22 



PRICE 



I use the terms "camber," "ramp," and "shoreface" for the three parts of the profile and 

 do not attempt to fix a "shelf break." Neither do I analyze the topography of the shelf slope. 



You will notice there is an overlap of the concave river profile on the continental shelf, 

 and overlap of the camber on the submarine canyon profile. Each high-energy zone begins in 

 the low-energy section of the preceding profile. 



Figure 13 : Map of wave energy and shelf conditions showing energy data computed from 

 wind data for six coastal stations. The energy of wave motion in the wind-wave is computed in 

 terms of horsepower-days per foot of crest advance and per foot of wave crest length for on- 

 shore winds on a deep-water basis. The figures were developed by Dr. Warren C. Thompson, 

 now of the Navy Post Graduate School of Monterey, California. He used regional wind data 

 taken at the coastal Weather Bureau Stations to compute the energy from each onshore wind 

 direction using a series of formulas and curves he developed. These values are shown in his 

 report in an energy wind-rose for each station. These shore data are considered to be appli- 

 cable to the adjacent edge of the continental shelf. The energy for the deep-water wave applies 

 at that point. 



At first we worked on the idea that it was mainly the onshore wind-wave movement which 

 was significant. When we thought about it more, we saw that offshore winds must also do ap- 

 preciable work on the continental shelf. As the fetch increases, the wind strength and wave de- 

 velopment increase gradually, and there is significant wave attack on the bottom. 



Charles L. Bretschneider, who has developed formulas and curves for computing bottom- 

 friction losses in wave motion, helped on this problem. He computed the total wave energy for 

 winds from all directions for one of the stations used by Thompson. His computation for Gal- 

 veston show 654 horsepower-days as a deep-water gross-energy figure as compared with 346 

 for onshore waves. 



Figure 14: Spectrum of Bretschneider' s wave-energy values along a profile of the shelf 

 off the Brazos delta and river mouth near Galveston. Energy along the bottom slowly decreases 

 landward up the steep shelf slope. Only 5% is "lost" in motion across the remote, deep camber 

 section of the continental shelf. A total of 20% is "lost" on reaching a depth of 100 feet. Ap- 

 proximately 75% of the energy reaches the outer edge of the ramp after crossing the broad but 

 low deltaic elevation which has absorbed some of the energy. However, in speaking of "lost" 

 energy, we are considering also changes in energy due to reduction in fetch of offshore winds. 

 On this nearly horizontal ramp, eight or ten miles long, only 10% of the energy is "lost." Thus, 

 in this example, 64% of the energy is left to be expended on the shoreface, the surface of the 

 steep, sandy barrier island with its offshore bar zone. As we will see, this flat ramp is a low- 

 energy zone. 



RIVER I 

 REGIME I I 



BARRIER 



- FUTURE RAMP 



ITY CURRENT 

 DELTA 



DIAGRAMMATIC BOTTOM PROFILE OF CONTINENTAL 

 SHELF 



figure 12. 



