Flood Generation Processes Research Articles

Integrated modelling for flood risk mitigation in Romania: case study of the Timis–Bega river basin

An integrated flood modelling approach has been applied in a demonstrator of a flood management system, which was developed within the framework of a collaborative project between Romania and the Netherlands. The developed demonstrator system had two objectives: (a) operational water management under extreme conditions when actions have to be taken quickly; (b) off-line analysis and design of flood mitigation measures and alternatives. This article presents the applied approach and the achieved results for meeting the second objective. The pilot basin for the development of the system was the Timis–Bega river basin, in which the rivers Timis and Bega were considered jointly. The system is based on modelling the flood generation and routing processes by combined development and application of hydrological and hydrodynamic models. The modelling system HEC-HMS was used for the hydrological model, HEC-RAS for the one-dimensional hydrodynamic model and SOBEK for the two-dimensional (2D) model used for downstream flood analysis and design of mitigation measures and alternatives. The 2D model includes alternatives of deliberate dike breaching as part of the analysis of the system response. The analysis presented is concentrated on a specific flood event that occurred in April 2005, which occurred due to dike breaches along the Timis river. The combination of models is first used for reconstruction of inundation patterns resulting during this flood event. Subsequently the models were used for testing flood mitigation alternatives of deliberate (planned) breaches of flood protection dikes located in the downstream part of the Timis river at the same location where they had occurred during the 2005 flood event, but at different times with respect to the arrival of the flood hydrograph. The demonstrated approach can enable decision-makers to analyse the behaviour of the physical system and design possible preventive and/or mitigation measures.

Seasonal characteristics of flood regimes across the Alpine–Carpathian range

SummaryThe aim of this paper is to analyse the differences in the long-term regimes of extreme precipitation and floods across the Alpine–Carpathian range using seasonality indices and atmospheric circulation patterns to understand the main flood-producing processes. This is supported by cluster analyses to identify areas of similar flood processes, both in terms of precipitation forcing and catchment processes. The results allow to isolate regions of similar flood generation processes including southerly versus westerly circulation patterns, effects of soil moisture seasonality due to evaporation and effects of soil moisture seasonality due to snow melt. In many regions of the Alpine–Carpathian range, there is a distinct shift in flood generating processes with flood magnitude as evidenced by a shift from summer to autumn floods. It is argued that the synoptic approach proposed here is valuable in both flood analysis and flood estimation.

Flash flood prediction using an uncalibrated hydrological model and radar rainfall data in a Mediterranean watershed under changing hydrological conditions

Summary Flash floods cause some of the most severe natural disasters in Europe but Mediterranean areas are especially vulnerable. They can cause devastating damage to property, infrastructures and loss of human life. The complexity of flash flood generation processes and their dependency on different factors related to watershed properties and rainfall characteristics make flash flood prediction a difficult task. In this study, as part of the EU-FLASH project, we used an uncalibrated hydrological model to simulate flow events in a 27 km2 Mediterranean watershed in Israel to analyze and better understand the various factors influencing flows. The model is based on the well-known SCS curve number method for rainfall–runoff calculations and on the kinematic wave method for flow routing. Existing data available from maps, GIS and field studies were used to define model parameters, and no further calibration was conducted to obtain a better fit between computed and observed flow data. The model rainfall input was obtained from the high temporal and spatial resolution radar data adjusted to rain gauges. Twenty flow events that occurred within the study area over a 15 year period were analyzed. The model shows a generally good capability in predicting flash flood peak discharge in terms of their general level, classified as low, medium or high (all high level events were correctly predicted). It was found that the model mainly well predicts flash floods generated by intense, short-lived convective storm events while model performances for low and moderate flows generated by more widespread winter storms were quite poor. The degree of urban development was found to have a large impact on runoff amount and peak discharge, with higher sensitivity of moderate and low flow events relative to high flows. Flash flood generation was also found to be very sensitive to the temporal distribution of rain intensity within a specific storm event.

A conceptual investigation of process controls upon flood frequency: role of thresholds

Abstract. Traditional statistical approaches to flood frequency inherently assume homogeneity and stationarity in the flood generation process. This study illustrates the impact of heterogeneity associated with threshold non-linearities in the storage-discharge relationship associated with the rainfall-runoff process upon flood frequency behaviour. For a simplified, non-threshold (i.e. homogeneous) scenario, flood frequency can be characterised in terms of rainfall frequency, the characteristic response time of the catchment, and storm intermittency, modified by the relative strength of evaporation. The flood frequency curve is then a consistent transformation of the rainfall frequency curve, and could be readily described by traditional statistical methods. The introduction of storage thresholds, namely a field capacity storage and a catchment storage capacity, however, results in different flood frequency "regions" associated with distinctly different rainfall-runoff response behaviour and different process controls. The return period associated with the transition between these regions is directly related to the frequency of threshold exceedence. Where threshold exceedence is relatively rare, statistical extrapolation of flood frequency on the basis of short historical flood records risks ignoring this heterogeneity, and therefore significantly underestimating the magnitude of extreme flood peaks.

