates were not computedbecause the sitehasthan less 10 yearsofpeak- periods of regulatedor channelizedsiteflows; excluded r, from regional analysisbecause flows were affected by regulation or [Q, recurrence[Q, interval flood discharge for yearsindicated; drainage DA, area; L, channel length;CSLOPE, channel slope; BSLOP Table 2. Table
40 Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina REG 100 CF 25 CF 2 CF ) 2 SHAPE (DA/L (ft/mi) BSLOPE (ft/mi) CSLOPE L (mi) —Continued E, basin slope; SHAPE, basin shape; CF, climate factorrecurrence for interval basinE, slope;basin SHAPE, shape; CF, ) flow record; *, duplicate map identification numberfor sites having separate 2 channelization; n.a., data not available] DA (mi 500 Q 200 Q 100 Q 50 Q 25 Q 10 Q 5 Q n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 143 21.61 112.60 411.88 .31 2.27 2.80 2.95 1 2 975 1600 2160 3070 3920 4950 6200 8250 14.9 7.41 98.65 246.33 .26 2.28 2.81 2.95 1 Q 2800 43703940 5530 7780 7120 115007380 8390 12900 18000 17700 24400 9740 25600 32500 11200 42600 32800 13200 59800 41400 144 51700 143 38.21 68300 21.61 40.37 112.60 347 464.09 411.88 .10 .31 37.92 2.27 2.27 56.29 2.80 2.80 464.22 2.95 2.95 1 1 .24 2.27 2.80 2.95 1 16500 24200 29700 37300 43300 49600 56300 65800 655 57.17 37.57 502.95 .20 2.27 2.79 2.94 1 r r r Recurrence interval discharges and basin characteristics for gaged rural sites in North Carolina North in sites rural gaged for characteristics basin and discharges interval Recurrence nc r r Map 354* 357* 352* 350351 5120 13600353354 7330 21400355 27400 8910356 1460 6860 36000 11000357 2070 9840 5340 43000 12700 8680 2500 12000 7350 50700 16700 14500 11500 3080 26500 14900 59100 16400 8750 13300 34100 17100 71300 3530 19000 10600 15700 44800 19500 12000 17600 4000 664 174 53700 22100 13500 19400 4490 63.31 63300 25600 49.80 15000 21300 5180 42.39 8.43 73800 17100 347 23900 475.41 430.10 89200 51.0 .07 .17 131 184 37.92 2.27 2.27 655 15.75 56.29 2.79 2.79 22.52 167.42 26.94 464.22 2.94 2.94 106.56 57.17 520.46 85.10 .24 1 613.81 1 37.57 .21 589.97 2.27 .26 502.95 2.27 .25 2.80 2.27 2.80 .20 2.27 2.95 2.80 2.95 2.27 2.80 1 2.94 1 2.79 2.94 1 2.94 1 1 329330331332333 1880 368334 2960 2870335 5440336 430 4550 5600 3790337 9290338 9430 146 5860 4970 468 5040 12400 16200339 12500340 224 9150 7720 26400 1060 17100 5940341 513 5990 17100342 12700 2050 34400 21200 9270 10500 283 7000 21000 546 15.7 2290 18100 46100 25700 11000 2930 14200 3350 25400 8140 366 3770 23000 23.5 1950 55900 30800 12800 577 19900 5180 4350 30300 9820 28500 2860 38400 66700 4940 15500 434 24900 29.2 37600 6560 5640 608 34900 78600 3530 30500 49.2 6660 104 43.3 37.1 506 7160 8520 44700 96300 60.8 648 36800 10.68 4430 8110 14.34 10100 37.92 43.5 183.96 8940 585 27.05 157 158.20 608 46500 5150 530.96 34.80 9710 9.2 11800 520.68 50.38 11700 0.43 50.3 410.63 699 39.09 484.24 73.77 .21 11500 92.1 5920 13700 2.21 4.90 .07 2.18 34.98 .08 23.71 23.1 57.6 2.77 14100 127.36 16400 19.38 2.17 6730 531.07 2.78 452.57 2.17 1.6 2.92 313.06 2.78 84.54 2.94 9.70 2.77 68.1 .10 .11 140 1 42.0 7880 .39 2.94 376.69 118.81 1 3.08 2.94 2.17 2.17 2.16 1 398.27 519.11 .24 1 11.66 2.77 2.77 25.26 .5 2.77 57.1 405.77 2.15 104.61 .25 2.93 2.93 2.94 406.56 2.77 6.86 .17 2.15 1 16.11 1 1.72 1 375.26 2.94 2.17 2.77 .31 997.21 31.33 1 2.77 .23 2.94 415.55 2.16 436.73 2.28 2.93 1 2.77 .18 .22 2.80 1 2.93 2.16 2.28 2.95 1 2.77 2.80 1 2.94 2.95 1 1 