Water Level Stations

Water level or “stage” is a relatively easy parameter to measure in the riverine environment and thus is commonly monitored as a means to compute discharge or flow based on the relation between stage and discharge. This relation is commonly known as the “stage-discharge rating”. Therefore, in many circumstances the selection of a stage or water level measurement site will be to a certain extent governed by the suitability of the site for making accurate discharge measurements. However, in some situations, stage is the primary hydrologic parameter of interest such as in determining the storage volume in reservoirs or for flood forecasting.

In either case, whether stage or discharge is the parameter of interest, the stage monitoring station should be located at a site on the river that is characterized by steady, uniform flow with stable physical channel features that result in readily discernible changes in stage associated with changes in discharge. Zones of high turbulence, eddies, and super elevation, such as on the outside of river bends, should be avoided.

The channel should be contained within a single channel with stable banks. The picture below shows a section of breading river, where a water level station should NOT be installed.

The channel should be relatively narrow in relation to the cross-sectional area so that the stage sensor will be submerged over the expected range of stage. Ideally the stage sensor should be deployed within 10-20 meters of the river’s historic high-water mark where the instrument shelter is located to house associated equipment without risk of flooding.

According to the WMO, the general course of the stream is straight for about 10 times the stream width, upstream and downstream from the gauge site. The picture below shows a site where there is a bend after bridge; hence it is NOT a good site for water level monitoring.

Apart from straight section, the approach velocities should be evenly distributed, and cross sections should be avoided where sediment deposition/scour is prevalent.

Establishing and maintaining datum is critical for stage monitoring as it provides the underlying basis for the calibration of the sensors. Thus benchmarks should be located within approximately 100 m of the proposed site to serve as the basis for gauge datum. If no such benchmarks currently exist at the site, plans should be made to establish a new benchmark using survey grade GPS equipment.





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