Report Number: 80
Year: 1997

Sizing of Surface Water Runoff Detention Ponds

Traditionally, stormwater management programs and criteria have focused on quantity issues related to flooding and drainage system design. Traditional designs are based on large rainfall-runoff events such as those having 2-year to 100-year return periods. While these are key criteria for management and control of peak flows, detention basins designed based on these criteria may not provide optimal treatment of the storm runoff. As evidenced by studies performed by numerous public and private organizations, the water quality impacts of stormwater runoff are more a function of the frequent daily rainfall-runoff events rather than the less frequent events that cause peak flooding.

Prior to this study there had been no detailed studies to characterize the variability of the more frequent rainfall events on Guam. Also there was a need to develop some design criteria that could be applied by designers, developers and agency officials in order to reduce the impact of stormwater runoff on the receiving bodies.

The objectives of this study were three-fold as follows.

  1. Characterize the daily rainfall-runoff events with respect to volume, frequency duration and the time between storm events
  2. Evaluate the rainfall-runoff characteristics with respect to capture volume for water quality treatment
  3. Prepare criteria for sizing and design of stormwater quality management facilities

The rainfall characterization studies have provided insight into the characteristics of rainstorms that are likely to produce non-point source pollution in stormwater runoff. The studies made concerning time intervals between storms have added insight into the period of time that pollutant debris will have to collect in urban runoff situations thus contributing to a better understanding of what levels of pollutants to expect in the stormwater runoff.

By far the most significant findings are the development of a series of design curves and equations that can be used in the actual sizing of stormwater detention and treatment facilities. This kind of information was not previously available and will be most valuable to those designing stormwater detention and treatment facilities and to those in Guam's governmental agencies who are regulating non-point pollution. If applied correctly, these design curves could lead to a reduction of non-point runoff to Guam's streams, estuaries and coastal environments.

Leroy F. Heitz
Shahram Khosrowpanah
Jay Nelson