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The study on optimization of sediment flushing efficiency from cascade reservoirs as mitigation to the secondary impact of volcanic hazard
Published in Jean-Pierre Tournier, Tony Bennett, Johanne Bibeau, Sustainable and Safe Dams Around the World, 2019
P.T. Juwono, F. Hidayat, R.V. Ruritan, A. Rianto, M. Taufiqurrachman
HEC-RAS 5.0 unsteady sediment model was used to model sediment flushing operation in Wlingi and Lodoyo cascade reservoirs. The model was constructed with the 24–26 March 2016 pre-flushing cross sections. The Wlingi dam was modeled with an inline structure and the gate operations were translated into gate time series. In HEC-RAS program, water surface profiles are computed from one cross section to the next by solving the Energy equation with an iterative procedure called the standard step method (US Army Corps of Engineers, 2016A and 2016B). The Energy equation is written as follows: Z2+Y2+a2V222g=Z1+Y1+a1V122g+he
Iterative Floodway Modeling Using HEC-RAS and GIS
Published in Saeid Eslamian, Faezeh Eslamian, Flood Handbook, 2022
Majid Galoie, Artemis Motamedi, Jihui Fan, Saeid Eslamian
There are several graphical and analytical methods available for the computation of water surface elevations in a 1D steady gradually varied flow, but two of them are most common: the direct step method and the standard step method. HEC-RAS uses the standard step method because of its applicability for both prismatic and non-prismatic channels. Furthermore, this method can be used for both sub-critical (Fr < 1) and super-critical (Fr > 1) flows. In the standard step method, water surface elevations are computed iteratively and based on the energy equation in the following form (Dyhouse et al, 2007): WS2=WS1+α1V122g−α2V222g+LS¯f+Ce,cα2V222g−α1V122g︸hL1−2
Estimation of water surface profiles using rating curves
Published in International Journal of River Basin Management, 2021
Mahmoud F. Maghrebi, Ghadeer Ali
HEC-RAS was developed by the US Army Corps of Engineers. HEC-RAS computes the water surface profile from the control section to the next section by applying the energy equation (Eq. 3) and solving it using the iterative procedure ‘standard step method’ (Brunner 2002). It uses the DCM to calculate the total conveyance and the velocity coefficient. The cross-section is divided based on the n-value break point and the conveyance for each cross section is computed. The total conveyance is . The kinetic energy coefficient is calculated as:where the subscripts t, lob, ch and rob refer to the whole cross-section, left overbank, main channel and right overbank, respectively.
Development of an analytical benchmark solution to assess various gradually varied flow computations
Published in ISH Journal of Hydraulic Engineering, 2021
The profile is also sketched in Figure 5 using HEC-RAS software (HEC-RAS 2010). The boundary condition should be given according to the flow regime in that software; if it is subcritical, water depth at downstream is needed, and water depth at upstream should be given while the regime is supercritical. However, in other numerical methods and the analytical solution developed in this study, there is no need to identify the flow regime. Here, since the M1 profile is sketched, water depth at downstream is given to the program. The analytical solution can be effectively utilized to synthesize and evaluate the performance of HEC-RAS for various step lengths. HEC-RAS software discretizes the spatial domain into equal parts according to the maximum distance between the stations defined by the user. Figure 5 shows the defined maximum distances for this example. Users of HEC-RAS should be cautious about the tolerance for HEC-RAS uses the standard step method for computation of water-surface profile in an iterative manner. The tolerance [cited as ‘tol’ in Figure 5] for in HEC-RAS is the maximum allowable difference between two consecutive computed values for at a station to stop the iteration process (HEC-RAS 2010). Undoubtedly, the difference between water depths at two successive stations should not be less than the defined tolerance. The minimum allowable tolerance in HEC-RAS 4.1 is 0.0001 m. In this example, for maximum distances of 20 and 200 m, the difference between water depths at two adjacent stations is more than the HEC-RAS default tolerance for , i.e. 0.003 m, and these two distance resolutions have led to good results. However, for the maximum distance of 2 m, the tolerance is more than the difference between water depths at two adjacent stations and, surprisingly, this quite small resolution has not led to more accurate results compared to the larger resolutions of 20 and 200 m (Figure 5). By defining the tolerance of 0.0001 m, the result is improved. For this example, using maximum distance of 200 m between the stations and the default tolerance of 0.003 m leads to acceptable results. However, as it can be observed in Figure 5, the profile computed by HEC-RAS gradually diverges from the analytical solution in the upstream direction. Upon computation of the water-surface profile for a length of approximately 2000 m, the error in water depth is found to be 1 cm.