Scale Model Study of Flow Behavior of Storm Diversion Structure

Abstract

The low-lying area behind the Sonargaon Hotel and the Hatirjheel lowlands, extending from the east of Tejgaon diversion road up to the Pragati Shwarani at Rampura, receive significant discharges through a number of major storm sewer outfalls. Illegal connections of both domestic and industrial wastewaters to the storm sewer network are usual case. As a result, during dry season, the storm sewers mainly carry significant flows of domestic sewage as well as industrial wastewater. The untreated domestic sewage and industrial wastewater drains through this low land via the Begunbari khal-Norai khal into the Balu river; the Balu river eventually discharges into the Sitalakhya river. During monsoon, the pollution level drops to some extent due to dilution of domestic sewage and industrial effluent by rainwater/ stormwater. Over the years, the lowlands behind Sonargaon Hotel and Hatirjheel have virtually been turned into wastelands. In order to manage this combined water flow, storm water diversion system can be introduced. Storm Diversion structure (SDS) is a structure which is used to separate dry weather flow from storm water flow. Working principle of SDS is that storm water combined with dry weather sewerage flow is used to divert excess flows received during storm events into nearby receiving water body (lake), thus relieving other hydraulic structures within the area and reducing the risk of flooding in urban areas. In this study, attempt has been made to study the hydraulic behavior of a Storm Diversion Structure (SDS) in laboratory model under various flow conditions. In addition the overflow gate operation at various flow conditions have also been studied. The flow condition of the receiving watercourse and within the structure chamber has been observed for various gate operations. For a typical scale model study outfall Q4 of hatirjel- Begunbari area located near Tejgaon diversion road has been selected. Physical modeling facility (46m x 11 m) of Department of Water Resources Engineering (DWRE), BUET has been used for this purpose. Froudian law has been applied to design the laboratory scale model. Based on the availability of the space and the discharge capacity in modeling facilities, an undistorted model of scale 1:4 has been selected. A total number of 13 test runs were performed. Total of eight different discharges and seven different overflow gate openings have been considered in the present study. The bypass conduit opening was fixed at 76.2 mm as that of the prototype condition. It reveals from model study that the gate heights should be maintained in such as way that the water flow through combined system and dry weather flow through bypass pipe can be maintained. To do so, the overflow gate height is to be decreased to increase the total flow of combined sewer and vice versa (that has to be increased for decrease of total flow of combined sewer). The Froude number at various components have been calculated and found that the flow is always subcritical in storm diversion structure and found supercritical flow at bypass pipe for the flow rate greater than the dry weather flow. A hydraulic jump was observed in the under flow bypass pipe. To check the prototype design, normal depth of the flow through bypass pipe has been calculated from the Froude number and depth of water at storm diversion structure. All the model results and observations have been transformed in to prototype designed condition and found satisfactory agreement.