NOAA's National Centers for Coastal Ocean Science

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The NCCOS scientist shown here is obtaining a detailed trace of the marsh surface with a GPS receiver mounted on wheels to maintain constant contact with the surface. This technique yields detailed Digital Elevation Models of the marsh surface, and was developed in partnership with NOAA’s NGS.

We have also compared sediment accretion rates in fringing marshes with and without oyster reefs to marshes planted behind stone sills or breakwaters, a design often described as a "living shoreline". We evaluate sediment elevation changes with a variety of approaches, including Surface Elevation Tables, survey equipment, and real–time kinematic GPS units. We are working with NOAA’s NGS to develop RTK GPS protocols for obtaining high–resolution digital elevation models of marsh elevations for areas remote from tidal datums. So far this work has shown us that stone sills are very effective sediment traps, and may in fact be a little too effective in that marsh elevations behind stone sills are higher than those in natural fringing marshes. The problem with that is that many of the ecosystem services provided by marshes (fishery habitat, sediment and nutrient removal from the water column) only occur when the marsh is flooded.

What we don’t understand so well at this point is just how different shoreline shapes and their varied exposures to waves may alter the ability of fringing salt marshes and/or oyster reefs to accrete sediments and still maintain their elevation relative to sea level. And we need a longer–term data set to understand the response of these shoreline habitats to storm events such as the well–known "nor’easters" and hurricanes all too common in this area.

So far we’ve focused on North Carolina. Are the North Carolina marshes representative, or not representative, of ecosystems we find elsewhere through the country? What "lessons learned" from this work can be applied elsewhere?

CC: Well, first, North Carolina marshes themselves exhibit a fair amount of variability in terms of shoreline erosion rates, sediment accretion rates, and projected responses to sea level rise. So I really wouldn’t want to suggest that we can even generalize across the whole state.

Intertidal oyster reefs at the base of Spartina alterniflora marshes, such as this in the Rachel Carson National Estuarine Research Reserve in NC, can enhance sediment accretion rates and prevent or slow shoreline erosion.

Having said that, the state clearly does not have areas that are subsiding at the rates found in Louisiana, for example, but we do have variable rates of subsidence between, for instance, the Albemarle–Pamlico region, the southern Outer Banks region, and the Cape Fear River region.

As for "lessons learned," studies from Chesapeake Bay, South Carolina (also see here), and Louisiana, for example, certainly have applicability to North Carolina estuaries on a regional basis. These include findings that salt marshes can maintain their elevations in response to sea level rise, assuming that they have adequate sediment supply and that relative sea level rise does not exceed roughly five millimeters or so per year. We also know from the Chesapeake Bay and elsewhere that hardened shorelines have an adverse impact on adjacent salt marshes and other shallow–water ecosystems, so those too are "lessons learned" that are applicable beyond the specific study areas.

We have several times mentioned sea level rise, and certainly the news media have had lots to say on that issue over the past several months. But how is potential sea level rise actually factored into your analyses, and are you reaching some general conclusions on potential impacts?

CC: We have a NOAA tide gauge here in Beaufort that tracks local sea level, and it’s been operating for decades, so it gives us a good picture of local changes in sea level.

We have also installed several temporary tide gauges near our study sites so that we can more accurately track the water depth and inundation time at those sites. We then analyze our data by the exact tidal elevation of each site. That allows us to model or predict how marshes might respond if their elevation is lower or the water higher.

CCFHR staff installing temporary tide gauges at a) Cape Lookout National Seashore Headquarters on Harkers Island and b) NC aquarium at Pine Knoll Shores. The tide gauges provide water level data that will be used to help model response of salt marshes to storm events and sea level rise. CCFHR followed a protocol for establishing the temporary tide gauges which was developed with the guidance of NOAA CO–OPS

We also work with academic scientists who run experiments in which we manipulate the tidal elevation of different shoreline habitats, artificially raising sea level and thus providing data for our predictive models. These experiments allow us to forecast consequences of different scenarios of sea level rise and the response of estuarine intertidal habitats such as salt marshes, oyster reefs and tidal flats. These kinds of predictive models, or forecasts, can help guide habitat restoration activities, for instance. And they help us in our efforts to prioritize conservation or shoreline stabilization efforts.

Let’s look for a moment at another issue that has attracted lots of media attention: With increasing coastal development and population, we’re seeing lots of examples of people artificially "hardening" their shorelines. What factors should residential and commercial land owners be considering as they evaluate whether to harden, how to harden, and how best to conserve their coastal shorelines? Are there practical alternatives to traditional hardening or rip–rap?

CC: Waterfront property owners will do well to consider their options in the following order: first, conservation; second, using vegetation to attenuate wave energy and stabilize the shoreline; and finally, hardening the shoreline.

