PERFORMANCE OF A SAND TRAP STRUCTURE AND EFFECTS OF IMPOUNDED SEDIMENTS, 

 CHANNEL ISLANDS HARBOR, CALIFORNIA 



by 

 R.D. Hobson 



1 . INTRODUCTION 



Sediment traps are sometimes used in conjunction with jetties to intercept 

 and collect littoral sand which might otherwise shoal in a navigation channel. 

 The trap is positioned to interrupt the natural flow of sand transported along 

 a coastline before it reaches the channel, and this sand is periodically dredged 

 and bypassed downcoast where it is reintroduced into the natural transport sys- 

 tem. A single updrift trap is used where longshore transport is dominantly 

 unidirectional whereas twin traps may be employed to protect a channel where 

 major transport reversals occur. A highly efficient trap that intercepts all 

 longshore transport also provides an ideal location for determining the physi- 

 cal characteristics and composition, as well as accumulation rates and patterns 

 of the sediments driven naturally by coastal processes. This study documents 

 the filling history and sedimentary characteristics of one sand trap and evalu- 

 ates how the results obtained may be useful to coastal engineers. 



The study area at Channel Islands Harbor, California (Fig. 1), includes the 

 "trap" as defined by the beach, the northern harbor entrance jetty and an off- 

 shore breakwater, and a "native beach" section located upcoast from the trap. 

 Channel Islands Harbor, which is approximately 80 kilometers north of Los 

 Angeles, has been the site of extensive field studies by the Coastal Engineering 

 Research Center (CERC) in documenting longshore transport rates and determining 

 accurate empirical relationships between nearshore wave thrust and sediment 

 transport. The harbor was constructed along this high energy coast in 1961 with 

 its entrance channeled between parallel shore-normal jetties located to the lee 

 of a shore-parallel offshore breakwater (Fig. 1). The configuration was chosen 

 to allow a protected harbor entrance to small boats regardless of the direction 

 of incoming waves. The configuration of the breakwater and jetties is also 

 unique because it acts as a nearly complete barrier to longshore transport. 

 Waves arrive at Channel Islands mainly from the northwest and west (Fig. 1, 

 direction 270°) due to shadowing of most other swell directions by the islands 

 and by Point Conception to the north (Herron and Harris, 1966). The result is 

 a generally southward drift which can be caught by a single sediment trap, 

 dredged, and then bypassed. Reversals can and usually do occur yearly when 

 swell from the southwest dominates (Fig. 1, swell direction 215°) but even 

 during these periods, the breakwater configuration still causes the trap to 

 continue filling from the north (Bruno, et al., 1981). Table 1 presents the 

 dredging history of the trap since construction and indicates that the average 

 yearly sand accumulation is about 720 000 cubic meters and that the trap usually 

 fills in about 1.5 years. 



II. DATA COLLECTION AND ANALYSIS 



Sediment cores, hydrosurveys, surface sediment samples, and a coordinated 

 set of process observations provide the basic data for this report. The coring 

 was performed specifically for the reported study whereas the other data were 

 collected in conjunction with the CERC study of sediment transport in the area. 



