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COMBINING HYDRAULIC FILLING WITH DIKING REINFORCES ERODED BEACH

Source: R.M. Buchanan, Vice President - Higgerson-Buchanan, Inc.

An experimental procedure for beach rehabilitation in the vicinity of Cape Hatteras was recently successfully completed. The principal construction tool was a 12-inch Ellicott® portable hydraulic pipeline dredge. Here is a report on how the project was carried out.

For several years  North Carolina had faced a growing problem of eroding beaches at Buxton, N.C. on Hatteras Island--a sliver of land extending north from Cape Hatteras lighthouse between the Atlantic Ocean and Pamlico Sound off the North Carolina Coast.

Basic to the problem was the need to maintain an existing duneline in view of the narrowness of the island (1,500 ft. in the 1.5 mile stretch requiring repairs).

So it was decided to split the job into two separate projects:

(1) Reinforcing the beach section in front of the existing duneline at the southern (or residential) section of the strip to be rehabilitated, and

(2) Building a "back-up" dike behind the deteriorated duneline at the northern section of the strip.

The second part of the job was the experimental portion: building "back-up" dikes to hold the duneline. These dikes (or retaining walls) would have to be strong laterally and topped off in a manner to retain proper, protective heights.

The plan was to provide 100,000 cu. yds. of fill to each section plus 20,000 cu. yds. as a stockpile at the south end for possible emergency use later on.

Reaching Shell Content
Since topping off of the northward reinforcing wall was essential to success of the job, the character of the fill material was examined in the vicinity of the borrow area off the island in Pamlico Sound.

The material was found to have a high degree of shell content, varying at different depths. The greater the shell content in the final layers of fill, the greater the stability of the fill. So it was essential to obtain excavating equipment with maneuverability, high capacity and the ability to pump material through extended pipeline lengths, as the plan was to fill and build the back-up dikes as we maneuvered into favorable positions for filling the beach areas in the southern residential section. Pipeline lengths up to 3,500, (over 1000m) feet would be needed.

A 12-inch hydraulic pipeline dredge in the line of Ellicott® "DRAGON" model portable dredges was found to meet all requirements. Capable of digging to a depth of 26 feet, the dredge can pump through pipelines varying in length up to the required 3,500 feet. It has an output capacity of up to 440 cu. yds. per hour, when pumping the kind of sand and small shell that would be encountered.

Digging Access Channel
The dredge, named CHESAPEAKE, began work digging an access channel. This enabled it to reach the borrow area opposite the northward stretch of land on which the dune back-up work needed to be performed.

An access channel 2,000 ft. long, 50 ft. wide and 6 ft. deep was excavated to reach the borrow area from existing deep water, and the dredge located offshore in the sound.

Then, as planned, work on the retaining wall section of the project began. The CHESAPEAKE'S capability to dig to 26 feet (8m) enabled it to obtain the material with the highest shell content, which was found at varying depths.
Discharging material to the farthest point of the fill section, the dredge pumped the sand, shell and water mixture through pipeline lengths up to 2,900 feet.

Varying Depths
Depth varied from 12 to 25 feet, depending on the location for the largest shell content in the sand. At the same time, the dredge was maneuvered into the best position for filling the beachfront area to the south.

After placement of 120,000 cu. yds. of material (20,000 more than originally planned), the dike was completed. 

The retaining dikes for the dike fill were built with a small bulldozer, which tended the dikes as the fill was placed, and dressed top and slopes when the job was finished. It was also used to close the discharge areas leading through the existing duneline to the beach.

With a few days weathering by rain and wind, the dike armoured itself with the shell fragments, which formed a glazed surface over the top of the entire filled area. According to Park Service officials, further erosion has been practically eliminated.

Beach Nourishment Job
In the meantime, the CHESAPEAKE had been maneuvered into position for providing the beach nourishment fill for the residential section. Here, material was dredged through 100-ft. pipeline lengths extending the full estimated distance of 3,500 feet.

Using a straight and level pipeline paid off in helping achieve this distance. At 3,200 feet from the dredge, we were able to hold a velocity in excess of 11 feet per second.

By careful attention to grade and adding pipe at the proper time, no additional earthmoving machinery was necessary for finishing, A Y-valve was used on both fills in order to minimize lost time in adding pipe.

In Winds and Tide
The dredge was operated in 40-to-50-MPH winds and sound tides up to five feet or more. Two crews worked 10 hours each with a small crew working during a four-hour down period making minor repairs, working pontoon lines, moving shore pipe, changing oil, etc. 
With the Cape Hatteras job completed, the CHESAPEAKE moved on to Stumpy Point, North Carolina, producing select material for use in building roads. Then, during the first part of June it moved on to Norfolk for placing 500,000 cu. yds. under a runway extension at the Municipal Airport. This contract involves the preliminary removal of 100,000 cu. yds. of unsuitable material.

By R.M. Buchanan, Vice President
Higgerson-Buchanan, Inc.
General Contractors
Norfolk, Virginia

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