SEDIMENT DISCHARGE ON THE STEEP SLOPE FIELD WHICH HAVE BEEN CONTRIBUTING TO THE MRICA RESERVOIR SEDIMENTATION
IN CENTRAL JAVA, INDONESIA

Sediment discharge due to the deforestation at upper Mrica watershed, Indonesia and turn to steep slope field has induced reservoir sedimentation at the lower.

Hydrological observations and laboratory experiments were conducted to understand the actual situation of sediment discharge and erosion velocity of upper Mrica watershed. The observations encompass sediment discharge monitoring at field, surface water sampling and discharge measurement at upper- and lower Merawu River and reservoir simultaneously. Monitoring of sediment discharge was carried out on two plots at upper part of field; Plot-A in planted area (vegetation cover ratio about 30%) of 0.5x16x25 m2, and Plot-B of new cultivated area 4x4 m2, both at the similar field slope gradient of 20o. Ground-erosion resistance tester was employed to analyze the critical erosion velocity of Merawu basin soil samples.

The result shows that rainfall amount near the critical for triggering sediment discharge at field is 42~58 mm/hour. Sediment discharge hydrograph at Plot-A and Plot-B shows well response, rise sharply near the peak of rainfall. Prevailing discharged materials are silt. After three hours peak rainfall, turbidity at lower Merawu River raise sharply to 780 mg/l. Laboratory experiments reveal that critical rotational frequency, Nc is 10~15 rpm while critical flow velocity, Ucp for this ranges is 7~10 cm/s. Average erosion velocity, E for critical rotation is 0.004~0.01 cm/min. The critical tractive force, tc is ranges from 0.2~0.5 Pascal and critical friction velocity, U*c is ranges from 1.5~2.2 cm/s. Erosion velocity obtained by the experiment is near to the actual ones. It can be estimate that February 21, 2004 rainfall event is around the critical rainfall event for triggering sediment discharge from the study field

Keywords: Mrica reservoir sedimentation; intensive tillage on the steep slope field; sediment discharge; hydrological observations; Ground-erosion resistance tester; erosion velocity; Central Java