US Army Engineer Research and Development Center

Vicksburg, Mississippi, USA

parchut@wes.army.mil

　

　

Most of the harbors and navigation channels in the world experience at least occasional shoaling problems, which result in loss of water depth needed for safe navigation of vessels. The major sources of shoaling include littoral transport, deposition of suspended sediment, sliding down of soft top layer along the slope of channel bank, sediment transport originating from adjacent areas, and slope instability. Catching sediment before it enters the project area is one of the effective methods for management of sediment deposition. Under favorable site and climatic conditions this can be achieved by providing a sediment trap at a carefully selected location.

　

Sediment traps do not catch all the sediment moving in the area. Hence dredging cannot be completely avoided, but the frequency and quantity of dredging can be significantly reduced. This increases harbor facility efficiency and results in significant cost savings on maintenance dredging. The sediment trap itself must be emptied periodically to keep it functional. Sediment traps cannot provide a general solution to channel sedimentation, however, they can be very effective in selected locations. Physical or numerical model studies are very useful in designing sediment traps. This paper describes the circumstances under which sediment traps can be effectively designed and operated for maximum benefit. Examples of sediment traps at Rollover Pass. Texas; Channel Islands Harbor, California, USA; Carolina Beach Inlet, North Carolina; and Visakhapatnam Port. India, are described. It is concluded that effective sediment traps can be designed at select sites for achieving reduction in maintenance dredging cost. Numerical and physical model studies, collection and analysis of field data on site parameters (particle size distribution of bed sediment, suspended sediment concentration, direction and magnitude of currents, bathymetry) and analysis of historical dredging records, are essential for a successful design and operation of sediment traps.

　

　

Most of the major estuarine, coastal and inland harbors in the world are connected to the sea. Ships from the sea access the berthing areas of these harbors through navigation channels. For the prevailing drafts of commercial vessels the depth of navigation channel varies from about 10 m to 15 m. Large oil tankers require greater depths of up to 20 m. After initial navigation channel dredging, most projects require periodic maintenance dredging, which is quite expensive. Port operating agencies all over the world have continually tried adopting various ways to reduce the amount, frequency, and cost of dredging.

　

Catching sediment before it enters the sensitive area is one of the effective methods for management of sediment deposition. Under favorable conditions of site and climate this can be achieved by providing a sediment trap at a carefully selected location. Sediment traps do not catch all the sediment moving in the area. Hence navigation channel maintenance dredging cannot be completely avoided, but the frequency and quantity of channel dredging can be significantly reduced. This increases the efficiency of harbor facilities and results in significant cost savings on maintenance dredging. The sediment trap must be emptied periodically through dredging to keep it functional. Although the volume to be dredged from the trap may sometimes offset any reduction in project shoaling, there can still be benefits. These include a) navigation is not disrupted by shoaling in the project, b) less frequent dredging usually reduces overall dredging costs, c) the trap can be intentionally located close to dredged material disposal areas, which results in reduced transportation time and cost. Sediment traps are not very common because they can be effective only at selected locations and cannot be provided as a general solution to channel sedimentation problems. Physical or numerical model studies are very useful in designing sediment traps.

　

　

Literature on the subject of trapping sediments is found under three main categories:

Five case studies along with details of sediment traps designed by the authors at two other projects are described in this paper.

　

　

When sediment-carrying currents flow normal or near normal to a navigation channel, the channel acts like a trap in collecting a part of the sediment crossing the channel and the remaining is bypassed. Larson and Kraus (2001) have given an analysis of the sediment-trapping phenomenon and a method to work out the trapping efficiency. The parameters that affect the efficiency of sediment trap are: depth of water inside and outside of trap, type and particle size of sediment, plan area of trap, alignment of trap relative to the predominant current direction, magnitude and distribution of current velocity, and mode of transport of sediment (bed load or suspended load). Sand traps are more efficient in catching bed load transport consisting of sand than in catching suspended sediment. Larger and deeper sediment traps may be required for trapping suspended sediment.

　

　

Parchure and Teeter (2002b) have conducted a review of potential methods adopted at several projects for reducing shoaling in harbors and navigation channels. The following case studies will illustrate the effective use of sand traps at a few projects. It is interesting to note that in the case of Delaware City Channel project, studies indicated that sand traps would have an adverse effect on shoaling.

　

A fixed-bed hydraulic model was used to qualitatively assess the relative merits of several shoaling-reduction proposals consisting of seventeen plans. One of the conclusions of the study was that a combination of three sediment traps and a deepened portion of Marcus Hook anchorage would materially reduce navigation channel maintenance from the Philadelphia Navy Yard to Marcus Hook.

　

Sediment traps are sometimes used in conjunction with jetties to intercept and collect littoral sand, which might otherwise cause shoaling in a navigation channel. The trap is positioned to interrupt the natural flow of sand transported along a coastline before it reaches the channel. This sand is periodically dredged and placed down coast where it is reintroduced back into the natural transport system. A single updrift trap is used where longshore transport is dominantly unidirectional whereas twin traps may be employed to protect a channel where major transport reversals occur. The Channel Islands sediment trap has functioned well as designed by trapping the bulk of littoral drift sediment.

