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"Sedimeter sensors designed and
built by Lindorm are a fantastic
tool for measuring water column
turbidity and siltation." (Gravity)

Siltation Monitoring BMP

Purpose: To monitor the sedimentological effects of a coastal construction project.

Environmental Issues Addressed: Siltation and light attenuation that negatively impact the ecosystem.

Applications: Any project where sediments are being moved or otherwise physically disturbed near a sensitive ecosystem, or where projects have the potential of changing the sediment dynamics in the nearshore. E.g.: beach restoration, beach re-nourishment, navigation channel dredging, coastal construction projects.

Description

The ecosystem is composed of living organisms, plus the abiotic environment in which they live. Apart from the living organisms, water contains dissolved matter and suspended matter. The suspended matter tends to sink to the bottom and become sediment. Coarse sediment such as sand can be transported along the bottom as bedload, whereas the finer fractions such as mud can be resuspended and transported in suspension by currents. While in suspension they impact the water quality, and attenuate the light. These processes affect dredging spill at borrow sites. Resuspension, as well as bedload transport, can also affect fill material placed on or near a beach, even long after the end of the project. Especially the finer fractions, and the non-clastic material that may be present (known as “loss on ignition”), have a strong potential for causing adverse impacts to offshore communities within a certain reach from the beach. The monitoring should quantify the effects and determine the impact distance.

Sediment monitoring may be designed to detect adverse conditions for the biota already before any serious harm has been done. On-line sediment meters near the work zone can provide real-time feedback about the potential for impact, so that work procedures may be adjusted to minimize negative impacts on the environment. Recording sedimeters in a wider zone around the project may be used to document the sediment dynamics before, during, and after the project. Such data may be used for quantifying the environmental impact of the project, and to discriminate between natural and project-induced effects. Geostatistical methods such as kriging can be used to interpolate between measurement stations, and to demonstrate that the station network has sufficient density to monitor the entire area with the desired confidence level.

Changes in sediment transport patterns caused by engineering projects may also result in channel siltation, beach erosion, and other geomorphological effects that can be monitored using real-time or recording sediment meters. The sediment monitoring plan may be integrated with the monitoring of bathymetrical changes in beach replenishment projects, channel dredging projects, and similar.

A sediment monitoring plan should be a concise document that describes the objective of the monitoring, the parameters to monitor, the type of sensors and their resolution, the area to monitor, the spatial resolution of estimates, the target confidence level of estimates, and the statistical and geostatistics methods to be used. If the system will be used for real-time operational control, the criteria for action should be defined. Also incident light, sediment concentration, currents, and waves may be monitored. The following subjects may be included in a sediment monitoring plan for a beach nourishment project:

Sediment Level - Near-Bed Sediment Concentration - Monitoring Schedule - Analysis, Statistics, Geostatistics - Organization, Responsibilities - Quality Control and Verification - Alert Levels and Contingency Plans - Deliverables, Reports

Literature

Erlingsson, U., 1991. A sensor for measuring erosion and deposition. Journal of Sedimentary Petrology, Vol. 61, Nr. 4, pp. 620–623.
Myers, J.C., 1997. Geostatistical Error Management. Van Nostrand Reinhold, ISBN 0-442-01429-5.
Thomas, S., and Ridd, P.V., 2004. Review of methods to measure short time scale sediment accumulation. Marine Geology, Vol. 207, pp. 95–114.

Disclaimer

This is a draft Best Management Practices document for how to monitor siltation. You may use it at your own discretion for any purpose aimed at facilitating environmental protection.

Further reading: Monitoring Siltation, by Dr. Ulf Erlingsson, provides a draft monitoring program for consultants, and advice on permit text for regulators.

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