Field And Laboratory Investigations Of Siltation Near Groins Under Tidal Bore

Groins are ordinary structure for river improvement, river regime control, reclamation works and bank protection works.Groins plays a role of the utmost importance in safety of dyke using the collective effect of groins that is flow-deflecting and siltation-promoting, flat-maintaining and bank protection. In this paper, effect of siltation-promoting and dyke-proteciting for low groins, especially for sheet-pile type groins are studied from movable bed model experiment, prototype observation and mechanics analysis. The mechanism of the effect on protecting dykes and flats in tidal estuary is probed into from the angles of hydraulic performance of tidal flow across groins and tidal bore head spread characteristics.The relation of dyke-protection low groins layout parameters that include length, height, cut angle and relative intervals and siltation-promoting effects under tidal bore to the erosion-preventing and siltation-promoting is investigated by means of movable-bed model experiment. Optimized parameters for low groins to be suitable for river reach with strong tidal bore are obtained. The erosion-preventing and siltation-promoting effect near groins for the all kinds of schemes of sheet-pile type groins including comparison of single row sheet-pile type and double row sheet-pile type groins, comparison of different groin spacings and comparison of different water permeability in strong tidal bore reach is systematically investigated by means of movable-bed model experiment. Design parameters are provide in order to develop new type sheet pile type groins which both weaken the bore strength and not to damage groin itself and to reduce engineering maintenance cost. Influence of runoff changes on erosion and siltation of river bed is analyzed by means of observation underwater topography. Change of flat height and forms near groins after constructing low groins are investigated from longitudinal direction, lateral direction and different groins respectively. Erosion-preventing and siltation-promoting effect for each several part near sheet pile type groins in strong bore reach and is analysed.The erosion-preventing and siltation-promoting mechanism is summed up. The differences of sheet-pile type groins and rockfill groins in erosion-preventing and siltation-promoting near groins are compared. Tidal head process of bore near groins and variable of flow velocity on the inside and outside of groins basin are investigated by field observations.The response of low sheet-pile type groins on lessening bore strength and changing dynamic factors of tidal bore is discussed. The spread law of tidal bore head through low sheet-pile type groins is made clear by means of experiment and field observation. On this basis, experience for existing groins is summarized. Impact of all parameters on tidal bore pressure on sheet-pile type groins so as to provide the key parameters for the design of sheet-pile type groins and to put forward some improving proposals about design and construction of sheet-pile groins and to give the technical guidance and experiences for the deepwater groining works which extensively apply to the lower reaches in Qiantang estuary and the strong tide reach in Hangzhou Bay, and for the coastal shore-protection works, siltation-promoting works for in the other strong tide estuaries.