SUCTION DESIGN CONSIDERATIONS FOR SAND BYPASSING BACKPASSING SYSTEMS

Bypassing and backpassing both involve significant dredging operations . In backpassing, beaches are replenished by offshore dredging, usuall y with conventional dredgers. Bypassing is typically employed to move sand supplied by littoral drift across an estuary or harbour entrance, thus preventing excessive erosion at the updrift side and erosion in the downdrift region. Fixed dredge plant and pipelines are often propo sed for bypassing. Both bypassing and backpassing generally involve hy draulic suction dredging, often from considerable depths. At these dre dging depths pumps must be set well below water surface level to avoid cavitation, and the required depth increases with solids concentratio n. For fine sands and velocities well above the limit of deposition, p ump settings can be calculated approximately by the equivalent fluid m odel. For other eases more sophisticated modeling is required. The exa mple calculations given in the paper show that for sand-water hows, en larged or sloping suction piping generally does not reduce the likelih ood of pump cavitation. However, submerged feeder pumps can be useful in this regard, for example centrifugal ladder pumps on dredgers used for backpassing. For bypassing plants, either a very deep single pump or multiple feeder pumps are possibilities. It is desirable to have th ese feeder pumps (probably jet pumps) discharge to an open sump before the main pump. This arrangement enhances stability and can increase s and concentration, leading to improved system efficiency.