WEIR SAND- BYPASSING SYSTEMS 



by 

 J. Richard Weggel 



I. INTRODUCTION 



The construction of jetties to provide safe navigation conditions at 

 harbors or tidal inlets along sandy coasts usually results in interruption of 

 the natural longshore transport of sand at the harbor or inlet. Sand that 

 previously found its way from an inlet's updrift side to its downdrift side 

 through natural processes is trapped in the updrift fillet or is diverted 

 offshore. The resulting starvation of the downdrift beach can cause serious 

 erosion unless measures are taken to transfer or bypass sand from the updrift 

 side to the downdrift beaches. Several sand-bypassing methods used in the 

 United States are discussed in Section 6.5 of the Shore Protection Manual 

 (SPM) (U.S. Army, Corps of Engineers, Coastal Engineering Research Center, 

 1977). 



The simplest method in concept, but in some respects the most difficult to 

 implement, is to remove the sand accumulated in the fillet of the updrift 

 jetty with a pipeline dredge and transfer it to the downdrift beach (Fig. 1). 

 However, the dredge may be difficult to operate in an area exposed to ocean 

 waves. This difficulty led to the development of fixed sand-bypassing plants. 

 These plants are usually constructed on the updrift jetty and are partially 

 protected from extremely large waves by the shallowness of the water in front 

 of them. Fixed bypassing plants are usually limited in the amount of sand 

 they can intercept and handle because of their lack of mobility. Another 

 method for bypassing sand from an updrift fillet is by use of a conventional 

 floating pipeline dredge to cut into the fillet from the ocean (described in 

 Section 6.521 of the SPM). The dredge operates within a lagoon in the fillet, 

 having closed the entrance channel behind it. After bypassing enough mate- 

 rial, the dredge again cuts a channel to the ocean and exits from the lagoon. 

 Experience in using this technique has been limited to a single instance at 

 Port Hueneme, California. 



At some harbors along an open coast with a shore-connected breakwater, by- 

 passing is performed by dredging the shoal that accumulates at the distal end 

 of the breakwater (Fig. 2). A dredge can usually operate in the quiet water 

 behind the shoal itself and move into the harbor if adverse weather threatens. 

 An example of this bypassing scheme is at Santa Barbara Harbor, California. 



Shore-parallel offshore breakwaters built updrift of inlets or harbor 

 entrances have also been used to establish a sheltered deposition area where a 

 dredge can operate to bypass sand (Fig. 3). In some cases, such as at Channel 

 Islands Harbor, California, the breakwater also serves to protect the harbor 

 entrance. The bypassing system at Channel Islands Harbor is described in 

 Section 6.522 of the SPM. 



This report discusses the sand-bypassing system that requires the con- 

 struction of a weir jetty. A typical weir-jetty system is shown in Figure 4. 

 In this system, a part of the updrift jetty Is depressed to form a weir 

 section across which sand is transported to a deposition area by waves and 

 tidal currents. A conventional pipeline dredge operating in the deposition 



11 



