5.521 Land-based Dredging Plants . 



a. Plant Considerations . This type of operation usually employs 

 a dredging plant at a fixed position near the beach from which the sand 

 transported alongshore is intercepted as it moves within reach of the 

 plant. Presently, plants are of the pump type and operate basically as 

 an ordinary suction dredge. Plants are positioned on an existing struc- 

 ture; however some are on an independent foundation. Moveable plants 

 located on a pier with capability of dredging along the length and on 

 both sides of the pier have been proposed, but none has been built to 

 date. Such a plant would have a much larger littoral reservoir or depo- 

 sition basin to accumulate the littoral drift during storm periods when 

 the rate of transport exceeds the pumping capacity of the plant. A plan, 

 using an eductor and pumps located in an impoundment area updrift of an 

 inlet and capable of being moved within that area, is a possible method 

 of bypassing for large-scale operations. Although not used for that pur- 

 pose, an operation of this nature was used at Los Angeles (El Segundo) , 

 California, to level ancient dunes well behind the shoreline and trans- 

 port the sand to the beach. 



Shore processes at a littoral barrier must be studied critically to 

 design and position a fixed bypassing plant. The average annual rate of 

 longshore transport moving to the barrier must be known. This annual 

 rate will normally be the controlling criterion for determining the capa- 

 city of the pumping plant. Tlie average annual impoundment of littoral 

 materials by the littoral barrier is equal to the minimum quantity that 

 must be supplied to the downdrift shores to achieve stability. Short-term 

 fluctuations of the actual rate of littoral material movement to the 

 barrier as on an hourly, daily, or weekly basis may be many times greater 

 or less than the estimated annual rate reduced arithmetically to an hourly, 

 daily, or weekly basis. Tlierefore, even though a bypassing plant may be 

 designed to handle the total amount of drift reaching a barrier on an an- 

 nual basis, there will be occasions during the year when the quantity of 

 sand reaching the barrier will greatly exceed the pumping capacity of the 

 plant and occasions when the plant may operate well below capacity due to 

 an insufficiency of material reaching the barrier. 



To establish design criteria, a detailed study must be made of the 

 beach profile updrift of the littoral barrier to determine the best 

 location for the plant along the profile. A comparison of foreshore pro- 

 files over a period of time will aid in predicting the future position of 

 the foreshore and allow a determination of the best position of the plant. 

 Location of the plant too far landward may result in a Zand-Zodked plant 

 when the rate of transport reaching the barrier in a short interval of 

 time exceeds the plant's pumping capacity. Such a location may also result 

 in large losses of material around the barrier. A location too far seaward 

 may result in ineffective operation until sufficient materials have been 

 impounded by the barrier and are within reach of the intake mechanism. 

 The disadvantage of the fixed position plant has led to consideration of 

 a movable dredging unit on a trestle with the capability of dredging a 

 long deposition reservoir on both sides. This would increase the capacity 



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