Study of Flood Effects due to Constuction of Bang Pakong Barrage, 2001

Sriwongsitanon, Nutchanart

Research, Department of Water Resource Engineering, Faculty of Engineering, Kasetsart University, Bangkok 10900, Thailand


Abstract

 

Since the Bang Pakong River has a seawater intrusion problem, it causes water shortages for public water supply, agriculture, aquaculture and industry, which are rapidly increasing in this region. It is therefore necessary to construct the Bang Pakong Barrage to solve this problem. However, the construction of Bang Pakong Barrage may effect the flow of water and particularly the flood effect. Therefore, river flow simulation and prediction of the Bang Pakong River and its tributaries is carried out using MIKE 11 mathematical model. MIKE 11 is a hydrodynamic model which is able to simulate the unsteady flow situation in open channel, which may be affected by backwater and tide. In the schematic orientation for river flow analysis, the upstream boundary controls are situated at the Prachin Buri River using the hydrograph at the KGT.3 gauging station, and at the Nakhon Nayok River using the hydrograph at the Bang Mao Regulator. The downstream boundary control is situated at the mouth of the Bang Pakong River using tidal data. The lateral flow estimation between upstream and downstream boundary controls is carried out using the SCS model and unit hydrograph technique (which is a module in MIKE 11 model). For the river flow simulation, it is necessary to calibrate and verify the model to get the roughness coefficients (Manning's n) for both the channel and floodplain for each cross-section. These roughness coefficients are then used in the river flow prediction of 2, 5, 10, 25 and 50 years return periods. Flood frequency analysis of the upstream boundary control and lateral flows are carried out using Gumbel distribution frequency analysis for those return periods. The results of the analysis showed that the flood levels of the Prachin Buri and Bang Pakong Rivers for both pre and post construction of the Bang Pakong Barrage for each return period are very similar. This is because the flood analysis have been carried out in the wet season, so it is necessary to open the regulators of the Bang Pakong Barrage to prevent flood effect of the Bang Pakong River Basin and its tributaries. The results of flood level analysis of the Prachin Buri and Bang Pakong Rivers for post construction of the Bang Pakong Barrage for the 50 years return period showed that for the 101 cross-sections of the Prachin Buri and Bang Pakong Rivers, there are 74 cross-sections which are overflow above the maximum left banks or left dikes. The average depth of the overflow is 0.48 metre. There are 46 cross-sections which are overflow above the maximum right banks or right dikes. The average depth of the overflow is 1.15 metres. The city areas which face the flood problems are Amphoe Prachin Buri, Amphoe Barn Sarng, Amphoe Muang Cha Cherng Sao, and Amphoe Bang Pakong. The maximum flood level above the maximum bank or dike is found at Amphoe Barn Sarng with 0.37 metre above the left river-side and 1.24 metres above the right river-side. Flood protection measures for the Prachin Buri and Bang Pakong Rivers for post construction of the Bang Pakong Barrage for the 50 years return period consist of two measures. These include the river modification of the Prachin Buri and Bang Pakong Rivers, and the dike construction or hightening the existing dike. For the measure of the river modification of the Prachin Buri and Bang Pakong Rivers, it is suggested to dredge the rivers (to be deeper). The dredging process is between km 41.62 to km 183.62 for 65 cross-sections-requiring 2.88 metres deeper. For the measure of the dike construction or hightening the existing dike, it is suggested to construct or highten the dike on the right river-side of 39 cross-sections for the average height of 1.48 metres, and on the left river-side of 78 cross-sections for the average height of 0.49 metre. After these measures are applied, there is no overflow above the maximum bank or dike for both river-side of the Prachin Buri and Bang Pakong Rivers.


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