lec 2 Discharge Flashcards
runoff/flow
overall volume of water that exits a drainage basin through either outflow cross section or other means.
Discharge
The volume passing a given cross-section of a river channel within a certain period of time. m3/s.
Why do we want to measure discharge (river runoff)?
Estimating drought-flows
Estimating flood frequency
Computing the load od the sediments.
Mass moved in river formula
conc of suspended sediment /solute X discharge.
Stream gauging
If we know the other components of the catchment water balance, we can estimate discharge.
Velocity Area Method
Q = v - A.
Q = Discharge
A = cross-sectional area of stream
v = av. velocity (use a float or current meter to measure)
Mean velocity is usually measured at…
0.6 of total depth.
Acoustic Doppler Current Profilers pros/cons
More efficient
More accurate
Can measure flow in unsteady conditions
More expensive
Ultrasonic Flow Meter
accuracy compromised by high suspended sediment concentrations or heavy weed growth, impeding signal.
Chemical Tracer Method
tracers must be…
non-polluting
readily dissolvable
detectable against background concentrations
affordable
Mariottes’s bottle
a device that delivers a constant rate of flow from closed bottles or tanks
Manning equation
provides an estimate of the average flow of velocity in an OPEN CHANNEL as a function of the SLOPE OF THE WATER SURFACE in the stream, and the hydraulic radius.
Stage-Discharge Relations
A stage-discharge relation is used to compute discharge in m3/s from stage (gauge height). The relation is developed over time by correlating discharge measurements made over a range of flows with stage.
Staff gauge placement criteria
The bed must be stable
the channel section should be straight
the channel should be free of plants
backwater effects should not occur
flow of the cross section must be smooth.
