Minimizing diurnal variation of downstream flow in hydroelectric projects to reduce environmental impact

Hydroelectric projects are generally operated as peaking power plant, particularly during lean period; consequently, diurnal variation of flow in the downstream of the dam is induced even in a run-of-the-river scheme. Although such deviation from the natural flow may have significant impact downstream, it generally goes unnoticed. Lower Subansiri Hydroelectric project, located on the Subansiri River of Assam, India, is one of the major hydroelectric projects proposed in the Northeastern part of India. Reservoir Simulation Model has been developed with the objective of assessing the extent of flow variation in the downstream due to operation of the Lower Subansiri Reservoir. Simulation has been carried out using a Standard Operating Policy proposed for the project. The study has revealed that during the peaking hour, the discharge downstream increases about eight times of the normal flow in the lean period and becomes almost zero during the non-operating period. While the high discharge poses erosion and flood threats, the low discharge may lead to adverse environmental impacts such as increase in pollutant concentrations. The possibility of adopting structural and non-structural measures for minimizing the deviation of flow from its normal condition is investigated and performance of these measures is compared based on seven performance criteria. The comparisons have revealed that the structural measure provide the best solution. As an alternative, the non-structural measure also promises notable improvement over the baseline standard operation scenario.

► Operation of hydroelectric project induces diurnal variations of flow downstream.
► Downstream impact due to diurnal variations is significant during lean period.
► Structural and operational measures are investigated to minimize diurnal variations.
► A regulating pond downstream can effectively minimize diurnal variations.
► Changing operation schedule can also be an alternative to reduce diurnal variations.