IMPROVED DESIGN CRITERIA FOR EXTENDING BRIDGE-LIKE INTAKES INTO OPEN CHANNELS

Extended water intakes are structures established at river sides to divert water for multiple uses and purposes. Previous studies have only focused on extending the intake to a sufficient water depth for ensuring permanent abstraction. They used physical and numerical models to predict the trend of the site riverbed morphological changes for identifying the lowest bed levels and biggest depths to which intakes could be extended. However, they have ignored other parameters that govern the extension. This paper aims to investigate those parameters and introduce a practical and scientific basis that helps estimate the minimum (min) offshore extension distance of a water intake. It is the distance between the point where the annual min water stage meets the channel bank and the point where a critical water depth is sufficient to submerge the suction pipe inlet without producing vortices that affect both the pumping system and channel bed. Based on the definition of the extension distance, the critical water depth under the min water stage at the channel cross section concerned has to be computed. By analyzing the min water stage, it was found that it is equal to the sum of a submergence depth, the diameter of the suction pipe inlet or (the strainer length), and a clearance distance above the channel bed. Doing further analysis, it was found that the submergence depth value depends on the water station discharge requirements, the water suction velocity at the pipe inlet, and the maximum height of a water wave generated by a moving navigation boat near the intake as well as the channel bed morphology. Also, the value of the clearance distance above the bed was found to be a function of the suction pipe diameter or (the strainer length). To compute the min extension distance, a Spreadsheet Model was developed to correlate all the concerned parameters and help study as many scenarios as possible to find the most economical and safest distance. Finally, the research concluded that the computation of the min extension distance is governed by a number of factors such as the min water stage, water station discharge requirements, water suction velocity at the pipe inlet, and maximum water wave height generated by a moving boat near the intake.