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Hydraulic parameters, including bed shear stress, are challenging to calculate for flash floods. Applying theoretical equations to strongly unsteady flows requires comprehensive and accurate flow data that are difficult to collect for natural events. To empirically evaluate the extent to which simpler calculations can reasonably predict shear stresses in rapidly changing hydrographs, and to determine how bed shear stresses differ for hydraulic bores propagating over dry beds compared to shallow water, we conducted laboratory flume experiments on idealized short-duration flash floods with bores. We compared the Saint-Venant shallow water equation, which theoretically captures the depth-averaged momentum balance of gradually varying unsteady flow to eight simpler methods, with assumptions that are not met in these flows. The Saint-Venant method predicts higher shear stresses immediately after bore arrival, but all of the simpler methods are nonlinearly correlated with the Saint-Venant method even when flow is rapidly changing. While none of the methods we evaluated should strictly apply to rapid changes in depth and velocity of bores, the correlations we found between methods just after bore arrival suggest that, for applications where shear stresses must be calculated but data are insufficient to apply the full Saint-Venant equations, simpler methods may provide meaningful shear stress constraints. Compared to flood bores propagating over a dry bed, bores propagating over shallow flowing water had steeper water surfaces but resulted in significantly lower bed shear stresses immediately after bore arrival. The common characteristic of tsunamis and waves moving onshore, and of dam breaks and flash floods occurring in rivers, is the presence of a sudden large increase in water depth and velocity termed flood bores. These result in an abrupt and large increase in shear stress (the force of the water on the bed), causing fatalities and extensive damage to infrastructures. We generated flood bores in a flume, with instrumentation that automatically monitored water depth, velocity, turbulence and water surface slope of bores arriving on a dry flume bed, with additional experiments of bores arriving on an existing shallow flow. We then calculated and compared the temporal change in shear stress by different methods, and found that, with the exception of the immediate arrival of a bore, shear stress calculated using simple assumptions is similar to the most demanding method. Another conclusion is that the shear stress is very large upon arrival of a bore on a dry bed—larger even than over a body of water. Hence, flood bores unexpectedly arriving on a dry bed are particularly dangerous to people, and their potential damage quantified by shear stress can be calculated using simplified formulas. [ABSTRACT FROM AUTHOR]

Although advanced physically based models have been presented in the last years, there is still much interest in equations for predicting the peak discharge. Indeed, simple relations are very useful for technical purposes, especially when rough calculations are acceptable. This paper presents new equations for assessing the peak discharge of the flood hydrograph caused by the gradual erosion of earth dams. The equations are obtained by a regression analysis carried out on the peak discharges provided by a physically based numerical model. The approach proposed in this paper has the advantage of taking into account some parameters usually neglected by common best-fit equations that have strong influence on the peak discharge. In particular, the equations discussed in this paper take into account the susceptibility to erosion of the dam, the shape of the reservoir, and the ratio of filling of the reservoir for failures caused by piping. The usefulness of the equations is shown through a comparison with the numerical model and with other regression relations by applying them to some real dam-breaching events. Finally, a sensitivity analysis is made in order to assess the influence of parameters that have been fixed in the present analysis, like dam crest width or abutment slopes. [ABSTRACT FROM AUTHOR]

Flash floods in ephemeral rivers are characterized by pronounced runoff dynamics, rapidly rising hydrographs, and infiltration through permeable river beds. In numerous countries, this infiltration-which is commonly referred to as transmission loss-is artificially increased by means of dams to promote groundwater recharge of local aquifers. However, flow dynamics are significantly altered by dam operation, i.e., initially pronounced process dynamics are reduced and standing/receding wave effects may occur in downstream river sections. For the adequate portrayal of such flow processes, this paper develops an integrated modeling system for flow routing in ephemeral rivers with groundwater recharge dams. The proposed system is based on a process-oriented description of flow, infiltration, and reservoir evaporation and allows for a robust application under a limited data situation, as usually encountered in arid and semiarid regions. Particularly, the proposed framework accounts for (1) the considerable loss of mass and momentum from the weakly dynamic flow downstream of a dam, attributable to transmission losses; (2) the transient character of transmission losses, which are nonlinearly depending on time and changing channel flow conditions; and (3) circumvents any numerical inconveniences associated with the modeling of dam release flow over initially dry beds by employing an analytical solution procedure of the governing flow equations. Following a comprehensive sensitivity analysis, relevant process parameters are estimated and the modeling system is applied for Wadi Ma'awil, Northern Oman. The application demonstrates both the system's accurateness and robustness for flash flood routing under transmission losses along the wadi, where a recharge dam causes strong flow retention. Therefore, the proposed modeling system can aid in deriving potential groundwater recharge rates, which is of high importance for a sound water resources assessment in the study area. [ABSTRACT FROM AUTHOR]

