Regional Flood Frequency Curve Research Articles

Regional Flood Frequency Analysis for Sustainable Water Resources Management of Genale–Dawa River Basin, Ethiopia

Regional information on stream discharge is needed in order to improve flood estimates based on the limited data availability. Regional flood estimation is fundamental for designing hydraulic structures and managing flood plains and water resource projects. It is essential for estimating flood risks during recurrent periods due to suitable distributions. Regional flood frequency analysis is crucial for evaluating design flows in ungauged basins, and can complement existing time series in gauged sites and transfer them to ungauged catchments. Hence, this study aims to perform a regional flood frequency analysis of the Genale–Dawa River Basin of Ethiopia using the index flood and L-moments approach for sustainable water resource management. Three homogeneous hydrological regions were defined and delineated based on homogeneity tests from data of 16 stream-gauged sites, named Region-A, Region-B, and Region-C. The discordancy index of regional data for L-moment statistics was identified using MATLAB. All regions showed promising results of L-moment statistics with discordance measures (discordance index less than 3) and homogeneity tests (combined coefficient of variation (CC) less than 0.3). L-moment ratio diagrams were used to select best fit probability distributions for areas. Generalized extreme value, log-Pearson type III, and generalized Pareto distributions were identified as suitable distributions for Region-A, Region-B, and Region-C, respectively, for accurately modeling flood flow in the basin. Regional flood frequency curves were constructed, and peak flood was predicted for different return periods. Statistical analysis of the gauged sites revealed an acceptable method of regionalization of the basin. This study confirms that the robustness of the regional L-moments algorithm depends on particular criteria used to measure the performance of estimators. The identified regions should be tested with other physical catchment features to enhance flood quantile estimates at gauged and ungauged sites. Henceforth, this study’s findings can be further extended into flood hazard, risk, and inundation mapping of identified regions of the study area. Furthermore, this study’s approach can be used as a reference for similar investigations of other river basins.

Regional Flood Frequency Analysis using Dimensionless Index Flood Method

Hydrologic designs require accurate estimation of quartiles of extreme floods. But in many developing regions, records of flood data are seldom available. A model framework using the dimensionless index flood for the transfer of Flood Frequency Curve (FFC) among stream gauging sites in a hydrologically homogeneous region is proposed. Key elements of the model framework include: (1) confirmation of the homogeneity of the region; (2) estimation of index flood-basin area relation; (3) derivation of the regional flood frequency curve (RFFC) and deduction of FFC of an ungauged catchment as a product of index flood and dimensionless RFFC. As an application, 1983 to 2004 annual extreme flood from six selected gauging sites located in Anambra-Imo River basin of southeast Nigeria, were used to demonstrate that the developed index flood model: , overestimated flood quartiles in an ungauged site of the basin. It is recommended that, for wider application, the model results can be improved by the availability and use of over 100 years length of flood data spatially distributed at critical locations of the watershed. Doi: 10.28991/cej-2020-03091627 Full Text: PDF

Revised Dicken’s method for flood frequency estimation of Upper Ganga basin

Flood discharge and its frequency is an important input to many infrastructure development, management and decision making models. However, for a number of potential sites, the data used for statistical analysis of flood discharge and its frequency, are either insufficient or unavailable. In such a situation, empirical formulae are the only alternative method to provide an estimate of design flood. Dicken’s formula is one of such empirical formulae used by practicing field engineers for flood estimation. In spite of its simplicity and applicability, Dicken’s formula is not capable of yielding flood volume at different desired frequency. In the present study an attempt has been made to modify the Dicken’s coefficient so as to accommodate the frequency component in flood estimation. For this purpose, observed flood discharge data sixteen gauging points of Ganga basin have been used to develop regional flood frequency curve, using probability weighted method, Index flood method, General extreme value and Wakeby distribution. Modified Dicken’s coefficient CT shows a sharp gradient for sub-catchments draining up to 5000 Km2. Further, CT value, ranges from 0.834 to 6.924. The CT –values are lower when calculated by Index Flood method while these are higher by Wakeby method. In all of these methods, CT –values do not approach the values of 11 or 14 as suggested for northern Indian catchments. An analysis of flood volume corresponding to CT –values of 11 or 14, reveals that these values yield high flood volume that correspond to return period of more than 1000 years and 3000 years respectively. These values are certainly very high for any rational design. Best fit distribution is decided, based on annual maximum discharge peak (ADF), Efficiency (EFF) and Standard Error (SE) parameters. Prediction accuracy of flood frequency analysis using revised Dicken’s formula is found to be above 85% for the best-fit distribution. General Extreme Value is found to be the best fit distribution for the Upper Ganga basin.

