The closed underground drainage system of a street is generally
designed to a storm frequency based on rainfall intensity and flow
concentration. The design varies based on peak runoff conditions and
infrastructure type being built, but ultimately the goal is for the total right
of way to be designed to handle the 100 year storm. The design of the street
should have the edge of the right of way below the first floor elevation of the
buildings along the street. The sidewalk and tree lawn should have a grade that
runs towards the street. The curb allows the flow off the tree lawn and into
the gutters which deliver the water to the closed underground system. The street pavement is
also designed to move the water to the gutter and to the closed underground system. If
the storm intensity and concentration exceeds the closed underground system, the right
of way, the area from property line to property line will hold up to sixteen
inches of water at the streets low point.
The design generally has an overland swale in the low area
that will deliver water exceeding the right of way capacity to an open drainage
system. The open systems in the City are generally detention ponds, ditches,
the creek or lake.