FORECAST-INFORMED EARLY RELEASE FOR FLOOD CONTROL AT THE BERINGIN SILA DAM, INDONESIA

Abstract

Early release (pre-event reservoir drawdown) is an operational measure for flood mitigation, especially for dams with ungated spillways where flow regulation during flood peaks is limited. However, uncertainty in short-term rainfall forecasts often leads to operational hesitation and suboptimal use of flood-control storage. This study develops and tests a forecast-informed early-release framework for the multipurpose Beringin Sila Dam while respecting the Annual Reservoir Operation Plan (RTOW) constraints. Short-lead (1-7 day) rainfall forecasts from the Indonesian Agency for Meteorology, Climatology, and Geophysics (BMKG) and The Weather Channel were transformed into inflow estimates using a rainfall-runoff model and then applied in reservoir operation simulations. Model calibration and verification yielded Nash-Sutcliffe Efficiency (NSE) values of 0.466 and 0.451, respectively, indicating moderate but adequate skill for operational decision support under data-limited conditions. Relative to conventional operation without early release, the forecast-informed strategy reduced peak outflow by approximately 15-25 %, lowered maximum reservoir water level by up to 0.8 m, and delayed downstream peak timing. These changes increased effective flood-control storage prior to flood arrival and reduced downstream flood risk, especially during the initial impounding stage. The proposed framework explicitly links real-time rainfall forecasts, inflow estimation, and RTOW-based operational rules for ungated spillway systems, providing a practical decision-support tool for improving flood management in similar multipurpose reservoirs.