Because of low interest rates, new developments seem to pop up weekly around the Lake Houston Area. The question often arises, “How will the development of new flood maps affect the development of new subdivisions?” Most people by now have heard that City building code revisions now require elevation at least two feet above the .02-percent-annual-chance flood (formerly known as 500-year flood). But does that mean two feet above the old floodplain or the new? Due to a little-known provision in the City’s floodplain regulations, it means the new floodplain even though the new floodplain maps are not official yet.

Timetable for Updating Flood Maps

Here’s what you need to know if you’re concerned that someone may be building future buyouts next to your neighborhood. First the timetable for new flood maps.

Even though FEMA won’t release the new flood maps officially for approximately another five years, developers should still be building to the higher standards associated with newly acquired data (in other words, the data on which the new maps are being built). See below.

Floodplain Regs Authorize City Engineer to Use Data Behind New Flood Maps

Section 19-4 of the City’s Floodplain regulations address Use of other flood hazard data to supplement the effective Flood Insurance Rate Map (FIRM). It states, “New elevation and flooding studies are undertaken by or under the auspices of FEMA and local political subdivisions, such as the Harris County Flood Control District. Upon determination that the data generated by such a study appears to be reliable and based upon sound engineering and surveying practices and further that the study’s data indicate that the effective FIRMs are FIRM is materially inaccurate, the city engineer may cause the study data to be administered for purposes of this chapter as though it were a part of the effective FIRM. Any such determination shall be issued in writing and a copy shall be placed on file in the office of the city secretary. The city engineer is authorized to utilize updated information from FIS and floodplain models in administering this chapter.”

Basically, that means even though the new maps have not yet been adopted, the City Engineer can require developers’ plans to reflect the new underlying data as though it were part of the current map. In this case, the new underlying data is already in hand.

MaapNext Website Describes New Data Improvements

Current floodplain maps will change greatly according to Harris County Flood Control. The district has already started releasing information on a new website called MaapNext (Modeling, Assessment and Awareness Project).

According to the MaapNext site, we now have updated data based on:

• County-wide impervious data developed from 2018 aerial imagery
• Completed flood risk reduction projects
• NOAA’s recently-released Atlas 14
• Updated terrain data

We also have new and better ways to model that data since the last survey after TS Allison:

• 2-Dimensional Hydraulic Modeling
• New hydrology method that better accounts for a watershed’s conveyance capacity
• Rain-on-Grid analysis that identifies previously unmapped urban flood risk.

And, we are beginning to develop better maps:

• Modeling results in GIS (Geographic Information Systems)
• New flood-risk data sets describe results in a variety of useful ways

Reportedly, the City is already requiring developers to act on the new “best available data” instead of waiting five years for the next maps.

Chapter 19 of the City Ordinances deals with dozens of other requirements for building in floodplains. But more on those in future posts.

Posted by Bob Rehak on 8/29/2020

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