[Acoustics] Edge estimation errors for Tethered Boat

[Randy Marsden]

Dear Francois,

The edge estimate should be measured as perpendicular distance from the
edge of water to the boat. This is true whether the boat is a tethered
boat or a powered boat. Unless the boat track is perpendicular to the
shore, using the along-track projected distance to the edge of water
will overestimate the edge distance and hence the edge discharge.

Randy Marsden
Water Resources Application Scientist
RD Instruments



Message: 3
Date: Fri, 5 Nov 2004 11:10:46 -0500
From: "Rainville,Fran?ois [CIS-WSB]" 	<Francois.Rainville at ec.gc.ca>
Subject: [Acoustics] Edge estimation errors for Tethered Boat
	deployments
To: <acoustics at simon.er.usgs.gov>
Cc: "Thomson,Chris \[CIS-WSB\]" <Chris.Thomson at ec.gc.ca>
	<644D07D3D59D8F408CD01AC2F833D8C61190A9 at cisxa.cmc.int.ec.gc.ca>
Content-Type: text/plain;	charset="iso-8859-1"

Edge estimation errors have likely been discussed in the past but I
would like feedback for this as it relates to tethered boat deployment
techniques. Also I would love to receive ideas on other methods being
developed by operators to measure their edge distances from bridges.

Background:
As is known, the ADCP software uses verctors to calculate flow within
the measured section while edges are computed using scalar values.
Therefore, whatever the path we take with the ADCP, the measured area of
flow will be corrected for any angle. However, the Q values at the edges
then depend on how the distance to shore is measured.

Problem: 
If I cross a bridge diagonal to a river with a tethered boat, should not
my estimate for the shore distance be perpendicular to the shore?  This
is a concern because of techniques being adopted to estimate edge
distances for tethered boats on bridges. A sighting perpendicular to the
bridge to measure the distance between the last ensemble to the shore is
sometimes used. This provides a cross section length in ways similar to
what was done when Current Meters were used at similar sites. But in
this case, my edge distances are then longer than if measured
perpendicular to shore (like when measuring with ADCP from a boat). I
might thus create a bias due to the deployment method employed. And this
will be particularly important for smaller streams where edges may
represent a greater percentage of the flow.  I am also curious about how
angled current near the shore will end up being factored in properly for
the edge mmt...

*- Can anyone confirm my concerns?
*- Did anyone look at the influence of edges on the total error for
tethered boat deployments?  
*- What are the methods being adopted by other operators to measure edge
distances from bridges?


Thanks

Francois Rainville



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