larger littoral reservoir or deposition basin to accumulate the littoral drift 

 during storm periods when the rate of transport exceeds the pumping capacity 

 of the plant . 



An installation using an eductor with pumps located in a weir jetty 

 impoundment basin has been in use since 1975 at Rudee Inlet, Virginia (Ch. 6, 

 Sec. V,l,c). This method, known as jet pump sand bypassing, dredges a large 

 deposition area by repositioning the pumps within that area. Richardson and 

 McNair (1981) describe the jet pump system and outline the necessary planning 

 and hydraulic design for such an installation. 



A critical study of shore processes at a littoral barrier must be made and 

 the variations in longshore transport moving to the barrier must be estimated 

 to design and position a fixed bypassing plant. The average annual impound- 

 ment of littoral materials by the littoral barrier is generally 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 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. Therefore, even though a bypassing plan may 

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

 annual basis, there will probably be occasions during the year when either the 

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

 plant or the plant will operate below capacity due to insufficient 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. Comparing foreshore profiles over a period of time will not only aid 

 in predicting the future position of the foreshore, but will also allow a 

 determination of the best position of the plant. Location of the plant too 

 far landward may result in a landlocked 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 an ineffective opera- 

 tion 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 of the littoral reservoir and reduce the possi- 

 bility of landlocking the plant. Mobility of a land-based dredging plant may 

 overcome some deficiencies of a fixed plant; however, it seems unlikely that 

 such a plant would be capable of bypassing all material when the rate of 

 arrival at the site is high. Therefore, some material would be lost around 

 the barrier. 



(2) D ischa r ge Line Considerations . The best alinement of the dis- 

 charge line from the fixed plant to the downdrift side of the littoral barrier 

 or inlet is controlled by local conditions. The discharge line must traverse 

 a channel maintained for vessel traffic; a floating discharge line is imprac- 

 ticable. If the line is positioned on the channel bottom, an allowance must 

 be made for the protection of the line against damage by pitching ships and by 

 maintenance dredging of the channel. Also, a submerged line may require a 



5-31 



