Drainage Parking Lot

Parking Lot Storm Drain Design

Substantial effort is spent designing a building to meet the needs of the client and effort is also expended making sure an adequate number of parking spaces are provided.  Designing grades for the parking lot and handling the runoff from a rainfall event are often left to less-experienced engineers and architects.  However, providing proper drainage for a parking lot can have a significant impact on the ability of the client to use the building and parking lot during a heavy rainfall event.  This course will provide engineers and architects that do not have extensive training in hydrology and hydraulics with the tools necessary to properly design storm drainage for a parking lot.

Rational Formula

For simple parking lots, the peak runoff can be easily computed using the Rational Formula.  While hydraulic engineers may argue that the Rational Formula is too simplistic to use for large storm drain systems, it is quite appropriate to use for a small parking lot.  The Rational Formula is:



Q = peak runoff rate in cubic feet per second (cfs)

I = rainfall intensity in inches per hour

A = drainage area in acres

A careful unit analysis will indicate that there should actually be a unit conversion factor included, however, the factor is almost one and therefore is ignored.  It is very important to note that the units of rainfall intensity (I) are in inches per hour.  This course will discuss both rainfall amounts (in inches) and rainfall intensity (in inches per hour).  It is important to carefully distinguish between these two variables.

Rainfall amounts will continue to increase as the duration of the storm increases.  Rainfall intensities, however, wil