UNIVARLATE LEAST SQUARES MUSKINGUM FLOOD ROUTING1

ABSTRACT: The Muskingum method of hydrologic flood routing is based on a graphical bivariate curve fitting procedure. The subjectivity of this procedure may lead to problems of reproducibility and provides no guarantee of model coefficients which minimize the error of estimation. A transformation is developed to express the Muskingum method as a univariate transcendental function suitable for numeric solution. Solution of this function minimizes the sum of the squared deviations between estimated and measured rates of discharge. Five textbook examples of the Muskingum method are evaluated by the univariate transformation. Error reductions range from negligible to tenfold. While the univariate transformation may not improve upon graphically-based results in every case, in no instances are the univariate results inferior; in many cases they are superior.     

FLOOD ROUTING THROUGH A FLAT,COMPLEX FLOODPLAIN USING A ONE-DIMENSIONAL UNSTEADY FLOW COMPUTER PROGRAM1

ABSTRACT: The routing of flood waves through the Central Basin of the Passaic River in New Jersey is complex because of flat gradients and flow reversals. The one-dimensional unsteady flow program DWOPER, developed by the National Weather Service, was used to simulate flood wave movement through the Basin. A historical event was used for calibration and two synthetic events were simulated. Boundary conditions consisted of discharge hydrographs at inflow points to the study area, local flow hydrographs at interior points, and a stage discharge relation for flow over the crest of a diversion dam at the basin outlet. Manning's n values were adjusted based on stage and discharge data for the historical event; however, verification data were not available for events comparable in magnitude to the synthetic events. Aspects of the investigation reported include techniques for characterizing the flow system, model calibration, techniques for representing a tunnel diversion, and simulation results.     

FLOOD EVENT MODELING - A STUDY OF TWO METHODS1

: Despite the advances in catchment modeling in recent years, engineers still face major problems in estimating flood flows. Application of unit hydrograph and runoff routing models to five United Kingdom catchments shows that either can be tuned to predict, on a test event, the routing effects of a catchment with equal accuracy. The larger remaining problem is the prediction of losses from rainfall and this study shows how alternative ways of describing the within event distribution of these losses can be an important factor controlling the success of the overall model. Other problems include the risks of extrapolation to larger events, baseflow separation methods, rainfall patterns, and inevitable errors in the data.     

MATHEMATICAL SIMULATIONS OF THE TOCCOA FALLS,GEORGIA, DAM-BREAK FLOOD1

ABSTRACT: Four dam-break models were selected for testing with an observed data set from the November 6, 1977, disaster at Toccoa Falls, Georgia. The Kelly Barnes Dam failure occurred with a 35-ft head of water and produced a peak discharge of 23,000 ft³/s. The selected models included: (1) Modified Puls (MP), (2) U. S. Army Corps of Engineers Gradually Varied Unsteady Flow Profiles (USTFLO), (3) National Weather Service's Dam-Break Flood Forecast (DBFF), and (4) U. S. Geological Survey's method of characteristics (MOC) coupled with a general purpose streamflow simulation (J879DB). Achieving a successful simulation was easiest with the MP model. The DBFF model required a moderate effort while the MOC-J879DB models required some data alterations and considerable effort. The USTFLO model failed to simulate this test case. In the stream segment near the dam, the computed peak stages were generally within 5 feet of the observed high water marks. Elsewhere, the peak stage results were much better, generally within 2 feet. The peak discharges computed by the models were generally within 20 percent of discharges estimated by slope area and contracted opening measurements, except near the dam where the MOC-J879DB model's results was 80 percent too high.     

COMPREHENSIVE FLOOD PLAIN MANAGEMENT1

Flooding and the susceptibility to flood damage inherent in all land uses constitute the flood hazard. Resolution of the hazard while properly recognizing flood plain environmental attributes within the context of overall community or area needs is the essence of comprehensive flood plain management. The traditional approach–flood control–has effected modification of only the flooding component of the hazard whether it be coastal or inland. Until recently Federal programs have overlooked the possibilities of modifying the susceptibility component, for which the major responsibility lies with non-Federal interests. Beginning with actions in the TVA area, the latter is now being strongly encouraged through Federal programs and actions notably the Flood Plain Management Services and Survey Programs of the Corps of Engineers, those stemming from Executive Order 11296, and those required for eligibility under the National Flood Insurance Act of 1968. Flood plain management objectives must be stated in planning, e.g., economic efficiency, reduction in threat to life and health, environmental improvement, and regional development, to permit proper evaluation of the optional means and approaches for achieving them.     

