Discharge

In selecting a site for a new stream flow discharge station the following considerations should be addressed in addition to those listed in the previous section for “Water Level” stations.

As previously mentioned the stage-discharge rating is the key tool used in converting field measurements of stage into computed discharge at a specific site. Stage is easily monitored using a variety of sensors, each having its own advantages and disadvantages and applicability under varying field conditions. Discharge is measured periodically over a wide range of flow conditions and the value is plotted against the measured stage at the time of the discharge measurement to construct the stage-discharge rating. The shape and stability of the channel at the gauge location has a direct impact on the shape and stability of the stage-discharge rating. Physical properties of the channel called “controls”, located downstream of the gauging station are what dictate the shape and stability of the rating curve. Common controls at low flows include bedrock or other stable channel bottoms and are called “section controls”. At medium and high flows, the stage-discharge relation is often controlled by the shape of the channel banks or a physical constriction such as a bridge. Typically, no single control is effective for the entire range of flow conditions but insteadthere exists what is referred to as acompound control. An example would be where a bedrock section control exists at low stages and the channel geometry controls the stage-discharge relation at medium and high stages. A control is stable if the stage-discharge relation it defines does not change with time; otherwise it is referred to as a shifting control.

Controls can be natural or artificial (manmade for flow measurement purposes). Artificial controls may be purposely built flow measurement structures such as weirs or flumes, which have a theoretical stage - discharge relation unique to the structure. Reservoir spillways, control weirs and anicuts frequently come into the ‘artificial’ category, even though they have not been intentionally built for flow measurement purposes, since it is often possible to derive theoretical stage-discharge relations for them. Structures, which have not been constructed for the purpose of flow measurement such as bridges, floodway channels and drifts, are not considered as artificial controls since they normally require full rating development. Stage-discharge gauging stations such as natural controls and non-purpose built structures, which require current meter gauging to define the stage-discharge relations are often referred to as rated sections.

The two most important attributes of a control are stability and sensitivity. If the control is stable the stage-discharge relation will be stable. It is also important that the control is sensitive, i.e. small changes in discharge should correspond to relatively small but easily measureable changes in stage. It is a primary requirement for stage-discharge gauging stations that the rating relation should be as sensitive over as wide a range of flows as possible. In other words, any change in the recorded water level should correspond to a relatively limited (in percentage rather than absolute terms) change in flow.

The site should be located outside the backwater zone of confluences with other rivers, tidal influence, another structures that impede the flow of water. The extent of the backwater reach L is approximately:

where:
L = approximate reach of the backwater effect
h_n = normal or equilibrium depth
S = slope of the river bed

1.1 Questionnaire

• Will data from this site meet the objectives of the monitoring network by providing a representative sample of the basin or river reach of interest? If not, what is the rationale for selecting this site over a more representative site.
• Will turbulence or superelevation of the water surface due to channel hydraulics or alignment have a negative effect on the accuracy of the stage measurement? If yes, look for an alternate site with a straight channel alignment for a distance equal to approximately five channel widths upstream of the monitoring site.
• Is the flow at the site confined to a single channel at all stages with no flow bypassing the gauging station location in a separate channel? If no, consider an alternate site with flows at all stages confined to a single channel.
• Is the channel at the gauging station location subject to significant scour and fill of the bed or does it have large amounts of aquatic growth during warm periods? If yes, consider an alternate site with a more stable channel.
• Are the banks near the proposed site above the maximum high water level so that data loggers and other associated equipment can be secured without risk of flood damage? If no, consider an alternate site.
• Is a stable natural control present in the form of a bedrock outcrop or stable riffle at low flows and a confining channel at medium to high flows? Is a pool is created by the control so that river stage can be effectively monitored at even the lowest flows? If no, consider an alternate site.
• Is the gauge located far enough upstream from the confluence of another river, constriction, or tidal zone to avoid being influenced by backwater? If no, consider an alternate site taking care to avoid the influence of backwater.
• Is there a good cross section for making discharge measurements located near the gauging site? Remember that it is not necessary that low and high flow measurements be made at the same location. However, there should be no gains or losses in flow between low and high flow measurement sections. If no, consider an alternate site with better choices for making discharge measurements.
• Is telemetry of data a requirement for this station? If yes, select the most appropriate type of telemetry based on the factors described in the Hydrological Information Manual.
• Is power available at the site and if not, would it be feasible to power the monitoring instruments and telemetry equipment with a solar panel and battery system?
• Is the site easily accessible by road to facilitate the operation and maintenance of the instruments and housing structure during all flow and weather conditions?
• Is the site is secure to avoid vandalism or theft of instruments and civil structures. If No, consider an alternate site.
• Has land ownership and security been considered? If no, these issues need to be addressed or an alternate site considered.