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Notes on river discharge measurements

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1 Notes on river discharge measurements
USGS permanent gaging stations current meters weirs and flumes methods for small creeks continuous tracer injection

2 USGS permanent stream gaging stations

3 USGS-style stream gaging station, with stilling well
and solar-powered recorder and data transmission

4 Crane and current meter for measuring river discharge from a bridge

5 Example of a stage-discharge rating curve
Note the change in river morphology after flood events, which means the rating curve must be re-established each year NOTE: It takes roughly 10 accurate readings over a range of discharges to create an accurate rating curve

6 For smaller streams, discharge measurements are collected manually, using a current meter and a calibrated wading rod. Many current meters use a mechanical propellor that spins in the water. The versions shown here use an electromagnetic sensor, with no wetted moving parts. Expensive (~ $4000).

7 Parshall flumes These are widely used for small streams or irrigation ditches. One reads the water level at the constriction, and refer to a table to calculate flow. They can be equipped with a stilling well, as shown above, which can harbor a float-type recorder, or some other type of water level sensor

8 Weirs A weir is a low dam or overflow structure built across an open channel. It has a specific size and shape with a unique free-flow, head- discharge relationship. The edge or surface over which the water flows is called the crest. Discharge rates are determined by measuring the vertical distance from the crest to the water surface in the pool upstream from the crest.

9 Methods for very low flows
Pipe and bucket create a dam and use a 2” or 4” PVC pipe to channel flow into a calibrated bucket. Time the flow. Often you can do this at culverts. Garbage bag sometimes there is not enough relief to make a dam… just channel the flow into a garbage bag for x seconds, and then dump the water into a calibrated bucket

10 Continuous tracer injection method
Used by USGS and others for detailed synoptic (downstream to upstream) sampling of a small stream At upper end of study reach, have a large reservoir full of a tracer (such as rhodamine dye, Cl-, or Br-) of known concentration. Inject the tracer at a constant and known rate into the creek for > 24 hours, to saturate the reach of interest with the tracer. Important that tracer concentration at lowest sampling station reaches steady state. Collect your synoptic samples quickly. Measure the concentration of tracer in each sample to determine the discharge at each station.

11 Flow calculation using continuous tracer injection
injection rate (mg tracer/min) Flow (L/min) = tracer concentration in sample (mg/L)

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