The amount of sand that needs to be stored in the updrift fillet can be 

 estimated from a mass curve constructed from the time history of longshore 

 transport at the site. Such a time series can be constructed from wave data 

 obtained by LEO observations, SSMO, or wave hindcast data. A representation 

 of the time series is shown in Figure 30. The upper curve in the figure 

 represents the time history of the longshore transport rate; the lower mass 

 curve is the integral under the longshore transport rate curve and represents 

 the cumulative amount of sand passing the observation point from the time 

 observations began. The line superimposed on the mass curve is a best-fit 

 straight line; its slope represents the net longshore transport rate at the 

 site. The deviation of the mass curve from the straight line corresponds to 

 the amount of sand needed in active storage to nourish updrift beaches during 

 reversals in transport. Figure 31 represents a similar curve for a site where 

 reversals are more frequent in contrast with a site where reversals are sea- 

 sonal. When reversals are short term and frequent, the amount of active 

 storage required in the updrift beach is generally less and the updrift beach 

 will be less variable in planform. There is less updrift shoreline retreat 

 during periods of transport reversal since the duration of reversals is 

 shorter. 



The minimum amount of sand to be transferred and the capacity of the depo- 

 sition basin can also be established from the mass curve. If bypassing is 

 performed biannually, the ordinate of the straight line on the mass curve (net 

 or average transport) at t = 2 years will give the minimum amount of deposi- 

 tion basin storage required. It also represents the minimum amount of sand 

 to be bypassed after a 2-year period. In addition, the mass curve provides 

 information on scheduling bypassing operations. If bypassing to the downdrift 

 beach is performed when the updrift beach is emptying (the trend of the slope 

 of the mass curve is negative), transport will be in the updrift direction and 

 sand placed on the downdrift beach will move toward the inlet. Bypassing 

 should be scheduled for those seasons when the trend of the mass curve slope 

 is positive to ensure that bypassed sand moves downcoast away from the inlet. 



A major problem in constructing the required mass curve is the avail- 

 ability of sufficient, reliable wave data to develop the time series of 

 longshore transport rates. Because the wave climate at a site may vary from 

 year to year, 1 or 2 years of wave records may not be enough to adequately 

 define the magnitude and duration of reversals. A minimum of 3 years of wave 

 data should be used and even then, conditions in any 1 year might differ 

 appreciably from conditions during the period of record. The designer should 

 investigate conditions that deviate from measured records to determine project 

 performance under extreme conditions. Questions such as, "How will the proj- 

 ect perform if the net longshore transport rate has been underestimated or 

 overestimated, or if the project experiences an extreme storm?" should be 

 asked and the consequences evaluated. 



VIII. WEIR SECTION LENGTH 



In general, weir section length should be established to extend seaward 

 beyond the normal breaker line. Most of the sand transported over the weir 

 moves across in a relatively narrow region close to where the weir, beach, and 

 waterline intersect. Preliminary results from laboratory tests indicate that 

 sand transported over the weir in this region moves as bedload. The amount of 

 sand transported varies with wave conditions and tidal stage. There is also 



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