Lecture 10-River Dynamics 4 Flashcards
Types of direct measurements
Volumetric gauging
Velocity-area gauging
Dilution gauging
Float method
Indirect measurement via stage discharge relationship
Empirical rating curve
Theoretical rating curve
Velocity area method
Velocity profiles will be different at different depths across the channel. With this method, we break the channel into segments and measure the depth and mean velocity for each segment. Then we can use this info to calculate the discharge. Needs to be efficient but also reduce error. Typically measure width of stream and divide by 20, then take measurements at those segments.
Sontek flow tracker
Set the Level of the instrument based on the water depth to measure mean velocity. Use a timer and the instrument sends out a sound up stream. It then uses the Dopler principle to measure the return of sound to the instrument.
Where is average velocity in the profile?
0.4 x depth of the bed, measured from the bottom (called the 0.6D method)
For more accurate measure average 2 measurements from 0.2 x depth and 0.8 x depth, typically used when flow depth is over 75 cm
Aquadock / Aqua dop?
More permanent, sits above the water and uses Doppler sound down through water to get velocity of the whole profile
Acoustic doper current profiler
On a float, can take across braided river, gets multiple returns to understand the whole profile. Used when its not safe to get in the river, so measures depth and distance across
How to take measurements
Stand downstream, behind the tape and at a slight angle off to the side of the instrument so as not to effect the flow, consider safety, tape out of the water
Site selection considerations
Accessible
Catch “all” flow
No back wash, no obstacles (vegetation, rocks, undercut bank )
Conditions suitable for technique
Section that is easily defined, stable and straight
Flow within limitations of the equipment (some need minimum flow to work)
Safety
Waders and lifejacket
Observers / spotters to help
Weather / sun screen / warm clothes
Aware of surroundings - trip hazards, river base (slippery), falling/hanging vegetation
Current meters (Pygmy and ott)
What we will use in lab
Counts rotations for a length of time
Float method (surface velocity)
Drop a float in and measure the time it takes to travel from A to B
Measurement distance should be > 10 times stream width
It will measure the surface velocity (which is faster than the mean ) a reasonable estimate to get average is 0.85 x surface velocity