FLOOD-RUNOFF FORECASTING WITH HEC1F1

HEC1F is a computer program for making short- to medium-term forecasts of uncontrolled flood runoff. The program employs unit hydrographs and hydrologic routing to simulate runoff from a subdivided basin. Estimates of future rainfall can be accommodated. Runoff parameters for gaged headwater subbasins can be estimated (optimized) in real time. Blending of calculated with observed hydrographs can be performed. HEC1F is a component of an on-line software system that includes capability for data acquisition and processing, precipitation analysis, streamflow forecasting, reservoir system analysis, and graphical display of data and simulation results. The conceptual framework for HEC1F is described, and application of the program is illustrated.     

Adapting a holistic approach to flood management in the Hawkesbury–Nepean region: complexities and perceptions of the agencies involved

This paper looks into the complexity of managing flood risks in the Hawkesbury–Nepean catchment, Australia. Several aspects are explored: (1) the complexities created by the way different agencies are involved in assessing flood risks; (2) different perceptions on acceptable flood risk level; (3) community engagement in defining acceptable flood risk level; (4) views on a holistic flood risk management plan; and (5) challenges of a centralised information system. This study concludes that the complexity of managing a large catchment is exacerbated by the difference in the way professionals perceive the problem. This has led to (1) different standards for acceptable risks; (2) inconsistent attempt to set up a regional-scale flood management plan beyond the jurisdictional boundaries; (3) absence of a regional-scale agency with licence to share and update information; and (d) lack of forums for dialogue with insurance companies to ensure an integrated approach to flood management.     

ASSESSING THE ACCURACY OF PALEOHYDROLOGIC INDICATORS,HARPERS FERRY,WEST VIRGINIA1

ABSTRACT: Gaging stations established in 1895 at Millville, West Virginia and in 1882 at Harpers Ferry, West Virginia record flows ranging from a maximum of 6,509 m³s^-1 to a minimum of 2 m³s^-1. Historical and botanical indicators were used to extend the systematic flood record of the Shenandoah River for a study reach approximately 7.5-km long. The long systematic record at the site provides a good opportunity to assess the accuracy of these sources of paleoflood information. Habitation of the area by settlers of European descent began in 1733, and historical flood records extend from 1748. Qualitative historical records from different sources were compared to yield the most complete flood history. The correlation between the various sources was extremely high. Botanical flood evidence preserved as adventitious sprouts, tree scars, and ring anomalies were documented in 37 trees. A flood chronology established from these data extended from 1896 to 1955. Botanical indicators provided an accurate, although incomplete, flood chronology. The ability to determine accurate flood stages from paleohydrologic indicators varied. Historical data yielded relatively accurate stages to within 1–2 m; only minimum values of flood stage could be obtained from botanical indicators. These results illustrate some of the strengths and weaknesses of paleohydrologic investigations in the eastern United States.     

STATISTICAL TERMINOLOGY: DEFINITIONS AND INTERPRETATION FOR FLOOD PEAK ESTIMATION1

ABSTRACT: Considerable effort is expended each year in making flood peak estimates at both gaged and ungaged sites. Many methods, both simplistic and complex, have been proposed for making such estimates. The hydrologist that must make an estimate at a particular site is interested in the accuracy of the estimate. Most methods are developed using either statistical analyses or analytical optimization schemes. While publications describing these methods often include some statistical measure of goodness-of-flt, the terminology often does not provide the potential user with an answer to the question,‘How accurate is the estimate?’ That is, statistical terminology often are not used properly, which may lead to a false sense of security. The use of the correct terminology will help potential users evaluate the usefulness of a proposed method and provide a means of comparing different methods. This study provides definitions for terms often used in literature on flood peak estimation and provides an interpretation for these terms. Specific problems discussed include the use of arbitrary levels of significance in statistical tests of hypotheses, the identification of both random and systematic variation in estimates from hydrologic methods, and the difference between accuracy of model calibration and accuracy of prediction.     

