January 13, 2004
D. M. Fitzgerald, G. A. Zarillo & S. Johnston
images, whereas sand yields weaker images and mud even weaker images. Side scan
surveys are also performed adjacent to coastal structures such as rock jetties to
determine areas of possible undermining and assess their vulnerability to collapse.
These maps provide a clear picture of scour holes and can even image individual
boulders dismantled from the structure. These surveys are most beneficial when
done in conjunction with multibeam surveys.
One of the clearest means of identifying pathways of sediment movement through
inlets and determining net bedload sediment transport directions in tidal inlet chan-
nels is by monitoring large bedforms and mapping their orientation. Side scan sonar
mosaics can provide a detailed bedform map of the channel bottom and can be used
as a first order assessment of sand transport calculations based on hydraulic data.
An indication of.the usefulness of side scan technology is shown at the jettied en-
trance of Saco River Inlet (Fig. 9). The two images in Fig. 9 were taken in the same
area of the channel and demonstrate how the channel bottom responds to changing
flow conditions. Similar maps have been used at other inlet to monitor the migration
of large sandwaves into navigation channel (Fenster and FitzGerald, 1996).
2.7. Acoustic doppler current profiler technology
2.7.1. General use
The Acoustic Doppler Current Profiler sensors use the Doppler effect to measure
current speed. In this case the Doppler shift is a change of frequency of a sound
wave when reflected from an object moving with a relative speed to the sound
source. ADCPs use the Doppler effect by transmitting sound at a fixed frequency
and measuring the backscatter returned from small particles present in the water
(small particles of sediment and organic matter). It is assumed that these particles
move with the same horizontal velocity as the water. The modern acoustic Doppler
current profilers are profiling current meters that can measure current speed and
direction in several spatial bins though the water column or across a channel. Its
predecessor was the Doppler speed log that measured the speed of ships through
the water or over the sea bottom at shallow waters. A redesign of the speed log to
measure more accurate velocity over a depth profile resulted in the first VM-ADCP
in the 1970s. ADCP technology advanced in the 1980s with the development of
different ADCP models (self-contained, vessel mounted, and direct reading). Doppler
signal processing has evolved over the years. Relatively simple processing algorithms
with phase locked loops have been used in speed logs.
In the first generation of ADCPs a narrow-bandwidth, single-pulse, autocorre-
lation method was used to extract velocity information. Broadband ADCP tech-
nology was developed in the early 1990s. Narrowband ADPCs measure frequency
shifts of single sound pulses. Broadband ADCPs measure phase shift of multiple
echoes over a broad range of frequencies. Each broadband sound pulse contains
shorter coded pulses. The phase ambiguity is resolved by autocorrelation methods.