YSI SonTek RiverSurveyor, System Manual

Page: 72 / 194

SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
60
4.3. River Condition Considerations
This section lists some river conditions that you may need to consider before conducting a dis-
charge measurement. Specifically, this section describes the following river conditions:
•
Slow and Fast Flows
•
Moving Bed
•
Suspended Sediment
•
Stratified Flows
•
Wind
4.3.1. Slow and Fast Flows
The velocity of the flow must be considered not only for safety reasons, but also for data quality.
If surveying in a fast flowing river, the wake produced by the ADP itself may cause cavitations
(air trapped in front of transducer). In fast flowing rivers, it is also more difficult to maintain a
steady course with the vessel, especially if a small vessel is being used. Fast flowing rivers also
tend to have more debris on the bottom, which may affect bottom tracking.
Slow moving streams, on the other hand, are easily traversed, but must be done in a slow man-
ner. Increasing the
averaging interval under such conditions is sometimes recommended to in-
crease the data quality. Refer to
Section 6
for more details about how to conduct a survey.
4.3.2. Moving Bed
A phenomenon known as moving bed , occurs when a layer of sediment on the riverbed is mov-
ing downstream with the water flow. This layer becomes indistinguishable from the actual river
bottom. This can make it difficult for the ADP’s bottom-track feature to determine the velocity
of the vessel (i.e., the ADP itself) over the ground. Moving bed is described in detail in §
3.3.5
.
4.3.3. Suspended Sediment
The material or sediment that is held in the water column is known as suspended sediment . Dop-
pler systems operate on the principal that there is a sufficient number of particles (or scatterers)
in the water to reflect the acoustic energy transmitted by the ADP back to the system.
If there are very few particles in the water, the signal return (or signal amplitude ) will be very
low (less than 60 counts). This can be seen when there is very little vertical variation in the sig-
nal amplitude data from the ADP to the bottom.
Conversely, if the river being surveyed has a very high suspended sediment concentration, the
acoustic signal may be attenuated enough so that by the time it reaches the bottom, it is not dis-
cernable from the background noise. Under such conditions, the signal amplitude will show a
rapid decay as distance from the transducers increases.
4.3.4. Stratified Flows
Stratified flow occurs when a layer of water with particular characteristics (density, temperature,
current direction) moves over another layer of water with different characteristics.
Bi-directional flow is a specific example of stratified flow when a layer of water moves over the
top of a different layer of water in the opposite direction. This phenomenon is most commonly
observed in estuaries when a layer of fresh water flows out to sea over a layer of salt water that
flows in and out of the estuary with the tide.