Land-Sea Interaction

DrGrahamGieseGraham GieseThe Land-Sea Interaction Program seeks to understand the physical processes responsible for coastal evolution at multiple temporal and spatial scales. Centennial scale changes to beach, barrier islands, and spits have been studied through multiple projects.

In the Marindin Project, Dr. Graham Giese (left) has partnered with Mark Adams of the Cape Cod National Seashore to resurvey over 200 transects that were first surveyed between 1887 and 1889. These transects were initially surveyed from onshore points across the dunes, bluffs, beaches and continued offshore for 1-2 km. They are all now within the boundaries of the Seashore.

Other current and recent projects include the study of tidal inlet evolution, documenting change in salt marshes, the relationship between eelgrass and coastal sediment transport.

MarineGeologyland-seainteraction_MarindinTransectsOngoing Projects

1. Marindin Project

This study was designed to examine current and past measurements of outer Cape Cod coastal features and identify trends that affect these landforms. Historical coastal profile measurements obtained by Henry L. Marindin between 1887 and 1889 are being analyzed and compared with newly acquired data to determine quantitative changes in coastal sediment volume. Current locations and volumes of sub-aerial shoreline features are being obtained through analysis of LIDAR and differential GPS data, while submarine forms will be determined from GPS-controlled bathymetric surveys.

The specific study objectives are:

a. Extend Marindin transects landward/seaward as found necessary to maintain their usefulness. Coastal erosion has claimed the origin and landward sections of some the original 19th century transects. In the case of others, recent measurements indicate that offshore sea bottom changes have extended into deeper water than that included in the original transects. Extension of these transects will ensure a high quality data set for future management-based studies.

b. Based on the comparison of modern and historic data, determine the century-scale change (area accreted/eroded and its distribution along-transect as well as linear shoreline advance/retreat) at each of Marindin’s 229 transects between North Chatham and Long Point in Provincetown. These data will allow for assessment of present and future threats to existing coastal facilities from shoreline erosion and the suitability of specific coastal locations for future infrastructure.

c. Calculate the annual sediment budget for the coast of outer Cape Cod and estimate the annual net alongshore sediment flux (direction and volume rate) at each of the 229 transects. Once the overall sediment budget for this single-source coastal system is known, expected near-future volumes of erosion/accretion can be specified for specific regions. Determination of net sediment flux at each transect will provide the database required for calibration of future oceanographic studies designed to calculate sediment flux from ocean-forcing functions, such as wave climate and sea-level change. This understanding will enable prediction of longer term future changes in response to specific changes in the forcing functions.

Comparison of High Water Line (HWL) and Marshline shoreline indicators along the same stretches of low-energy bay shoreline. (A) HWL transects and (B) marshline transects. Black is erosional, white accretional and gray represents transect within the range of uncertainty.

Comparison of High Water Line (HWL) and Marshline shoreline indicators along the same stretches of low-energy bay shoreline. (A) HWL transects and (B) marshline transects. Black is erosional, white accretional and gray represents transect within the range of uncertainty.

2. East Harbor Tidal Restoration

The Center is cooperating with the US Army Corps of Engineers and Cape Cod National Seashore in the analysis of the proposed redesign of the culvert connecting East Harbor and Cape Cod Bay. The Center conducted topographic surveys, collected beach profiles and vessel-based bathymetric surveys on and near Beach Point, Truro in the early winter and late spring of 2010.

These surveys provided accurate, up-to-date information for the USACE’s numerical models that will be run to determine potential impacts to various-sized culverts to the downdrift beach and nearshore ecosystem. The only bathymetric data available to the USACE were those taken from the most recent NOAA nautical chart of the area (Chart 13249), the data fronting Beach Point were collected between 1900 and 1939.

3. Rates of Change in Salt Marsh Ecosystems

Change in Marshline and HWL. A) 1938 Aerial Photograph. B) 2005 Aerial Photograph. Inset shows location within Pleasant Bay. The dashed and solid orange lines are the 1938 marshline and HWL respectively. The dashed and solid black lines are the 2005 marshline and HWL respectively. Inset, arrow indicates approximate location in Pleasant Bay.

Change in Marshline and HWL. A) 1938 Aerial Photograph. B) 2005 Aerial Photograph. Inset shows location within Pleasant Bay. The dashed and solid orange lines are the 1938 marshline and HWL respectively. The dashed and solid black lines are the 2005 marshline and HWL respectively. Inset, arrow indicates approximate location in Pleasant Bay.

Rates of shoreline change are rarely calculated for low-energy coastal embayments. The belief that most of the rates generated from such a study would fall within the range of uncertainty, and thus yield little usable data, have discouraged many investigators from pursuing studies. In fact, the USGS is currently working on a national shoreline change study but are not looking at low energy coastal embayments in any detail.

As part of his doctoral research, a subsequent technical report and a peer-reviewed journal article currently in preparation, Mark Borrelli looked at shoreline change in a low energy coastal embayment. In Pleasant Bay, Chatham, Massachusetts he compared rates of shoreline change using the most prevalent shoreline indicator for studies using historic aerial photographs, the High-Water Line (HWL), with a new shoreline indicator that represents ‘the most-basinward edge of marsh vegetation or marshline.

The delineation of the marshline from aerial photographs was more objective, repeatable and reliable and thus more scientifically defensible than the visual cues associated with the HWL. Further, several sources of uncertainty in calculating rates of shoreline change are eliminated or significantly reduced when using the marshline.

Less than 25% of the lagoon shoreline with fringing marsh examined using HWL exhibited statistically significant shoreline change as compared to more than 56% of that same shoreline when using the marshline.

The marshline allows the investigator to quantitatively assess changes in salt marsh habitat related to surface area, fringing marsh width, shoreline orientation and marsh appearance and disappearance. Further, a better understanding of the evolution of fringing marsh can contribute to investigations involving low-energy sediment transport, sea level rise, water quality, predator-prey relationships, filtration and sequestration of contaminants and overall ecosystem health.

4. Tidal Inlet and Tidal Delta Evolution

Tidal inlets have a profound influence on sediment transport pathways, water quality, wave and tidal current and other coastal processes. Understanding these systems has been the subject of many studies here at the Center. Currently several papers are either in press or in preparation regarding the evolution of the tidal inlet at Chatham Harbor, Massachusetts. Dr. Graham Giese has several publications regarding this system and has contributed to ongoing studies.

A paper to be delivered at Coastal Sediments 2011 looks at the counter intuitive increase or continued instability of a newly formed inlet in 2007 that was assumed to decrease the hydraulic inefficiency of the previous inlet that was formed in 1987.

Two other articles are in preparation that use Chatham Harbor as an example to illustrate: 1) the relationship between an inlet’s flood-tidal delta as a indicator of inlet stability; and 2) the influence a relatively small anthropogenic alteration can have on a highly energetic system.

5. Sediment Transport and eelgrass (Zostera Marina)

The seafloor mapping program is collecting data that will provide Center investigators with the information needed to address questions regarding the interaction between eelgrass and bedforms.

Eelgrassandbedforms