Imperial Beach Model Study 



14. Curren and Chatham (1977) conducted a model study on Imperial Beach 

 which is located on the Pacific Ocean 5.6 km north of the Mexican border and 

 17.7 km south of San Diego, California. It is primarily a recreational beach, 

 with a 366-m-long fishing pier situated in the approximate center of the study 

 area. Two groins, 226 m and 122 m long, are located 899 and 495 m north of 

 the fishing pier, respectively. The Tijuana River is believed to be the main 

 historical source of sediment for Imperial Beach. However, construction of 

 the Morena and Barret Dams in Cottonwood Creek and the Rodriquez Dam in the 

 Rio de las Palmas causes river sediments to be trapped behind the dams without 

 ever reaching the coast. In addition, the lack of recent floods has caused a 

 shortage of sediment reaching the mouth of the Tijuana River. Therefore, 

 there is a decreased quantity of sediment available for longshore transport to 

 Imperial Beach and beach erosion has increased. Two groins in the area, con- 

 structed between 1959 and 1963, have been ineffective in stabilizing the 

 beach. 



15. A l:75-scale hydraulic model was constructed (representing about 

 13.5 sq km in the prototype) and used to investigate the arrangement and 

 design of alternative proposed structures for preventing erosion of the 

 Imperial Beach shore . 



16. Model tests were conducted using a crushed-coal tracer material for 

 existing conditions and the improvement plans under various wave conditions. 

 The structures proposed for Imperial Beach consisted of (a) single detached 

 breakwaters at the -4.6 and -3.0 m contours, (b) segmented breakwaters at the 

 -4.6 and -1.5 m contours, (c) a single detached breakwater segmented by low 

 sill sections at the -3.0 and -1.5 m contours, and (d) various groin plans. 



17. Model test results for existing conditions indicated that both 

 north- and south-directed longshore currents would be interrupted at regular 

 intervals by strong rip currents. These rips transported significant quanti- 

 ties of sediment offshore where it was either (a) transported longshore on the 

 bar, (b) lost in deep water, or (c) transported back shoreward by low- 

 steepness waves. Rip currents occurring in the model were similar to those 

 observed in the prototype. The five-groin plan tested in the model created 

 strong rip currents for almost all incident wave conditions. The five groins 

 were not only ineffective in trapping tracer material but contributed to 



C15 



