Coasts 



_ 31 



sediment contributed by tributaries. Where 

 streams or landslides contribute too much 

 sediment to the beach for the waves to move 

 immediately, a delta or bulge results; simi- 

 larly, trunk streams may be dammed or shal- 

 lowed by sediment from a tributary or a 

 landshde. In both examples the effects are 

 felt far up and down the beach and far up 

 and down the stream. 



If the gradient of the water surface of a 

 section of stream having a flood plain is arti- 

 ficially reduced by the building of a dam, the 

 water entering the reservoir builds a delta 

 whose head extends the same distance up- 

 stream that the original valley was at grade. 

 At the same time the water leaving the res- 

 ervoir is underloaded so that it erodes the 

 flood plain far enough downstream until its 

 load again reaches capacity. If the down- 

 stream area was formerly at grade, the 

 stream must incise a new valley all the way 

 to its mouth. Thus, for a graded stream 



valley the dam produces extensive deposi- 

 tion upstream and erosion downstream 

 (Mackin, 1948; Gould, 1953). An exactly 

 parallel series of changes occurs when a groin 

 or breakwater is built across a beach that is 

 in equilibrium with the environment. The 

 sand that normally would pass through the 

 area is dammed up by the structure so that 

 prograding of the beach eventually occurs 

 far up-beach. Sand down-beach from the 

 groin continues its down-beach movement, 

 but the structure built across the beach pre- 

 vents the normal replenishment of this sand 

 by more from up the beach; thus erosion 

 occurs down-beach from the structure, at 

 first close to it and then at a greater distance. 

 This sequence of events has occurred after 

 the building of practically every such struc- 

 ture across southern CaUfornia beaches — 

 Santa Barbara, Santa Monica, Redondo, 

 Seal Beach, Mission Beach, and Camp 

 Pendleton. 



Figure 31. A channel cut by 

 a rip current near Scripps 

 Institution of Oceanography, 

 La Jolla. Contour interval is 

 0.5 feet. The letters C and T 

 indicate loose cobbles and 

 outcrops of terrace alluvium, 

 respectively. The insert 

 shows the changes observed 

 in a resurvey of part of the 

 area 12 days later. 



100 200 300 



CONTOUR INTERVAL 

 0.5 FEET 



