Steady flow of data helps authorities manage floods
Devastating floods dominated headlines in 2007 around the world -- from rural China to the rolling meadows of England, rampant rivers wreaked destruction. But just as significant as the scope of the damage was the value of the information that researchers and policymakers were able to collect on the floods. A steady flow of data on water level and velocity helped experts understand the events, and provided a vital foundation for planning for upcoming floods...
• Fast, reliable water level and velocity information proves vital worldwide
By Steve Werblow
Devastating floods dominated headlines in 2007 around the world -- from rural China to the rolling meadows of England, rampant rivers wreaked destruction. But just as significant as the scope of the damage was the value of the information that researchers and policymakers were able to collect on the floods. A steady flow of data on water level and velocity helped experts understand the events, and provided a vital foundation for planning for upcoming floods.
The residents of Kuala Lumpur, Malaysia are no strangers to floods, which rush out of the nearby mountains via the Klang, Ampang, Gombak, Keroh, Bunus and Batu rivers and meet in the concrete metropolis that spreads around their confluences. On June 10, 2007, the city was inundated for hours as the rivers -- which together normally flow at approximately 15 cubic meters per second -- surged to 360 cubic meters per second at their confluence just west of downtown. The Klang River peaked at over a meter above its banks and flooded the city.
It was a perfect illustration of why the Malaysian government was so eager to complete construction of the revolutionary SMART Project, a basin-and-tunnel system designed to capture, store, and move up to 4 million cubic meters of water away from downtown Kuala Lumpur.
Awesome Project Demands Data
What captivates many people about the SMART Project -- which stands for Stormwater Management and Road Tunnel -- is the tunnel itself. Just under 12 meters in diameter and approximately12 km long, the tunnel is designed to serve as a massive underground storage vessel and floodwater pipeline when capture basins at ground level threaten to fill, says Bruce Sproule, International Manager for Greenspan Technology Pty Ltd., the engineering and systems integration firm that designed the project's flood detection and automated management system. In full flood-protection mode, the tunnel stores water, then transports it to the Kerayong River downstream of the city to safely drain it. For the majority of the time, however, a 3-km section of the tunnel offers an alternative motorway for drivers seeking to avoid Kuala Lumpur's traffic-choked downtown. When floods are imminent, the tunnel is evacuated and traffic is re-routed.
Forecasting those conditions and tracking floodwaters makes Greenspan's flood detection system just as awe-inspiring as the tunnel itself. Accurate and timely water level, velocity and flow data are vital to the operation of the tunnel, the protection of the city, and the safety ofthousands of drivers.
Greenspan's Hydrographic Field Technicians Ben Noble, Clem Williams and Faizal Yusoff deployed 22 rain gauges, 50 pressure sensors coupled to gas-bubble systems to gather water level data, and 16 SonTek Argonaut acoustic Doppler current meters to offer real-time water level and velocity data. The Argonauts are positioned at specific heights to be ready for high-flow situations, notes Sproule, the result of exacting research beforehand to predict conditions in the system and channel surveying that includes 80,000-point cross-sections for every measurement site.
Floods in Kuala Lumpur are strongly impacted by tidal action just a few kilometers downstream, where the Klang meets the sea. As river levels rise and the backwater effect gains in importance, the Greenspan model shifts over from the level data from the pressure gauges to the water level and velocity information flowing from the Doppler units. The Argonauts use acoustic beams to measure the actual water level and velocity of real parcels of water, rather than relying on the calculated depth:discharge relationships that pressure sensors are calibrated to assume. As a result, explains Sproule, "it's more accurate information. If you have tidal influence or a backwater effect, you can get hysteresis, and depth:discharge data isn't as accurate."
Sproule says the acoustic Doppler current meters were a natural fit for the project. "We have a lot of experience using SonTek equipment, and it was the easiest and most accurate to incorporate into this project," he explains. "We had an eight-man stormwater monitoring team in Singapore using SonTek equipment for 14 months. We know what it does and doesn't do -- the exact distance the beams will cover, the angles and the diffraction."
To keep the data flowing as floodwaters surge, each Argonaut outputs the data to Greenspan's models and SCADA system. Some stations are connected by Ethernet and report every minute; others, connected by high-speed VHF link, broadcast their data at five-to-10-minute intervals. At the control center, a team led by Greenspan Project Director Mark Wolf and Project Manager Marc Schmidt views the data as it's integrated with rainfall information and run through proprietary discharge and velocity models.
