conditions at the weir and also to decrease the tendency for adverse currents 

 to form. The amount of wave action on the leeward side of the weir is also 

 reduced because of the better wave attenuation characteristics of a rubble 

 weir. Figure 8 shows a typical rubble-weir section. 



Toe Protection 



///A&///^///^W^/k&W 



Foundation Blanket 

 J I I L 







Ift) 



25 



30 



Note: Dimensions and Stone Size will 

 Vary with Local Conditions 



Figure 8. Typical weir cross section. for a rubble-mound weir. 



Rubble structures provide some flexibility in adjusting the weir to accom- 

 modate unforseen transport condition variations for which the system may not 

 have been designed. Since the current ability to accurately and adequately 

 describe the longshore transport environment is poor, flexibility in adjusting 

 the transport characteristics of the weir section is highly desirable to cope 

 with reversals and anomalies in the period of record. Early attempts to 

 achieve flexibility by use of king piles with removable panels were unsuccess- 

 ful, mainly because of structural problems with removing and inserting panels 

 after the structure settled in response to wave and soil forces. These prob- 

 lems do not arise with rubble construction although any modification of a 

 rubble-weir section is costly and requires the use of heavy construction 

 equipment. Cost was a factor in selecting rubble weirs for both Murrells and 

 Little River Inlets where estimated construction costs for sheet-pile weirs 

 exceeded the cost of the selected rubble-weir configuration. The use of 

 rubble for the weir section is strongly recommended. 



The performance of the north jetty at St. Lucie Inlet, Florida, suggests 

 the possibility of designing a permeable jetty that functions as a weir. An 

 advantage of such a system is the increased wave protection afforded to a 

 dredge operating in its lee. St. Lucie Inlet, however, is not typical since 

 the deposition area Is not in the lee of the updrift jetty but in a spit that 

 develops inside the inlet (Fig. 9). Sediment passing through the weir is 

 carried by flood currents into the inlet where it is deposited in a spit which 

 grows along the inlet shore into the bay. The spit is dredged periodically 

 and the sand bypassed. The effectiveness of the system is partly attributable 

 to the local inlet geometry and to the particular wave conditions at the site. 

 However, a permeable weir may be more costly because of the additional stone 

 required to fill the weir section. 



The choice of weir type and alinement largely depends on the (a) site- 

 specific conditions, (b) location of existing shoals, (c) availability of a 

 suitable deposition basin area either adjacent to the weir or farther back 

 in the inlet, and (d) relative cost of the various alternative types of 

 construction. 



22 



