What Is a Drainage Study and When Does Your Project Need One?

If you've ever submitted a development application to a city or county in Texas and been told you need a drainage study, you're not alone. It's one of the most common requirements developers and property owners encounter — and one of the least understood. In this post I want to explain exactly what a drainage study is, what goes into one, and how to know whether your project needs one before you get deep into the planning process.

What Is a Drainage Study?

A drainage study is an engineering analysis that evaluates how stormwater runoff behaves on and around your property — both before and after your proposed development. The goal is to answer a fundamental question: will your project cause flooding, erosion, or drainage problems for your property or your neighbors?

The study looks at things like how much rain falls on your site during a design storm event, how fast that water runs off, where it goes, and whether the downstream drainage system can handle it. The engineer uses that analysis to design drainage infrastructure — things like storm drains, culverts, swales, detention ponds, and drainage easements — that manages stormwater safely and in compliance with local requirements.

A drainage study is not just a permit box to check. Done right, it's a planning tool that protects your investment, reduces your liability, and keeps your project from getting sent back during plan review.

What's Actually In a Drainage Study?

While the exact contents vary by jurisdiction and project type, a typical drainage study for a development project in McLennan County or the surrounding Central Texas area includes:

A watershed delineation — mapping the area of land that drains to your site and through it. This establishes the scope of the analysis and identifies where water is coming from upstream.

Hydrologic analysis — calculating how much runoff is generated by the design storm events. In Texas this typically means the 2-year, 10-year, 25-year, and 100-year storms. Engineers use methods like the Rational Method for smaller areas or the NRCS Unit Hydrograph method for larger, more complex watersheds.

Hydraulic analysis — evaluating the capacity of drainage features like culverts, channels, and storm drains to carry that runoff safely. This is where software like HEC-RAS comes in for more complex situations involving floodplains or open channels.

Detention analysis — if your jurisdiction requires it, calculating the size of a detention facility needed to limit post-development runoff rates to pre-development levels.

A drainage report and exhibits — a written report documenting the methodology, assumptions, calculations, and conclusions, along with drainage area maps, grading plans, and infrastructure details stamped by a licensed Professional Engineer.

When Does Your Project Need a Drainage Study?

This is the question we get most often from developers and property owners, and the answer depends on your jurisdiction and the scope of your project. Here are the most common situations where a drainage study will be required:

New subdivision platting. In McLennan County and most surrounding jurisdictions, any new subdivision plat requires a drainage study as part of the engineering submittal. This applies whether you're platting 5 lots or 500.

Commercial site development. Most cities require a drainage study for any new commercial development or significant redevelopment that changes the impervious cover on a site. In Waco, the 2025 Stormwater Design Criteria sets clear thresholds for when full drainage analysis is required.

Projects near a floodplain. If your property is adjacent to or within a FEMA-mapped floodplain, a drainage study is almost always required — and may need to include a detailed HEC-RAS hydraulic model to demonstrate no adverse impact to flood elevations.

TxDOT driveway permits. If your project requires a driveway access permit from TxDOT, a drainage study or at minimum a drainage analysis is typically required to evaluate the impact on roadway ditches and culverts.

Culvert replacements and channel improvements. Any time you're modifying an existing drainage feature — replacing a culvert, regrading a channel, or adding a detention pond — engineering analysis is required to demonstrate the modification is adequate and doesn't create downstream problems.

When in doubt, ask early. One of the most common and costly mistakes we see is developers who wait until they're deep into site planning — or worse, into permitting — to find out a drainage study is required. A quick conversation with your engineer at the start of a project can save significant time and money.

How Long Does a Drainage Study Take?

For a typical residential subdivision or commercial site in McLennan County, a drainage study generally takes two to four weeks from the time we have the survey data and site plan in hand. More complex projects involving large watersheds, floodplain analysis, or HEC-RAS modeling can take longer.

The biggest factor that affects timeline is data availability. Having a current boundary and topographic survey, a preliminary site plan, and any available as-built information on existing drainage infrastructure ready when you engage your engineer makes a significant difference.

How Much Does a Drainage Study Cost?

Cost varies based on project complexity, watershed size, and what the jurisdiction requires. A straightforward drainage study for a small subdivision or commercial site typically runs in the range of a few thousand dollars. Projects requiring HEC-RAS modeling, FEMA floodplain analysis, or coordination with TxDOT or the City will cost more.

The right way to think about it: a drainage study is a fraction of the cost of a project delay, a redesign forced by plan review comments, or a drainage problem discovered after construction. It's one of the highest-value engineering investments you make on a development project.

How Merritt Engineering Services Can Help

At Merritt Engineering Services we prepare drainage studies for subdivision, commercial, and municipal projects throughout McLennan County and Central Texas. We know the local jurisdictions, the review process, and what it takes to get a drainage study approved the first time.

If you're planning a development project and aren't sure whether you need a drainage study — or you know you need one and want to get started — contact us. We're glad to talk through your project and give you a straight answer on what's required and what it will take.