Data from the SMART project are also helping scientists and officials better understand the local river system. For instance, the Greenspan team showed that after a flood event, the Klang stores a surprising amount of water in its groundwater table and releases it over a longer period of time than originally assumed. Information like that will help fine-tune management of the tunnel and the floodwaters it captures.
Moving Bottom Creates Challenges
Just 16 days after Kuala Lumpur was flooded -- and halfway around the world -- the British Midlands experienced some of the worst summer flooding in 150 years. The Derwent River peaked at more than 257 cubic meters per second, well above the previously recorded high of 167 cubic meters per second.
That extraordinary flow was dramatically underestimated using a traditional moving boat measurement method, notes Nick Martin, Services Engineer for SonTek/YSI Hydrodata in Letchworth, Herefordshire, England. In fact, the moving boat method estimated flow at approximately 90 cubic meters per second -- an under-recording of 65 percent. Such a dramatic underestimate could have devastating effects on cities and farms downstream.
The low estimate was caused by the dramatic re-suspension of the riverbed during the flood, explains Martin. The movement of the bed sediments can confuse the readings taken with the moving boat method, significantly lowering the calculated flow. To get a more accurate reading, Martin used a SonTek RiverCat, a catamaran-mounted Doppler profiling system that he deployed from a bridge on a fixed length of rope. Using the company's Stationary Measurement System software, he was able to account for both the highly turbid conditions near the bottom, as well as the movement of the water column. His experiment provided important data to British hydrologists for a once-in-a-lifetime flood event, and demonstrated an accurate method of collecting reliable flow and velocity data over fast-moving bottoms -- or where bottoms are weedy or water is highly turbid.
Big River, Big Challenges
Measuring water level and velocity in a river system is really put to the test in the Lower Mississippi River, the main artery of the U.S. inland shipping network. Though storm surges from Hurricanes Katrina and Rita in 2005 made world news, a flood in the same region in May 2007 garnered relatively little media coverage. However, the 2007 flood illustrated the day-to-day challenges of maintaining the safe flow of shipping on one of the world's busiest waterways.
Towboats on the lower Mississippi commonly push 30 barges at a time, a half-mile-long string of cargo vessels with no brakes. Water level and velocity are matters of life or death on the river, which is one reason that the data collected by the U.S. Geological Survey (USGS) is carefully scrutinized by river traffic managers.
"We collect it for discharge purposes, but [dispatchers] piggyback on it and relay it to the captains so they know how much horsepower they need and how fast they have to go to maintain control," says hydrologist Todd Baumann of the USGS office in Baton Rouge, La. Bauman's team has three Argonaut SLs on the Mississippi/Atchafalaya system near Baton Rouge, and plans to set two more in the Old River and Red River to monitor discharge from those key tributaries.
"Historically, we used electromagnetic point velocimeters at those sites," Baumann says. "When Doppler technology came out, we switched to it because it's such a broad spatial sample. With a point velocity sample, you're just looking at one point. With an acoustic sample, you're looking at a 50, 60, even a 300-foot swath of the river. You get a far better idea of what's actually happening out there. We can actually sample the center of the channel without being in the middle of it. We're sampling areas we couldn't sample before."
Baumann points out that the acoustic Doppler current meters can find the bottom themselves, allowing researchers and traffic dispatchers to see true flow data in widely varying conditions across the river. "So much of the river is dredged, so if you look at the cross section, there's great variability," he notes. "It may be 40 feet deep on one side for the ship channel and 20 feet deep on the other.
"Another huge benefit for us is that 99 percent of the time, the instrument is attached to a bridge structure," Baumann adds. "You're going to get flow interference from the bridge -- you can get a significant increase coming around that pier, especially in a big river. With the acoustic Doppler current meters, we can block out the pier influence."
The other big influence in Baumann's area is tidal action from the Gulf of Mexico downstream. Like Sproule's team in Kuala Lumpur, Baumann can't rely on a water level/discharge method to estimate the flow.
"We can actually measure the Mississippi River flowing upstream," Baumann says. "When you're in the tidal zone, there's no way to accurately gauge discharge without water velocity -- it's essentially meaningless."
Whether it's along the Mississippi, under a medieval bridge in England, or beneath the ultra-modern streets of downtown Kuala Lumpur, SonTek's acoustic Doppler technology has proved itself under the worst that Mother Nature could dish out in 2007 -- and helped people, from towboat captains to civic leaders, prepare for the next season of flooding.
About the Author: Steve Werblow is a freelance writer based in Ashland, Oregon in the United States. He covers agriculture, resource industries and water issues. He can be contacted via e-mail: firstname.lastname@example.org