During a visit to the area in October 1976, longshore currents were meas- 

 ured using dye as a tracer. A minor storm was then occurring from the north- 

 west with wave heights of 1.2 meters and periods of 7 seconds. The water depth 

 along the seawall increased from 1.4 meters at the northern end to 1.7 meters 

 near the southern end of the wall. Because of this depth, waves were not break- 

 ing before striking the seawall. Longshore current measurements taken about 2 

 meters from shore at stations 14, 28, and 44 (see Fig. 9) are given in Table 13. 

 The southward-moving current was about twice as fast near the southern end of 

 the seawall as north of it, and more than three times faster than the current 

 just south of the downdrift cut where the beach begins. Therefore, the current 

 is capable of moving more sediment at the downdrift end of the seawall than it 

 is either updrift of the seawall or south of the downdrift cut. Although the 

 amount of material moved depends on the width of the current, its effectiveness 

 is clearly evident in the complete absence of a beach in front of the seawall. 



Table 13. Longshore current measurements 

 in reach B, 16 October 1976. 



Station 



m/s 



ft/s 



44 (north of seawall) 

 28 (along seawall) 

 14 (south of seawall) 



0.30 

 0.58 

 0.18 



1.0 

 1.9 

 0.6 



According to Silvester (1977), the effect of the seawall should be localized 

 between the shoal and the downdrift end of the wall. In this specific case, 

 the affected beach appears to be lengthening, partly because of the measures 

 taken to stabilize the cut and partly because of the reduced sediment supply 

 caused by the wall. 



Using the aerial photo data, the volume of material lost in the downdrift 

 cut was estimated and compared to the volume of material removed from the sedi- 

 ment supply by the seawall. The computations (Table 14) are based on the period 

 between 20 March 1973 and 2 3 November 1974 when the seawall was completed, the 

 backing bluff had stabilized, and the beach in front had disappeared. Expected 

 rates of recession for the seawall and downdrift cut were computed from the re- 

 cession of reach A during the same period but were adjusted by a factor based 

 on the relative recession rate between each section and reach A in 1971, before 

 the seawall was constructed. Therefore, the expected rate for the downdrift 

 cut is low relative to reach A. Average elevations for each section were sub- 

 jectively determined from topographic maps and a few ground measurements. 





Table 



14. Conp 



arisen 



of volumetric losses behind and adjacent 



to seawall. 





Section 



station 



Elevation' 



Length 



1971 blaff 

 recession 



(m) 



Factor^ 



Bluff recession rate 



20 Mar. 1973 to 23 Nov. 1974 



(m/yr) 



Total volume change 

 20 Mar. 1973 to 23 Nov. 1974 

 (niVyr) 



Expected 



Actual 



Expected 



Actual 



Difference 



Reach A 



51 to 111 



12 



1,646 



3.6 



1.0 



4.6 



4.6 



90,859 



90,859 







Dune 



42 to 46 



13 



152 



8.1 



2.3 



10.6 



3.1 



20,946 



6,126 



-14,820 



Seawall 



23 to 41 



12 



579 



2.9 



0.8 



3.7 



1.5 



25,708 



10,422 



-15,286 



Downdrift 



14 to 22 



9 



274 



0.8 



0.2 



0.9 



9.1 



2,219 



22,441 



20,222 



cut 



1 to 13 



7 



396 



1.8 



0.5 



2.3 



3.6 



6,376 



9,979 



3,603 



Total 





















- 6,281 



^Estimated from topographic maps and some field data. 



^Ratio of the recession rate of each section to the reach A rate for November 1970 to November 1971. 



NOTE. — Ratio of volume differences: dovmdrift cut (stations 1 to 22) 

 to dune and seawall (stations 23 to 46) 23,825/30»106 = 0,8. 



59 



