Analysis of drainage capacity as a flood control effects in laweyan sub-district

Laweyan sub-district is located in the Laweyan sub-district, Surakarta City, and passes by a Premulung river. Flooding is often causing inundation around the riverbank area. Due to the high rainfall intensity, the discharge at the confluence of the two rivers is causing overtopping of embankment in many sites. It happened to the Premulung River and the Brojo River. As an effort to control floods, it must analyze the existing drainage system. It is an effective way to determine the performance of a drainage system. The drainage system depends on the contour maps of the Laweyan sub-district on the flood-affected area. The capacity of the drainage system is calculated by Hec-Ras. Reducing the volume of runoff is analyzed based on the Manning's formula. The result shows that the drainage channel has ability to reduce flooding. The flash flooding volume of the Premulung rivers due to Q10 is 3750 m³/hr. Based on Q25, 4272.5 m³/hr, based on the maximum of two-consecutive days rainfall, 5722.8 m³/hr, the flooding caused inundation in 3 sites with duration for a long 23 hours. The existing drainage in normal conditions with a capacity of 118653.26 m³ / hour can reduce water runoff from rivers.