WMS:Travel Times from Map Data

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The Time Computation coverage can be used to create arcs representing flow path segments when computing time of concentration or lag time for a basin or reach. Within a basin the time of concentration or lag time is usually determined by combining the time of travel across one or more flow path segments. Travel time equations are generally functions of the length and slope of the flow path segment as well as surface roughness (i.e. Manning’s roughness coefficient) and channel shape and roughness. Since length and slope (providing there is a background TIN or DEM) are easily determined from arcs, the time computation coverage provides a simple and powerful method for computing basin time of concentration and/or lag time. Travel times between consecutive outlet points may also be computed using the same tools.

If having developed a watershed model from a TIN or DEM, it is possible to have WMS automatically create flow path arcs from selected points using the Node→Flow Arcs and Stream→Flow Arcs commands.

The process of computing a time of concentration involves two primary steps.

1. Compute travel times for individual arc segments.

2. Combine the travel times of all arcs within a basin to compute the time of concentration for the basin, or combine arcs between outlet points to compute the travel time along a stream reach.

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