Turbidity Monitoring Establishes Baseline Data Prior to Construction Activities

SUCCCESS CASES

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Overview

The Pono Kai Seawall is located on the eastern coastline of the island of Kaua’i. It was significantly damaged from years of exposure to high tides, heavy rains, and pounding surf. The seawall was to be repaired by constructing a sheet pile wall with a rock toe and adding beach nourishment. However, the repair project could result in suspended sediment that might degrade water quality in the area around the project site, possibly causing harm to wildlife and vegetation. During the construction, water quality had to be continually assessed to determine any degradation of water quality.

Pono Kai, Hawaii

Challenge

Water quality was monitored along approximately 1,000 feet of shoreline from the rock jetty at Waika`ea Canal to a point about 100 feet north of the north end of the sheet pile wall. The offshore boundary is less than 100 feet from the shoreline. Water depth in the nearshore areas was shallow, ranging up to about 5 feet deep. Nearshore ocean waters and ocean bottom were the areas of interest. The construction of the new sheet pile wall, rock toe, and beach nourishment required a minimum of 3 to 4 months.

Solution

To establish a more realistic baseline for comparison with construction measurements, turbidity was measured for 10 days over a 2-week period before construction started. To meet the monitoring goal, water quality was assessed by measuring turbidity with four Manta probes. This was the most resource-effective method for collecting turbidity measurements because fewer labor hours were needed for sampling. A Manta water quality probe ran 24 hours per day without human intervention. The probe was downloaded once per week and collected much more data than could be obtained from a grab sample. The site was visited once a week to download water quality data.

Results

Regular monitoring of water quality using the Manta probes assured there were no adverse effects to wildlife or vegetation during construction. The project was successfully completed in the proposed time.