Flood-frequency Characteristics Research Articles

Linking flood frequency to long‐term water balance: Incorporating effects of seasonality

Derived flood frequency models can be used to study climate and land use change effects on the flood frequency curve. Intra‐annual (i.e., within year) climate variability strongly impacts upon the flood frequency characteristics in two ways: in a direct way through the seasonal variability of storm characteristics and indirectly through the seasonality of rainfall and evapotranspiration which then affect the antecedent catchment conditions for individual storm events. In this paper we propose a quasi‐analytical derived flood frequency model that is able to account for both types of seasonalities. The model treats individual events separately. It consists of a rainfall model with seasonally varying parameters. Increased flood peaks, as compared to block rainfall, due to random within‐storm rainfall time patterns are represented by a factor that is a function of the ratio of storm duration and catchment response time. Event runoff coefficients are allowed to vary seasonally and include a random component. Their statistical characteristics are derived from long‐term water balance simulations. The components of the derived flood frequency model are integrated in probability space to derive monthly flood frequency curves. These are then combined into annual flood frequency curves. Comparisons with Monte Carlo simulations using parameters that are typical of Austrian catchments indicate that the approximations used here are appropriate. We perform sensitivity analyses to explore the effects of the interaction of rainfall and antecedent soil moisture seasonalities on the flood frequency curve. When the two seasonalities are in phase, there is resonance, which increases the flood frequency curve dramatically. We are also able to isolate the contributions of individual months to the annual flood frequency curve. Monthly flood frequency curves cross over for the parameters chosen here, as extreme floods tend to mainly occur in summer while less extreme floods may occur throughout the year.

Impulse response of a linear diffusion analogy model as a flood frequency probability density function

The impulse response of a linear convective-diffusion analogy (LD) model used for flow routing in open channels is proposed as a probability distribution for flood frequency analysis. The flood frequency model has two parameters, which are derived using the methods of moments and maximum likelihood. Also derived are errors in quantiles for these parameter estimation methods. The distribution shows that the two methods are equivalent in terms of producing mean values—the important property in case of unknown true distribution function. The flood frequency model is tested using annual peak discharges for the gauging sections of 39 Polish rivers where the average value of the ratio of the coefficient of skewness to the coefficient of variation equals about 2.52, a value closer to the ratio of the LD model than to the gamma or the lognormal model. The likelihood ratio indicates the preference of the LD over the lognormal for 27 out of 39 cases. It is found that the proposed flood frequency model represents flood frequency characteristics well (measured by the moment ratio) when the LD flood routing model is likely to be the best of all linear flow routing models.

Flood frequency analysis for the Red River at Winnipeg

This paper reviews the flood frequency characteristics of the Red River at Winnipeg. The impacts of persistence in the flood series on estimates of flood quantiles and their associated confidence intervals are examined. This is done by generating a large number of data sequences using a mixed noise model that preserves the short-term and long-term correlation structures of the observed flood series. The results reveal that persistence in the data series can lead to a slight increase in the expected flood magnitude for a given return period. More importantly, persistence is shown to dramatically increase the uncertainty associated with estimated flood quantiles. The 117-year flood series for the Red River at Winnipeg is demonstrated to be equivalent to roughly 45 years of independent data.Key words: flood frequency, extreme events, simulation, historical data.

REGIONAL FLOOD FREQUENCY ANALYSIS IN SOUTHERN ONTARIO USING L-MOMENTS

Annual flood series from unregulated rivers in southern Ontario are examined with a view to testing the efficacy of the generalized extreme value distribution to represent all flood frequency characteristics in the area. Series are characterized by their sample L-moments, the higher orders of which are more reliable than product moments, facilitating both the selection of the appropriate probability distribution and the estimation of its necessary parameters. The generalized extreme value distribution is found to provide a good fit to the diversity of annual flood characteristics found in the area and to furnish a reasonable regional flood frequency distribution. The spatial variations of the derived parameters may be successfully explained in terms of potential geographic and hydrologic controls on precipitation input, flood generating processes (snowmelt or rainfall), and land use

An investigation into the physical causes of scaling and heterogeneity of regional flood frequency

Peak discharge data from catchments in the central Appalachian region of eastern United States suggest that the coefficient of variation of annual flood peaks, CV[Q], is not constant, as implied by the index flood method but varies with catchment size in a complex manner [Smith, 1992]. Gupta et al. [1994] have interpreted the data as indicating that for catchments smaller than a critical threshold size, CV[Q] increases with increasing catchment size, while for larger catchments CV[Q] decreases with catchment size. Our analysis of the same discharge data suggests further that the spatial heterogeneity of these flood frequency characteristics, for example, mean annual flood,E[Q], and coefficient of variation, CV[Q], between catchments in the region is also not constant but varies systematically with catchment size. The spatial heterogeneity of E[Q] appears to decrease with catchment size, while heterogeneity of CV[Q] appears to mirror the observed scaling behavior of CV[Q]. These observations have been made based on statistical analysis of empirical flood data without being underpinned by a physical theory to explain them. In this paper, motivated by these observations and by the need for a physical theory of regional flood frequency, we develop a simple derived flood frequency model. On the basis of insights provided by the model, we relate the increase of CV[Q] with catchment size for small catchments (smaller than a threshold size) to the scaling behavior of the ratio of storm duration to catchment response time. On the other hand, we connect the decrease of CV[Q] with catchment size for larger catchments to the spatial scaling of rainfall (excess) intensity. Our simple model also permits us to separate the relative contributions of catchment routing response and rainfall intensity to the scaling behavior of E[Q]. Our results also lead us to hypothesize that the heterogeneity ofCV[Q] between catchments is primarily due to the heterogeneity of catchment response time, while the heterogeneity of E[Q] is primarily due to the heterogeneity of runoff generation processes.

