Waco's 2025 Stormwater Criteria: What Developers Need to Know

If you are developing land in Waco or its extraterritorial jurisdiction, there is a document you need to know about: the City of Waco's new Stormwater Design Criteria, adopted January 22, 2025. This updated criteria replaces the old Storm Drainage Design Manual — a document that dates back to 1959. The new criteria represents the most comprehensive overhaul of Waco's stormwater engineering requirements in over 60 years.

At Merritt Engineering Services, we work on land development, subdivision planning, drainage studies, and municipal infrastructure projects throughout McLennan County and Central Texas every day. In this post, I want to break down the key changes, explain what those changes mean for developers and property owners, and help you understand what to expect when you bring a project to us.

A Tale of Two Documents

Before diving into specifics, it's worth appreciating just how much the world has changed since 1959. The old manual was focused primarily on sizing storm sewers, gutter flow, culverts, and drainage channels using the Rational Method and basic hand calculations.

The 2025 Stormwater Design Criteria is a fundamentally different document — a comprehensive 120+ page policy and engineering framework that addresses not just how to size a pipe, but how development should interact with the entire watershed, including water quality, floodplain management, low-impact development, detention requirements, site grading standards, and erosion control.

Key takeaway for developers: The 2025 criteria is not just an updated pipe-sizing manual. It's a comprehensive development policy with significant implications for how you plan, design, and permit a project in Waco.

Design Storm Frequencies: A Major Shift

One of the most significant technical changes involves design storm requirements. Under the old 1959 manual, storm sewers were typically designed for 2 to 5-year storm events, and culverts or channels were sized for 5 to 25-year events.

The 2025 criteria upgrades this across the board. The Secondary Drainage System — curbs, gutters, inlets, and storm drains — must now be designed for the 25-year storm. Beyond that, every development must identify and design a 100-Year Conveyance Flowpath: a defined route through which the 100-year storm can travel safely within dedicated easements or rights-of-way.

In practical terms, storm drain systems in Waco must be meaningfully larger than the old manual required, and every development must demonstrate a safe overflow path for major storm events — a concept the 1959 manual simply didn't contemplate.

The No Adverse Impact Requirement

Perhaps the most consequential addition in the 2025 criteria is the formal No Adverse Impact standard. A development cannot cause increased flooding to buildings or roadways, increased erosion, higher 100-year water surface elevations, expansion of the floodplain boundary, or concentration of drainage onto adjacent properties.

To achieve this, developers must provide stormwater detention limiting post-development peak discharge to pre-development levels for the 2-year, 25-year, and 100-year storms. There is a Fee-in-Lieu of Detention option available in certain cases, but on-site detention will be the typical requirement for most developments.

Site Grading and Finished Floor Elevations: New Standards

Section 10 of the 2025 criteria introduces detailed site grading standards that were entirely absent from the old 1959 manual. This section affects every development in Waco and its ETJ and has direct implications for how you lay out your site from day one.

For residential projects, the criteria sets specific requirements for swale grades, minimum lot grades toward the street or drainage easements, and prohibitions on directing drainage from one lot onto an adjacent lot without proper easements.

For non-residential projects, the standards address parking lot grading, building pad design, and drainage flow patterns across the site.

These requirements are not optional and they are being applied during plan review. Getting your grading strategy right early — before you finalize your site plan — saves costly redesign later. This is one of the most common areas where we see projects sent back during City review.

How Merritt Engineering Services Can Help

At Merritt Engineering Services, we have deep experience with drainage design, subdivision engineering, and municipal coordination throughout McLennan County and Central Texas. We work with the 2025 criteria on active projects and understand both the technical requirements and the practical implications for developers, builders, and property owners. Whether you need a full drainage study for a new subdivision, help navigating detention requirements for an infill development, stormwater quality design for a commercial project, or simply want to understand how the new criteria affects your land before you commit to purchasing it — we are here to help. Contact Merritt Engineering Services in Waco to discuss your project. We serve clients throughout McLennan County and the surrounding Central Texas region.

Merritt Engineering Services LLC | Waco, Texas | TBPE Firm F-23159 | Charlie Merritt, PE #110118