Opportunities for Process-Based Management of Rivers Below Water Diversions: Exploring and Exploiting Coupled Alteration of Flow and Sediment Regimes

Abstract:

In mountainous, rural catchments throughout the world, small to medium-sized rivers are being developed for diversion hydropower. Previous study indicates that in the absence of careful management, consequences to rivers downstream of diversion hydropower dams can be severe. However, patterns of hydrologic alteration below diversion hydropower dams have not been well described. In addition, diversion hydropower projects typically require some form of sediment management to maintain relatively small impoundments at diversion sites. Investigation of principal coupled processes, notably flow and sediment dynamics, in the context of alteration by diversion schemes may elucidate potential for working with natural processes to mitigate consequences to rivers.

This analysis characterizes hydrologic alteration by diversion hydropower dams, investigating flows below 31 diversion dams. Diversion for hydropower production generated distinctive signatures of hydrologic alteration in sample rivers. For instance, flow signals were characterized by reduced low to moderate flows and steep transitions between low and high stages. In a number of rivers, small floods were reduced in number and size. However, effect to magnitude and timing of large floods was minimal across all rivers investigated. Sediments were stored in impoundments and released in pulses annually through lower sediment gates. The hydrologic signatures observed were strongly linked to diversion designs and may be transferable to other rivers developed with similar infrastructure.

Understanding characteristic flow alterations due to diversion hydropower, in combination with cycles of sediment storage and release, may inform management practices for simultaneously maintaining reservoirs and downstream channels. In particular, investigating altered flood magnitudes and frequencies in the context of geomorphic work may indicate potential for maintaining natural hydrogeomorphic processes. Detailed study of sediment pulse transport under the characteristic modified flow regime is necessary to inform recommendations for integrating flow and sediment releases below diversion dams.