The Short Answer
A drainage plan is an engineered document that shows how stormwater is collected, conveyed, and released from a site so that runoff does not damage the property, neighboring properties, or public infrastructure. It maps inlets, pipes, swales, detention or retention facilities, and discharge points, and it demonstrates that the site handles rainfall safely. A licensed civil engineer prepares the drainage plan, which is required for most site development permits.
What a Drainage Plan Includes
Drainage Areas and Flow Paths
The plan divides the site into drainage areas and traces how water moves across each surface toward collection points. It accounts for roofs, pavement, landscaping, and how each contributes runoff. Getting these flow paths right is the core of drainage design.
Collection and Conveyance
Inlets, catch basins, area drains, swales, and storm pipes collect runoff and carry it to where it needs to go. The plan sizes these elements so they can handle design storms without overflowing or backing up onto the site.
Storage and Release
Many jurisdictions require that a developed site not release stormwater faster than it did before development. Detention basins, underground storage, or retention facilities hold water temporarily and release it at a controlled rate. The drainage plan shows these facilities and how they meet local requirements.
Discharge Point
Every drainage system needs a legal, controlled outfall — a public storm system, a channel, or an approved on-site facility. The plan documents where water leaves the site and confirms that the receiving system can accept it.
Why Drainage Design Matters
Poor drainage causes flooding, erosion, pavement failure, foundation problems, and disputes with neighbors. A properly engineered drainage plan protects the investment in the building and site while satisfying the reviewing agency. It also protects downstream properties from receiving more or faster runoff than they can handle.
How It Relates to a Drainage Report
The drainage plan is the graphic document; a drainage report is the supporting calculations and narrative that justify the design. Depending on the jurisdiction and project size, one or both may be required. We prepare the plan and, when required, the accompanying report.
When Do You Need a Drainage Plan?
You typically need a drainage plan when you add or modify impervious surfaces — new buildings, parking lots, additions, or redevelopment — because those changes alter how much and how fast water runs off the site. If your project changes the site's surface or drainage patterns, or if the reviewing agency requires it, a drainage plan is part of the permit set.
Frequently Asked Questions
Is a drainage plan required for small projects?
It depends on how much impervious area is added or altered. Small projects below a jurisdiction's threshold may not require one, but many do. We confirm the requirement for your site.
Who prepares a drainage plan?
A licensed civil engineer prepares and stamps the drainage plan and any supporting report, taking professional responsibility for the design.
Can a drainage plan reduce flooding on my property?
A well-designed system manages runoff to protect the site and neighbors, but it must be based on accurate survey and site data to perform as intended.