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Beach Morphology Monitoring Program
Nearshore Bathymetry
It has historically been very difficult and expensive to collect data in the highly dynamic sub-aqueous region and only a few coastlines in the world have sufficient nearshore data to quantify this variability. A new system, based on the Coastal Profiling System developed by Oregon State University, is now being used in the CRLC. This system was designed to collect bathymetric data in energetic, nearshore environments. It consists of a highly maneuverable watercraft (a wave runner) that is equipped with an echo sounder, GPS receiver and antenna, and an onboard computer. Kilometer-spaced cross-shore transects have been collected for most of the littoral cell revealing important information about variability in nearshore beach slope, sandbar size and sandbar location.
Since sandbars dissipate wave energy and provide a buffering capacity to protect the sub-aerial beach and the shoreline, both the temporal and alongshore variability of nearshore morphology (i.e. position, height and length of sandbars) may create regions (in time or space) of vulnerability or resilience along the coast. Although the exact time and space scales associated with how bar variability affects shoreline position is not precisely known, this morphological link is suspected to act over longer time scales than shoreline fluctuation itself. Therefore, nearshore bathymetry should be considered a longer-term area of interest for coastal management (see monitoring program design).
Following a pilot effort in 1998, hundreds of cross-shore profiles were collected along the CRLC in 1999 using a second generation Coastal Profiling System, a survey system with sub-decimeter vertical accuracy. Each profile extends from the shoreline to a deep-water limit ranging between 10 and 16 m (MSL). Many of the bathymetric profiles are combined with topographic surveys, extending the profiles landward through the dune fields. Sandbars are the most prominent morphologic features along much of the CRLC and the spatial variability of bar properties is striking. The nearshore morphology exhibits between 0 and 4 bars, ranging in height from approximately 0.2 m to a remarkable 6.0 m as measured from the seaward crest to landward trough. The bar crest position varies from approximately 100 m from the shoreline for inner swash bars to 1000 m from the shoreline for outer bars. When outer bars are present, the water depth at the outer bar crest ranges from 5.0 to 8.5 m (MSL). Crest depths are 3.0 to 5.0 m for the middle bar and 0.0 to 1.5 m for the inner bars.
This extensive nearshore bathymetric data set reveals strikingly different large-scale coastal behaviour among the four sub-cells of the Columbia River littoral cell. Nearshore morphologic features along North Beach and Long Beach are large and three-dimensional while along the Grayland Plains and Clatsop Plains bars are more linear and smaller in magnitude. These observations are surprising in that the deep-water wave climate is uniform along the entire region (Tillotsen and Komar, 1997), suggesting that sediment supply, sediment characteristics, antecedent morphology, or flow patterns may dominate in forcing these regional differences.
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References
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Tillotsen, K.J. and P.D. Komar, 1997. The wave climate of the Pacific Northwest (Oregon and Washington): A comparison of data sources, Journal of Coastal Research, 13, pp. 440-452.
