Evaluation and modification of some empirical and semi-empirical approaches for prediction of area-storage capacity curves in reservoirs of dams

The storage capacity of reservoirs is gradually reduced due to sediment accumulation that causes changes in the area-storage capacity (ASC) curves. Establishing these curves and predicting their future change is an important issue for planners, designers and operators of dams. Many empirical and semi-empirical approaches have been suggested for establishing and predicting the future changes for these curves. In this study four empirical and semi-empirical methods were evaluated and three of them were modified to be used for the prediction of changes in the ASC curves due to sedimentation, based on the existing sedimentation survey data for 11 reservoirs in the USA. For evaluation, these approaches were reviewed and used to determine sedimentation depth and establishing the ASC curves for the Mosul dam reservoir (MDR), which is the biggest hydraulic structure on the River Tigris in northern Iraq. MDR started operating in 1986 with a storage capacity of 11.11 km3 and a water surface area 380 km2 at normal operation stage (330 m a.s.l.). The results obtained from these methods were evaluated using observed bathymetric survey data that had been collected in 2011 after 25 years of the operation of the dam. The evaluation results showed three methods had presented more accurate results for estimating water depth or sedimentation depth at dam site with percentage error about 1.06-3.30%. Whilst for establishing ASC curves, one method presented good agreement result with survey data. Furthermore, ASC and sedimentation depths at dam site of MDR for periods 50, 75, 100 and 125 years were estimated using the modified approaches and the area reduction method. The results of the modified methods provided reasonable agreement when compared with the area reduction method proposed by the U.S. Bureau of Reclamation and the agreement became better with an increase in time period.