Estimating design hydrographs at the basin scale: from event-based to continuous hydrological simulation

ABSTRACT Design hydrographs are widely used in practical hydrologic engineering problems. Typical applications adopt event-based (EBM) methods, using rainfall-runoff models to convert design hyetographs into design hydrographs. Uncertainties include the definition of antecedent conditions and the assumption of equivalence between hyetograph and hydrograph return periods. An alternative is to use continuous simulation (CSM) methods, by forcing a rainfall-runoff model with long precipitation series, and directly analyzing the output discharges. To better understand uncertainties in the EBM method and differences between CSM and EBM ones, we applied a hydrological model in the Itajaí-Açu river basin to compare a CSM method with 730 different simulations of an EBM one, considering different basin antecedent conditions and design hyetographs (10- and 50-years). Results indicated that the EBM method leads to a large range of design discharges depending on the antecedent condition. CS-based 10- and 50-years maximum discharges corresponded to percentiles between 30% and 50% of the EBM estimates. Higher discharge variation occurred in sub-basins with larger maximum soil water storage. Our conclusions agree with the literature, which points towards CSM-based methods to estimate design discharges.

Flood mapping in small ungauged basins: a comparison of different approaches for two case studies in Slovakia

Flood mapping is a crucial element of flood risk management. In small and ungauged basins, empirical and regionalization approaches are often adopted to estimate the design hydrographs that represent input data in hydraulic models. In this study, two basins were selected in Slovakia and different methodologies for flood mapping were tested highlighting the role of digital elevation model (DEM) resolution, hydrologic modeling and the hydraulic model. Two DEM resolutions (2 m and 20 m) were adopted. Two hydrologic approaches were employed: a regional formula for peak flow estimation and the EBA4SUB model. Two hydraulic approaches (HEC-RAS and FLO-2D) were selected. Different combinations of hydrologic and hydraulic modeling were tested, under different spatial resolutions. Regarding the DEM resolution, results showed its fundamental importance in the low relief area while its effect was secondary in the moderate relief area. Regarding the hydrologic modeling, results confirmed that it affects the results of the flood areas in the same way independently of DEM resolution and that when using event-based models, the hydrograph shape determination is fundamental. Regarding the hydraulic modeling, this was the step where major differences in the flood area estimation were found.

Strupczewski Method for Parametric Design Hydrographs in Ungauged Cross-Sections

Nonparametric hydrographs, constructed by the method suggested by Archer, are usually used for developing parametric design hydrographs. Flow changes in time are described by the UPO ERR Gamma complex function, which denotes a Gamma curve reformulated to have a Unit Peak at the Origin (abbreviated to UPO), supplemented by the Exponential Replacement Recession (ERR) curve. It may be observed, that this solution does not work in some areas of the upper Vistula and middle Odra catchments when the times of the rising limb of a hydrograph are higher than the times of the falling limb, i.e. when the skewness coefficient approximates 0.5 or higher values. Better results can be achieved with the function suggested by Strupczewski in 1964. It is a solution which uses two parameters of the flood hydrograph. The objective of the present paper is to assess the Strupczewski method by comparing it with a complex UPO ERR Gamma function for gauged cross-sections in the upper Vistula and middle Odra catchments. The assessment was carried out for 30 gauged cross-sections (15 in each river catchment). The parameters were optimized for width-hydrograph descriptors W75 and W50, designed by the Archer method, and for the skewness coefficient s. Optimization using only two width-hydrograph descriptors aims to test how the Strupczewski method works for cross-sections for which the values of width-hydrograph descriptors W75 and W50 are known. The assessment of both methods was carried out with reference to a nonparametric hydrograph constructed by the Archer method. The results of these assessments suggest that the Strupczewski method may be used not only for gauged cross-sections, but also for ungauged ones.

A software package for predicting design-flood hydrographs in small and ungauged basins

In this study, software for estimating design hydrographs in small and ungauged basins is presented. The main aim is to propose a fast and user-friendly empirical tool that the practitioner can apply for hydrological studies characterised by a lack of observed data. The software implements a homonymous framework called event-based approach for small and ungauged basins (EBA4SUB) that was recently developed and tested by the authors to estimate the design peak discharge using the same input information necessary to apply the rational formula. EBA4SUB is a classical hydrological event-based model in which each step (design hyetograph, net rainfall estimation, and rainfall-runoff transformation) is appropriately adapted for empirical applications without calibration. As a case study, the software is applied in a small watershed while varying the hyetograph shape, rainfall peak position, and return time. The results provide an overview of the software and confirm the secondary role of the design rainfall peak position.