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They can increase their conservation efforts through various land–use choices, for instance trying to reduce the need for shoreline stabilization by siting structures away from eroding shorelines. Most states have minimum building setbacks from the high water mark; that setback should be observed as a minimum. Optimizing your water view may seem like a good idea during construction/purchase, but during your first hurricane or nor’easter, you might really wish you had your house further back. There are also published erosion rates for many estuarine shorelines, and coastal property owners should consult them prior to buying waterfront property.

If some sort of shoreline stabilization is desired, the maintenance or restoration of natural shoreline vegetation can be very effective. Just a 10–foot buffer of salt marsh vegetation can reduce wave energy by 50 percent, and wider buffers are even more resistant to erosion. Salt marshes also trap sediments, and can actually increase their baseline elevation given adequate sediment supply.

Fringing salt marshes bordered by oyster reefs provide a variety of ecosystem services, including shoreline stabilization, fishery habitat, primary and secondary productivity, and removal of sediments and nutrients from the water column. Shoreline stabilization structures should be designed to minimize the adverse impacts on intertidal habitats.

In addition, a shoreline with upland bushes, salt marsh vegetation and oyster reef provides a tremendous amount of erosion protection, and also essential fish and bird habitat. An added bonus is the decrease in storm water runoff and the resulting improvement in water quality associated with such a shoreline.

Where some shoreline hardening is still determined to be necessary, the property owner should seek to minimize impacts on adjacent property and offshore habitats. Hardened shorelines reflect, rather than absorb waves, although some structures do better than others. Bulkheads should be brought as far landward as possible to minimize the impact on estuarine ecosystems, and the height of breakwaters or sills should be minimized as much as possible to reduce adverse impacts.

Q: A 2006 report from the National Academies of Science’s National Research Council appears to suggest that shoreline management strategies need to be individualized or customized to address varying ecosystem settings. Is there really no standardized approach to mitigating shoreline erosion? Are your scientific findings pointing to a need for regional strategies?

Results of the Wave Exposure Model (WEMo) for Carteret County, NC.

CC: I don’t think there is a "one size fits all" approach to managing shoreline erosion, although there are some general concepts that hold true.

Wave energy, shoreline morphology, geology, sediment supply and vegetation can vary greatly even within a single estuarine ecosystem. A regional strategy would certainly help identify methods appropriate for given energy environments within a region, and it could help to narrow the choices so that property owners aren’t starting from scratch.

We are stratifying, or characterizing, our sites by the wave experience they experience. Other researchers here at the lab (Mark Fonseca and Amit Malhotra) have developed a Wave Exposure Model (WEMo), which combines historic wind data, bathymetry, and calculated fetch from 40 compass points to generate a site–specific wave energy estimate. This is an important first step in understanding shoreline erosion dynamics, and in designing appropriate shoreline protection.

NCCOS staff measuring wave heights and changes in light attenuation associated with boat wakes impinging on a) a submerged stone sill fronting a restored salt marsh and b) a vertical bulkhead. Instrumentation is deployed on PVC poles, which include a pressure sensor, light sensors, and a multispectral attenuation/absorption meter.

With Fonseca and Malhotra we are also addressing another important aspect of estuarine shoreline erosion–boat wakes. This was identified in the National Academies’ report as an important element that is currently poorly managed. We are beginning research to look at how different shoreline types reflect, refract and/or attenuate boat wakes, and to assess the factors influencing sediment resuspension associated with boat wakes. Sediments suspended into the water column by waves or boat wakes reduce the amount of light that penetrates the water column. In turn, that reduced amount of light means there is less light available to the seagrasses, macroalgae and microalgae which are so important to estuarine ecosystems. In shallow estuarine environments, sediment resuspension associated with boat wakes and waves may be an important control on overall water quality and estuarine productivity.

What do you say to those who think the challenges of managing our coastal shorelines and controlling erosion are just too great? Those who fear the situation is perhaps hopeless?

NCCOS staff and student volunteer mapping vegetation and Spartina alterniflora density on Harkers Island, NC. Portions of this north–facing shore have experienced significant erosion of the fringing salt marsh.

CC: Oh, I don’t think it’s hopeless at all. But there’s little question that the research is pointing to a need for society as a whole to make some tough decisions about the cost–benefits of shoreline stabilization.

If sea level rise does rise a foot over the next 50 years, as some models are suggesting is a possibility, there will be a large number of property owners trying to maintain a particular shoreline location "no matter what." Society, politicians, and resource managers will have to make decisions on how to balance the loss of property vs. the adverse impact to public trust resources resulting from further hardening of shorelines.

My hope is that we as scientists will provide good science–based information to allow informed decisions. That’s what brings us to work every day.