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USGS Open-File Report 2004-1443, Operation Manual: Time-Series, Storm-Activated Suspended Sediment Sampler Deployed in the Coastal Ocean


Skip past Table of Contents to main text Title Page
Abstract
Introduction
Operational Software Summary
WTS-Tr & WTS-P
Getting Started
WTS-Tr Maintenance/
Field Tests
WTS-P Maintenance/
Field Tests
Conclusions
Bibliography
Tables
Figures
Appendices

Introduction


The instrumentation described in this manual is designed to collect marine suspended sediment "in-situ" during a wide range of oceanic conditions. The unique feature of this instrument package is that it can collect samples during storms when collection by conventional methods from a surface vessel is impossible. Integrated sensors, which monitor either the pressure generated by surface wave height or the turbidity caused by resuspension of bottom sediment, identify storm conditions. The instrument also collects samples during non-storm conditions on the basis of a programmable time schedule.

The goal of this instrument development is to improve our understanding of the fate and transport of sediments and associated contaminants. Both suspended and fine-grained bottom sediments have reactive surfaces that adsorb many contaminants from seawater. Knowledge about the concentration and composition of particulates under different hydrodynamic conditions is a critical component of sediment transport models This instrumentation has application to many coastal areas adjacent to metropolitan areas around the United States and the world where contaminants are of concern. Massachusetts Bay has been the site of initial use because of an existing USGS program to provide basic scientific information to support management decisions related to the $4 billion upgrade to Boston's sewage treatment system. (Bothner and others, 2002; see also http://woodshole.er.usgs.gov/project-pages/bostonharbor/.)

This report summarizes the operating procedures, maintenance, observations, and test results of two generations of suspended sediment samplers that were developed by McLane Laboratories of North Falmouth, Massachusetts, according to USGS specifications. The first version, referred to as WTS-Tr (McLane Research Laboratories model AT-WTS Mark 5-18), is a water transfer system activated on the basis of a transmissometer signal. The second unit, known as WTS-P (model WTS 6-24-47FH), is activated by changes in pressure caused by passing waves. Both sampling devices collect suspended matter by sequentially pumping a known volume of water through individual 47 mm diameter filters positioned along a valved manifold. The schedule of sampling events is selected using internal software to collect samples on a fixed time interval or in response to storm events according to a pre-determined threshold value from an external sensor or a combination of both.

The WTS-Tr receives input from an external transmissometer, which generates a lower output voltage as the turbidity of the water increases. Mounted on a tripod 2 M above the seafloor at 30 M water depth, increasing turbidity is usually a result of storm-generated surface waves that can resuspend bottom sediment. As a storm progresses and the time averaged voltage readings fall below a pre-determined user-defined threshold, an algorithm contained in the WTS software initiates a sampling event representing the onset of the storm. The software continues to monitor changes in turbidity and samples again soon after a storm maximum is identified and again at the end of storm when the voltage is nearly back to pre-storm levels. At the end of the storm sampling sequence, the software will recalculate the scheduled sampling events for the remaining time of deployment unless the schedule is overridden by another storm event and a similar sampling sequence will be initiated.

The WTS-P identifies storm events by using a piezoelectric pressure sensor (transducer) to monitor pressure changes caused by storm generated surface waves. Unlike the transmissometer sensor, the pressure sensor is not affected by biofouling. The software algorithm collects pressure readings over time and calculates the standard deviation of pressure (PSDEV) caused by surface waves generated by storm-induced winds. Surface waves and sediment resuspension are highly correlated (Bothner and others, 2002). The running time-averaged PSDEV trends are the basis for determining the first and subsequent sampling events during a storm cycle. If the averaged PSDEV values exceed a pre-determined threshold value, the software identifies the onset of a storm event and triggers a sampling sequence. The next sample is collected as the averaged PSDEV value begins to decrease immediately after the storm maximum. Three additional samples are collected after the maximum as the storm continues. After the storm has passed, the software will recalculate the remaining scheduled pump events in a manner similar to that of the WTS-Tr unless another storm event initiates an additional sampling sequence.

The two units have different styles of intake ports. The WTS-Tr has a separate port at the tip of each of the 18 individual filter holders. The WTS-P unit has a single entry port located on the manifold that is sequenced to each of the 23 filter holders. The WTS-P unit is designed to flush the entry port and connecting tube with filtered water before and after each sample is collected in order to minimize potential cross contamination between samples. Specifications and additional details for the WTS-P instrument type are available from the McLane Research Laboratories web site http://www.McLaneLabs.com, and McLane Research Laboratories User Manual, 1992.

skip index. Title Page  /  Abstract  /  Introduction  /  Software Summary  /  Getting Started  /  WTS-Tr Maintenance  /   WTS-P Maintenance  /  Conclusions   /  Bibliography   /  Tables   /  Figures   /  Appendices

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