o 



^ 



NORTH 

 JETTY 



SIUSLAW 



SOUTH 

 JETTY 



SCALE IN FEET 



1000 



3000 



> 



Figure 16. Layout of Siuslaw River jetties and spurs 



The jetty extensions and spurs from the mouth of the Siuslaw River 

 were monitored during the period September 1988 through September 

 1993 (Pollock et al. 1995). The objectives of the monitoring program 

 were to (a) determine if the spurs effectively deflected sediment, (b) iden- 

 tify shoaling patterns near the jetties, (c) compare prototype conditions 

 against those predicted by a physical model study, (d) evaluate the effec- 

 tiveness of the system in reducing maintenance dredging requirements, 

 and (e) evaluate the impact of the jetties on the surrounding beaches. 



An inspection of the jetty system on 26 June 1997 revealed that some 

 repairs were needed. On the north jetty head, a void was observed on the 

 channel side of the structure. The area was steep, and it appeared that 

 stone had been pulled downslope by wave action as remnants could be 

 seen seaward of the area. About 20 m (65 ft) shoreward of the jetty head, 

 a low area in the center of the crest existed; and approximately 27.5 m (90 

 ft) from the head a void existed on the channel side of the structure. In 

 several areas along the jetty extension, it appeared that the center portion 

 of the crest had subsided and the elevation was irregular. The stone along 

 the slopes, however, appeared to be up to required elevation. At a point 

 about half the length of the jetty spur, it appeared a stone had been up- 

 lifted leaving a small void (Figure 17). With the exception of the jetty 

 head, the slopes on the north jetty extension and jetty spur were in good 

 condition. Inspection of the south jetty extension indicated a similar situ- 

 ation at the jetty head on the sea side of the structure. The slope was very 

 steep, and it appeared that stone had been pulled downslope by wave 

 action (Figure 18). The head of the south jetty spur also was damaged on 

 its shoreward side as evidenced by a lack of stone above the water surface 

 (Figure 19). In addition, immediately shoreward of the jetty spur head, a 

 void existed on the shoreward side of the spur (Figure 20). The side 



18 



Chapter 2 Project Descriptions and Inspection Results 



