Gauging stations are normally sited in river sections characterised by their ability to maintain a reasonably stable relationship between river level and flow. However, this relationship may be disturbed by changes to the hydraulic characteristics of the gauging reach, for example due to changes in the bed profile following a flood or the seasonal impact of aquatic plant growth.
Such circumstances are relatively common and necessitate an ongoing review, and updating, of the rating. For ultrasonic gauging stations, which have found wide application since the lates, a stable relationship between river level and flow is not a necessary requirement. Flows are computed on-site where the times are measured for acoustic pulses to traverse a river section along an oblique path in both directions. The mean river velocity is related to the difference in the two timings and the flow is then assessed using the river's cross-sectional area.
Accurate computed flows can be expected for stable river sections and within a range in stage that permits good estimates of mean channel velocity to be derived from a velocity traverse set at a series of fixed depths. Accuracy can be compromised by high suspended sediment concentrations or heavy weedgrowth which can impede the acoustic signal, or by thermal stratification in the water column — serving to deflect the acoustic beams.
Flow data from electromagnetic gauging stations may also be computed on-site. The technique requires the measurement of the electromotive force emf induced in flowing water as it cuts a vertical magnetic field generated by means of a large coil buried beneath the river bed or constructed above it.
If a big storm hits, the river stage could rise to 15 or 20 feet, sometimes very quickly. This is important because, from past records, we might know that when the stage hits 21 feet, the water will start flowing over its banks and into the basements of houses along the river -- time to tell those people to move the dog's bed upstairs!
With the advent of modern computer and satellite technology, the USGS can monitor the stage of many streams almost instantly. Since some streams, especially those in the normally arid Western U. Recreational users of streams, such as kayakers, also use "real-time" stream-stage data to tell them if certain streams are at the right height for kayaking.
The USGS can now gather data on river stage and even produce graphs showing stage as the rain is falling. Skip to main content. ADCP methods have become more popular in recent decades as technology advances. They work by transmitting sound into water and measuring the return signal to measure velocity and the cross-sectional area of the river. The ADCP device is commonly attached to a small boat which is guided manually or controlled remotely across the river.
Other ADCP devices can be operated to measure the velocity in a vertical subsection, like the current meter. Structures, in the form of notches and weirs, are engineered to measure flows and are ideal on small rivers. The water level height and mathematical formula are used to calculate the flow, based on the design of the weir. At some hydrometric stations, structures such as weirs and channel controls have been constructed to facilitate the measurement of river flows.
These structures stabilise river channel conditions and help to overcome the problems caused by weed growth. Data on water levels and flows for hydrometric stations are on HydroNet.
How we measure water level and flow of rivers Read about how the EPA collects river levels data. River levels We collect information on water level height also referred to as stage from hydrometric stations using a variety of automated recorders. Example of water levels in the River Nore at Hydrometric station Deryduff River flows Although water level height is valuable information on its own, we are also interested in volumetric flows.
0コメント