Muqaddas, Zaiful; Zaenal Kusuma; Runi Asmaranto; and Bagyo Yanuwiadi
Abstract:
The Kamoning watershed is the largest watershed in Sampang Regency, East Java, Indonesia with a catchment area of 354,215 km2. Main River Length 143.61 km. Almost every rainy season floods occur in the downstream area (Sampang city), in the area near the river mouth, a narrowing of the river occurs with a capacity River Qks = 136.5 m3/sec. If heavy rains occur in the upper watershed by recording an average of 45 mm / day bulk stations, river floods will occur in the city which becomes flooded. Based on Land Cover data or forest vegetation area in the Kamoning watershed is only 9.97% or 3,530 Ha. Regulation of the Minister of Forestry of the Republic of Indonesia Number 32 of 2009 requires a minimum area of vegetation above 30%. From the existing condition with a vegetation area of only 9.97%, the flood discharge value for Q2th of 161 m3 / sec has exceeded the river capacity of only 136.5 m3 / sec. The analysis results for Q5th, Q10th and Q25th and Q50th consecutive years obtained values of 240.6 m3/sec, 317.3 m3/sec, 446.1 m3/sec and 569.2 m3/sec. A land change scenario model was made by increasing the area of forest or plant vegetation and reducing the area of the moor with scenario 1 increasing forest area by 20%, scenario 2 of forest area being 30% and scenario 3 of forest or vegetation area being 40%, and scenario 4 of forest area to 50% of the watershed area, using the HEC-HMS 3.5 software with the Soil Conservation Service (SCS) method, the peak discharge value is reduced by vegetation expansion, in scenario 3, the peak Q2th discharge value becomes 134.4 m3/sec <136.5 m3/sec below the river capacity value, meaning that the addition of 40% vegetation in the Kamoning watershed will reduce the frequency of annual flooding and be safe against flooding equivalent to the Q2th return period. This model can calculate the value of flood reduction per village and per 1 hectare of plants can also determine the need for crop area per village and per hectare as well as the budget, so it is more applicable in the field implementation because the forest area is calculated for each village in the watershed area.
Keywords: Watershed, Reducing Runoff, River Capacity Value and Soil Conservation Service (SCS)
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