Runoff Hydrograph Analysis of HEC-RAS 2D Flow Hydrodynamics Meteorological Rain-on-Grid on Observed Watershed: A Case Study of Wiroko Sub-Watershed

Abstract

In contemporary hydrological analysis, numerical models have emerged as an alternative solution for water resources planning and management, particularly in forecasting extreme events and mitigating disaster risks. HEC-RAS 2D flow hydrodynamics enables the modeling of rainfall with input from meteorological boundary condition data. Precipitation within the studied watershed can be modeled as point rainfall or area-averaged rainfall in the rain-on-grid feature of HEC-RAS 2D flow hydrodynamics. In this study, the HEC-RAS 2D flow model utilizes the diffusion wave equations (DWE) for simulating unsteady flow routing. When modeling a complex watershed, the 2D hydrodynamics model has become a viable alternative, especially concerning the watershed's physical characteristics. This study aims to assess the reliability of HEC-RAS 2D flow hydrodynamics and compare its results with the hydrograph data from the observed watershed in the Wiroko Sub-Watershed of the upper Wonogiri Dam. In this research, extreme rainfall events of 100 mm in the first hour are simulated and compared with the hydrograph data from the observed watershed. Based on the numerical model results in the Wiroko Sub-Watershed, it was determined that conforming to the land use with a Manning roughness value of 0.12 (designated as developed area-medium intensity) resulted in a peak discharge (Qp) difference of 0.8%. Meanwhile, the time to peak (Tp) value exhibited a discrepancy of 1.93 hours longer between the numerical model and the observed hydrograph.