Design Manual

A local stormwater design manual is the technical companion to a community’s stormwater management ordinance. It provides designers, engineers, and plan reviewers with the specific, detailed guidance needed to select, size, design, construct, and maintain stormwater treatment practices (STPs) at development sites. While the ordinance establishes the legal authority and requirements for managing stormwater, the manual provides the engineering criteria and procedures to ensure that those requirements are met consistently and effectively across the jurisdiction.

The chapters of a comprehensive design manual follow the logical lifecycle of a development project. The process begins with establishing the required performance targets using unified sizing criteria for water quality, channel protection, and flood control. Designers can then incorporate better site design techniques to earn stormwater credits, which reduce the required treatment volume. The proposed stormwater management plan is then submitted for approval through a defined local review process. Once a plan is approved, detailed construction specifications guide the proper installation of practices. Finally, a robust maintenance program ensures the long-term performance and functionality of the installed stormwater infrastructure.

Sizing Criteria

Sizing criteria establish the minimum required performance for stormwater management at a development site. A unified approach defines five key management volumes and peak flow rates that must be controlled: the Water Quality Volume (WQv), Recharge Volume (Rev), Channel Protection Volume (Cpv), Overbank Flood Protection Peak Flow (Qp), and Extreme Flood Protection Peak Flow (Qf). These criteria work together to protect groundwater resources, prevent stream channel erosion, and mitigate downstream flooding. Learn more about the methodology and application of these standards in the Sizing Criteria chapter.

Stormwater Credits

A stormwater crediting system provides an incentive for designers to use a range of non-structural and site-level design techniques that reduce the generation of stormwater runoff. By incorporating practices such as conservation of natural areas, rooftop disconnection, or use of permeable pavers, a designer can earn credits that reduce the total water quality or channel protection volume that must be treated by structural STPs. The Stormwater Credits chapter details eligible practices and the methodology for calculating volume reductions.

Local Review Process

An effective and predictable review process is essential for the successful implementation of a stormwater management program. This process outlines the specific requirements for plan submission, the checklists used by reviewers to ensure compliance with the ordinance and manual, and the procedures for inspections and final approval. A well-defined process provides clarity for both designers and regulators. The Local Review Process chapter provides a framework for these administrative procedures.

Construction Specifications

The long-term performance of any stormwater practice depends on its proper installation. Construction specifications provide detailed, enforceable standards for materials, workmanship, and construction sequencing. These specifications help prevent common failures such as soil compaction in bioretention areas, improper installation of underdrains, or damage to infiltration surfaces during construction. The Construction Specifications chapter provides model language for a wide range of stormwater practices.

Maintenance

Stormwater practices are infrastructure assets that require routine and long-term maintenance to function as designed. An effective maintenance plan includes schedules for inspection, routine tasks like sediment and debris removal, and protocols for non-routine repairs. This requires clear legal and financial frameworks, such as dedicated maintenance easements to guarantee access and the use of performance bonds to ensure accountability. The Maintenance chapter provides guidance on creating enforceable and effective maintenance programs.

The design manual provides the technical “how-to” for implementing a stormwater program. It is directly supported by the legal framework established in a model stormwater management ordinance. To assist with the calculations required by the sizing criteria, engineers can use a variety of design calculators. For guidance on choosing the most appropriate practice for a given site, consult the BMP selector tool.