Monitoring of the Turag River Dyke in Dhaka City Using Ground Penetrating Radar (GPR): A New Approach of Flood Risk Reduction in Bangladesh

Abstract

AbstractThe focus of this study is placed on inspecting the Turag River dyke in Dhaka by using the Ground Penetrating Radar (GPR) which is often damaged during the monsoon season due to erosion caused by flooding and overtopping. The dyke was inspected to identify the weak zones at several locations by using GPR which is a non-destructive investigation technique. It requires no physical penetration into the ground, can perform quick survey and hence low cost involved compared to other surveys.GPR survey has been conducted in three selected stretches of the dyke. These selected stretches are Mirpur Sluice Gate, Rostampur Cross Dyke and Prottasha Bridge areas. GPR investigations on these particular portions of the dyke have done to monitor the health of the dyke and to locate any disturbed or weak zones which may need strengthening before monsoon.Radargram of Mirpur Sluice Gate area shows four distinct reflection layers of pitch (tar) mixed with brick; brick chips with sand; pre-existing footpath and compacted soil. Compacted soil below the pre-existing footpath formed by silty clay to clay materials found with slight variation of materials types, materials compactness and wetness, indication comparative weak zones in silty clay to clay. The lines representing the reflections from the ground layers suddenly end from both sides in the radargram indicating the existence of the sluice gate. In Rostampur Cross Dyke three subsurface layers can easily be identified from the radargram. The uppermost unit is mainly composed of brick chips and silty clay materials. Below the top layer, lithology of second layer has not been identified. Underneath the second layer the main dam materials are silty clay to clay. Variation of materials, compactness and wetness are also observed here. Radargram from GPR measurement on the highway beside Prottasha Bridge clearly shows that the bridge structure of the section is different from the adjacent sections at both sides. Reflections from pipes or cables under the highway are strongly visible. Although dyke materials are comparatively fine textured and wet but results were satisfactory and encouraging.KeywordsGPRDykeRadargramCompactness