ANALYSIS OF DRAINAGE CAPACITY AS A FLOOD CONTROL EFFECTS IN LAWEYAN SUB-DISTRICT

Abstract

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 118653.26m3 / hour can reduce water runoff from rivers.