Unless economical and practical factors dictate otherwise, rubble-mound 

 structures should be used for jetty construction. 



3. Weir Structure . 



Factors required in designing the weir section of a jetty include deter- 

 mining weir length, orientation, elevation, construction type, and location of 

 the landward end of the weir section. The weir length is selected so that it 

 extends through the normal surf zone and thus intercepts most of the sand in 

 transport along the beach. Laboratory experiments and qualitative field 

 observations indicate that much of the sand transported over a weir structure 

 crosses near the beach face and that the beach profile adjacent and updrift of 

 the weir adjusts and flattens to allow significant bedload transport over the 

 weir in the region where the beach, weir, and waterline intersect. The loca- 

 tion of maximum transport on the beach face changes with tidal stage. In the 

 breaker region adjacent to the weir, suspended sediments are also carried over 

 the structure. The amount of transport over the weir in this region is prob- 

 ably sensitive to the weir elevation, tidal stage, and level of wave activity. 

 To intercept this transport under most conditions, the weir section should 

 extend beyond the normal breaker location. Protection for a dredge from wave 

 action in the deposition basin is also a factor in establishing weir length. 

 The weir section allows some waves to overtop it and at times may allow exces- 

 sive wave action in the deposition area which would suspend bypassing opera- 

 tions. To minimize this downtime, the weir section should be as short as 

 possible. Weir elevation also influences the level of wave activity in the 

 deposition basin. The length of the weir section on existing weir jetties 

 varies from about 580 to 1,800 feet (see Table 1 which presents the character- 

 istics of the weir sections and wave conditions at six existing weir jetties 

 in the United States). The length of these weirs reflects designer concern 

 for "sanding-in" of the weir section. Sanding-in may occur during storms when 

 large quantities of sand reach the jetty but are not efficiently transported 

 over the weir into the deposition basin. Observations of the performance of 

 existing weir jetties suggest that this may not be as serious a problem as 

 first believed. In any event, the design of weirs in areas where large 

 quantities of sediment may be transported during short time periods should 

 consider the possibility of sanding-in. 



Weir elevation is established with sediment transport, wave attenuation, 

 tidal range, and tidal current considerations in mind. Sediment transport 

 considerations dictate that the weir be as low as possible. In fact, for 

 transport purposes, the weir could possibly be omitted altogether and only the 

 bottom armored to fix the beach at a desired profile. The weir would thus 

 serve as a template for the updrift beach. The necessity of wave protection 

 for a dredge operating in the deposition basin behind the weir, however, 

 requires that the weir elevation be as high as possible. A compromise weir 

 elevation must be established that will functionally serve both sediment 

 transport control and wave protection. The elevation of existing weirs is 

 given in Table 1. Generally, the weir elevation has been set at the mean tide 

 level (MTL) in areas where the tidal range is about 2 to 5 feet (Atlantic 

 coast) and at mean low water (MLW) in areas with a relatively low tidal range 

 (gulf coast). This appears to be a satisfactory compromise, but one that 

 should be investigated in the design of any weir-jetty system. In regions 

 with a large tidal range (12 to 15 feet) weir jetties are generally not a 

 viable alternative since the transport of sediment over the weir is limited to 

 only a small part of the tidal period. 



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