The effect of the flood recession time on the local scour depth at bed sills in gravel deposits is examined. Experiments were carried out to study the development of scour holes under time-varying hydraulic conditions with no upstream sediment feed. Triangular-shaped hydrographs, having recession times up to three times the duration of the rising limb, were used. Traditionally, the peak water discharge in any flood event is used as a design value in estimating the final depth of scour formed by a flood. This approach is overly conservative when the flow hydrograph is steep, i.e., during the occurrence of flash floods. The actual reduction of the scour depth from this estimated value is dependent on both the characteristics of the flood event and the characteristics of the stream. The results show that the maximum potential scour depth can be achieved only for hydrographs with long recession times, while the rate of this process can be estimated as a function of the ratio between a characteristic flood time and the steady-state temporal scale of scour development. A method is proposed for the prediction of the scouring process under unsteady flows in terms of two dimensionless temporal parameters. Results obtained for clear-water boundary conditions can be extended to sediment-supply tests if specific supply input conditions hold. [ABSTRACT FROM AUTHOR]

The temporal development of clear-water local scour depth at bed sills in uniform gravel beds is considered. Experiments are presented on the development of scour holes under unsteady hydraulic conditions, with the triangular-shaped hydrographs tested being of different durations and different rates of flow variation. Based on the experimental results and a theoretical framework, a method is given for the definition and prediction of the scouring process under unsteady flows in terms of a dimensionless temporal parameter. A “flash flood” is here defined as an event for which the scour doesn’t attain its potential magnitude, i.e., the equilibrium value for the peak hydrograph flow rate. This flood nature is dependent on both the characteristics of the flood event itself and the characteristics of the stream. A quantitative measure of what constitutes a flash flood is given in terms of the identified temporal parameter. Results show that the ratio between the final scour depth and the potential scour depth at a bed sill for a given hydrograph can be estimated as a function of the identified temporal parameter. [ABSTRACT FROM AUTHOR]

This paper proposes a dam-breach model which predicts, in a simple but physically based manner, not only the peak discharge but the whole outflow hydrograph and breach development. The following aspects are taken into account: the geometry of the embankment, the shape of the reservoir, the hydraulic characteristics of the flow through the breach and its erosive capacity, and the shape of the breach. The model needs only one calibration parameter and can be easily applied to real cases. The application of the model with a single value of the calibration parameter produced excellent results in the simulation of 12 historic earthfill dam failures, with a discharge range covering three orders of magnitude. [ABSTRACT FROM AUTHOR]

A sensitivity analysis of the model presented in the companion paper is made and, on the basis of the results a criterion is proposed for the choice of values to assign to the side slopes of the breach, in order to use the model for prediction. Moreover, the sensitivity analysis shows that the outflow hydrograph and its peak value depend not only on the dam height and the stored volume, but also on the vertical distribution of the water mass in the reservoir. The model is compared with some previously published methods and the disadvantages, limitations, and errors that can be made using parametric models and predictive equations are pointed out. Finally, easy to use equations interpolating the numerical results of the model are provided that predict not only the peak discharge but the whole outflow hydrograph for overtopping failures. [ABSTRACT FROM AUTHOR]

An analytical model of flood wave propagation is used to study the sensitivity of dam-breach flood waves to breach-outflow hydrograph volume, peak discharge, and downstream-channel bed slope. Dimensionless parameters are identified for discharge and distance along the channel. A dam-breach Froude number is defined to enable analysis through a wide range of site and flow conditions. It is found that, at a certain distance downstream, the attenuated peak discharge is independent of the magnitude of the discharge at the breach site. This attenuated peak discharge is termed the “ultimate discharge,” and the distance associated with it, the “ultimate distance.” These ultimate values are a function, primarily of the downstream-channel bed slope and, secondarily, of the breach-outflow hydrograph volume. [ABSTRACT FROM AUTHOR]