Flow Assessment Downstream of a Hydroelectric Project in an Ungauged Area

Hydropower dams can induce flash floods, leading to a severe cataclysm in flood-prone areas at downstream regions. On the catchment scale, flooding is not contributed solely by the reservoir releases, and there can be significant flow contributions from tributaries downstream of the dam. The major challenge in estimating the lateral flow contribution is that most tributaries are ungauged and situated in inaccessible areas. To overcome this inconsistency and to increase the precision of downstream flood warnings, a modeling framework was developed to quantify the flow contribution by ungauged tributaries to the mainstream using the drainage area ratio (DAR) method. The model parameters were estimated using optimization algorithms, and the best parameters were selected based on the error metrics. The modeling framework constitutes a reservoir operation model and hydrodynamic model developed in MATLAB version 2020b environment with the ease of coupling the two models. The estimated flow from the lateral tributaries based on the optimal model parameters of DAR and hourly inflow hydrographs were incorporated into the model. Two scenarios were analysed with and without lateral flow from ungauged tributaries. Results impart that the flood peaks have increased by more than 75% with the incorporation of the lateral flow. The model was validated with downstream stage and discharge data. The results indicated that the magnitude of the model generated and actual flow data were in the same range.

Flooding downstream due to sudden release from a hydropower dam is a matter of serious concern worldwide. To evaluate the potential flooding situation downstream, a dam release is generally routed by a hydrodynamic model. However, because hydropower dams are mostly located in remote areas, the tributaries located at inaccessible downstream areas remain ungauged and, therefore, obtaining precipitation/streamflow data of such tributaries become difficult. In absence of downstream flow contribution, the water level obtained by routing the reservoir release underestimates flood magnitude. The dam release flood falls in the high-hazard category because of its suddenness characteristics and, therefore, adverse consequences of underestimation cannot be overemphasized. This paper presents a framework that couples a reservoir operation model, a hydrodynamic model, and a simplified area–proportionate model to estimate downstream tributary contribution, so that a more reliable estimation of the downstream flood situation can be made. The modeling framework has been tested in the Ranganadi Hydropower Project situated in northeastern part of India. The coupled model can be applied to any reservoir with proper calibration of model parameters. By applying this model, a disaster manager would be in a position to disseminate in advance a more reliable downstream flood warning.