The Short Answer
A drainage report is generally required when a project changes the amount or rate of stormwater runoff enough to affect the site or downstream systems — most commonly when new impervious area is added, when a project exceeds a jurisdiction's disturbance threshold, or when the reviewing agency requests supporting calculations for a drainage design. The report documents the hydrology and hydraulics that justify the drainage plan, and a licensed civil engineer prepares it.
What Triggers a Drainage Report
Added or Modified Impervious Area
Adding roofs, pavement, or other hard surfaces increases runoff. Once the new or modified impervious area crosses a local threshold, most jurisdictions require a report demonstrating that runoff is managed and does not increase discharge to neighbors or public systems.
Disturbed Area Thresholds
Many agencies tie the requirement to the total area of land disturbed by grading and construction. Exceeding that area triggers stormwater management requirements, including a report and often erosion and sediment control provisions.
Detention or Water Quality Requirements
Where a jurisdiction requires detention (controlling release rate) or water quality treatment, a report is needed to size and justify those facilities. The calculations prove the design meets the applicable standard.
Agency Request
Even when thresholds are not clearly met, a reviewer may request a report to confirm the drainage design is adequate — particularly on sites with known flooding, sensitive receiving waters, or complex conditions.
What a Drainage Report Contains
- A description of existing and proposed site conditions.
- Hydrologic calculations estimating runoff for design storms.
- Hydraulic calculations sizing pipes, channels, and structures.
- Detention or retention sizing and water quality provisions where required.
- A demonstration that post-development runoff meets the applicable standard.
When It Applies
Expect a drainage report on most commercial site development, parking expansions that add pavement, additions that increase impervious area, and redevelopment. Interior-only projects that do not change site surfaces usually do not require one. Because thresholds and standards vary widely between jurisdictions, we verify the local requirement for your specific site early in the process.
Frequently Asked Questions
What is the difference between a drainage plan and a drainage report?
The plan is the drawing that shows the drainage system; the report is the written calculations and narrative that justify it. Many permit submittals require both.
Who prepares the report?
A licensed civil engineer prepares, stamps, and takes responsibility for the drainage report and its calculations.
How do I know if my project needs one?
Send us your project address and scope. We check the local thresholds and tell you whether a report is required and what it must include.