Sediment Flushing

Worldwide large dams are being discouraged on environmental, social or political grounds. Small-to-medium-sized cascade reservoirs with the hydropower projects are being preferred. However, the flushing of sediments by drawing down the reservoir is of immense interest to increase the useful life of a reservoir. In our research, we applied a depth-averaged, Godunov type model to simulate the 2D flushing. The solution technique employed possesses shock-capturing property and is especially suitable to dam-break accompanied by intense sediment transport phenomenon. The retrogressive erosion and flushing out a large volume of sediment in a very small time interval with creation of transitory flow conditions was tackled by this technique. We applied the model to two experiments; the first was a physical model study, while the second case pertained to a laboratory flume. The model validation was made through the bed topography change and the computation of the flushing efficiency. The model was in good agreement with the bed changes demonstrating its suitability to reproduce the phenomenon of retrogressive channel as well as lateral erosion. The accompanying sensitivity analysis focused on evaluating the influence of a number of parameters critical to a successful flushing simulation exercise.

Sediment deposition in a reservoir decreases storage capacity and effects many other parameters of the reservoir adversely. It reduces benefits and useful life of a hydro power project that have huge socioeconomic impacts. Flushing is one of the techniques to remove sediments from reservoirs. This study investigates sediment accumulation, transportation and flushing using both the physical and numerical modeling. Gulpur Hydro Power Project (HPP) on Poonch River in Pakistan was chosen for this purpose. The geometry, cross-sections and other physical attributes of the Poonch River were prepared and hydraulic structures were placed on the basis of topographic survey using AutoCAD. Physical model of scale 1:40 was developed at Nandipur Research Station in Pakistan. After base test the model was used to get data for various scenarios of sediment flows. HEC-RAS was used for numerical simulations. Delta profile and flushing were simulated. Delta modeling was made for hourly time step for 20 years of sediment deposition with average discharge conditions, whereas, suitable flushing durations were predicted for various flushing discharges to de-silt yearly deposited sediments. Simulation showed that life of the un-sluiced Gulpur HPP is about 14-15 years. To enhance the life of project, annually 4-5 days are required for flushing with 250 m 3 /s discharge.

A large area of axis sediment (>500 m2) may be annually removed from the head of Monterey Submarine Canyon with the first onshore storm of the fall/winter storm season. In this scenario, flushing events are followed by accumulation of sediment and organic debris—especially macro-algae—in the shallow axis. Net accumulation of this fill material increases during the calmer spring and summer until the next fall-flushing. The benthic community at a canyon axis station was characterized by highly fluctuating populations of opportunistic polychaete worms and gammarid amphipods, primarily Capitella spp., Armandia brevis, and Orchomene pacifica. The canyon axis community was very different from communities living at two other stations where sudden flushing does not occur—an adjacent sloping-wall station and a sandflat station. Sloping-wall and sandflat stations harbored more and longer-lived species, larger individuals, and a less-variable population structure during a year of sampling. The Loma Prieta earthquake in the fall of 1989 triggered small sediment slumps on the canyon walls, but it did not trigger axis-flushing. The usual seasonal flushing of the axis occurred 2 weeks after the earthquake with the arrival of the first storm. Benthic communities were reduced in abundance inside earthquake-induced slumps; however, the slumped areas were rapidly colonized by Prionospio pygmaea, a polychaete opportunist common to the sandflat. Surprisingly, the physical and biological impacts of the earthquake were much less severe than the seasonal axis-flushing associated with storms. Observations of sediment-flushing combined with measurements of benthic community changes in Monterey Canyon head represent a step towards an ecological model of mass wasting with implications for the continental shelf and slope and possibly the deep sea. © 1997 Elsevier Science Ltd