Thesis (Ph.D.) - Cairo University - Faculty of Engineering - Department of Civil Engineering

Dredging is an excavation activity usually carried out underwater, in shallow rivvers, seas or fresh water lakes with the purpose of gathering up bottom sediments and disposing them at a different location. This technique is often used to keepwaterways navigable. It is also used as a way to replenish sand on some public beaches, where sand has been lost due to coastal erosion Two major categories for dredging exist; (I) mechanical, and (ii) hydraulic. Examples include off cutter -suction, bucket wheel, clamshell, and water injection. Mechanical dredging initiates a lot of pollution problems, negative environmental impacts, suspension, and leakage of oil beside to its dependence on massive equipment with high cost. On the other hand, traditional hydraulic dredging causes a lot of disturbance effects which may extend to large areas outside of the dredging zone. The main objectives of the study are to: (I) establish an enhanced controlled hydraulic dredging technique which entrains sediment motion through creating a flow field with inward radial pressure gradient thus creating a spiral boundary current along the bed, which in turn simulates the action of a vortex vacuum cleaner that removes materials from flooring ( air flow), the study aims to remove sediment from lake bottom (water flow), and (II) study the effect of vortex strength on the efficiency of dredging and maximum scour depth for non-cohesive soils. To investigate the vortex dredging technique and its effect on dredging efficiency, both physical and numerical models are designed and tested. Several physical models have been constructed and tested at the Irrigation and Hydraulics lab, Cairo University. The final physical model consists of a cylinder with total height of 980 mm. The inflows through two tangential pipes with diameter of 21mm are used. A cone of diameter 180mm and height 200mm represents the inlet to a 40mm suction pipe to dredge the removed soil. The initial depth of soil is 280 mm from the bottom of cylinder. The pressure head variations inside the physical model are measured using nine bottom radial peizometers. Two different sand grain sizes of D50=0.5 mm, and 0.9 mm are tested

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