Question

Describe how a rating curve is obtained for a gaged station. Worth 10 marks so please answer with explanations/diagrams worthy of 10 points

Answer 1

Describe how a rating curve is obtained for a gaged station. Worth 10 marks so please answer with explanations/diagrams worthy of 10 points.

A rating table or curve is a relationship between stage and discharge at a cross section of a river. In order to convert water height into a volume of water (or “discharge”, usually expressed as cubic feet per second), USGS hydro graphers must establish a relationship between them. This stage-discharge relationship is called a rating curve. It’s developed by making frequent direct discharge measurements at stream gaging stations.

Gaging stations are facilities used by hydrologists to automatically monitor streams, wells, lakes, canals, reservoirs, or other water bodies. Instruments at these stations collect information such as water height, discharge, water chemistry, and water temperature.

The US Geological Survey has gaging stations at thousands of locations across the United States. Each dot on the map represents one USGS stream gaging station in their real-time stream flow data system. Image by USGS.

One diagram of the Gaging station is as shown below:-

These stations collect information about the stream and transmit it to the USGS via a satellite communication system. The data is then processed and delivered to the public via the internet.

Image by USGS.

Measurements from these stations are useful for a wide variety of

• flood prediction,
• water management,
• measure stream flow
• recreation, and navigation purposes.
• USGS realizes the public demand for this information and allows anyone to use it.

There are many different types of stream gages. The pictures below will give you some idea of their variety, expense, size, and technical basis. All of these gages measure the stage of the stream (height of the water above a datum). Stage values can be converted into discharge values if a rating curve has been established for the stream at that location.

Staff Gage

This is a simplest stream gage . A "giant ruler" mounted on a pier, bridge support, post or column. Stage height is read manually.

Staff gage in a stream.

Wire Weight Gage

Permanently mounted on the side of a bridge, a wire weight gage has a weight, a reel of wire, and a manual crank. The weight is lowered until it touches the water. A calibrated spool accurately determines how much wire was required for the weight to reach the water. This number is used to calculate the stage of the stream.

Wire weight gage.

Vertical Pipe Gage

This type of gage is mounted above a pipe that penetrates the bottom of the stream or sediments along the stream's bank. Water flows into the pipe through perforations or through the sediment and fills it to the same level as the water in the stream. Pressure sensors or a float/wire system is used to determine the height of the water. Some of these gages record the height of the water in a memory - which is then downloaded to a laptop during occasional visits by the hydrologist to that place .

Vertical pipe gages.

Gage House

A permanent house that holds stream gaging equipment - typically a gage of some type, a computer, and a satellite uplink. Frequently a stilling well or a vertical pipe is beneath the gage house .

Images on this page are from USGS and NOAA.

Photo of a gage house.                                

Diagram of a gage house.

Obtaining and updating rating tables:

The process to keep rating tables up to date is different from office to office. Most WFOs receive updates to USGS rating tables automatically from the RFC through Hydromet. Such a process ensures that all offices (WFOs and RFCs) are using the same ratings, which is essential to operating the hydrology program. For river locations rated by other agencies (counties, USBR, etc..), or for USGS ratings not updated by the RFC through Hydromet, WFOs must work in coordination with the RFCs to obtain the latest ratings from the concerned agencies, and must ensure everyone has been provided with this latest rating (neighbouring WFOs, backup WFOs, RFCs, etc..).

Rating curves usually have a break point, which is around the stage at which the river spreads out of it's banks, or it could be at a lower stage if the river bed cross section changes dramatically. Above that stage, the river does not rise as fast, given that other conditions remain constant. This is illustrated by a change in slope in the rating curve. On this figure the break point appears to be around 6-7 feet.

POINTS TO BE UNDERSTAND ON THIS TOPIC

• Prediction of stage-discharge relation or a rating curve is of immense importance for reliable planning, design, and management of most of the water resources projects.
• Discharge rating curves may be simple or complex depending upon the river reach and flow regime.
• These relations are typically developed empirically from periodic measurements of the stage and discharge.
• These data are plotted on the graph to define the rating curve for the stream.
• Measurement of discharge by this method involves a two- step procedure.
• Once the stage-discharge relationship is established, the subsequent procedure consists of measuring the stage and obtaining the discharge corresponding to the stage from the stage-discharge relationship.
• The rating curve on a man-made structure is always different with the rating curve made on the natural streams because in the man-made channel, there is always a constant discharge and on the natural streams, the discharge is changing with time.
• For new gauging stations, many discharge measurements are needed to develop the stage discharge relation throughout the entire range of stream flow data.
• The selected cross section of each canal is divided into segments and velocity by current meter and discharge by mid-section method is measured, then the stage discharge data is plotted on graph to obtain a rating curve and made best fit curve
• The laboratory practical is performed on a rectangular weir to obtain a rating curve between stage and discharge, generate best fit curve and calibrate it.
• The relation between stage and discharge can be affected by a number of factors that can change shape and position of the rating curve.

1. The factors that affect the rating curve include

• changes in channel cross section,
• scour and silt,
• growth and decay of aquatic vegetation,
• debris jams (an accumulation of logs and other organic debris which blocks the flow of a stream),
• variable backwater,
• rapidly changing discharge
• , discharge to or from over bank areas,
• including additional parameters,
• such as an estimate of the water surface slope or the rate of change of the water surface at the gauge.

I AM TRYING TO GIVE MY BEST, AND I BELIEVE THAT YOU ARE LEARNER IN THIS TOPIC SO I TRIED TO EXPLAIN ON SIMPLEST WAY ,AS MUCH AS POSSIBLE .THANK YOU.