“Raining on my parade” – Design storm selection and its impact on compliance costs

The selection of an appropriate design storm is one of the most critical decisions that will be made during the evaluation of a collection system, yet its impact is only now being realized and evaluated. This paper will present a case study that will demonstrate the process of selecting a design storm and how this selection can and will impact the cost of regulatory compliance. The selection of a design storm has a major impact on SSO abatement. Of major concern is the development of the rainfall distribution or shape of the design storm and the intensity of the associated rainfall. This is used to determine the level of protection desired and resulting costs. This case study will demonstrate that there are many ways to interpret the meaning of what comprises a particular design storm. Regulators “hands-off” policy in the “approval” of design storms will be discussed and the possible adverse impact. Collection system managers, design consultants and regulators should have a new appreciation of the importance of design storm selection and the significant impact on the cost for achieving wet weather compliance.