Study On Sedimentation Hazards And Mitigation Measures For Port And Navigation Channels On Silty-Type Coasts

China has coasts with long coastlines,along which silty-type coasts are widely distributed.Although those coasts share their common features,the characteristics in terms of the features related to sedimentation are not completely identical,e.g.,the coastal morphology,the source of sediment,the sea bed composition features,the mechanism of sedimentation,and morphology evolution.The construction of a port on the silty-type coast is unavoidably confronted with the common problem of sediment deposition.A methodology was proposed for the classification of typical silty-type of the domestic coast.The main features of sediment movement possessed by different coasts were analyzed.Prediction formula of sediment deposition for the outer navigation channel of port was improved by the introduction of a new formula for estimation of the bottom sediment concentration.A concept of the return period of storm deposition was proposed for navigation channel.The features of sediment motion on silty coast were analyzed,and the corresponding mitigation measures for sediment deposition were proposed based on engineering practices.The main conclusions are summarized as follows.(1)From the viewpoint of coastal morphology,the silty coasts were categorized in the present study as plain-type,estuarine-delta-type,and radial-sand-ridges-type.From the viewpoint of topography equilibrium,the silty coast was classified as depositive-type,equilibrium-type,and erosive-type.Further study indicated that a silty coast behaves more like a sandy coast if the sediment composition of the sea bed incorporates coarser silt or even fine sand,whereas behaves more like a muddy coast if the sea bed composition involves more fine silt or mud.Therefore,a method was proposed to classify the silty-type coast into the fine-sand silty type and muddy-silty type from the view of bottom composition and features of sediment motion.A coast is classified as fine-silt type if it has a median grain size beyond 0.06 mm,a fraction of cohesive sediment below 15% and a bottom slope below 1:1000.The sedimentation on this type of coast combines the transport patterns of both longshore current and tide-driven current.A coast was categorized as muddy-silt coast if it has a sediment grain size below 0.06 mm,a fraction of cohesive portion beyond 15%,and a bottom slope below 1/1000.(2)The formulae for estimation of sediment deposition in navigation channels on silt coasts were summarized,in which sediment motion is assumed in three patterns,namely the suspended mode,the bed load mode and the high concentration mode.The formulae were discussed based on the measured data of Huanghua Port.The results indicated that the bottom sediment concentration estimated by the van Rijn formula is close to the results measured in the field.Based on the results of estimation,sediment deposition volume was estimated using the three-mode formula and the results basically agree with the field measurements and the formula was proved better than the formulae available.(3)Sediment motion on the silty coast is generally governed by the role that waves play a role of disturbing sediment while tide current plays a role in transporting sediment.Besides this basic feature,each type of coast has its own characteristics and features.The fine-sand silty coast exhibits some what sandy coast.Through numerical calculation and analysis,it is believed that besides the amount of sediment transport inside the surf zone corresponding to the wave-induced longshore current,the amount of sediment transported by tidal current outside the surf zone should also be taken into account.Through discussion on some typical formulae of longshore sediment transport,it is found that the tide-induced sediment transport should be taken into consideration for a silty-type coast and reasonable results can be obtained by using the flow velocity results from a shallow-water model combined with a current-based sediment transport formula.(4)The modeled results of current field using the combined numerical models of a wind generation wave solver,a shallow water equation solver and a sediment transport equation solver were discussed.The results of waves and currents were introduced as input into some typical analytical formulae of longshore sediment transport with focus on the applications of the three formulae van Rijn’s formula,Wang and Zhou’s formula and Sun’s formula.The calculation indicates that Wang and Zhou’s formula is easier to use in calculation of sediment transport process during a storm than the other two formulae,because Wang and Zhou’s formula uses local wave and current parameters as input and the results also agreed well with the measurement,while the formulae of van Rijn and Sun share the similarity of using characteristic wave and current parameters as input,which is difficult to determine due to the complexity.From the results it was also found that both wave height and water depth play an important role in sediment transport,leading to that sediment transport rate gains the maximum value in the condition of large wave height on shallow water depth.(5)By summarizing the domestic experiences in port construction,a concept of return period for storm deposition was proposed and related standards on the protection and maintenance measures for navigation channels as well related design principles and methodology were proposed.For a newly-developed port,from the viewpoint of deposition mitigation effect,operation safety,and investment interest,a10-year return period is reasonable for the design of the countermeasures for a navigation channel against storm deposition.For a port reaching a large scale,the standard against storm deposition can be lifted reasonably based on assessment of the operation requirement,real deposition amount of the channel and allocation of the dredging facilities of the port.For a fine-sand silty coast,as its beach slope is relatively steeper,only a relatively shorter dike is required,allowing a choice of relatively higher standard of protection measure.(6)A strategy to combine emerged and submerged dike was proposed to adapt to the difference of various types of silty coasts for deposition mitigation as well as for reducing disturbance on the flow field.The applicable condition of a new type of sediment resisting curtain was analyzed and a selection method for main parameters of the silt curtain was proposed.The Theoretical formulae in the design of silt curtain were validated using physical model test data.The curtain force formula for the condition under waves was modified using experiment data.