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FOREWORD Experience has shown that most of the more serious flooding situations are "created." Usually, this occurs from the transportation of more storm water to a given area than can be carried away. Years ago, engineers determined that it was economical to construct sewers to transport and dispose of stormwater runoff. While this concept is basically sound and the approach logical, this method of handling storm water runoff is not effective unless the entire watershed adheres to a uniform policy and follows established guidelines, Unfortunately, the only communities that are usually concerned are those in downstream areas where floodiT\g occurs. This lack of concern leads to ever-increasing flooding problems unless well-conceived, cooperative stormwater drainage and flood control programs are undertaken throughout the entire watershed and pursued diligently. Regardless of what decisions are made in a community concerning how to handle stormwater runoff, there results a selection of space for its temporary storage. It may be decided to store it in ponds, on flat roofs, on parking lots - or perhaps no decision will be made, In the latter instance, the stormwater may find a resting place in highway underpasses, yards or within buildings. The concept of temporarily storing excess storm water runoff to prevent local flooding, and then releasing it at a regulated rate to prevent flooding downstream, is an important fundamental principle in stormwater management. It is extremely important in areas of gently-sloping terrain. It is possible to determine the rate at which stormwater can be routed through a watershed without creating problems. Since this safe capacity will normally be far below the anticipated inflow of stormwater from the watershed for the return frequency rainfall chosen for d(ljlign purposes, it is required that the excess be stored. The ideal design begins with storing of excess storm water runoff as close as possible to the ridge lines that divide and subdivide a watershed. In other words, it is best to hold the stormwater as close as possible to the point where it falls as rain for as long as necessary and to take precautions so as not to create more problems than have been solved. This concept of stormwater management is most effective when practiced on the higher, more dominant land to prevent the lower, more servient land from flooding. This report presents the findings of an exhaustive study made of all aspects of on-site detention of excess urban runoff as it relates to reducing local flooding, soil erosion, siltation and pollution. Hopefully, the information presented herein will be useful to all persons who have some responsibility for planning, designing, providing, financing, regulating, operating or maintaining urban drainage systems. A special acknowledgement is made of the fine assistance received from Stifel W. J ens who prepared background reports of several projects involving on-site stormwater detention. These were used in the preparation of this report. A great deal of help in writing the draft of this report was provided by John Reindl who was, at that time, a graduate student in the Environmental Engineering l'rogram at the University of Wisconsin - Milwaukee. Assistance in editing the final draft and in obtaining information from public agencies and practicing engineers was provided by Richard H. Sullivan, Assistant Executive Director for Technical Services, American Public Works Association. Typing and many other tasks were carried out faithfully by Audrey Poertner. The many persons who participated in this study are listed on the preceding pages. Herbert G. Poertner Engineering and Research Consultant v