Analysis on flood generation processes by means of a continuous simulation model

Abstract. In the present research, we exploited a continuous hydrological simulation to investigate on key variables responsible of flood peak formation. With this purpose, a distributed hydrological model (DREAM) is used in cascade with a rainfall generator (IRP-Iterated Random Pulse) to simulate a large number of extreme events providing insight into the main controls of flood generation mechanisms. Investigated variables are those used in theoretically derived probability distribution of floods based on the concept of partial contributing area (e.g. Iacobellis and Fiorentino, 2000). The continuous simulation model is used to investigate on the hydrological losses occurring during extreme events, the variability of the source area contributing to the flood peak and its lag-time. Results suggest interesting simplification for the theoretical probability distribution of floods according to the different climatic and geomorfologic environments. The study is applied to two basins located in Southern Italy with different climatic characteristics.

Simulation of Water and Energy Fluxes in an Old-Growth Seasonal Temperate Rain Forest Using the Simultaneous Heat and Water (SHAW) Model

In the Pacific Northwest (PNW), concern about the impacts of climate and land cover change on water resources and flood-generating processes emphasizes the need for a mechanistic understanding of the interactions between forest canopies and hydrologic processes. Detailed measurements during the 1999 and 2000 hydrologic years were used to modify the Simultaneous Heat and Water (SHAW) model for application in forested systems. Major changes to the model include improved representation of rainfall interception and stomatal conductance dynamics. The model was developed for the 1999 hydrologic year and tested for the 2000 hydrologic year without modification of the site parameters. The model effectively simulated throughfall, soil water content profiles, and shallow soil temperatures for both years. The largest discrepancies between soil moisture and temperature were observed during periods of discontinuous snow cover due to spatial variability that was not explicitly simulated by the model. Soil warming at bare locations was delayed until most of the snow cover ablated because of the large heat sink associated with the residual snow patches. During the summer, simulated transpiration decreased from a maximum monthly mean of 2.2 mm day21 in July to 1.3 mm day21 in September as a result of decreasing soil moisture and declining net radiation. The results indicate that a relatively simple representation of the vegetation canopy can accurately simulate seasonal hydrologic fluxes in this environment, except during periods of discontinuous snow cover.

Investigation of the hydrological processes using chemical and isotopic tracers in a small Mediterranean forested catchment during autumn recharge

Flood generation processes are studied in a small mountainous Mediterranean catchment using natural chemical and isotopic water tracing. The study takes place during the autumn of 1996 for three successive floods of uneven magnitude. The flow resumption following the summer period is a prime opportunity to establish the impact of the soil moisture status on the catchment response. A dynamic scheme of the catchment behaviour and mechanisms is obtained. The combined use of different tracers highlights the speed of the water transfer from the hillslope to the catchment outlet as well as the predominant contribution of short residence time water (current water and previous rain water). Study results show that subsurface drainage through the soil macroporosity is one of the dominant processes in the formation of streamflow generation. This is in keeping with the soil's high permeability. Hydrograph separations demonstrate that pre-event water is the major contributor to streamflow. However, with time, a rise in the proportion of direct rainwater is observed, thereby highlighting the impact of the spread of soil saturation on the speed of water transfer and on the expansion of the contributing areas. Thus, for the last flood event (13/15-10), almost a third of the streamwater volume comes from the event rainfall and 50% from the first autumn rain event (19-9). The contributing areas reached 14% of the total catchment surface. A δ 18O simulation in shallow subsurface runoff is carried out, at the slope scale, with the use of a simple model. The results demonstrate that this subsurface runoff came from the mixing of the rain event's successive fractions without involving pre-existent water stored in the upper soil layer. For this autumnal flow resumption, the catchment mechanisms can be explained by a two-reservoir system, each geochemically distinct from the other. One is a surface reservoir primarily containing rainwater and the other is a subsurface reservoir made up of pre-event water from earlier rainfall. The contribution of an underground long residence time water is negligible.

Transformations for improved convergence of distributions of flood maxima to a Gumbel limit

The application of the asymptotic extreme value theory to flood maxima requires the assumption that the asymptotic condition has in fact been achieved. An alternative approach is to assume that non-Gumbel distributions of annual flood maxima are subasymptotic with respect to a Gumbel limit. Given this assumption, a power transformation and an exponential transformation are proposed to improve the convergence to the limit Gumbel distribution. An estimation procedure is given for obtaining the transformation constants. The transformations are particularly helpful if the flood generation process can be approximated by a truncated Weibull exceedance distribution combined with a Poisson exceedance frequency.