Cary, NC Environmental Rules: Stormwater Management (2026)
Key Facts
- Standard
- no net increase, 1-yr design storm
- Exemption threshold
- under 10% net increase in peak flow
- Calculation methods
- Rational Method, TR-55, Putnam, USGS
- Applies to
- new development, redevelopment
Summary
New development in the Town of Cary cannot increase peak stormwater runoff leaving the site during a one-year design storm, under the town's Land Development Ordinance.
There shall be no net increase in peak stormwater run-off flow leaving a development from pre-development conditions for the one (1)-year design storm. New developments are required to minimize damage to subject streams caused by storm flows.
(A) Calculation of Peak Flow Acceptable methodologies for computing pre- and post-development conditions for the one (1)-year design storm include:
(1) The Rational Method (used when the drainage area is two hundred
(200) acres or less)
(2) The Peak Discharge Method as described in USDA Technical Release Number 55 (TR-55) (drainage area is two thousand (2,000) acres or less)
(3) The Putnam Method (drainage area is greater than two thousand (2,000) acres)
(4) USGS Regression Equations, where applicable The same method must be used for both the pre- and post-development conditions.
(B) Exceptions to Peak Flow Control Developments with less than ten (10) percent net increase in peak flow of the one (1)-year design storm are not required to control peak flow for the one (1)-year storm from the site. However, if the net increase in peak flow from the new development is greater than ten (10) percent, the entire net increase from pre-development one (1)-year design storm peak flow must be controlled.
Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: 2023 S-87_S-57: Current through local ordinance 2022-Code-05, passed 11-17-2022 and effective 11-17-2022 | LDO: Current through local ordinance 2022-LDO-04, passed 11-17-22 and effective 11-17-22).
Full Breakdown
Cary, North Carolina's Land Development Ordinance bars new development from causing a net increase in peak stormwater runoff leaving the site, measured against pre-development conditions for the one-year, 24-hour design storm. Developers calculate pre- and post-development flow using the Rational Method, TR-55, the Putnam Method, or USGS regression equations depending on drainage area, and projects under a 10% net increase are exempt from further peak-flow control.
Violations & Fines
Development plans that fail to demonstrate no net increase in peak runoff are not approved by the Town; ongoing noncompliance on a built site is enforced as a Land Development Ordinance violation.
Frequently Asked Questions
Does Cary limit how much runoff a new development can send downstream?
Are small Cary projects exempt from peak flow control?
Sources & Official References
Other rules in Cary
Compare Cary to another location·View the North Carolina environmental rules overview
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Stormwater Management in Nearby Cities
How other cities in Wake County handle stormwater management.