Measurement and quantification of a sedimentation budget for a reservoir with regular flushing

This paper provides a case study of successful removal of reservoir sedimen t by empty flashing. The main aim of this study was to determine: (i) sedim ent inflow to the Cachi reservoir; (ii) the pattern and rate of deposition in the reservoir; (iii) the erosion within the reservoir during flushing; a nd (iv) sediment outflow at a downstream hydrological station during flushi ng. The results are integrated as a sediment budget for the reservoir; The drainage basin of the Cachi reservoir in Costa Rica is 785 km(2). The sediment budget for the period between two flushings indicates a sedime nt inflow to the reservoir of c. 930 000 tonnes and a net accumulation of c . 133 000 tonnes, or 14.3% of the sediment inflow. The sediment throughflow was also c. 133 000 tonnes up to the erosion phase of the flushing, and th e outflow during flushing including bed load was 663 000 tonnes, i.e. 71.4% of the sediment inflow. Thus, the flushing of the reservoir was highly eff ective in releasing sediment. The balance of the sediment budget makes it probable that the major compone nts of the sediment budget are of the right order of magnitude. The main pr inciple to obtain correct sediment loads by the rating-curve technique was to collect sediment data from a large number of storm events at equal time intervals during rising and falling stages. This was achieved by turbidimet er recordings. Because of the wide scatter of data, sediment rating curves were developed on mean sediment loads in discharge classes to avoid the bia s of log regressions. The trap efficiency and sediment throughflow were determined with Sundborg' s physically based sedimentation model. The throughflow amounted to 20% of the suspended sediment inflow, which compares well with the empirical budge t. The depositional pattern in the Cachi reservoir was surveyed with side-s can sonar, repeated echo-soundings, and by excavating pits in deposits when the reservoir was empty. Deposition occurred mainly in the old river chann el, indicating that the major part of the sediment is transported by densit y currents. The volume of eroded material in the reservoir during scour valve release w as estimated by echo-soundings before and after the flushing. Dry bulk dens ity of the reservoir deposits was determined by X-ray radiographic and dens itometric analysis. The radiographic analyses indicated a dry bulk density of 0.4 g cm(-3) of the loose material in the thalweg, and 0.7 g cm(-3) of t he whole deposition depth on the terraces. Copyright (C) 2000 John Wiley & Sons, Ltd.