B.1.3 Selecting a Site for the Gauging Station

B.1.3.1 Pre-Reconnaissance Work

To select an appropriate site for establishing a gauging station, the objectives and the purpose of the station must first be clearly identified (Section B.1.1). Also, to some degree, the site is determined by the type of station.

The watercourse and basin should be studied in some detail prior to making any field trips. Obtain available maps-topographical, geological, and water licence-and recent air photos. If possible, talk to people who are familiar with the watercourse and region, e.g. residents, water users, loggers, First Nations, other technical specialists, etc. Ask about the characteristics of the watercourse, including flood history, and find out if any activities are under way in the area that might affect the watercourse, e.g. logging, bridge construction, reservoirs, diversions.

Ideally, sites for a gauging station should be uniform, and have the following characteristics:

• Straight, aligned banks.
• Good current meter measuring sites, e.g. single channel, no undercut banks, minimal obstructions, no turbulence, no slow-moving pools (deadwater), no eddies.
• Reasonable means of access.
• No tributaries between gauge and metering sites.
• No swamps downstream or in vicinity of gauge.

Study the maps and air photos. Look for the characteristics of a suitable site, realizing that no site will be perfect. Select two or three potential locations and prioritize them.

Prepare a plan to reconnoitre the site identified as having the most potential, but be prepared to move on to other potential sites if the first one does not work out.

Before setting out on a field trip, be sure to check if written or verbal permission is needed from the land owner or manager to access the property.

B.1.3.2 Field Reconnaissance and Site Selections

To complete the selection of a site for the gauging station, take a field trip to the potential site. The field trip is an opportunity to make a detailed evaluation of the site in terms of the objectives and the characteristics listed above (Section B.1.2.2).

Reconnaissance should include careful observation of the following:

• Low Flow Conditions.

Look for a stable well-defined low water control. A raised culvert invert at a road crossing can be an ideal site for a gauging station, particularly in areas with flat gradients.

If a stable control is not available, consider the feasibility of building an artificial low water control. Investigate the options, and gather and record preliminary survey data.

If a site with a movable streambed must be accepted, i.e. one with a mobile granular channel, it is best to locate the gauge in as uniform a reach as possible, avoiding any channel obstruction that may intensify scour and fill.

Where the watercourse emerges from an area of steeper gradients onto an alluvial fan, the reconnaissance must include streamflow measurements to determine where the seepage of water into the alluvium becomes significant. The station should be located upstream of the area of water seepage if the maximum yield of the watercourse is required.

• High Flow Conditions.

To determine the magnitude of maximum discharge, look for evidence of past flooding, e.g. trash lines. Consider how easy it would be to access the gauge at all stages, and if it will be possible to position the recorder stilling well so as to avoid damage from high water levels or velocities and floating debris.

• Flow Measurement Conditions.

Ensure that the site has a suitable location for measuring discharge, either with a current meter or by other means. Although the station will be designed to accommodate discharge measurement, it is usually best to make observations that will cover all the options for obtaining a stage discharge record. Make enough observations, including making photographs, to prepare reasonable alternative conceptual designs and cost estimates.

Ideally, the metering section should have the following characteristics:

1. Fairly uniform depth and velocity with parallel flow lines across the section.
2. One or more locations at which the full range of flows can be measured with available equipment. For flows that can not be waded, it is more economical to use an existing bridge or culvert for high flow measurements. In the absence of a suitable structure, a site for a metering bridge or other alternative method should be available. See section B.2, Constructing a Gauging Station and Figure B-8, Trout Creek metering bridge.
3. Reasonable proximity to the gauge. This is particularly important in manual gauge installations where rapid changes in stage may occur during the course of a measurement and several gauge readings would be required.
4. No inflows or outflows between the gauging point and the measurement section. If this is unavoidable, an auxiliary station will be required to gauge these flows.
5. No aquatic growth or vegetation. Both the low flow and high flow measurement sections should be clear of aquatic growth. The portions of the bank subject to inundation during high flows should be cleared of any vegetation that could affect the measurement.
6. Backwater effect. Where the site is affected by a variable backwater, care must be exercised to remain above the instrument's threshold velocity.

Record the results of the reconnaissance in a report, with supporting survey data. The report will be a key reference in making decisions regarding the design, costing, construction, and operation of the gauging station.

Figure B-8. Trout Creek metering bridge.