344345346347348 2940 123 4610 6190 9400 254 8880 2500 5890 13500 10800 3440 16400 7710 378 13400 4100 20400 9220 584 15500 23500 4960 10900 17700 26700 778 12600 5630 19900 30200 1010 15200358 6320 23200 35000359 1290360 86.5 7040 323 436 1740 20.22 19.9 8040 944 34.17 123.81 2310 52.12 36.9 435.46 1290 1.6 5.76 51.9 3640 6.48 .21 394.79 408.05 51.6 1530 16.68 2.64 2.28 4660 .28 .16 491.80 2.80 63.83 2.24 1850 74.8 467.80 2.27 6120 2.95 456.46 2.80 2.80 .25 1 2090 .19 2.96 95.5 7320 2.94 2.27 2.28 1 1 2340 2.80 119 8630 2.80 2.95 2.95 2600 10100 1 147 1 12100 2960 190 44.4 13.8 12.93 .5 8.53 173.28 346.36 606.86 1.78 601.47 595.52 .26 .19 474.30 2.26 2.27 .20 2.79 2.79 2.27 2.93 2.94 2.80 1 1 2.94 1 352 343 349 (fig. 1) number identification identification Table 2. Table [Q, recurrence interval flood discharge for years indicated; DA, drainage area; L, channel length; CSLOPE, channel slope; BSLOP years indicated; REG,region; nc, flood-frequency estimates werecomputed not becausethe siteless hasthan 10 years of peak- periodsofregulated or channelized siteexcluded flows;r, from regional analysis because flowswereaffected by regulation or
Tables 41 REG 100 CF 25 CF 2 CF ) 2 SHAPE (DA/L (ft/mi) BSLOPE (ft/mi) CSLOPE L (mi) —Continued E, basinslope; SHAPE, climate factor basin shape; for CF, recurrence interval ) flow record; duplicate *, mapidentification number for siteshaving separate 2 channelization; n.a.,data not available] DA (mi 500 Q 200 Q 100 Q 50 Q 25 Q 10 Q 5 Q n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 1571 85.95 21.06 472.39 0.21 2.28 2.79 2.94 1 2 Q 1640 20908900 12500 2470 14800 3070 17400 3600 19100 20800 4220 22400 4940 24400 6070 406 190 47.41 26.23 15.70 50.04 368.28 384.09 .18 .28 2.31 2.29 2.81 2.81 2.96 2.96 1 1 10800 17500 22400 29100 34500 40100 46100 54400 1571 85.95 21.06 472.39 .21 2.28 2.79 2.94 1 r r Recurrence interval discharges and basin characteristics for gaged rural sites in North Carolina North in sites rural gaged for characteristics basin and discharges interval Recurrence nc r Map 365* 361* 365366 11200 15700 4210 18900 6530 23100 8290 26400 10800 29800 12800 33400 15000 38400 17400 406 20900 47.41 104 15.70 368.28 22.63 .18 50.23 2.31 435.97 2.81 .20 2.96 2.31 1 2.81 2.95 1 361 362363 2930 1450 4150 2420 5010 3190 6160 4330 7070 5300 8020 6380 9010 7570 10400 9360 42.0 37.6 19.16 86.51 7.91 480.21 150.30 474.21 .11 2.29 .61 2.80 2.29 2.95 2.81 1 2.96 1 364 (fig. 1) (fig. number identification identification Table 2. Table years indicated; REG, region; nc,flood-frequency estimates were not computedbecause the sitehasthan less 10 yearsofpeak- periods of regulatedor channelizedsiteflows; excluded r, from regional analysisbecause flows were affected by regulation or [Q, recurrence[Q, interval flood discharge for yearsindicated; drainage DA, area; L, channel length;CSLOPE, channel slope; BSLOP
42 Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina APPENDIX The mean square sampling error, MSEs,0, for an ungaged site with basin characteristics given by the row The value of the mean square error (MSE ) at a s vector x0=[1 log (DA0) log (IA0) log (RQ500)], for specific site can be estimated as follows: Denote the example, is calculated as column vector of n logarithms of observed peak- discharge characteristics at n sites in a region by Y. For TΛ–1 -1 T example, MSEs,0 = x0 {X X} x0 ,
