Flood Zoning in Songkram Basin

Songkram river is the main rever in the Sakon Nakorn River Basin with 465 km. in length cover 12,700 km 2 of watershed area and located in the north of the northeast of Thailand. Songkram River is a tributary of Meakong River and meandering. In the rainy season flood plain the area is always flooded because during this period high river discharge together with the back water effect from meakong River and very mild channel slope can cause severe flooding of the flood plain. Flood problems in Songkram river basin can be decrease by appropriately management, which required flood zoning map for guideline. This study is aimed to classify flooded area of different severity. Regional flood frequency method was employed for calculating flood flow at various locations and return periods. The flood discharge, ( Q r ) of a specified return period, T can be compute from standardized discharge, ( q r ) of the same return period and mean annual peak discharge, ( Q m ) at the same location. The equation is Q r = q r Q M , where q r = 0.8532 – 0.1935 ln[ln(1-1/T)], Q M = 6.0816 x 10 6 A 0.625 P 1.4044 . The height of flood flow can be calculated from peak discharge by using the method of steady non-uniform flow. Since the slope of the Songkram River is very flat and the water level at the confluence with Mekong River controls the flood flow, therefore the gradually varied flow was used for flood flow profile calculation. The Manning’s roughness coefficient was derived as n = 0.035, and the bend local head loss coefficient was derived as C L = 0.01. The flooded area predictions for the return period of 2, 10, 50 and 100 years are 452, 908, 1202 and 1290 km 2 respectively. They were also mapped.