Environmental Engineering Reference
In-Depth Information
to the velocity-area method and volumetric measurements, tracer-dilution methods
tend to have a larger degree of uncertainty due to lack of complete mixing, loss of
tracer to groundwater, sensor calibration issues, and insufficient time for complete
capture of tracer. Other methods likely have greater degrees of uncertainty.
Errors associated with the stage-discharge rating curve are expected to be
reasonably small for well-maintained and calibrated artificial control structures such
as weirs and flumes. However, rating curves for naturally-controlled gaging stations
can be greatly affected by short- and long-term changes in channel conditions. It is not
uncommon to have root-mean-squared (RMS) errors of rating curves exceeding 20%,
particularly for those locations that are susceptible to changes in density and extent
of vegetation or shifts in boulders or gravel bars. Therefore, as stated earlier, it is
important to visit stations frequently to conduct maintenance, make manual discharge
measurements, detect any shifts in the rating curve, andmake appropriate adjustments.
Errors in the stage measurement (seeWetland stage section) also affect discharge data,
and should be kept at a minimum.
The formation of channel ice in cold regions subject to freezing temperatures can
have a major effect on the stage-discharge relation by causing backwater due to
increased flow resistance. The backwater effect is dependent on the quantity and
nature of the ice, as well as the amount of discharge, which necessitates frequent
discharge measurements, particularly during freeze-up and thaw when the flow is
highly variable. Rantz (
1982
) describes procedures for making discharge measure-
ments in ice-covered streams and adjustments to stage-discharge rating curves
to account for the backwater effect (Rantz
1982
:360-376). If it is not feasible to
maintain ice-free condition or to measure discharge frequently, the record may be
regarded as “seasonal”, with no data available during the ice-covered period.
3.6.8 Summary
Surface-water flow can be a major component of a wetland water budget, and the
uncertainty of surface-flow measurements can dominate the cumulative uncertainty
of a water-budget calculation. Measurement accuracy of 10 % or better can be
achieved at well-maintained gaging stations with appropriate flow control provided
the stage-discharge rating curve is frequently checked and adjusted. However,
stream channels in many wetland settings are ill-defined and conditions can be
highly variable over time. If it is necessary to measure discharge in an undesirable
location, the observer should strive to obtain estimates of measurement error and
uncertainty, which can be reflected in the water-budget calculation.
3.7 Diffuse Overland Flow
During snowmelt or storm events, surface runoff generated within the drainage area
may directly reach the wetland by flowing over the land surface without entering
stream channels. This process is called diffuse overland flow. Since it is practically
