Hydrodynamic Modelling as a Study for Brantas River Watershed Recovery Effort (Study Case: Kali Surabaya and Kali Mas)

Water plays a crucial role in sustaining life and supporting human activities yet increasing industrialization and residential expansion continue to threaten its quality, especially in river systems. Kali Surabaya and Kali Mas, as part of the main streams in the Brantas River watershed, serve vital ecological and socioeconomic functions for East Java. However, rapid development has led to significant water quality degradation. This degradation challenges the achievement of key Sustainable Development Goals (SDGs), particularly SDG 6 (Clean Water and Sanitation), SDG 11 (Sustainable Cities and Communities), and SDG 14 (Life Below Water). This study uses the hydrodynamic model to simulate flow characteristics in the Kali Surabaya and Kali Mass and integrates module for water quality modeling to assess the distribution of BOD, DO, NH4, and NO3 across seasons. The model is validated using the error bias comparison method to verify its content resemblance to real life conditions. The results show that water quality tends to decline downstream, particularly during the dry season, with BOD levels reaching 6.3 mg/L, NH4 concentrations exceeding 0.65 mg/L in the downstream and DO dropping to 2.2 mg/L in the upstream, hence placing those areas in class 3 to class 4 under Indonesian standards PP No. 22/2021. Based on these findings, targeted restoration measures are proposed for each river segment, including aeration systems in the upstream areas, waste load reduction in midstream zones, and community-based cleanups and urban buffers in downstream urban areas.