River gauge system improving
A network of rapid deployment gauges is being installed at various points along the Souris and Des Lacs rivers in preparation for the upcoming snowmelt and runoff season. The gauges will provide additional river flow information vital to monitoring and forecasting.
Three of the temporary gauges were installed earlier this week. One at Minot’s Broadway Bridge, a second on the Des Lacs River bridge at Burlington and the third on N.D. Highway 5 west of Mohall, where the roadway crosses the Souris. Two more temporary gauges are scheduled to be deployed next week. They will be placed on the Des Lacs River just below the Des Lacs Refuge, and at the Logan bridge on the Souris.
“It is all part of a rapid deployment network. The State Water Commission purchased the units with a grant from Homeland Security. We install them,” said Brad Sether, United States Geological Survey Water Science Center in Bismarck. “It’ll give forecasters better information.”
River gauges became an issue during the high water days of the flood of 2011. At times gauges were deemed unreliable due to heavy flows or gathering debris. There were also stretches of the river from which no reliable information was available to river forecasters. In 2011 temporary river gauges were installed at the locations listed above other than the one on Highway 5. The Highway 5 gauge helps fill a lengthy void in data from where the Souris enters North Dakota to the next gauge downstream at Baker’s Bridge.
Temporary gauges were removed after the 2011 flood and were not used on the Souris or Des Lacs drainages in 2012.
Although the Broadway Bridge river measurement remains of primary interest to many Minoters, no forecasting agency has maintained a permanent gauge at that structure. Prior to the 2011 flood, when a temporary gauge was installed at Broadway Bridge, river stage readings were obtained by lowering a weighted line from the top of the bridge to the surface of the water below and then calculating the river’s height.
Because there has been no automated river gauge at Broadway Bridge, no accurate chart of flow rate has been devised to coincide with various river stages. The hope for this runoff season is that a series of calibrations of flow, cubic feet per second, will be made manually so that a relationship between river height and cfs can be charted.
“We’ll establish a stage discharge relationship,” explained Sether. “Then a given gauge height will develop a flow measurement, or rating.”
Like the permanent gauges in place at several points along the Souris, the rapid deployment gauges rely on satellite technology to regularly transmit river information to the USGS and National Weather Service.
“The new gauges are only good for stage, not flows,” added Allen Schlag, NWS hydrologist in Bismarck. “Only after we collect flow measurements, such as at Broadway Bridge, can we develop a stage rating table. Flow is very much subject to channel conditions.”
Ice on rivers also creates a reliability problem for automated gauges, often leading to an inflated perception of cfs. A river height chart, with corresponding cfs projections, has been established at most reporting points. However, cfs accuracy is not assured until rivers are free of ice.
“We use the cfs data only when the ice is completely off,” explained Schlag. “The USGS is empirical data. At Broadway Bridge it is more theoretical.”
Although additional river gauges on the Souris and Des Lacs are either in place or about to be installed, the action is considered precautionary and are not an indicator that high flows on either drainage are imminent. The current Probalistic Outlook issued by the NWS shows little chance of the Souris getting out of its banks in the coming weeks with the exception of the immediate Baker’s Bridge area and annually troublesome areas downstream from Minot.
No date has been given by the NWS for an updated runoff forecast for the Souris. When a new outlook is released it will be based on information supplied from a variety of sources, including aerial gamma surveys of snowpack water content throughout the drainage and information relayed from both permanent and temporary gauges.