Lecture 6 - Streamflow Flashcards
The flow of water in an open channel (or discharge) is defined as the volume of water passing a specified point in a given interval of time
Streamflow
The ————from a particular watershed can be measured at a particular gauging station by measuring the stage at the station and then converting the stage to an associated discharge using a rating curve.
Streamflow
generally measured by observing stage or elevation above a specified datum, in a channel and then relating stage to discharge via a rating curve.
Streamflow
are plots of water level vs. discharge.
Rating curves
The stage of a river is the height of the water surface above an established reference plane.
Water Stage
Requirements for the Watercourse During Measurement:
The watercourse has to be straight for at least 100 m upstream and downstream from the point of measurement for a regular velocity distribution in the water. No backwater effect. Close to the point of measurement, measurements of velocity have to be near as possible so stage-discharge curves can be compiled.
This gage may consist of a vertical scale attached to a bridge pier, piling, wharf, or other structure that extends into the low-water channel of the stream.
Staff Gage.
Lowered structure surface,
to the and readings are taken as a function of time through a storm event.
Wire-Weight Gage.
Uses a small amount of cork inside an enclosed staff gage. The cork floats as the water rises, and some adheres to the stick at the highest level reached by the water.
Crest Gage.
The only direct system measurement, which can measure the water stage continuously and automatically.
Float Gage.
Used mainly as an inside stilling well reference gage for a water level recorder and consist s of a float attached to a counterweight by means of cable or a stainless steel tape.
Float Gage.
Based on the measurement of pressure needed to produce bubbles against water pressure.
Bubble-Type Pneumatic Gage.
Mainly used when the location is not suitable for a float gage.
Bubble-Type Pneumatic Gage.
Most common method of measuring the stage of a river is through the use of a stilling well.
Stage Height
are located on the bank of a stream or on a bridge pier and are topped by a shelter that holds recorders and other instruments associated with the station.
Stilling wells
has a sensor located at either a fixed point above the water surface or a fixed point below water surface.
Ultrasonic Water Level Gage.
Determination of the discharge of a river every time is laborious and time consuming that’s why it is a common practice to establish a relationship between the stage and discharge (rating curve) at a particular station.
Rating Curves
DISCHARGE/STREAMFLOW MEASUREMENT
Velocity-Area Method. Slope-Area Method. Sluice gate and Spillway. Weirs and Flumes. Dilution Gauging. Floats.
Obtain a near instantaneous value of the discharge of a river. The discharge Q of a river being measured by means of measuring velocities (vi), representative for the part Ai of the area (A), in a suitable cross section.
Velocity-Area Method.
is used to the measure velocity in a channel.
Current Meter
Usually used in computing flood discharges. This method is less accurate and application is usually considered as an ad hoc method if other methods are not feasible.
Slope-Area Method.
High-water or flood marks along reach of channel permit determination of the water-surface slope at the time of peak and cross sections of the channel are determined by leveling or sounding.
Slope-Area Method.
The area and hydraulic radius are then calculated and the discharge is computed using the Manning’s formula.
Slope-Area Method.
Usually used in computing discharges at dams.
Sluice gate and Spillway.
It has the same hydraulic properties as the orifice.
Sluice gate and Spillway.
Calibration tests are advisable if accurate measurements are to be determined since its coefficient of discharge varies widely
Sluice gate and Spillway.
Usually used for flow measurements in small streams
Weirs and Flumes.
Such structures have the advantage that they are less sensitive to the downstream conditions like the channel roughness and backwater effect.
Weirs and Flumes.
These devices are rated on the basis of laboratory calibration, although the rating may be checked in place with current meters.
Weirs and Flumes.
Usually used in very small streams in the mountainous areas.
Dilution Gauging.
Rough measurements of discharge can be made by timing the speed of floats.
Floats.
During peak flows, floats are the most appropriate instruments used for measuring discharges since they are cheap and easy to improvise.
Floats.
