TOADS on the Chicago Sanitary and Ship Canal near Lemont, Illinois (USGS streamgage 05536890). Labelled: barge-detection camera, small gage house, upstream notch, staff gage, acoustic velocity meter (AVM), acoustic Doppler velocity meter (ADVM), and temperature and specific-conductance strings. USGS.
The problem
Hydroacoustics have made U.S. Geological Survey streamflow measurements faster and better. But someone qualified still has to be there to make one. So during a flood, flow is measured only now and then, and usually after the peak has passed.
The USGS answer is the Trolley Operated Automatic Discharge System (TOADS), which measures streamflow with nobody on site.
What it does
A TOADS measurement is like a moving-boat measurement, where a boat crosses bank to bank reading water speed at different depths beneath it. Instead of crossing, the instrument moves up and down through the water, profiling speed across the river at several depths.
- Saves time and money.
- Better flow ratings from many targeted measurements across a range of conditions.
- Safer than a boat when the river is dangerous.
- Triggered on demand: by river level, flow or time of day, and repeated at set intervals through floods, droughts or any event of interest.
How it works
The heart of it is an acoustic Doppler velocity meter (ADVM) on a vertical I-beam set into the bank, reaching from above ground to the channel bottom. A small chain-driven cart with an electric motor moves the ADVM up and down. A datalogger in the gage house starts each measurement when set conditions are met.
Figure 2. The canal's cross-section, with TOADS and the part of it the ADVM measures: five depths, each split into ten segments. USGS.
- Measure the water. From its "home" position near the streambed, a sensor reads the water level. The datalogger works out the water's depth and where 30, 40, 50, 60 and 70 percent of it lie — five measuring depths.
- Profile. The ADVM climbs the beam, stopping at each depth to measure water velocity across the channel. Each crossing profile is split into 10 equal segments, and two-dimensional velocity is recorded for each of the 10 segments at each of the 5 depths.
- Compute. Back home, the sensor takes a final water-level reading, and the datalogger calculates flow from the measured velocities, using adapted USGS methods, and stores it.
- Repeat at programmed intervals whenever the chosen conditions occur.
On this canal, two extras protect the data and the gear: a camera shows when barge traffic may have upset a measurement, and a notch in the canal wall keeps barges from damaging the system.
Why it matters
TOADS takes more, and better-timed, measurements for less and with less risk to field crews. It can be installed temporarily or permanently and adapted to sites with different channel shapes. It is one of the new tools the USGS says are improving flood assessment and watershed management.
Sources
Based on Trolley Operated Automatic Discharge System (TOADS)—An Automated System for Horizontal Profiling of Water Velocity and River Discharge Measurements, USGS Fact Sheet 2021–3008, USGS Central Midwest Water Science Center, U.S. Geological Survey; a work of the United States government in the public domain. The figures are taken from the fact sheet's PDF.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
1
0
0
0
Comments
