Acoustic Monitoring of Dredging-Related Suspended-Sediment Plumes

ERDC TN-DOER-E7

April 2000

Acoustic Monitoring of Dredging-

Related Suspended-Sediment Plumes

PURPOSE: This technical note presents the results of acoustic monitoring of suspended-sediment

plumes during dredging operations. Monitoring was conducted in Mobile Bay, Alabama, during

dredging by a hopper dredge and in Boston Harbor, Massachusetts, during dredging by a clamshell

dredge. The results show how acoustic monitoring can be used with other instrumentation to provide

valuable data characterizing the extent and dynamics of suspended-sediment plumes. Data pre-

sented in this technical note provide input for modeling of suspended-sediment plumes, evaluation

of dredging methods, and environmental assessments.

BACKGROUND: All dredging operations create some form of suspended-sediment plume. To

evaluate the environmental effects of the suspended sediment, the location, extent, and physical

properties of the resulting plumes need to be determined. Numerical models of dredging-related

suspended-sediment plumes (e.g., SSFATE, Johnson and Parchure 1999) can be used to predict the

physical properties of the plumes for more locations and for a greater number of environmental

conditions than could be practical using field measurements. However, there is a need to monitor

a number of test cases to improve and validate the models. A critical parameter in any dredging-

related suspended-sediment model is the source term (i.e., the amount of sediment being suspended

by the dredge). Different dredging methods produce different amounts, and create different

temporal and spatial distributions of suspended sediments. Monitoring conducted in Boston Harbor

produced data on the relative amounts of suspension caused by three different types of buckets used

by a clamshell dredge.

Interpretation of biological responses to various degrees of exposure to dredging-related suspended-

sediment plumes requires quantifying physical properties of the plumes and determining the location

of the biological resource relative to the impacted areas. Acoustic methods of making these

determinations are useful because they can make measurements rapidly over a wide area. For this

reason, they can potentially provide data on which to base the determination of cause and effect in

situations where temporal variations are significant, as they are in dredging and dredged-material

disposal operations. One of the intended objectives of the monitoring in Mobile Bay was to test the

efficacy of combining data from an acoustic Doppler current profiler with data from a fishery

hydroacoustics echosounder to examine the interactions between fishes, current flow, and sus-

pended sediment plumes.

INTRODUCTION: The presence of suspended sediment can be determined acoustically by

sending an acoustic signal through the water and measuring the acoustic energy returned. Sediment

particles in suspension in the water column will scatter some of the transmitted acoustic signal,

returning a portion of the scattered signal to the instrument. The acoustic energy returned to the

instrument is referred to as backscatter. The strength of the backscatter is a function of the size of

the sediment particles and the amount of sediment in suspension. Theoretically, it is possible to