Forecasting Floods

March 12, 2008

About the author: Kris Cauwenberghs is vice director of operational water management for the Flemish Environmental Organization. Cauwenberghs can be reached by e-mail at [email protected].

The Flemish environmental agency Vlaamse Millieumaatschappij (VMM) has launched a flood forecasting website, based on Wallingford Software’s FloodWorks software, which allows the public to access detailed information on current and forecast flood levels and rainfall.

The website links to FloodWorks models, which constantly receive new telemetry data from observations of precipitation, stages and flows and the results of weather forecasting for up to the next two days. Maps, graphics, tables and text, including an interpretation by hydrologists, will help system users assess a particular area’s flood risk in real time.

The central dispatching unit in Brussels.

Building a Program

This is the second forecasting system that VMM has developed. Five years ago, it undertook a major modeling exercise for the Demer river basin. AMINAL, the organization’s name at the time, commissioned a flood forecasting system designed to give operational flood forecasters the information to provide fast and accurate flood warnings. This covered the key flooding areas of the 2,275-sq-km basin, including the towns of Aarschot, Zichem and Diest in Flanders, Belgium.

VMM had large flood control reservoirs ahead of the cities and wanted more information—data that could explain what would happen when these were full. As a result, the group created two complex hydraulic models of the 300 km of waterways, which included real-time data from telemetry (i.e., the position of gates along the waterways).

The current program is designed to make all its information available to the public on the VMM website. Flanders has 11 basins, and VMM acknowledges that it will take a great deal of funding and time to cover the entire area. The organization anticipates seven years for completion.

Because it will take so long, VMM started with a simplified forecasting system in 2006, which was just the hydrology and not hydraulic models but still complex. Staff input radar rainfall data from three Belgian radar systems and calibrated it in real time with the rain gauge data.

The website now has publicly accessible models for the OBM-Dender and OBM-Centrale regions (hydrological forecasts for the Flanders region and hydraulic forecasting for the Dender basin). Clicking on an area brings up topographic maps with fully hydraulic data so that areas at risk of flooding can be identified.

Interface of radar interpretation software.

Extending Coverage

The modeling is extremely complex. For the Dender basin alone, there are seven hydraulic models of waterways, which include not only cross-sections of spill units but also floodplain sections and gate information. The seven models contain 13,000 nodes, including spills, breaches, pumps, weirs, storage areas and flood compartments.

The new models have been running on FloodWorks 6.0, still on a clustered server since June 2007. These also generate hydrological forecasts rather than flood maps, forecasting the zones and times in which regional and local flooding may occur in Flanders. The predictions are based on passing thresholds for rainfall, water levels and unit discharge levels in liters per second per hectare. There are alarm modes both for basins and subcatchments and real-time updating from telemetry at numerous critical points along the area’s river courses.

The long-term plan is to extend the amount of detail in the Dender basin to every basin and river where flooding is a potential hazard. To date, 10% of the region’s waterways are online.

About the Author

Kris Cauwenberghs