Nearshore Bathymetry
The nearshore bathymetry in and around Kill Devil Hills has similar characteristics to many coastal beaches on the east coast of the United States. The slope is generally moderate and contours are usually shore-parallel. The very nearshore is often characterized by a nearshore bar (0.5-2m depth) ranging from 10-100m from shore and and offshore bar (3-5m depth) another 100m or more beyond the nearshore bar. However, there is alongshore and cross-shore variability in this typical profile in some locations, most notably in the northern third of KDH beach. Here the nearshore is characterized by irregular bathymetry extending further north. This variability appears to cause the surf zone bathymetry to often behave quite different than the southern more "normal" two-thirds of the beach.
An image showing the bathymetric variability of KDH Beach collected by the amphibious LARC. The northern and southern extents are indicated. Courtesy of USACE FRF.
The nearshore bathymetry is something that is pivotal in understanding surf zone processes, especially rip currents (Brander, 1999; Brander and Short, 2000; MacMahan et al., 2005; etc). However, the bathymetry nearshore and particularly in the surf zone is constantly changing and can be very difficult to measure. To best interpret the constantly changing bathymetric conditions in the surf zone and the more stable bathymetry slightly offshore (0-2km offshore) we have employed four different types of data.
LARC Survey Data
Bathymetry profiles collected by the amphibious LARC owned and operated by the USACE Field Research Facility have been made from 2004 to 2006 in the area of KDH Beach. This data provides high resolution bathymetric profiles from the beach face to over 2km offshore. This data is being used for ADCP deployment as well as historical analysis of the very nearshore and surf zone bathymetry. An example of the LARC bathymetry profiles can be seen in the image above showing the KDH beach.
Image courtesy of USACE FRF
Swath Bathymetry Data
An image showing the swath bathymetry near KDH Beach. The northern and southern extents are marked. The ADCP is the FRF ADCP located in Kitty Hawk. Courtesy of USACE FRF.
Somewhat similar to the LARC data, the FRF has also collected swath bathymetry data for the nearshore region encompassing KDH. The swath bathymetry is collected via the GeoSwath system which can collect bathymetry at a swath width 8-12 times the water depth. This added coverage enables the system to measure bathymetry at a higher resolution when compared to the LARC profiles, which is helpful for mapping the highly variable region in the northern third of KDH. This data is being used for ADCP deployment and for analyzing bathymetry variability.
BASIR (Bar And Swash Imaging Radar)
The BASIR is a mobile X-band radar designed by Jesse McNinch that maps the nearshore bathymetry including the nearshore bar and swash zone (McNinch, 2007). The method provides high resolution mapping of the nearshore by inferring the bathymetry by applying linear wave theory to the radar measured wave parameters. This method is limited to times when wave heights are large enough to begin breaking on the nearshore bar, and thus the BASIR can only be used at certain times. Despite this limitation, the BASIR has been shown to collect data extremely fast (about 3 hours start to finish for 10km of beach) and provide well resolved bathymetry of the nearshore. The BASIR thus provides us with a means to resolve the extent and depth of the nearshore bar quickly and accurately.
A Google Earth image showing the BASIR inferred bathymetry at KDH. Courtesy of Jesse McNinch, USACE FRF.
Beach to Bar Level Profiles
The last method we will be using to obtain detailed nearshore bathymetry information is by using a level and a level rod to do beach-bar profiles on a bi-weekly basis at 7 different locations along KDH Beach. This method was chosen because it is low cost and simplistic yet highly accurate when done properly. In addition, there needed to be more recent and consistent bathymetry data than that is provided by the above methods. The 7 locations are roughly 1km apart and were chosen due to their locations near guard chairs, and because there are stable objects in the dune line for backsights (i.e. stairways, buildings). This data will provide an indication of the changing beach topography and nearshore bathymetry both alongshore at a given time and throughout the summer.
An example of level profiles collected at the Clark St location in KDH.
Picture of Greg taking measurements with the level.