Project Profile: Preserving Hawaiian Coastline with a Low-Impact Sediment Barrier

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Earthworks Pacific Inc. is a heavy civil and underground utility contractor based in Lihue, HI. Founded in 2003, the company has a reputation for excellent work in both the public and private sectors throughout the Hawaiian Islands.

EPI was contracted to start work on the new Makahuena Estates Subdivision in Poipu, on the island of Kauai, in October of 2016. The future neighborhood of custom homes was planned along the shoreline, with a beautiful view of the Pacific Ocean.

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Alyssa Carveiro, project engineer, prepared the initial stormwater pollution prevention plan (SWPPP) for the project. Bryan Davidson, project manager and superintendent at EPI, made modifications to the SWPPP based on field conditions. He then purchased and installed BMPs and oversaw inspection, along with Brandon Ezaki and Jordan Wilson, both project engineers with EPI, for the project until its completion in July of 2017.

The Challenge
The Makahuena Estates Subdivision was to be built on a pristine shoreline property, and the natural landscape needed to be preserved at all costs. The property had several different types of ground conditions that varied from silt to solid basalt rock. The BMP chosen for the project needed to be installed on these different ground surfaces while still providing a continuous barrier to keep sediment-laden water trapped onsite.

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DuraWattle can be installed on soil or rock surfaces.

EPI did not want to use silt fence or similar trenched-in products for the project. Installing silt fence on the solid basalt rock would have required special equipment and more labor hours for installation, and it would have caused irreparable damage to the existing ground along the shoreline.

The project drawings called for straw wattles or a silt fence alternative. However, if the alternative barrier chosen was trenched into the rock per the details, then the removal of the product would have undoubtedly permanently damaged the landscape. The project needed a BMP that could be installed on hard surfaces without damaging the ground surface when removed.

Additionally, substantial grouted rock walls up to 13 feet tall were to be built less than 10 feet from the perimeter of the project by the subcontractor, T. Masaniai Contractors. The perimeter control BMP would need to withstand traffic and heavy construction activity without compromising its overall sediment retention effectiveness.

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Crews operated equipment near the rock walls and perimeter

The Solution
Davidson first learned about DuraWattle in Erosion Control magazine. After doing his own research, he purchased one pallet of the product from Exacta Sales in Honolulu, HI.

According to his research, DuraWattle was the best alternative to silt fence that could provide a continuous sediment control barrier across multiple surface types. The product also solved the issue of a low-impact installation; the recommended hard surface installation of DuraWattle would keep the basalt rock shoreline intact while keeping sediment trapped onsite.

DuraWattle is also designed to be driven over by heavy traffic. Subcontractors would be operating excavators near the perimeter when constructing the grouted rock walls. The product would be able to bounce back and remain effective during heavy rain events even after being driven over by excavators.

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Uneven terrain

The Installation
When 1,800 feet of the product arrived compressed on one pallet at the site, the lightweight bundles were moved and distributed by hand around the job site.

According to Davidson, 1,650 feet of the product was installed at the Makahuena subdivision by a three-man team in 15 hours. He says that installing DuraWattle on soil took longer than installing silt fence on soil; however, the overall installation cost less than if he had installed the BMPs that the plans originally specified. “The DuraWattle was 60% cheaper than trenching into the solid rock to install the silt fence alternative,” he notes.

The installation team used the provided pins and washers to install the product on soil and powder-actuated fasteners with lath when installing on the solid basalt rock. For hard-surface installation, the product is usually secured to the ground using wood planks or steel beams. However, the basalt rock was too uneven, and Davidson had to secure the fasteners directly into the tail section and “backfill” over the tail section with 3- to 6-inch gravel.

This project was being carried out during Kauai’s rainy season, and large rain events could be expected at any time. Augmenting the DuraWattle with gravel over the tail section was necessary to keep sediment onsite. Without the tail section sealed securely to the ground, any heavy concentrated flow would have undermined the DuraWattle at the uneven sections of rock and caused a massive BMP failure, especially given the project’s proximity to the ocean.

Ec1805 48The Outcome
The project experienced two large rain events in a nine-month period. Both events saw over 5 inches of rain in less than 24 hours. The heavy rain could have been catastrophic for EPI’s team, as well as the adjacent Pacific Ocean, but the company’s good housekeeping practices kept the site secure during both events.

The gravel helped to dissipate the stormwater flow enough to allow the DuraWattle’s high-flow filter medium to retain sediment and create sheet flow offsite. Davidson’s team needed to re-backfill the tail section in a few locations and remove the built-up silt in other areas. However, Davidson was very impressed with the performance of the BMP combination.

Although the DuraWattle saw little traffic from heavy machinery, Davidson still had to replace a few 12-foot sections after they had been torn by large excavators that turned their steel tracks while on top of the DuraWattle.

After the rock walls were installed and the site was stabilized, Wilson removed the DuraWattle with little difficulty or damage to the product. The basalt rock and pristine shoreline were preserved. “It was difficult to tell where DuraWattle had been installed after we finished removing it,” says Davidson.

EPI plans to reuse the DuraWattle on future sites. After removing the product, the team bundled it with rope and stored it in a pile.

Despite heavy rain events and heavy construction activity near the perimeter of the site, EPI created an innovative combination of BMPs that worked. The team kept the site in compliance while preserving the beautiful Hawaiian shoreline for both the future residents and the public to enjoy. Ec Bug Web

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