Holistic Environmental Flow Assessment by Building Block Method in Inaccessible Himalayan River Basins

Abstract

Estimating the environmental flow (E-flow) of a river stretch prior to the construction of a hydroelectric project (HEP) is essential to conserve riverine ecosystems. The Building Block Method (BBM) is a widely used holistic technique for E-flow assessment (EFA) that uses hydrological flow information as fundamental to understand flow–ecology relationships. However, the application of methods like BBM is restricted in inaccessible river basins of the Himalayas due to data limitations such as flow, cross-section, and slope. This work presents a hybrid approach by supporting BBM with 90% dependable year analysis and hydrodynamic modeling to encounter the lack of data in EFA. Three case studies are demonstrated at the upcoming HEP on Tawang River, located in the Eastern Himalayan region. Ecosystem components like river biodiversity, river hydraulics, cultural requirement, livelihood requirement, and ecosystem function are considered as blocks of BBM. The input flow for hydrodynamic routing is fed by seasonal water availability, which is found by examining daily flow data with 90% dependable year analysis. Hydrodynamic simulation for multi-release scenarios is used to determine appropriate E-flow requirements for blocks of BBM at a critical stretch. A final comparison of seasonal release policy by existing ministerial recommendations with estimated E-flow requirement shows over-release in monsoon and under-release in remaining seasons in the case studies. Consequently, the new release policy is framed based on estimations of E-flow requirements.