Application of Index Procedures to Flood Frequency Analysis in Turkey1

Abstract: This study investigates the regional analysis of annual maximum flood series of 48 stream gauging stations in the basins of the West Mediterranean Region in Turkey. The region is divided into three homogeneous subregions according to both Student‐t test and Dalrymple homogeneity test. The regional relationships of mean annual flood per unit area‐drainage area and coefficient of skew‐coefficient of variation are obtained. Two statistically meaningful relationships of the mean flood per unit area‐drainage area and a unique relationship between skewness and variation coefficients exist. Results show that the index‐flood method may be applicable to each homogenous subregion to estimate flood quantiles in the study area.     

ADAPTATION OF FLOOD PEAKS AND DESIGN HYDROGRAPHS FROM GAGED TO NEARBY UNGAGED WATERSHEDS1

ABSTRACT: Equations were developed to transform peak flows and to adapt design hydrographs and unit hydrographs from gaged watersheds to ungaged watersheds with similar hydrologic characteristics. Dimensional analysis was used to develop adjustment equations for peak flow and time base, and these equations were reinforced with results from regional flood frequency research. The authors believe that the use of these transformation equations should yield more reliable flood peak values and hydrogrphs than the common use of empirical flood estimating curves or equations.     

A NONTRADITIONAL METHODOLOGY FOR FLOOD STAGE-DAMAGE CALCULATIONS1

ABSTRACT: This paper presents a new methodology to calculate economic losses from hypothetical, extreme flood events, such as the Probable Maximum Flood. The methodology uses economic data compiled from already-available secondary sources, such as U.S. Census data on magnetic tapes, utilizing microcomputer and other electronic media. Estimates of land elevations are obtained from topographic maps, and flood elevations axe estimated using, for example, a dam breach and flood routing (DAMBRK) model (Fread, 1984). The calculations are performed at a disaggregate spatial scale, by various land use and industrial classification categories. The basic areal units are city blocks (for urbanized areas), enumeration districts, and Census tracts. Depth-damage functions, which provide an estimate of damages as a proportion of the existing value of the structure, are estimated statistically. Computer software (called DAMAGE) is used to combine the economic, flood elevation, and depth-damage information to compute economic losses for different possible flood stages and for different inflow events. Two case studies are presented as illustrations of the method.     

AN EVALUATION OF FLOOD FREQUENCY ESTIMATES BASED ON RAINFALL/RUNOFF MODELING1

ABSTRACT: An evaluation of flood frequency estimates simulated from a rainfall/runoff model is based on (1) computation of the equivalent years of record for regional estimating equations based on 50 small stream sites in Oklahoma and (2) computation of the bias for synthetic flood estimates as compared to observed estimates at 97 small stream sites with at least 20 years of record in eight eastern states. Because of the high intercorrelation of synthetic flood estimates between watersheds, little or no regional (spatial) information may be added to the network as a result of the modeling activity. The equivalent years of record for the regional estimating equations based totally on synthetic flood discharges is shown to be considerably less than the length of rainfall record used to simulate the runoff. Furthermore, the flood estimates from the rainfall/runoff model consistently underestimate the flood discharges based on observed record, particularly for the larger floods. Depending on the way bias is computed, the synthetic estimate of the 100-year flood discharge varies from 11 to 29 percent less than the value based on observed record. In addition, the correlation between observed and synthetic flood frequency estimates at the same site is also investigated. The degree of correlation between these estimates appears to vary with recurrence interval. Unless the correlation between these two estimates is known, it is not possible to compute a weighted estimate with minimum variance.     

PROBLEMS OF REESTABLISHING COMMERCIAL RECREATION BUSINESSES IN NEW YORK FOLLOWING HURRICANE AGNES1

ABSTRACT: Forty-two commercial campgrounds and 34 commercial marinas were studied to determine the amount and types of flood-related damages incurred from Hurricane Agnes, and to evaluate the institutional measures available to help management overcome these losses. Economic losses incurred by firms due to inability to operate, and declines in the number of recreationists, were over twice the magnitude of losses suffered via direct physical damages from flooding. The Federal Flood Insurance Program, as presently constituted, is of very little use to firms having structural investments in and over water. Most firms were complimentary of the Small Business Administration's loan programs. It appears that active programs of communication and promotion are needed following restoration of regions involved in natural diasters to reestablish the tourist industry of those regions.