2 Wave Focusing by 
Shrewsbury Rocks 
This analysis examines the potential that the hot spot at Monmouth Beach is 
caused by differential longshore sand transport rates caused by wave transforma- 
tion over the Shrewsbury Rocks. Hot-spot erosion at Corps of Engineers beach- 
fill projects at Ocean City, Maryland, and Folly Beach, South Carolina, has been 
linked to differential longshore sand transport caused by irregularities in the 
nearshore bathymetry. Consequently, there is a possibility that wave transfor- 
mation over the Shrewsbury Rocks could be responsible for the observed hot 
spot at Monmouth Beach. This chapter examines the available wave data and 
determines what areas of the beach are down-wave of the rock outcropping 
under wave conditions observed since construction of Contract 1A. 
Wave Statistics 
Wave data for the project region are available from two gauges. The gauge 
nearest the beach fill is located at Long Branch, New Jersey (40.30° N, 
73.97° W), in a water depth of approximately 32.8 ft(10 m). A slope array was 
deployed at Long Branch in 1994 and replaced with a directional wave gauge 
(DWG) in 1995. Wave data are available from January 1994 - March 1996. 
These data were collected at a rate of 1 Hz for 1,024 sec. Samples were 
collected each hour during expected storm conditions and once every 4 hr for 
low-wave-height conditions. Wave periods and directions were not recorded for 
wave heights less than 0.7 ft (0.2 m). Numerous gaps appear in the wave record 
(30 percent of the gauge deployment). The longest gap was mid-November 1994 
to mid-July 1995, when the slope array failed and was replaced with the DWG. 
Wave directions are also missing from January to mid-March 1994. Statistics 
from the Long Branch gauge are given in Appendix Al. The statistics include 
breakdowns of the wave height, period, and direction by month; wave height and 
period by direction; and the mean and maximum wave heights for the duration of 
the gauge deployment. Statistical tables and mean heights were generated using 
data taken at 4-hr increments (0, 400, 800, 1200, 1600, 2000 Greenwich mean 
time (GMT)) to evenly weight storm and nonstorm conditions in the statistics. 
The statistical values have not been adjusted to account for the nonrandom gaps 
in the data. The maximum recorded wave height was 13.8 ft (4.2 m) on 
Chapter 2 Waves Focusing by Shrewsbury Rocks 
