Page - 648 - in Book of Full Papers - Symposium Hydro Engineering

have as result a steepening of the reservoir invert. Therefore, density current venting might be a feasible option in the future. RESCON-2 indicated that a reduction of the sedimentation by approximately 10% might be possible through density current venting in the future. RESCON-2 indicated that a standard hydraulic dredger with capacity of 115000 m³/a, which is within the technically feasible limits of dredging in reservoirs, is the only technique that can sustain the active storage at the pre-impoundment levels. The time path of reservoir gross and active storage is plotted in Fig. 3 with the orange line. Dredging however is associated with high operation costs as well as environmental considerations about the disposal of the dredged. RESCON-2 calculated for flushing a Sediment Balance Ratio (SBR) of 3.1, i.e. the sediment removal capacity of the specified flushing event will be approximately 3 times higher than the expected annual sediment deposits. This indicates that flushing is a technically feasible solution for Moragolla reservoir. The Long-term Capacity Ratio (LTCR) will be 0.64, i.e. 64% of the reservoir gross storage can be preserved sustainably. Its efficiency however drops to 50% with regards to active storage. The time path of reservoir gross and active storage is plotted in Fig. 3 with green lines. Environmental considerations associated with the expected high sediment concentrations downstream of the reservoir that would endanger the fish habitat of Mahaweli River together with the high energy generation losses due to the necessary full water level drawdown had as result that this technique was evaluated as not acceptable. The unregulated part of the catchment area draining into Moragolla reservoir is mostly forested or covered by tea gardens which are one of the major economic activities in the project region. It was considered therefore, that measures such as reforestation or change of the agricultural practices with purpose the reduction of the surface erosion are not technically feasible. The effect of dispersed check dams in the catchment area on the sediment inflow was not part of this study and has to be investigated more in detail. The evaluation of the river morphological features indicated that the reach of Mahaweli River extending approximately 2 km upstream of the reservoir headwaters is characterized by pronounced river meandering and sandy river bed. Limited sand mining is already performed by the local communities at this location. Numerical simulations with the one-dimensional model HEC-RAS indicated that the construction of a relatively short check dam with height of approximately 2.5 m would have as result an intensification of the retention of the sand particles in this river reach. A sediment trap at this location can retain annually approximately 60,000 m³ of sediment, corresponding to 40 % of the expected total mean annual sediment inflow into Moragolla reservoir. This can be translated as full retention of the material transported as bedload and retention of the coarsest 20% of the material transported in suspension. The necessary requirement for the above described sediment trap performance is the regular and continuous restoration of its trap efficiency by means of sand mining. The impact of this sediment inflow reduction is that approximately 10% of the gross storage and 50% of the pre- impoundment active storage is preserved sustainably. Except from the sediment inflow reduction an important aspect is that the sediment entering now the reservoir is finer since the coarsest part will be retained in the sediment trap. This has as 648