Modeling results for the existing condition produced an onshore movement 

 of the coal tracer for small waves of low steepness with longshore transport at 

 the shoreline. For high-steepness waves, the coal tracer moved seaward 

 forming a bar at the most seaward breaker zone. This material migrated north 

 or south depending on wave direction. The high-steepness waves re-formed 

 and broke a second time near the shoreline, resulting in a second nearshore 

 zone of longshore transport. The detached breakwater plans tested included a 

 single 1,494-m-long structure with varying crest elevation and a segmented 

 breakwater system consisting of four 203-m-long segments with 203-m gaps. 

 Each plan was tested both with and without groins at the northern and 

 southern extremes of the beach. Movable-bed model tests showed that the test 

 plans without groins (Figure 42) generally resulted in erosion of the shore on 

 the updrift side of the model beach and loss of material from the downdrift 

 side indicating inadequate protection of the beach fill. Tests with groins at 

 each extreme (Figure 43) showed a reduction of the amount of coal leaving 

 the beach area and a fairly stable shore. 



Imperial Beach model study. Imperial Beach is located on the Pacific 

 Ocean coasdine 5.6 km north of the Mexican border and 17.7 km south of 

 San Diego, California. It is a recreational beach with a 366-m-long fishing 

 pier located in the center and normal to the beach. Two groins, 226 and 122 

 m long, are located 899 and 495 m north of the fishing pier, respectively. 

 The main sediment source for Imperial Beach is the Tijuana River, and 

 construction of Morena, Barret, and Rodriquez Dams has trapped the river 

 sediments behind the dams. Lack of river flooding has also been cited for the 

 shortage of sediment reaching the mouth of the Tijuana River. The decreased 

 amount of sediment available for longshore transport to Imperial Beach has 

 caused increased beach erosion. Two groins that were constructed between 

 1959 and 1963 were ineffective in stabilizing the beach. 



Froude model testing in a 1:75 scale physical model was conducted to 

 evaluate the arrangement and design of alternative structures for the 

 prevention of Imperial Beach erosion. Crushed coal was used as a tracer for 

 modeling the existing condition and proposed new structures under various 

 wave conditions. The proposed new structures consisted of (1) a single 

 detached breakwater at the -4.6- and -3.0-m depth contours, (2) segmented 

 breakwaters at the -4.6- and -1.5-m contours, (3) a single detached breakwater 

 segmented by low sill sections at the -3.0- and -1.5-m contours, and (4) vari- 

 ous groin locations. 



The model results for existing conditions showed that both north- and 

 south-directed longshore currents were interrupted at regular intervals by 

 strong rip currents that transported significant quantities of sediment offshore 

 where it was either lost in deep water, transported alongshore on a bar, or 

 transported shoreward by low steepness waves. These model rip currents 

 were similar to observed prototype currents. A five-groin plan resulted in 

 strong rip currents for almost all wave conditions and was ineffective in 

 trapping tracer material. A nine-groin series was effective in trapping tracer 

 material, but significant quantities of stone would be required for construction. 



Chapter 3 Tools for Prediction of Morphologic Response 



73 



