DESIGN OF RETENTION PONDS AS A FLOOD CONTROL MEASURE IN SANGKAPURA SUB-DISTRICT, BAWEAN ISLAND

Sangkapura Sub-district on Bawean Island is highly vulnerable to flooding due to high
rainfall intensity, land-use changes, and topographic conditions traversed by three main
rivers. Hydrological analysis was conducted using the Gumbel method based on 10 years of
maximum daily rainfall data, resulting in a design rainfall of 219.93 mm/day for a 20-year
return period. The planned flood discharge calculated using the Rational Method for four
watersheds was 13.03 m³/s, 19.56 m³/s, 8.70 m³/s, and 21.69 m³/s, with a total volume of
564,341.41 m³.The river capacity, determined using Manning’s equation, was 90.61 m³/s for
the Legung Tete River, 127.40 m³/s for Legung Tete B, 61.40 m³/s for Gunung Mas, 1.50
m³/s for the drainage channel, and 156.14 m³/s for the Gudeg River, with a total volume of
524,525.12 m³. Based on the analysis, the effective storage requirement of the retention pond
was 42,285.05 m³. The design consists of three retention ponds with capacities of 15,000 m³,
15,000 m³, and 21,450 m³. Combined with river capacity, the total storage reached
575,975.12 m³.The outlet structure designed using the orifice equation produced gate
dimensions of 3.40 m, 1.00 m, and 3.70 m. Stability analysis showed a slope safety factor of
4.84 (>1.5), indicating that the retention pond design is technically feasible and effective in
mitigating flood risk in Sangkapura Sub-district.