A stochastic study of floods in Canada: frequency analysis and regionalization

Peak stream discharge is a hydrologic parameter that is very important for the determination of flood risk, design of engineering works, and management of water resources. In this study, some 237 stream records across Canada were analyzed using the theory of stochastic processes applied to extreme values. This model, based on partial duration series analysis, was applied to each stream record, considering the time of occurrence of floods to be a Poisson process. In addition, exceedances (values above a given discharge level or truncation level) were considered to be independent random variables identically distributed over a one-year time interval. After this frequency analysis of each stream record, a regionalization of the flood frequency characteristics in Canada was performed using two different approaches: multiple regression analysis and index-flood method. A comparison of the two approaches was carried out by examining mean relative error and root-mean-square error. It was determined that the level of difficulty in applying the stochastic flood model was not the same across Canada. Moreover, error associated with the index-flood method is mainly due to error in estimating low return floods. Key words: flood, partial duration series, regional hydrology, index-flood method, low return flood.

Towards the use of catchment geomorphology in flood frequency predictions

AbstractA further development of a topography‐based model of catchment hydrology (TOPMODEL) is described and applied to the problem of predicting flood frequency characteristics. The model can simulate infiltration excess, saturation excess, and subsurface runoff contributions to peak flows. Catchment geomorphology plays a central role in predicting the nature of the hydrological response. Using stochastic rainfall and initial condition inputs based on measured data, the model satisfactorily reproduces the mean hourly flow flood frequency growth curve for the Wye catchment, but not the mean number of peaks greater than 3mm h−1 each year. Suggestions for further improvements are made.

El Niño and annual floods on the north Peruvian littoral

A three-component mixed Gumbel distribution satisfactorily models the observed annual flood frequencies of rivers in northern Peru which display highly variable annual peak-flood characteristics corresponding to three sets of ocean-atmosphere conditions. Physical support for the “a priori” subdivision of the series is provided by consideration of the regional offshore oceanic-atmospheric conditions. Exceptionally warm waters (El Niño) cause extensive heavy rains; unusually cold waters (anti-El Niño) restrict both the quantity and distribution of precipitation. Model parameters reveal marked spatial trends in the severity and extent of flooding during any set of offshore conditions and geographic shifts in the regional flood-frequency characteristics between such conditions.

FLOOD FREQUENCY CHARACTERISTICS OF SOME ARIZONA WATERSHEDS1

ABSTRACTThe flood frequency characteristics of 18 watersheds in southeastern Arizona were studied using the log‐Boughton and the log‐Pearson Type 3 distribution. From the flood frequency study, a generalized envelope for Q100 for watersheds 0.01 to 4000 mi2 in area has been produced for southeastern Arizona. The generalized envelope allows comparisons to be made among the relative flood characteristics of the watersheds used in the study and provides a conservative estimate of Q100 for ungaged watersheds in the region.

A derived flood frequency distribution using Horton Order Ratios

A flood frequency distribution is derived from assumed climatic distributions and the geomorphologic unit hydrograph (GUH) model of catchment response. The derived distribution is tested on two Appalachian Mountain catchments and shows good agreement with historical data. Because the GUH is rooted in catchment shape and geomorphology, this derived frequency distribution may be a promising vehicle for studying the effect of catchment shape on flood frequency characteristics. Furthermore, it is felt that the question of catchment homogeneity and similarity, necessary for regional flood estimation, can also be studied through the GUH derived flood distribution.

Discussion of “Urban Flood Frequency Characteristics”

Discussion of “Urban Flood Frequency Characteristics”

Urban Flood Frequency Characteristics

This paper presents the results of a study concerning the occurrence and factors affecting floods from small urban watersheds. Based on Log-Pearson Type III flood frequency analysis on four urban watersheds located in Texas, urbanization has significantly increased peak discharge. However, data on the Waller Creek watershed (Austin, TX) and Wilbarger Creek watershed (near Austin, TX) suggest that the effects of urbanization on peak discharge decreases after a certain recurrence interval. Data from 27 urban watersheds in Texas, combined with 33 urban watersheds located in Virginia, Maryland, Delaware, Illinois, and Mississippi, form the basis for the derivation of empirical flood frequency equations for small urban watersheds. These equations relate the flood discharge for recurrence intervals of 2.33, 5, 10, 20, and 50 yr. to physiographic, urban, and climatic factors. The significant urban factors were found to be the impervious cover and a channel urbanization factor.

Flood Frequency Estimating Techniques for Small Watersheds

This report describes a literature search undertaken to identify hydrologic methods suitable for defining the flood frequency characteristics of streams of less than 20 sq mile drainage area. Results of comparative testing on 11 small watersheds using digital computer simulation, regional flood frequency analysis, Potter's statistical procedure, and an arbitrarily standardized version of the rational method are reported. For the streams tested, digital simulation using the Hydrocomp Simulation Program was most successful in reproducing the flood frequency curves determined from the historic streamflow. The study reaffirmed the basic problems involved in frequency analysis of short flow records on small streams. Further research to resolve some of these problems is recommended. A program of research and development to adapt digital simulation to the derivation of design flows for highway culverts is also proposed.