National Park Service

Gulf Coast Network (GULN)

Coastal Dynamics Monitoring

Gulf Islands Sand Bar Gulf Islands Sand Bar

GULN Coastal Dynamics Monitoring Protocol Summary

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Importance

Geomorphology (primarily surface form, topography and elevation) is a very high priority vital sign for monitoring on our two barrier island parks, GUIS and PAIS. Landform provides the physical foundation for all park surface ecosystems and natural resources, and it changes dynamically in response to normal system processes, acute storm events, and anthropogenic actions. The primary objectives of the GULN Geomorphic Change Monitoring Protocol are to detect and document substantial physical changes and displacement in the surface and near sub-surface geography of coastal parks, including recording land subsidence and land-form changes that are occurring on our coastal parks as a result of climate changes and changes in sea-level, storm events, and on-going anthropogenic activities such as dredging, canalization, water withdrawal and flow modifications.

This section summarizes aspects of the GULN approach to monitoring coastal geomorphology. Development of this protocol will proceed in collaboration with the USGS, and modeled after the Barrier Island Comprehensive Monitoring (BICM) program currently under development for monitoring barrier islands in Louisiana. We are interested in developing an approach that is consistent with other similar efforts in the northern Gulf of Mexico. Many technical aspects, including sampling design spatial resolution and coverage, are currently tentative and under development; complete designs and discussion will be presented in detail in the finalized protocol documentation.

The GULN approach to monitoring geomorphic change is based, in large part, on the airborne pulse LiDAR (Light Detection And Ranging) technology that we will use to address multiple vital signs across network parks. For geomorphic monitoring, LiDAR will be used to create a 3-dimensional bald-earth topographic model of the mineral and water surface of sampled coastal landscapes. Additional uses of LiDAR will be discussed in the next sub-section.


Parks to be Monitored

  • Gulf Island National Seashore (GUIS)
  • Padre Island National Seashore (PAIS)

LiDAR Technology and Two Levels of Sampling Design

Airborne LiDAR collects elevation data on sample points on a landscape by bouncing red or green-light laser pulses off of the sampled substrate and measuring changes in reflection intensity and return time to the sending unit aboard the aircraft. Key elements in the reflection signature from each laser pulse indicate the elevations of the first return or canopy-top elevation, and the under-lying bald-earth mineral ground (or in some cases water) reference surface relative to sea level. Sampling is performed by flying a linear flyway at a fixed height (relative to sea level) over the landscape and taking readings (sample measurements) with laser pulses in a raster scan design similar to that used to generate the image on a typical television screen. Pulse data are recorded simultaneously with GPS coordinates and sampling time that provides sampling location and order information.

LiDAR sampling involves two levels of sampling design . The actual LiDAR surveys generate a Primary or technology-generated sampling design during the physical sampling event. This primary design is a dynamic, high-resolution virtual systematic grid, sampling of a flyway belt transect across the landscape. The flyway dimensions are set by aircraft altitude, arc of scan across the flyway, and the length of flyway. For example, a typical flyway may be approximately 240 meters wide by n meters long (n = some discretionary length). In this primary design, the sample frame may be anything from one flyway to a combination of parallel flyways covering the whole park. For coastal geomorphic monitoring, the sample frame may include near-shore waters and directly-abutting adjacent lands. Sample units may be anything from one flyway to the entire park. Actual sampled area is determined by the cumulative length of parallel flyways flown during one sampling occasion.

The virtual grid is anchored on or referenced to fixed-location ground stations that are initially subjectively selected (locations permitted according to park management preference). Both ground stations and all sampled points in the virtual grid will have GPS coordinates, allowing the entire sample and database to be accurately georeferenced on the park map. As ground stations are permanent and fixed, the virtual grid can be re-located to cover the same sample frame each time (high degree of grid relocation for repeated measures ), but individual sample points within the grid have a low probability of being re-sampled in any given subsequent sampling occasion.

Sampling points are distributed roughly evenly within the grid with a user-defined mean spacing that can be adjusted by changing the over-flight altitude and pulse frequency of the laser. Typical sample spacing within the grid is about 1.5 x 2.0 meters, yielding very large sample sizes (and high sampling intensity) (approximately 25,000+ points per square kilometer) composed of very consistent and reliable machine-logic-generated and recorded data.

After primary sampling is performed over the entire sample frame or park, the datasets composed of all elevation data with geospatial locator and time records are available for diverse secondary or a posteriori sampling design development (Figure 4.1.). For example, the total island sample may be stratified to analyze areas of interest (geospatial stratification), and elevation zones of interest (vertical stratification). It may also be sub-sampled to focus on select classes of features (targeted sampling). Another important feature of very large sample sizes is the ability to combine a posteriori adjacent sample points to create new sample-point sizes (footprints) with mean values and associated error terms. This allows one dataset to be used in a variety of sampling designs and analyses addressing questions at different scales and resolution. Of particular interest is that combined footprint sample points gain higher reliability of being effectively re-sampled for monitoring over time, as each foot print can assume the mean geospatial location of its several points. This feature will allow us to effectively track shapes, volumes, and locations of objects, such as small sand dunes, over time.


Sampling Intervals, Revisit Schedules, and Panels

The GULN program has yet to finalize the sampling interval, frequency and date-scheduling of geomorphic monitoring. One possible model we are currently considering is that GUIS and PAIS could be treated as one panel to be sampled one time every three years. Sampling dates would be approximate windows, determined by weather and flight availability with a revisit design of type [1-2]. We further anticipate that one or both parks may receive additional sampling runs (adaptive sampling) following acute storm events if park management requests assessment of storm impacts on island geomorphology and resources.

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Last Updated: May 18, 2017 Contact Webmaster