On the relationship between flood and contributing area

AbstractAlthough it is well known that the vast majority of the time only a portion of any watershed contributes run‐off to the outlet, this extent is rarely documented. Also, the power law form of the streamflow and contributing area (Q‐Ac) relationship has been known for a half century, but it is uncommon for it to be quantified, and time series of contributing area extensive enough to calculate its frequency distribution are almost non‐existent. Data from the Canadian Prairies, where there are extensive estimates of contributing area during the median annual flood, imply that the power law coefficient for any Q‐Ac curve is a function of flow magnitude and return period. These data also suggest that regional flood frequency curves are a construct of Q‐Ac curves from individual basins. This paper will discuss research that attempted to reproduce the Q‐Ac curves for the La Salle River Watershed with a semidistributed numerical hydrological model, MESH‐PDMROF. The model simulated streamflow reasonably well (Nash Sutcliffe values = 0.62) compared with published examples of comparable models applied in the region. Estimates of the coefficient and exponent of the Q‐Ac power law function ranged from 0.08–0.14 and 0.9–1.12, respectively. These exponent values were lower than those of regional flood frequency curves and support the theory that regional flood frequency curves are a construct of Q‐Ac curves. Simulations of the area contributing to the median annual flood were lower (0.3) than those derived from independent topographic analysis (0.9) described in earlier literature though there is uncertainty in both these estimates. This uncertainty was extended across the flood frequency distribution and may be too large to definitively verify the study hypothesis.

Use of L-Moments Approach for Regional Flood Frequency Analysis in Sicily, Italy

Extremely great floods are among environmental events with the most disastrous consequences for the entire world. Estimates of their return periods and design values are of great importance in hydrologic modeling, engineering practice for water resources and reservoirs design and management, planning for weather-related emergencies, etc. Regional flood frequency analysis resolves the problem of estimating the extreme flood events for catchments having short data records or ungauged catchments. This paper analyzes annual maximum peak flood discharge data recorded from more than 50 stream flow gauging sites in Sicily, Italy, in order to derive regional flood frequency curves. First these data are analyzed to point out some problems concerning the homogeneity of the single time series. On the basis of the L-moments and using cluster analysis techniques, the entire region is subdivided in five subregions whose homogeneity is tested using the L-moments based heterogeneity measure. Comparative regional flood frequency analysis studies are carried out employing the L-moments based commonly used frequency distributions. Based on the L-moment ratio diagram and other statistic criteria, generalized extreme value (GEV) distribution is identified as the robust distribution for the study area. Regional flood frequency relationships are developed to estimate floods at various return periods for gauged and ungauged catchments in different subregions of the Sicily. These relationships have been implemented using the L-moment based GEV distribution and a regional relation between mean annual peak flood and some geomorphologic and climatic parameters of catchments.

Estimation of Design Flood Hydrograph for an Ungauged Watershed

Based on the physiographical features of the studied watershed of 102.5 km2 in northern Ontario and the meteorological data of one nearby climatic station, through the combination of the regionalization of flood and the HEC-1 modeling, this article presents an approach that may be used to determine the desired peak flows for the ungauged watershed. The empirical equations used to determine the desired peak flows were developed by the Cumming Cockburn Ltd. (CCL) who has carried out the studies on flood regionalization by using flow data at 380 gauging stations in Ontario. CCL has proposed four methods to estimate peak flows, namely 1) regression method describing multiple linear relationships between flood flow and related parameters; 2) index flood frequency curve; 3) regional flood frequency curve and 4) isoline map for unit peak flow. The desired peak flows for studied watershed determined by CCL methods were used to calibrate the HEC-1 model for the rainfall-runoff simulation for this watershed which consists of 15 subwatersheds. Through slight adjustment in the CN number used in the HEC-1 model, the calibrated HEC-1 model could be used for rainfall-runoff simulation for this ungauged watershed. This approach could be recommended for hydrological design and watershed management for ungauged watersheds provided the analyses of flood regionalization could be conducted. In addition, comparing to the Bavaria forest region, Germany, some questions have been discussed in this article.

Regional Flood Formulas for Seven Subzones of Zone 3 of India

Regional flood frequency curves are developed by fitting the L-moment based generalized extreme value distribution to annual maximum peak flood data of small-to-medium size catchments of the seven hydrometeorological subzones of Zone 3 and combined Zone 3 of India. These seven subzones cover an area of about 1,041,661 km2. Relationships developed between mean annual peak flood and catchment area are coupled with the respective regional flood frequency curves for derivation of the regional flood formulas. The regional flood frequency curves developed for each subzone together with at-site mean annual peak floods may be used for gauged catchments, whereas for ungauged catchments, regional flood formulas developed for the respective subzones may be adopted for obtaining rational flood frequency estimates.

