ADCP Discharge Measurement Procedure

Or: How to get the best data every time

The Steps

1. Run the wizard – use best estimates of river depth and speed

2. Stationary measurement – ALWAYS

3. Find and mark the edge starting locations

4. Move slowly and smoothly

5. Watch the data

6. Make at least four discharge measurements that match within 5%

The Stationary Measurement

• Always the first measurement

• Make at deepest, fastest part of river

• Measure for at least 5 minutes, 10 minutes is better

• Hold position as well as possible

WHY??

• Allows ADCP temperature sensor to cool/warm to temperature of the water

• Gets data to determine correct velocity profile for extrapolation

• Obtains data to look for bottom track bias – sometimes called ‘moving bed’

• Verifies that all is working properly

Temperature Sensor

• Speed of sound is computed from temperature and salinity

• Doppler velocity is proportional to speed of sound

• To get accurate velocity, I need accurate temperature

Velocity profile

• To get the most accurate discharge, I need accurate extrapolation

• To get accurate extrapolation I need to know the velocity profile

• To know the velocity profile I need good data

• The stationary test gives me good data for the velocity profile

Look at Velocity Profile

• Average stationary file

• Record Calculated depth from Composite tabular table

• Copy Earth Velocity Magnitude and Direction table and paste into spreadsheet

• Do power law fit to data

• Use power law for extrapolated layers

Example

Velocity vs Normalized Elevation

y = 0.6217x0.2749

R2 = 0.9801

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Elevation above bottom

Vel

oci

ty (

m/s

)

Velocity

Pow er (Velocity)

Power law exponent

Bottom Track Bias

• May occur if velocity, shear, or sediment load is high

• ALWAYS appears as movement opposite the direction of the flow

• The only reliable test is the stationary test

• Easily determined from stationary test data

Bottom Track Bias II

• At the end of the stationary test divide the Distance MG by the Time

Navigation (Btm)Boat Speed 0.000 [m/s]Boat Course 228.22 [°]Water Speed 0.491 [m/s]Water Dir. 9.45 [°]Calc. Depth 1.53 [m]Length 23.75 [m]Distance MG 0.27 [m]Course MG 226.17 [°]Time 1218.32 [s]4-Jan-07 14:01:35

Distance MG/Time =0.27m/1218s = 0.00022 m/s

Since this is much less than water speed, no need to worry. If this is More than 1% or water speed then Use DGPS or loop method to correct

Find the Edges

• Where is the edge?

• It is where you can reliably get water velocity data in at least two cells

• Find that spot on both sides and mark the tagline or bridge, drop a marker, or pick a landmark.

• Accurately measure the distance to the edge – don’t guess it, measure it!

Now we can measure the discharge

• Start pinging (F4)• Start recording (F5)• Wait for at least ten ensembles • Accelerate slowly away from the bank• Cross smoothly at a speed that will take at least

three minutes to reach the other side• Slow down smoothly• Don’t overshoot the edge!• Hold for at least ten ensembles – the stop (F5)

Did you get the message?

SLOW

and

SMOOTH

Am I slow enough?

• After two measurements, Hit F12

• Look at the Std/Avg in the table

• If this is more than 0.04, then slow down!

• You should be able to get 0.02 on most rivers!

• If you do not, then you are going too fast or you are not moving smoothly

Example

File Name # Ens. Start Time Total Q File Name # Ens. Start Time Total Q

[m³/s] [m³/s]

Chk13_004001r.000 210 11:11:26 2.351 Check13-167a000r.000 193 11:12:17 2.275

Chk13_004003r.000 203 11:26:12 2.384 Check13-167a001r.000 230 11:19:02 2.288

Chk13_004004r.000 190 11:31:49 2.326 Check13-167a002r.000 180 11:25:30 2.332

Chk13_004005r.000 193 11:47:26 2.381 Check13-167a003r.000 202 11:40:33 2.46

Chk13_004006r.000 169 11:52:31 2.358 Check13-167a004r.000 168 11:46:07 2.493

Average 193 2.36 Average 195 2.37

Std. Dev. 16 0.024 Std. Dev. 24 0.101

Std./| Avg.| 0.08 0.01 Std./| Avg.| 0.12 0.04

He moved Slow and Smooth!

He didn’t!

Why does moving slow and smooth matter?

• Because the bottom track ping and the water velocity ping do not happen at the same time

• If the boat changes speed or direction during the time between the water and bottom pings, then you add noise to the measurement

How can I tell!

• Watch the boat!

• Try keep the boat pointed at the same angle to the flow during the entire crossing except near the edges where the speed and direction of the boat should change very SLOWLY and SMOOTHLY