Suction Design Considerations for Sand Bypassing/Backpassing Systems

Abstract

Bypassing and backpassing both involve significant dredging operations. In backpassing, beaches are replenished by offshore dredging, usually 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 proposed for bypassing. Both bypassing and backpassing generally involve hydraulic suction dredging, often from considerable depths. At these dredging depths pumps must be set well below water surface level to avoid cavitation, and the required depth increases with solids concentration. For fine sands and velocities well above the limit of deposition, pump settings can be calculated approximately by the equivalent fluid model. For other cases more sophisticated modeling is required. The example calculations given in the paper show that for sand-water flows, enlarged or sloping suction piping generally does not reduce the likelihood 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 these feeder pumps (probably jet pumps) discharge to an open sump before the main pump. This arrangement enhances stability and can increase sand concentration, leading to improved system efficiency.