was narrow, with a dangerous shoal and 

 rapid tides. This situation continued 

 through the nineteenth century. In 1888 

 the U.S. Army Corps of Engineers re- 

 ported that there was no reason to im- 

 prove the channel entrance. Fifteen 

 years later, the north jetty was propos- 

 ed to control the ebb current (Terich 

 and Komar 1973). The north jetty was 

 completed to a length of 5,400 ft (1,646 

 m) in 1917 at a cost of $776,000. It 

 incorporated 429,000 tons (389,180 met- 

 ric tons) of stone. It was extended 300 

 ft (91 m) in 1933 (Terich and Komar 

 1973). No cost information on the exten- 

 sion was found. 



By 1921, four years after the north 

 jetty was constructed, the channel had 

 migrated to a new position against the 

 ietty, and dredging was later required 

 to keep it open (Kieslich and Mason 

 1975). The hazardous channel conditions 

 ultimately led to the construction of a 

 second, longer jetty on the south side 

 of the entrance beginning in 1969 

 (Terich and Komar 1973). The south 

 ietty, completed in 1974, cost about 

 $11.3 million (Anderson 1975). Total 

 volume of stone used is not known, but 

 Anderson (1975) reports that it was 

 considerably more than had been esti- 

 mated. This underestimation of material 

 required was largely due to problems 

 encountered during construction. 



The jetty was built on the natural 

 sand bottom. Though allowances were 

 made for moderate sand loss due to the 

 crosscurrent scouring during construc- 

 tion, the magnitude of this loss was 

 grossly underestimated. At the halfway 

 point in construction, the entire quan- 

 tity of bedding material had been used. 

 Strong currents around the advancing 

 end of the jetty were washing out bed- 

 ding material and sand to a depth of 30 

 ft (9 m) for about 300 ft (91 m) beyond 

 the jetty tip. This problem was solved 

 in part by eliminating bedding material 

 and dumping large 200-lb (90- kg) to 

 5-ton (4.5-metric ton) core stone direct- 

 ly on the sand bottom and by working 

 double shifts to accelerate the construc- 

 tion process (Anderson 1975). 



Kieslich and Mason (1975) state 

 that design objectives for jetties are to 



minimize undesirable effects of wave ac- 

 tion on navigation and to eliminate the 

 necessity for artificial channel mainte- 

 nance. The latter is usually achieved 

 by either preventing littoral drift from 

 entering a channel or concentrating ebb 

 currents so that their natural scouring 

 action is enhanced. Apparently neither 

 of these objectives has been achieved 

 by the Tillamook jetties. The channel 

 required dredging a few years after the 

 north jetty was constructed (Kieslich 

 and Mason 1975). In 1975, the Portland 

 District, U.S. Army Corps of Engineers 

 prepared an environmental impact state- 

 ment for miscellaneous activities, includ- 

 ing channel dredging, in Tillamook Bay. 



Another factor in the construction 

 of the south jetty was local desire for a 

 means of halting erosion of Bayocean 

 Spit. Following the extension of the 

 north jetty, erosion apparently acceler- 

 ated on the long, narrow sand spit 

 (Terich and Komar 1973). Few records 

 were kept previously, so it is unknown 

 whether or not the construction of the 

 north jetty increased erosion of the 

 spit. It is known that the three-fathom 

 contour moved 1,500 ft (457 m) closer 

 to the spit between 1885 and 1939. This 

 caused increased nearshore wave energy 

 and concomitant erosion potential (Terich 

 and Komar 1973). Historical records 

 show definite changes in the shoreline 

 both up and downdrift of the bay mouth 

 following construction of the north jet- 

 ty. Up drift sand accretion occurred 

 behind the jetty, while the shoreline of 

 the downdrift spit retreated due to ero- 

 sion (Komar et al. 1976). The spit 

 eventually became so narrow that a 

 storm in 1939 opened gaps which allow- 

 ed the sea to enter the bay. In 1952, a 

 storm broke through and left a 0.8 mi 

 (1.2 km) breach near the broad south 

 end of the spit. This was later diked, 

 but for some time there were essentially 

 two entrance channels into the bay 

 (Terich and Komar 1973). Recent infor- 

 mation seems to indicate that erosion of 

 the spit has slowed since construction 

 of the south jetty (U.S. Army Corps of 

 Engineers, Portland 1975). 



The effects of the Tillamook jetties 

 on the biota of the area can only be 

 inferred since no quantitative before- 



Ill 



