In 2019, Montgomery County posted a minor update to a drainage criteria manual first published in 1989. However, in February 2024, the County published a draft of a comprehensive new update to the county’s drainage standards. The 124-page manual is a thorough rewrite that brings the County’s standards up to date and more in line with surrounding areas.

Some Major Revisions

Because the manual is targeted to engineers, it gets very complex very fast. However, it is obvious that Montgomery County took pains to remedy some of the shortcomings of the past. For instance:

Section 1.4

Addresses approved hydraulic modeling software packages. It’s not that the packages are revolutionary. It’s that it requires the use of industry-standard packages. The previous manual did not; it only recommended certain packages. According to one flood plain manager I talked to, this will greatly facilitate the checking of plans. (See Page 10.)

Section 1.5

Specifies “No Adverse Impact” from drainage. The County says, “…one property owner should not be allowed to adversely affect the rights of other members of the community.” This protects the rights of neighboring property owners and also protects the natural systems that provide flood mitigation benefits to the community. It specifically prohibits flooding of downstream neighbors.

Significantly, No Adverse Impact includes impacts other than water surface elevation. They include peak flow, velocity and flow type. (See Page 11.)

Section 2.2.1

Mandates use of certain roughness coefficients for use in different situations. This affects calculations for the speed of runoff. They are critical for determining how fast and high flood peaks will build. In the old manual, calculating the roughness was somewhat subjective. (See pages 16, 48 and 50.)

Section 3.3

Unifies the discussion of erosion controls (both structural and natural) in great detail.

Section 5.3

Requires more storm drain inlets along roads by reducing spacing to 500 feet from 750.

Section 5.4

Requires land plan and street layout changes to improve stormwater conveyance in 100-year events.

Section 6.2

Stipulates that all projects shall mitigate and attenuate the runoff for 5-, 10- and 100-year storm events.

Section 6.3

Requires detention for all proposed developments, regardless of size. It also specifies how the amount will be calculated, and minimum rates (acre feet per acre) for different sizes and types of developments. In addition to other things, it specifies that post-development runoff rates shall not exceed pre-development rates.

Section 7.2

Discusses acceptable floodplain mitigation measures. For instance, “Natural channel clearing to reduce friction losses within the floodplain shall not be allowed as an acceptable floodplain mitigation measure.”

Maintenance

The name and contact information for the entity responsible for maintenance of detention facility, channels, and storm sewers must be listed on construction drawings under the proposed guidelines. “Maintenance shall be the responsibility of the private property owner and not the County,” says the draft manual in various sections.

Still Doesn’t Completely Embrace Harris County Recommendations

From a public protection standpoint, the changes in the new drainage criteria manual may not be revolutionary, but they are a vast improvement according to several hydrologists and a flood forecaster I consulted.

Does it incorporate the minimum drainage standards that Harris County urged surrounding counties to adopt after Hurricane Harvey? Some, but not all of them.

The table below summarizes Harris County’s recommendations and the recommendations reflected in Montgomery County’s draft drainage criteria manual.

Comparison of Recommendations
Measure	Harris	Montgomery
Use of Atlas 14 Rainfall Standards	Yes	Yes
Minimum Detention Rate	.65 acre feet/acre	.55 acre feet/acre
Prohibit Hydrographic Timing	Yes	Yes
No Net Fill in 500-Yr Floodplain	Prohibited	Still allowed
Finished Floor Elevation	At 500-yr flood elevation	100-yr flood elevation + 1 ft.

Atlas 14, Minimum Detention Rates, Hydrologic Timing

Montgomery County already uses Atlas 14 rainfall rates as the basis for all its drainage calculations. That will continue if the new draft drainage criteria are adopted. That’s great.

Even better though is this news. With a few minor exceptions, Montgomery County will require a minimum detention rate of .55 acre-feet per acre. It’s not quite up to Harris County rates, but it’s WAAAAAY better than nothing.

Previously, many developments used hydrographic timing surveys to avoid building any stormwater detention at all. But if adopted, the new drainage criteria will prohibit the use of such surveys. They will require detention for all new developments.

Timing surveys encouraged developers to get their water to bayous as fast as they could, which built flood peaks faster and higher for people downstream.

Fill, Finished-Floor Elevations

Sadly, Montgomery County’s draft drainage criteria would allow fill to be brought into the 500-year flood plain. In extreme events, this could constrict the width of the floodplain and cause a rise in flood levels.

The proposed finished-floor-elevation requirements in MoCo are also less stringent than Harris County. That, however, is the sort of thing more likely to affect Montgomery County residents than those downstream.

I remember owning a new home in the Dallas area back in the early 1980s that was supposed to be 1-foot above the 100-year flood plain. Within three years, it flooded. A new survey by the Army Corps of Engineers found that because of insufficiently mitigated upstream development during those three years, we were actually ten feet below the 100-year floodplain instead of a foot above it.

Montgomery County is also revising its subdivision regulations. They may have more to say on this topic.

Complete Manuals and Summary of Drainage Revisions

To see the old and proposed new drainage criteria manuals, see these links:

• 1989 Version with 2019 updates
• 2024 Draft Manual

Also see Montgomery County’s Summary of the Revisions.

For easy future reference, you can also find these documents under the Regulations tab in the reports page.

As of this writing, it is not clear when the new manual will go before Commissioners Court for approval.

Posted by Bob Rehak on 4/27/24

2433 Days since Hurricane Harvey

For the more technically inclined, Montgomery County provided this summary of significant revisions.