DESIGN OF ENCLOSED HARBORS TO REDUCE SEDIMENTATION

Craig H. Everts

Abstract


Sedimentation may be an important problem when quantities of suspended material are carried into an enclosed harbor on a flooding tide. In order to forecast future maintenance costs, two methods for predicting the sedimentation rate prior to harbor construction are proposed: 1) a sedimentation tank to be placed at the proposed harbor site, and 2) a mathematical model which uses sediment and hydraulic data collected at the harbor site. Certain considerations in the design phase of a project may effect a reduction in harbor sedimentation. If feasible, the harbor may be sited in a region where suspended sediment concentrations are low and sediment sizes (settling velocities) are small. Proximity to river sediment sources may be a factor. Conversely, a harbor site in a clear-water river adjacent to a sediment-laden estuary may be desirable if bedload transport during freshets would not be a problem. Settlement of suspended material may occur in the channel which connects an enclosed harbor basin with navigable waters. This material may subsequently be resuspended and carried into the basin thereby increasing the sedimentation rate. To reduce that rate the channel should be designed as short as possible. A sill in the channel may also be used to reduce initial excavation costs and the sedimentation rate. Flotation for vessels in the basin will be provided at all times, but movement into and out of the harbor will be reduced to times of higher water. In high latitude areas where harbor use is limited to periods when ice cover is absent, the sedimentation rate may be reduced using a channel closure structure during non-use periods. Winter sedimentation rates can be predicted using the mathematical model for summer conditions, and when ice thickness is known.

Keywords


sedimentation; enclosed harbor; harbor design; sedimentation reduction

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