A Coastal Engineer Answers

Written by Ken Humiston, Humiston and Moore Engineers | 15 March 2009

T-groins are used to prevent beach erosion.

A reply to the guest editorial that charges some consultants with threatening beaches.

Debate between Orin Pilkey and the coastal engineering community has been going on for a long time. The first thing anyone who read Dr. Pilkey’s editorial (March 2, 2009 Angler Alert) should understand about the debate is that it is not over whether or not dredging sand from coastal inlet shoals causes impact to adjacent beaches. I have had the pleasure of personal discussions with Dr. Pilkey on several occasions an assure everyone that we are in complete agreement on this issue. In fact the engineering community has studied the cause and effect relationships of this complicated dynamic process very carefully in order to gain a much better understanding of how it works.

Why do engineers study dynamic inlet processes? Because inlet shoals are routinely dredged for a variety of reasons, the most frequent one being navigation.

Clam Pass is a small inlet in Collier County which is the only tidal connection to a 500 acre bay and mangrove wetland preserve. For many years this small inlet had closed on a frequent basis. Inlet closure was associated with degraded water quality in the bay, fish kills, and large areas of mangrove die-off, (conditions pictured on left). Dredging the flow-ways improved tidal circulation throughout the preserve, and the improved flow resulted in stronger inlet tidal currents that generate enough force to scour sand from the channel to prevent inlet closure. There has been significant recovery in the mangrove die-off areas and no inlet closures or fish kills since the 2000 dredging. (photo on right)

Three T-groins on on the south end of North Captiva Island at Redfish Pass were placed there in 1998. That shoreline had been eroding at more than 50 feet per year but it hasn't gone anywhere since, and there has been no sand placement or maintenance of the structures, so from an engineering standpoint they have been extremely successful. The Redfish Pass T-groins are a favorite spot for some of the local fishing guides, attracting and holding redfish and many other species. Structures like these are basically nearshore artificial reef and provide habitat for a wide range of marine organisms over a broad spectrum of the food chain.

Dredging for the purpose of navigation improvements is necessary to maintain channels for recreation, commerce, and national defense. The debate between Dr. Pilkey and the engineering community is therefore more over design issues, or whether or not dredging is justified at all, than it is over the extent to which dredging causes impacts to adjacent beaches.

In the early days of dredging for navigation, dredged sand was actually dumped off shore just to get rid of it. That practice was discontinued during the 1980’s because by that time it had become apparent that removing sand from the littoral system was damaging beaches. Since then the state has required that beach quality sand dredged from inlets be placed on adjacent beaches where impacts occur. The engineering community helps those entities who wish to maintain navigation channels to do so under best management practices that minimize impacts to beaches, and to mitigate for impacts that may be unavoidable.

Dr. Pilkey’s editorial deals more with mining of sand specifically as a source for beach restoration rather than navigation maintenance dredging. The impacts to beaches from this practice is similar to impacts associated with dredging navigation channels because both have the potential for interrupting natural sand transport past inlets. As with maintenance dredging, early mining of sand from inlet shoals was less discriminate than it is today because today there is a much greater understanding of impacts that resulted from that practice.

The Gasparilla Island project restored three miles of beach with approximately one million cubic yards of sand. The sand was taken from the ebb shoal at Boca Grande Pass, however, it was selectively mined in a way that simply widened the natural channel at a point between two and three miles offshore and the volume dredged was only approximately 3% of the sand stored in the shoal. Detailed analysis showed that the dredging would not cause impacts to adjacent beaches. The project restored beach along an eroded shoreline which in many places had been armored with seawalls and rock revetments. The primary environmental benefit from this project is restoration of sea turtle nesting habitat, but the project also provides significant recreational and storm protection benefits. A second phase of this project will add a segmented breakwater to control erosion near the south end of the island where sand from the beach is washed into Boca Grande Pass. Adding stability and reducing sand loss with the structures will reduce the need for renourishment in the future, thereby reducing the environmental disturbance that occurs during construction of projects like this, and reducing long term maintenance costs.

Environmental issues are often of vital importance on engineering projects in the coastal environment. There are in fact unavoidable environmental impacts from these projects, and there have been cases where impacts were severe. However, those are the exception rather than the rule. For the most part environmental impacts are short term and insignificant in comparison to resulting long term benefits. Without Florida’s beach nourishment projects, miles of beach would turn into armored shoreline of seawalls and rock revetments.

Dr. Pilkey is not the only one who would prefer to see development retreat from the shoreline when threatened by erosion, however, realistically there are few places where that will happen, and engineered solutions are often the only realistic alternative.

The Naples T-groin project was completed early in 2000 to reestablish and stabilize beach in an area where the shoreline had for years been armored with rock rip-rap and a seawall several hundred feet north of Gordon Pass in Collier County. Since construction of the two T-groins, the rock and seawall have remained buried behind a beach that has been recolonized with native dune vegetation, and has been restored as sea turtle nesting habitat. A minor amount of sand was placed at the time of groin construction, however, no renourishment of this project has been necessary since then. The groins are constructed with a unique low profile design that allows longshore transport to continue, and there has been no downdrift impact in the form of beach erosion of the type that often occurs with other types of coastal structures. The low profile also means sea turtle hatchlings reach water before they can encounter the structure which so far has prevented the trapping of hatchlings in the manner that has been documented for a high profile T-shaped groin project on the east coast.

The level of success of any individual project is going to be related to how well it is designed, which is the role of the coastal engineer. As with any profession, doctors, lawyers, and even in Dr. Pilkey’s own field of geology, some are more successful than others. To cite several examples of projects that did not perform to design expectations as a basis for criticizing an entire profession is not credible.

What one may be certain of is that in spite of the challenges associated with engineering projects that deal with many complex and at times uncertain variables of the coastal environment, the vast majority of dredging and beach restoration projects completed in Florida have been very successful. Environmental and economic benefits of these projects are well documented.

We don't profess what we do is perfect. One of the issues that is still being debated is potential impacts of structures such as T-groins to migrating sea turtle hatchlings; whether the structures would obstruct migration or concentrate predatory fish. Maybe someday we will have more science to answer these questions.