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Coastal Morphology & Morphodynamics
When waves break close to the shore, they dissipate their energy both into the water column and in the form of friction along the sandy bottom. The resulting water flows cause sand grains to roll and hop along the seafloor or even lift the grains up into the water column, sometimes carrying them far distances. When many waves cumulatively act to move sand in a similar manner, the result is a change to the shape of the beach and the underwater sandbars, known as “morphology”. Morphology can change rapidly if lots of wave energy is dissipated in a short amount of time (storm conditions) or slowly over many years as a result of persistent but less forceful variations in waves and currents. The FRF has tracked morphologic changes of the nearshore, beach, and dune for more than 40 years, creating one of the most unique datasets of coastal change in the world–in fact the beach near the pier in Duck is probably the most studied beach in the world. Learn more below about our “Survey Record”, some of the common “Sediment Transport” mechanisms observed and its effect on the beach shape, as well as how our beach is affected by ecological processes known as “Eco-morphodynamics”
Since 1981, the FRF has been conducting detailed surveys of the nearshore bathymetry (below the water surface) and beach topography (above the water). The fort-nightly to monthly efforts over the last four decades have created a dataset with more than 1000 surveys. No dataset in the world covers the same spatial and temporal extent. State-of-the-art survey techniques have always been employed, providing highest available accuracy using the CRAB or the LARC. The survey vessels repeatedly record data along the same lines oriented from the beach out to deep water, providing our researchers with “beach profiles”. These data are being used to refine theories of nearshore morphologic change, and develop and test numerical simulations of coastal change.
In order to make accurate predictions about large-scale coastal changes that impact our communities, we need to understand the small-scale sediment movements that are constantly occurring at the beach. Each wave exerts a forward fluid velocity (flow) under the crest, and a reverse velocity under the trough, such that sandy sediments are pushed towards the beach and pulled back away every couple of seconds as each wave approaches the shore. The size of the wave and the water depth it is passing over will affect the magnitude of those two currents, which means a sand grain is not always moved back to the same place with each passing wave. For example, small waves lead to slower velocities, which sometimes cannot overcome the cohesive forces from neighboring sand grains and gravitational forces that hold the sand grains in place. As flow speeds increase they overcome a threshold for motion, and begin to move more and more sand grains. This is one reason why changes on the beach are more noticeable after storms. However, even slight changes from the average wave height can drive substantial sediment transport and morphology change if those conditions are persistent. Larger waves in the winter typically stir more sediment into the water column, while also creating a more consistent undertow current that moves that sediment into deeper water. As a result the beach is often narrower and a sandbar is located in deeper water during the winter. When wave heights are small, that sediment is moved back towards the shore, widening the beach and moving sandbars closer to the beach. The FRF has hosted numerous large-scale experiments contributing to our fundamental understanding of the sediment transport occurring under breaking waves. In addition, since the character and size of sediments vary in space and time, samples are collected weekly on the beach, monthly offshore and quarterly across the FRF property to better understand complex sediment distributions and ultimately help improve the physics in predictive modeling efforts.
The currents created by waves dissipating energy into the water column can also carry large sediment quantities. Longshore sediment transport is the movement of sand grains along the beach due to nearshore currents created by waves breaking at an angle to the shore. Any sand grain lifted into the water column by waves experiences the alongshore currents such that it falls back to the sand bed slightly further down the beach. Cumulatively, this process tends to smooth sandy coastlines. It’s the dominant mechanism redistributing sediment from beach nourishments, and over millennia it creates whole landforms such as sand spits and capes that are characteristic of sandy coasts around the world. The FRF supports several constantly operating instruments to measure nearshore currents.
In addition to pioneering oceanographic measurement techniques in the water, the FRF is a leader in developing novel ‘remote sensing’ technologies to monitor coastal environments from out of the water (i.e. no wetsuits, no seasickness)! From a fixed location on dry land, absent of pummeling waves and saltwater, remote sensors such as Lidars, Radars, and cameras can provide high frequency wave, current, depth, and beach elevation estimates for over thousands of points over thousands of feet. If deployed from an Unmanned Aerial Vehicle (UAV) or satellite, this coverage can extend even farther. These tools can provide measurements more often and with more coverage than traditional in-water sensors with less time-consuming installation and risk to personnel. This is particularly true during storm events, the most disruptive and dynamic coastal conditions. Scientists and engineers at the FRF use this technology to answer tough science questions as well as develop innovative tools for US. Army coastal reconnaissance and USACE district beach project monitoring.
Lidars are instruments that rapidly emit and receive a series of safe laser pulses reflected off objects. With great precision, the instruments can measure distance of these objects (such as waves and beach shape) from the sensor and knowing the position of the sensor, their absolute position. The FRF houses the first continuously operating lidar system to measure beach elevation and nearshore wave parameters hourly with data processed and made public in near real-time. In addition, the FRF developed and operates CLARIS (Coastal Lidar and Radar Imaging System), a mobile Lidar system on top of a 4x4 Vehicle, which can scan over 40+Km of coastline in a few hours. The FRF is continuously evaluating new Lidar systems for coastal applications as this emerging technology develops. Access these data here.
We can infer many coastal processes with our eyes. For example, waves typically break over sandbars; when waves break, they produce white foam; thus, we can assume where we see persistent white foam there is wave breaking and potentially a sandbar! Using video imagery and principles of photogrammetry, FRF researchers in collaboration with Oregon State University have developed techniques to digitize and map images of the coast and extract important observations on coastal processes. For over 30 years, six cameras on top of the 100 ft Argus tower have monitored the FRF coastline providing shoreline position, depth, and current estimates hourly. Currently, FRF researchers are adapting this technology to for more ubiquitous video sources such as web, trail, and cellular phone cameras to monitor our coastlines across the country. Access these data here.
Unmanned Aerial Systems (UAS) are small aviation vehicles that can be piloted remotely and host a variety of sensors such as Lidars and Cameras. Flown over 1-3 kms, these systems can map beach elevation with high resolution with a flight time of less than 10 minutes and few personnel. FRF researchers are leaders in applying novel analysis techniques to UAS data, such estimating depths from
Nearshore bathymetry at the FRF is characterized by regular shore-parallel contours, a moderate slope, and barred surf zone (usually with an outer sandbar in water depth of about 4 m and inner bar in depths between 1 and 2 m). This pattern is interrupted in the immediate vicinity of the pier where a trough runs under much of the piers length, ening in a sour hole at the pier’s seaward end where depths are up to 3.0 m greater than the adjacent bottom. All data is publicly available on the CHL THREDDS Server (https://frfdataportal.erdc.dren.mil/).
Coastal dunes and beaches evolve at various time scales in response to wave and wind processes. Dunes serve as the primary line of defense from flooding and storm impacts and often protect valuable infrastructure, homes and roadways. Feedbacks between ecology and sediment transport can play a crucial role in the development of dune and beach morphology. Plant characteristics such as density, form, and root depth can all affect sedimentation patterns and dune evolution as wind blown sand is transported landward from the beach. This process is particularly relevant behind beach nourishments, where larger dunes have grown through eco-morphodynamic feedbacks and ultimately provided more protection from extreme coastal storms along the eastern seaboard. The specific ecology and plant establishment can in turn affect the erosion of the dune by acting as structural support and resistance. Dune evolution studies are possible at the FRF due to the intentionally undisturbed backshore, allowing natural processes of ecology emergence and recession to interact with both wind and wave sediment transport processes. Researchers at the FRF use a variety of methods to study beach/dune eco-morphodynamics including lidar (CLARIS), 3D imagery, vegetation sampling and land cover mapping.