UNCERTAINTIES IN MATHEMATICAL MODELING OF SEDIMENT FLUSHING IN POL–RUD RESERVOIR

Deposition of sediments is the major factor in decreasing the useful life of reservoirs. In some regions of the world like Gilan state in north of Iran, sediment yield is very high, say more than 1000 Tonn/yr.Km2, which forces designers to allocate a considerable portion of the reservoir storage to sediments as dead volume. However, it is possible to decrease the volume of sediments trapped in the reservoir in certain conditions. Flushing operation is a hydraulic method for decreasing the amount of sediments trapped in reservoirs by evacuating the incoming or previously trapped sediments by water flow from the reservoir through an appropriately–designed reservoir outlet. Pol–Rud is a medium capacity reservoir planned to be constructed in Gilan state in north of Iran. According to the preliminary studies it is anticipated that the reservoir will face severe sedimentation and lose 2% of its total storage annually. The major purpose of the reservoir is to regulate water for irrigation demands. The reservoir will typically impound water only for five months a year and can be emptied for the rest of the year for flushing operation. In order to assess the effectiveness of sediment flushing operation and determine some parameters needed for designing the hydraulic structures, a state–of–the–art mathematical modeling study was performed. Major goals of the study were to determine 1) Total sediment deposition in the reservoir during its 50 years useful life, 2) Optimum level for sediment flushing outlet, 3) Optimum capacity for sediment flushing outlet and 4) Safe (sediment–free) level for irrigation and bottom outlets of the reservoir. The mathematical modeling study encompassed three sub–models. These were a semi–2D long term model for sediment deposition and erosion in the reservoir, a short term semi–2D model for sediment transport in the reservoir, and a mathematical model for sediment deposition in the reservoir during water impoundment period. Also, Wallingford Institute's experimental method was used as a check for mathematical modeling results. All modeling operations were carried out in design mode to account for uncertainties associated with sediment behavior in water flow and to produce reliable results for direct use in reservoir design. Two different scenarios were considered for operation of the reservoir; these were regular (periodic) and irregular (selective) flushing operations. The results show that the proposed sediment flushing operation can evacuate up to 50% of incoming sediments from the reservoir. Also, appropriate level for irrigation and bottom outlets found to be about 27 meters above the flushing outlet level and optimum level and capacity for the flushing outlet were determined as well.