The river gage at US59 and the West Fork of the San Jacinto River is being upgraded. A new Acoustic Doppler Velocity Meter (ADVM) should be operational by Monday, May 14, 2018, according to Jeff Lindner of Harris County Flood Control.
Concerns About Previous Gage
Some flood forecasting professionals suspected that the previous meter was not accurately predicting flood height because of several factors at this location. They felt sedimentation, scouring at the base of the bridge, backwater from Lake Houston, and unsteady stream flow all affected the “rating curve.”
How Rating Curves Work
Rating curves show the correlation between the river discharge (flow volume as measured in cubic feet per second) and the river stage (height). Velocity multiplied by the area of the cross section of the river for any given height equals the discharge rate. Such rates are expressed on a mathematical curve that correlates height and discharge.
These curves can change frequently, especially on a river where both sedimentation and scour can result in a change in the amount of flow at a location, as it does at US59.
Need for More Accurate Gage
One meteorologist suspected that – because the rating curve had shifted – we were seeing higher river levels with less water than pre-Harvey. That impacted the forecast accuracy for the Humble gage. A river forecaster noted that in the last two flood events, initial forecasts were “underdone” another indicator of a shifted rating curve.
According to the U.S. Geological Service (USGS), ADVMs are indispensable for backwater-influenced gages. The USGS in recent years has built hundreds of index-velocity gages with an ADVM for the measurement of streamflow. They are especially valuable in reaches where unsteady (varied, nonuniform) streamflow is prevalent that prevents the development of a conventional stage-discharge rating.
ADVMs deliver real-time flow data that would help us better understand what is going on with the rating curve and have more accurate forecasts for the Humble/Kingwood area.
An ADVM measures water velocity by using the Doppler principle applied to sound transmitted under water. Acoustic Doppler systems rely on SONAR, which uses sound waves to determine the distance to targets. They bounce acoustic signals of a known frequency off sediment in the water and measure the shift in frequency when the signals return. By measuring the time between the original pulse and the return signal, forecasters can compute the velocity of flow.
Velocity is crucial in computing the volume of water flowing past a measurement station. River forecasters use the formula:
- Q = quantity (cubic feet/second)
- V = average velocity for the cross section (ft/s)
- A = the area of the cross section of the river.
The faster the velocity for any given cross section, the more water that is flowing past that point.
Lindner cautions that although the new gage will begin collecting data immediately, it will take the river rising and falling several times to get enough data to accurately predict flow rates at different levels.
“A river is always in flux,” says Lindner. “The USGS has already published a new post-Harvey rating which takes into account sedimentation from Harvey near this location.”
Dredging along the West Fork would likely change that rating curve again. “The rating will have frequent changes over the next several months, as the shape of the river evolves both from natural and man-made causes,” said Lindner.
USGS has already sent the new post-Harvey rating for US59 at the West Fork to the West Gulf River Forecast Center to incorporate into their modeling efforts.
For those interested in learning more about gages and flood forecasting, the USGS has an excellent high-level, non-technical intro to measuring stream discharge. Scientists and engineers may be interested in a more technical discussion of the advantage of ADVM’s in developing more reliable real-time discharge estimates.
Posted by Bob Rehak, May 13, 2018
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