Detailed study of inlets through barrier islands on the U.S. Atlan- 

 tic and gulf coasts shows that the shape of the shoreline at an inlet can 

 be classified in one of four characteristic planforms. (See Figure 4-58, 

 adapted from Galvin, 1971,) Inlets with overlapping offset (Fig. 4-59) 

 occur only where waves from the updrift side dominate longshore transport. 

 Wliere waves from the updrift side are less dominant, the updrift offset 

 (Fig. 4-59) is common. Where waves approach equally from both sides, in- 

 lets typically have negligible offset. (See Figure 4-60.) IVhere the sup- 

 ply of littoral drift on the updrift side is limited, and the coast is 

 fairly well exposed, a noticeable downdrift offset is common as, for exam- 

 ple, in southern New Jersey and southern Delmarva. (See Hayes, et al., 

 (1970.) These planform relations to littoral processes have been found 

 for inlets through sandy barrier islands, but they do not necessarily hold 

 at inlets with rocky boundaries. The relations hold regionally, but tem- 

 porary local departures due to inlet migration may occur. 



4.82 FIELD STUDY 



A field study of the problem area is usually necessary to obtain 

 types of data not found in the office study, to supplement incomplete 

 data, and to serve as a check on the preliminary interpretation and corre- 

 lations made from the office data. Information on coastal processes may 

 be obtained from wave gage data and visual observations, sediment sam- 

 pling, topographic and bathymetric surveys, tracer programs, and effects 

 of natural and manmade structures. 



4.821 Wave Data Collection . A wave-gaging program yields height and 

 period data. However, visual observations may currently be the best 

 source of breaker direction data. Thompson and Harris (1972) deter- 

 mined that 1 year of wave-gage records provides a reliable estimate of 

 the wave height frequency distribution. It is reasonable to assume that 

 the same is true for wave direction. 



A visual observation program is inexpensive, and may be used for 

 breaker direction and for regional coverage when few wave-gage records 

 are available. The observer should be provided with instructions, so 

 that all data collected will be uniform, and contact between observer 

 and engineer should be maintained. 



4.822 Sediment Sampling . Sediment sampling programs are described in 

 Section 4.26. Samples are usually surface samples taken along a line 

 perpendicular to the shoreline. These are supplemented by borings or 

 cores as necessary. Complete and permanent identification of the sam- 

 ple is important. 



4.823 Surveys . Most engineering studies of littoral processes require 

 surveying the beach and nearshore slope. Successive surveys provide 

 data on changes in the beach due to storms, or long-term erosion or 

 accretion. If beach length is also considered, an approximate volume 

 of sand eroded or accreted can be obtained which provides information 

 for the sediment budget of the beach. The envelope of a profile defines 



4-147 