A worldwide comparison of regional flood estimation methods and climate

A summary of the results of regional flood frequency analyses that have been carried out in many tropical and sub-tropical countries is presented. The results provide a set of flood estimation methods which should be of immediate usefulness in practical applications, bearing in mind the errors of generalization. By defining the representative values of the mean annual flood and of the flood frequency curves for each region, an attempt has been made to develop relationships between the methods and climatic characteristics. These show that adequate relationships can be obtained based solely on the median annual average rainfall and the median catchment area for the regions. In particular, the steepness of the regional flood frequency curve can be estimated reasonably well from these two parameters. The relationships confirm that, broadly speaking, for humid regions the “average” flood—the flood that occurs regularly every one or two years—is relatively large, but the flood frequency curve is not very steep; rare floods (occurring once in every 100 to 1000 years) are not very much larger than the average flood. Conversely, in arid regions, the average flood is small, but rare floods can be extremely large multiples of the average. These relationships provide a useful initial indication of the type of regional flood frequency estimation method likely to be found in tropical and sub-tropical regions throughout the world.

FLOOD-FREQUENCY ANALYSIS FOR THE SANTA YNEZ RIVER AND ADJACENT REGION, CALIFORNIA

A regional flood-frequency curve is presented for the Santa Ynez River and adjacent areas in southern California. This curve was developed to improve estimates of return periods for very extreme floods on the Santa Ynez River and to evaluate the likelihood that Bradbury Dam, with its present spillway capacity of 4500 cms, would be overtopped. Peak-flow data from 31 gaging stations in the region were used in the analysis. The flood-frequency curve was fit using an index flood method and the assumption that floods in this region follow the log-Pearson Type III distribution. Estimates of the parameters for this distribution were found to be independent of basin size. The resulting flood-frequency curve for the Santa Ynez River region is relatively steep: the discharge from the 100-yr flood is about 10 times that of the mean annual flood, and of the 1000-yr flood, about 20 times that of the mean annual flood. The discharges of the 100-yr and 1000-yr floods on the Santa Ynez River are 1150 cms and 2300 cms, re...

Regional flood frequency analysis in arid and semi-arid areas

Dimensionless regional flood frequency curves use a number of records from a reasonably homogeneous area to overcome the variability of site records. The problems posed by short individual records and the variability of estimates are particularly acute in arid areas. Regional flood frequency curves in a number of semi-arid and arid areas have been assembled to illustrate their extreme slope and skewness, and the mutual similarity of these curves. Annual maxima from 162 stations with annual basin rainfall below 600 mm are assembled from NW Africa, Iran, Jordan, Saudi Arabia, Botswana and South Africa, and compared with records from Australia, southwest USA and Russia. The general extreme value (GEV) curves from each region, fitted by probability weighted moments (PWM), are compared with the combined curve. Since floods in arid areas, if nowhere else, result from storm intensity exceeding soil infiltration, the similarity of regional flood curves may indicate similar storm characteristics in different regions. The mean annual flood (MAF) for each catchment has been related to both basin area and mean annual rainfall by regression in an attempt to provide a preliminary means of estimating the scaling factor, MAF, for catchments with no flow data. Whilst in many of the regions studied, insufficient data are currently available to permit reliable estimation of the MAF by such an approach, it is hoped that the regression equations presented may permit estimation of the MAF for Iran, Saudi Arabia and Yemen, Queensland and more tentatively South Africa and Botswana. In addition, a combined regression equation for all semi-arid and arid regions is presented. This combination of estimated MAF and regional flood frequency curves permits flood magnitude with a known risk of exceedance to be derived within arid zones of the world even where local flow data are unavailable. Specific problems associated with the analysis of flood records from arid areas include gauging problems and measurements, low or zero annual maxima in a number of years, and the suitability, or otherwise, of the mean annual flood as the appropriate scaling factor for the dimensionless curves. In some cases, the 5-year flood may be a more appropriate scaling factor.

Regional Exceedance Probabilities

Construction of a regional flood frequency curve is based, as a rule, on fitting this curve to representative quantiles. In a regional sample of floods the probability of extreme values corresponding to return periods, that exceed the record lengths, is much larger than that of individual series, used to determine the representative quantiles. The probabilities of exceedance of regional extremes can be calculated straightforward in case of independent data, applying the theory of order statistics. For regionally dependent data one can define an equivalent number of independent regional series and then utilize the theory for independent data. This approach is exemplified with flood data from Norway.

FLOOD FREQUENCY ANALYSIS FOR SMALL WATERSHEDS IN SOUTHERN ILLINOIS1

ABSTRACT: Twenty‐two gaging stations were selected for developing a regional flood frequency curve for small (area less than 2 square miles) watersheds in southern Illinois. Five probability functions were compared, and the extreme value type I function was selected to develop the regional flood curve. The curve was generated with the index flood method and also another empirical method that related the function parameters to the watershed area. Estimated peak discharges with various return periods were compared with the results obtained from multiple regression analysis.