　

The sediment trap in Carolina Beach Inlet has functioned fairly well but was located too close to the main flow through the inlet to be completely effective. Studies showed that relocation of the sediment trap seaward of and away from the main channel should greatly enhance its overall sand trapping ability. Jarrett (1988) has made the following recommendations. "Sediment traps in tidal inlets should be located in areas removed from the concentrated tidal flows. For example, an ideal location for a sediment trap would be in the area of an existing interior shoal that is fed with littoral material moving off the inlet shoulders. In the case of Carolina Beach Inlet, much of the trap was located in the area of concentrated tidal flows and, as a result, the trap only filled to about 66 percent of its dredged capacity. The trap should also be dredged as deep as possible, but not deep enough to create problem with sloughing of the adjacent shorelines into the trap."

　

Various plans to reduce heavy siltation in the harbor area of Front River were examined in a physical model (US Army Engineer Waterways Experiment Station, 1963). The recommended plan consisted of a sediment trap in the lower portion of Back River, and a tide gate structure in Back River upstream of the trap. The gates would be closed during ebb tide, forcing more flow down through Front River. This would flush sediments downstream in the navigation channel. The gates would be opened during flood tide, allowing normal flow up through Back River. This would divert sediments from the navigation channel into the Back River sediment trap. Relocation of the sediment deposition area not only reduced shoaling in the harbor area but also resulted in dredging operations closer to available disposal areas. Navigation channel shoaling was reduced by about 30 percent. The trap functioned very well for several years. The gates had to be removed later for environmental reasons.

　

The Tidewater Oil Company, Delaware Refinery, at Delaware City explored the possibility of reducing shoaling at their facility. Six plans were developed consisting of dikes and two locations of a sand trap. The results of investigations were interesting. They showed that all the plans tested had an adverse effect on total shoaling in the company channels. If the plans were implemented, total shoaling was expected to increase by amounts varying between 4% and 42% per year depending upon the plan.

　

　

Rollover Pass is a narrow man-made channel, which connects the Gulf of Mexico and Rollover Bay. The Gulf Intracoastal Water Way (GIWW) crosses the Rollover Bay on the north side of Rollover Pass. The U.S. Army Corps of Engineer District, Galveston, maintains a navigation channel, 40 m wide and 3.6 m deep within the GIWW for commercial barge traffic. Over the past several years considerable siltation has been taking place within the GIWW in the vicinity of Rollover Pass area and periodic dredging is required for maintaining navigable depths. The U.S. Army Engineer Research and Development Center (ERDC), Vicksburg examined ways to reduce siltation of the channel. The objective of the study was to construct a working numerical model of the Rollover Pass area and to use the model for design of a sediment trap, which would be feasible and effective in reducing the frequency of dredging in GIWW. Parchure et al (2000) have described the design of a sediment trap at this location. The hydrodynamic model code RMA2, available at ERDC, was used to calculate the hydrodynamics of the system with this two-dimensional numerical model. The model was verified using hydrodynamic field data. Velocity patterns under selected tidal conditions were generated.

　

Sediment data collected from site included bed samples, water samples and past dredging records. Results of analysis of bed samples are given in Figure 1. It was seen that the reach of channel directly in the path of flood currents through the inlet consisted of sand whereas on both sides of this reach the bed material consisted of fine sediments. It was noticed that the average composition of the bed samples consisted of 30% sand, 50% silt, and 20% clay. Coarse sediment appeared to be traveling from the sea all the way to the GIWW whereas East Bay has been the source of finer sediment. Analysis of dredging records showed (Fig.2) that a length of GIWW between sections 2136 and 2166 had a much higher rate of sediment deposition. Typical flow patterns in the vicinity of the proposed sediment trap for the flood and ebb currents obtained on the numerical model are shown in Figures 3 and 4 respectively. Computation and analysis of bed shear stress patterns were used along with the velocity data to estimate where and by how much sediment deposition is expected to occur. Several alternative sediment trap layouts in terms of location, shape, size and depth were used for evaluation. The recommended layout is shown in Figure 5.

　

The recommended sediment trap layout has a length of 915 m and a width of 120 m and it is separated from GIWW by a distance of 60 m. The 120 m width would be needed not only for obtaining better trapping efficiency but also for providing adequate room for maneuvering a dredge inside the trap. Recommended design depth in the trap is 2.75 m, which is expected to be adequate for safe dredging operation. The width and depth of the trap may varied in the future, if found necessary and advantageous.

　

It is recommended that the new sediment trap be dredged over a smaller area. Its effectiveness should be monitored over the next two years after construction. Expansion of the trap over larger areas in the next two phases should be done later, if experience shows that the first phase is having the desired effect. The proposed trap is expected to catch the excessive sediment accumulating between sections 2136 and 2166 and prevent formation of a local hump, which at present necessitates more frequent dredging. Hence the sediment trap is expected to reduce the frequency between consecutive dredging operations and the average annual cost of dredging.

　

The recommended trap configuration has the following features. It is not connected to the GIWW over its entire length. It does not include construction of any structures. It provides one connection with GIWW for a dredge to enter. The trap does not permit a "flow-through" hydraulic condition. Phasing of dredging work for future expansion is easy and feasible. The sediment removed for making the trap should be deposited on the eroding beach, provided it is suitable for beach nourishment. Environmental impacts of the sediment trap were not examined in the study. The Galveston District has accepted the recommendation and is proceeding with plans for construction. Field data on its functioning will be available after the trap is constructed.