Λ logQ50, 1 in which is the (n by n) covariance matrix associated Λ logQ with Y. The diagonal elements of are model error Y = 50, 2 , variance, γ2 , plus the time-sampling error for each site "" i (i=1,2,3,...n), which is estimated as a function of a logQ50, n regional estimate of the standard deviation of annual peaks at site i, the recurrence interval of the dependent variable and the number of years of record at site i. The off-diagonal elements of Λ are the sample covariance in which, Q50,i, represents the observed 50-year peak at the ith gaging station in the region. Further, let X of the estimated t-year peaks at sites i and j. These off- represent a (n by p) matrix of p-1 basin characteristics diagonal elements are estimated as a function of a augmented by a column of ones at n gaging stations and regional estimate of the standard deviation of annual B represent a column vector of p regression coeffi- peaks at sites i and j, the recurrence interval of the cients. dependent variable and the number of concurrent years of record at sites i and j (Tasker and Stedinger, 1989). The (p by p) matrix {XTΛ–1 X}-1 for each equation is For example, given in Appendix table 1. The mean square error of a prediction, in log (base 10) units, at specific ungaged sites can be estimated as ()() () 1 log DA1 log IA1 log RQ501 a 1 log()DA log()IA log()RQ50 b MSE = (γ2 + MSE ). X= 2 2 2 and B = 1 . p,0 s,0 "" "" "" b2 1 log()DA log()IA log()RQ50 b n n n 4 The standard error of a prediction, SEprediction, in percent, can be calculated as
The linear model can be written as {}5.302 × ()MSEp, 0 0.5 SEprediction = 100 e – 1 . Y=XB.
Appendix 43 T –1 –1 Appendix Table 1. Matrix {}X Λ X for the equations in table 5 (p. 11) [These matrices can be used to compute the standard error of prediction and prediction intervals as explained in the text. Numbers are given in scientific notation, for example, 0.43958E–01 = 0.43958 x 10-1 = 0.043958]
Hydrologic area Blue Ridge-Piedmont Coastal Plain Sand Hills 2-year recurrence interval 0.15029E–02 –0.53920E–03 0.28189E–02 –0.90910E–03 0.96765E–02 –0.43179E–02 –0.53920E–03 0.26847E–03 –0.90910E–03 0.42844E–03 –0.43179E–02 0.24592E–02 5-year recurrence interval 0.17447E–02 –0.60050E–03 0.33220E–02 –0.10290E–02 0.11983E–01 –0.52941E–02 –0.60050E–03 0.28874E–03 –0.10290E–02 0.45946E–03 –0.52941E–02 0.29971E–02 10-year recurrence interval 0.20021E–02 –0.66987E–03 0.39046E–02 –0.11856E–02 0.13840E–01 –0.60545E–02 –0.66987E–03 0.31419E–03 –0.11856E–02 0.51365E–03 –0.60545E–02 0.34066E–02 25-year recurrence interval 0.23859E–02 –0.77570E–03 0.47979E–02 –0.14331E–02 0.16439E–01 –0.71059E–02 –0.77570E–03 0.35450E–03 –0.14331E–02 0.60449E–03 –0.71059E–02 0.39674E–02 50-year recurrence interval 0.26993E–02 –0.86340E–03 0.55348E–02 –0.16401E–02 0.18505E–01 –0.79370E–02 –0.86340E–03 0.38866E–03 –0.16401E–02 0.68241E–03 –0.79370E–02 0.44084E–02 100-year recurrence interval 0.30284E–02 –0.95634E–03 0.63113E–02 –0.18599E–02 0.20648E–01 –0.87971E–02 –0.95634E–03 0.42534E–03 –0.18599E–02 0.76611E–03 –0.87971E–02 0.48635E–02 200-year recurrence interval 0.33710E–02 –0.10538E–02 0.71209E–02 –0.20902E–02 0.22864E–01 –0.96843E–02 –0.10538E–02 0.46416E–03 –0.20902E–02 0.85453E–03 –0.96843E–02 0.53320E–02 500-year recurrence interval 0.38420E–02 –0.11886E–02 0.82349E–02 –0.24083E–02 0.25894E–01 –0.10896E–01 –0.11886E–02 0.51827E–03 –0.24083E–02 0.97751E–03 –0.10896E–01 0.59708E–02
44 Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina