The Role of Coastal Processes

cp1Barrier islands like those found on the Outer Banks of North Carolina represent land forms that are subjugated to an array of environmental factors including variable winds, waves, and episodic storm events.  Rodanthe is a back-barrier area, meaning it is on the sound side of the islands.  This makes it susceptible to different conditions or processes than the ocean side.  These environmental conditions impact what goes on both above and below the water and have the capacity to bring about rapid change to the habitats that are found therein.

 

A team of geologists at the UNC Coastal Studies Institute are evaluating wave and current processes with regards to sediment dynamics in the research area at Rodanthe.  This area is referred to as Hatteras Flats and is characterized by being both very flat and shallow.  What these researchers want to know is how the processes of wind and waves move sediment on the Hatteras Flats.  Scientists are trying to discover what options there are to mitigate the refilling of the channel due to these processes.

PSES

 

 Sound Science: The Pamlico Sound is the largest of North Carolina’s estuaries measuring 80 miles in length and 15-20 miles in width, providing a water surface area of 3,000 square miles.

 

Sediment Dynamics

 

When geologists talk about sediment dynamics, they are talking about particles that are transported by water flow.  Sediment dynamics is the attempt to understand how these particles move around, what causes them to move and also what contributes to their retention.  If scientists can figure out a way to inhibit sediment dispersal, then it paves the way for understanding a way to disrupt the degradation of shoreline caused by back-barrier erosion.   By understanding the sediment dynamics unique to the defined research area in Rodanthe, scientists hope to offer suggestions to the NC DOT about their options for dredging and spoil deposition.

Research Methods: Field and Lab Work

IMG_6064cj  sediment (1 of 1)

 

 

 

 

 

The first step in the approach to understanding sediment dynamics in the research area is to first understand the sediment.  That means recording its type (whether its sand or silt), size (medium or fine), and the amount of organic material present (lots of vegetation, little or none).  All of these factor into the capacity for the sediment to be re-suspended, or swept up into the water column and able to be carried away.

In addition to knowing the characteristics of the sediment, scientists need to know the characteristics of the processes.  That means they need data on the wind, water depths, tide movement, water clarity and the rate of erosion.  If you combine knowledge of the variety of sediment present with knowledge of processes, then an educated suggestion can be offered concerning where to place the sediment, where not to place it, or how long DOT can expect to re-dredge a particular area.

Before conducting any field research, the geologists spent time in the lab pouring over maps like the one below, which use aerial images of the coastline going back decades.  By looking at these images and comparing how the coastline looked and changed over the years, the scientist can learn a lot about the effects tides, wind, and weather have historically wrought on the back barrier.  In getting a sense for this past, the scientist are better informed to make judgements based on modern data about what is going to happen in the future.

aerialimage copy

This map details shoreline change in the study area at Rodanthe spanning nearly 70 years.

 

Rodanthe Coastal Processes Results

Several pieces of data from shorelines to waves and currents were collected over an 18 month period and show a dynamic back-barrier environment. Shorelines are eroding across the region, and over recent years there have been large changes in the shoreline type. Waves and currents were also great enough that sediments across the shallow back-barrier flat are frequently in motion. The movement of sediments likely removes locally deposited organic content and mud.

The shoreline data from the Rodanthe region showed an area that was dominated by erosion (Fig. 1).

Figure 1: Shoreline change rates on decadal- (1949-2015; left) and annual (2012-2015; right) timescales. Note the widespread erosion during both timescales.

The long-term erosion rates were mostly between 0 and 1 meter per year, although some areas showed erosion as high as 2 meters per year.  One of the shoreline features that were analyzed was shoreline type (Fig. 2).

Shoreline type was classified by three categories: marsh, sediment bank, and modified. Five regions were delineated based on their modern (2015) shoreline types. Two of these regions showed severe marsh loss and an increase of sediment banks. One of the two with marsh loss also saw a major increase in shoreline modification (i.e. seawalls, rip-rap).

Data collected on waves and currents showed that there were times that sand-sized sediments were remobilized, or moved around the region. This would happen when winds were blowing a moderate breeze (~20 mph) or greater. The wave and current data demonstrate that storm-related winds, waves, and currents will effectively move sediments around the back-barrier region.

The sediments further from the Rodanthe shoreline are on average medium-fine sand. These sediments had very low organic content and very low mud-sized material. In areas with submerged aquatic vegetation, there is often more organic material and mud. The resuspension and transport of the Rodanthe sediments are likely removing organics and muds initially deposited in the region.

Figure 2: Shoreline type change for each region during all time steps. Note the increase in modified shoreline type in Region 4, an area of high erosion rates.

Time series bathymetry in the ferry channel was also analyzed (Fig. 3) and found to shoal over certain time spans (~1-2 years). Several of these shoaling periods were times the Outer Banks were impacted by a tropical cyclone. With future storms, the channel will likely shoal again, which would require dredging prior to emergency ferry access.

The collection of data portrays a dynamic back-barrier environment characterized by shoreline erosion and change in shoreline type associated with the high erosion rates. Sediments in the back-barrier are often remobilized and transported both within and to areas outside of the region. This research will help inform ferry channel management for maintaining access to the island when necessary.

Figure 3: Example measured bathymetry for 2000 in the emergency ferry channel approaching Rodanthe. Sectors were defined to evaluate spatial change in the channel. These maps were created for several years between 1995 and 2014.