TRCA Real Time Gauging v.2

Flood Warning and Forecasting Program Watershed Response Project - Project Sheet

TRCA's Flood Forecasting and Warning Program (NexFlood)

Flood Forecasting and Warning

TRCA as responsibility for flood forecasting and warning within the geographic boundaries of the TRCA jurisdiction. In the past, flood forecasting and warning was based on the application of simple relationships between rainfall and the amount of runoff from the land surface. This approach, while appropriate for its time, made use of the tools available and was thus limited to coarse, uniform predictions of flood risk. Since that time, there have been improvements in weather forecasting and data available, rapid development in GIS systems and use in integrating GIS and watershed modeling, and advances in computing power that now make rapid assessment of watershed response and flood risk due to rainfall possible.


Development of a flood forecasting and warning system that is able to operate in a continuous manner to provide rapid assessment of flood potential would require functionality to acquire rainfall data, simulate the response of the land surface to the rainfall and estimate the amount of surface runoff generated, convey the surface runoff within the network of drainage channels, streams and rives, and determine the extent of flooding as a result of conveyance of surface runoff that exceeds the capacity of the drainage network. The NexFlood system will provide continuous estimation of watershed response to actual and forecasted precipitation, with mean elapsed time between successive estimates being 10 to 15 minutes. Partitioning of functionality in the system is reflected in the various components (phases) of the NexFlood system.

Radar Acquisition and Forecasting

Hydrologic Modeling

The response of the land surface to incident rainfall is simulated using hydrologic models. This model simulates many of the physical processes occurring in watersheds, such as evaporation, infiltration, and overland flow. The watershed models will provide estimates of the amount of rainfall that is converted to surface runoff that then finds its way into our streams and rivers.

Hydraulic Modeling

The transport of surface runoff due to excess rainfall on the watersheds is simulated using flow routing that is capable of handling a variety of flow situations.

Flood Limits

Once flood depths have been computed, the NexFlood will access customized GIS interfaces and routines to determine the limits to which floodwaters will spread across the landscape, using as its basis a digital representation of the terrain elevations within the TRCA watersheds. This will be done dynamically as the flood depths and velocities continue to change in response to changes in incident precipitation and forecasted precipitation,

Flood Vulnerable Areas and Structures

Structures at risk of inundation by floodwaters are automatically identified by the NexFlood system, using a database of flood vulnerable areas and roads prepared by TRCA. A list of affect structures can also be generated for further use in emergency operations.

Benefits of NexFlood

The NexFlood system will enable Flood Warning Staff to have continually updated information on the potential for flooding, and the location and magnitude of potential impacts on homes, roads and other structures that may be affected by floodwaters. This information would enable TRCA to provide added value in flood warnings and messages to our municipal partners, and could be used during emergency operations to better allocate available resources.

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