Question

1) Describe how you can use measurements of stream height (stage) to estimate discharge and what pieces you need to do so. (I’m looking for you to describe both the curve and one method for making discharge measurements)

2) If a flood occurs and erodes the channel where we are measuring stage, can we still use the same relationship between stage and discharge? Why or why not?

Answer 1

Measurements of Stream Height (Stage) To Estimate Discharge

The discharge of a stream is the product of its velocity (V - length of travel per unit of time such as feet/second) times depth of the water (D - unit of length) times width (W of the water - units of length). (Make sure all all three lengths are expressed in the same unit.)

Discharge = V x D x W

If length is measured in feet and time in seconds,

Discharge has units of feet3/sec or cubic feet per second (cfs).

A stable stream cross section is divided into a number (8-10) of smaller units of equal width and uniform bed conditions. Velocity is measured at 60% (from the surface) of the depth of each unit and then multiplied by the unit area to give the unit discharge.

e.g. If the depth of the stream is 50 cm, then the 5 dm mark on the sliding rod should be opposite the 0 cm mark on the wading rod. The flow meter will now be at 20 cm on the wading rod (4/10 of the distance from the stream bed to the water surface).

Step 1: Selecting the Channel Location

The area to be monitored should be a stable stream channel or at least not significant alteration. The portion of the stream to monitor should not be a braided section of the stream or an meander area. If possible, the section to be monitored should have flow that is parallel to the stream channel orientation and not within a pool area or other area altered by structures that may create backwater areas or reverse the flow of the water. The site should be accessible   

Step 2. Developing a Cross-Section of the Site and Establishing a Reference or Staff Gauge

These methods utilize a velocity-area approach to measuring stream flow (or volume of water passing a set point in a given period of time). That is, to determine flow, the monitors combine information about stream area with information about the velocity of the water at their site.

If possible, the section of the stream used for this measurement should be relatively stable, i.e., not actively downcutting or meandering. In addition, it would be advisable to select a stream where the flow is nearly parallel to the stream channel and not immediately after or before a meander or rapid. When developing the cross-section, it may be necessary to remove or relocate some of the stream channel bottom material to create a more uniform bottom. It is also advisable to extend the cross-section to a point that is above the flood level for the stream.

Step 3. Measuring Interval and Depth

Monitors measure the area of the stream at their site by measuring the width across the stream and the depth at several locations across the measured width. For a small stream, the width interval between measurements is typically 6 inches. For larger streams, intervals of 1 - foot can be used.

Stream discharge can be measured using

(1) volumetric gauging, (2) float gauging, (3) current metering, (4) dilution gauging (constant injection or gulp methods), (5) structural methods, and (6) slope-area methods. The choice of method depends on the characteristics of the stream and on the application.

1. Current metering:

In this method, the stream channel cross section is divided into numerous vertical subsections. In each subsection, the area is obtained by measuring the width and depth of the subsection, and the water velocity is determined using a current meter. The discharge in each subsection is computed by multiplying the subsection area by the measured velocity. The total discharge is then computed by summing the discharge of each subsection.

With current metering the rotation of a current meter's impeller gives the local water velocity following application of a calibration equation (called a rating equation). To cope with the vertical distribution of velocity, measurements should be made at different depths (D) in the water profile. If only two depths are used for measurement, then an average of 0.2D and 0.8D gives a good representation of the profile velocity, or 0.6D if only one depth is used (Hewlett, 1982 ). The transverse (i.e., 'across-river') distribution of velocity can be characterised by first dividing the channel cross-section into a number of 'segments'. The edge of these segments are called 'verticals' and these are the locations at which the measurements to calculated the profile-average velocity should be made .The 'Mean-Section Method' or 'Mid-Section Method' can then be used to calculate the discharge for each segment

2. If a flood occurs and erodes the channel where we are measuring stage, can we still use the same relationship between stage and discharge? Why or why not?

No, we cannot use same measurement It will affect stream-water velocity are the size of sediments on the stream bed — because large particles tend to slow the flow more than small ones — and the discharge, or volume of water passing a point in a unit of time (e.g., m3/second). During a flood, the water level always rises, so there is more cross-sectional area for the water to flow in; however, as long as a river remains confined to its channel, the velocity of the water flow also increases.

the nature of sediment transportation in a stream. Large particles rest on the bottom — bedload — and may only be moved during rapid flows under flood conditions. They can be moved by saltation (bouncing) and by traction (being pushed along by the force of the flow).

Smaller particles may rest on the bottom some of the time, where they can be moved by saltation and traction, but they can also be held in suspension in the flowing water, especially at higher velocities. As you know from intuition and from experience, streams that flow fast tend to be turbulent (flow paths are chaotic and the water surface appears rough) and the wate.

may be muddy, while those that flow more slowly tend to have laminar flow (straight-line flow and a smooth water surface) and clear water. Turbulent flow is more effective than laminar flow at keeping sediments in suspension.

The faster the water is flowing, the larger the particles that can be kept in suspension and transported within the flowing water. the relationship between grain size and the likelihood of a grain being eroded, transported, or deposited is not as simple as one might imagine