Runoff Methods Flashcards
What is volumetric gauging? What type of situation is it best for?
You fill a bucket with a known volume and time how long it took, through unit conversion get m^3/s.
Works for low discharge and in small streams
What is the velocity area method?
You measure the stream velocity and multiply that by the pipe surface area to get m^3/s.
What are the two major challenges with the velocity area method?
Channel geometry changes over time and space making cross sectional area measurements hard
Flow velocities can vary significantly (inside of stream faster than outside of stream due to friction)
How do you solve the challenges posed by the velocity area method?
Use transects, get measurements of depth and velocity at discrete points, find area by multiplying depth and width, multiply that area with the velocity to get the discharge at each point and add up to get total discharge of river
What are three considerations about depth we must follow?
Velocity measurement should be at 0.6 times depth for streams <0.75m deep.
For deeper streams, take average of 0.2 times depth and 0.8 times depth.
For streams with complicated bottom topography, more samples are necessary.
as velocity changes with depth
What are the four drawbacks of the velocity area method?
Doesn’t work well in complex channels
Difficult/dangerous in large rivers
Difficult/dangerous during floods
Time consuming
What is the basic instrument used in the velocity area method?
You have stick with measurements and propeller at the bottom, propeller measures velocity and stick measures depth, set up transect and do it along the transect
What is the ADCP? How does it work?
Is an instrument that measures flow velocity, is the acoustic doppler current profile, works by sending high frequency sound pulses up through the water column… Those sound pulses reflect off of particles moving in the water, and their speed and direction changes the frequency of the returned soundwave (due to the Doppler effect). Length of time it takes to get the returning frequency gives you depth, so it can tell you velocity, direction, and depth.
What is dilution gauging?
You dump a tracer at some point upstream time how long it takes to get donwstream and the concentration of it downstream and use C1V1=C2V2 to get the volume of water over time.
What are the ways to do the dilution gauging method?
Slug injections and constant rate injections (which you don’t need time for)
What are some considerations of how to do dilution guaging?
Need to pick an appropriate distance downstream, if too far tracer might be too dilute if too close tracer might not mix
What are the 5 tracer considerations you must have?
Readily soluble
Zero or low concentration in natural stream
Inert (no “sinks” or reactions)
Easy to detect at low concentrations
Harmless
What is a stage measurement?
You drill a hole beside a river and water fills the hole to the water level.
How can you combine stage measurements with other measurements of discharge to create a rating curve?
Step 1. You put a stage in
Step 2. Send someone to measure stage (water level) and measure the discharge, plot against eachother and both together will create a rating curve
What is a rating curve?
an empirical model that connects stage to discharge
