54 



Test of Hypotheses 



To analyze various parts of the weir jetty system, the monitoring program 

 was designed to address the specific hypotheses introduced in Chapter 3. The 

 results of the analyses to test these hypotheses are given below. 



a. Hypothesis No. 1: The weir-jetty system has minimal impact on adjacent 

 beaches. The time frame of 1984 to 1990 encompasses the building of the 

 jetties at the mouth of the river. During this time, over the first 3 km updrift of 

 the inlet, the shoreline position prograded an average of 20 m. Then, between 

 1990 and 1992, the shoreline eroded an average of 6.5 m. These data and a 

 numerical model by Heilman (1995) indicate that the shoreline adjustment to 

 the presence of the jetties has been relatively minor. Thus, the weir-jetty 

 system has had minimal impact on the adjacent beaches. 



b. Hypothesis No. 2: The weir and impoundment basin should be on the 

 northeast side of the inlet. Analysis of the wave data indicates that the strongly 

 predominant sediment transport direction is to the southwest (see Figures 13 

 and 14). This is in agreement with other published information, including: 

 Watson (1968), McGowen and Brewton (1975), and Heihnan (1995). Thus, 

 the proper location for the weir and impoundment basin is on the northeast side 

 of the inlet. 



c. Hypothesis No. 3: The weir is in the proper cross-shore location, is at 

 the correct elevation, and is the proper length. Visual observations during the 

 field experiments showed that the active portion of the surf zone (the breaker 

 zone) approximately coincided with the weir section of the jetty. The fact that 

 this occurred without a major adjustment of the shoreline position indicates that 

 the weir was properly located. In addition, analysis of the profiles across the 

 impoundment basin showed that the basin filled reasonably uniformly, with 

 only somewhat more rapid filling at the landward end. This also indicates that 

 the weir is in the proper location and is of the proper length. Attempts to 

 measure the cross-shore divergence of the longshore transport rate during the 

 field experiments did not yield conclusive results. Thus, quantitative support 

 for this conclusion is not available; however, qualitatively, the weir appears to 

 have been properly designed. 



Concern has been expressed that the sill depth of the weir is too low and 

 that this has led to excessive infilling of the impoundment basin. It is true that 

 if the elevation were raised, the impoundment basin would fill at a slower rate. 

 However, the sand not trapped in the basin would still go somewhere. It would 

 either cause the upstream beach to prograde, or it would lead to faster shoaling 

 around the jetty tips. The fact that the beach is fairly stable near the weir 

 suggests that the sill depth is at the proper elevation. 



d. Hypothesis No. 4: The impoundment area is large enough and the 

 anticipated dredging frequency is correct. Prior to jetty construction, the 

 littoral drift rate was estimated to be 230,000 cubic meters/year, and the 



Final Draft Chapter 6 Evaluation of Project Design 



