MoCo Adopts New Drainage Criteria Manual 8 Years After Harvey
Montgomery County has finally updated its drainage criteria manual…eight years after Hurricane Harvey caused widespread flooding that exposed shortcomings in its previous manual which dated to the 1980s.
Unanimously Approved
MoCo Commissioners approved the new manual unanimously this morning. The draft of the new manual was first proposed in early 2024. That followed a minor update in 2019 from the county’s old 1988 drainage criteria manual in effect at the time of Harvey.
The new manual does not adopt all of the minimum drainage recommendations proposed by Harris County for areas draining into Harris, though it is a vast improvement over the previous iteration.
Minimum Drainage Requirements Proposed by Harris County
The minimum drainage requirements proposed by Harris County included five key measures:
1. Using Atlas 14 rainfall rates for sizing storm water conveyance and detention systems.
2. Requiring a minimum detention rate of 0.55 acre-feet per acre of detention for any new development on tracts one acre or larger. A single-family residential structure and accessory building proposed on an existing lot is exempt from providing detention.
3. Prohibiting the use of hydrograph timing as a substitute for detention on any project, unless it directly outfalls into Galveston Bay.
4. Requiring “no net fill” in the current mapped 500-year floodplain, except in areas identified as coastal zones only.
5. Requiring the minimum Finished Flood Elevation of new habitable structures be established at or waterproofed to the 500-year flood elevation as shown on the effective Flood Insurance Study.
Major Changes in New MoCo Drainage Criteria Manual
MoCo’s new drainage criteria manual includes some, but not all, of those recommendations.
| Comparison of Recommendations | ||
| Measure | Harris | Montgomery |
| Use of Atlas 14 Rainfall Standards | Yes | Yes |
| Minimum Detention Rate | .55 acre feet/acre | .55 acre feet/acre for areas greater than 20 acres (see page 57) |
| Prohibit Hydrographic Timing | Yes | Yes, but with limitations (see page 68) |
| No Net Fill in 500-Yr Floodplain | Prohibited | Still allowed |
| Finished Floor Elevation | At 500-yr flood elevation | Requires drainage be maintained one foot below lowest finished floor elevation for 100-year event (See page 54) |
It’s not perfect. But it’s a vast improvement. Assuming the county enforces them.
For ease of future reference, you can find Montgomery County’s new, updated Drainage Criteria Manual on the Reports page under the Regulations tab.
Suggestions by MoCo Resident
Montgomery County resident Chad Price addressed Commissioners Court before the vote. He applauded most of the updates in the manual. However, he also urged commissioners to adopt ALL of Harris County’s minimum requirements.
Price emphasized the uncertainty surrounding rainfall rates, the increasing frequency of storms that exceed predicted maximums, and flood maps that have yet to be updated to reflect Atlas-14.
He made two excellent points:
- We must not design drainage systems based on outdated data.
- Better flood regulations are not about stopping growth—they’re about making sure growth is sustainable and safe.
In that regard, Price urged commissioners to update building codes, require smarter drainage planning, preserve natural floodplains, and use science-based floodplain mapping. He said, “These steps will reduce long-term costs to taxpayers, protect property values, and most importantly, safeguard our communities.”
Next Up
Montgomery County’s Floodplain Manager currently shows floodplain regulations adopted in 2014. That’s before the Tax Day, Memorial Day, Harvey, Imelda and May Day 2024 floods. Given the thousands of homes in MoCo that flood repeatedly, there may be some opportunities for improvement in those floodplain regs.
Posted by Bob Rehak on 8/26/25
2919 Days since Hurricane Harvey
The thoughts expressed in this post represent opinions on matters of public concern and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the Great State of Texas.
