Structural BMPs for Stormwater Treatment Control – Selection and Design Criteria for Urban Drainage System

Stormwater management for reducing pollution has shifted focus on “starting from the source”. The tenets of low impact development (LID), better site design and “smart growth” concepts are conveyed with the goal of producing site plans that reduce impervious areas and runoff volume, also address water quality. The LID based stormwater management program conveys the message that end of pipe structural Best Management Practices (BMPs) are the least desirable method for stormwater pollution controls, though these treatment BMPs play an important role. In the past decades, structure BMPs have proven successful in controlling stormwater runoff pollution. However, their success is offset by a prevalence of inadequate and inconsistent design criteria, which have resulted in selection and installation of units whose performance does not meet expectations and the units ultimately fail to protect receiving water bodies and aquatic environment from stormwater pollution to the degree intended.

This paper proposes evaluation guidelines and the implementation of defined design criteria for flow-through separation treatment processes. Definitive hydrology guidelines using accepted modeling methodologies is a significant factor in determining the appropriate size, potential performance efficiency as well as often overlooked hydraulic analyses of the flow through these treatment systems. Implementation and adherence to a set of definitive sizing guidelines is a relative simple endeavor that could initiated throughout North America to improve water quality. The flow-through units are typically installed as part of the urban drainage system. The hydraulic analysis of the flow-through system is critical to ensure the proper hydraulic function of the entire drainage system. Unfortunately the hydraulic analysis of the flowthrough system, especially the non-public domain units, is inadequately addressed. This paper will present sample hydraulic analyses and proposed hydrology and hydraulic design criteria for the flow-through system in urban drainage design.

A flow-through separation unit removes particulates and associated contaminants from stormwater runoff through physical separation of particulates from the aqueous phase by either gravity, screening and/or filtration. The performance of this type of system is typically evaluated by particulate removal, measured analytical methods such as total suspended solids (TSS) method or suspended sediment concentration (SSC). Evaluation of a manufactured flow-through system has shown that the system performance is a function of particle size, influent concentration, water temperature and flow rate. These important design parameters need to be explicitly addressed in the guidelines to ensure uniform, consistent design methodologies that are independent of specific type of separation unit being considered.