This tutorial demonstrates how to construct a hydrologic model of an urban catchment and use it to compare stormwater runoff under both pre- and post-development conditions. Only catchment runoff will be considered. Models of this type are very common in practice. Many local stormwater ordinances and agencies require that new developments limit peak runoff flows relative to those under pre-development conditions.
The figure below is a contour map of a 29-acre natural catchment area where a new residential development is planned. This undeveloped area is primarily pasture land that has a silt loam soil type.
The following figure shows the proposed development for this site. With the exception of the depressions located in the parkland area, no major changes in topography are expected. This implies that future streets will, in general, follow the natural slope. However, the residential lots will be graded toward the street at a slope of 2% so they can drain easily. The developed site will drain to a stream through a culvert under the street located on the southeast side of the site, which is considered to be the outlet point of the catchment.