Adding a cubic yard of fill dirt requires adding a cubic yard of detention.  A square foot of impervious surface must have a cubic foot of detention.  Why?

Engineers chose a target to design for.  For flooding, the typical design goal is the 24-hour 100-year event, which for most of the watersheds in Houston equates to about 13+” in 24 hours.  To make the numbers easy, a foot of water is assumed to run off an impervious surface.  Hence, a cubic foot of detention is needed to store the runoff from every square foot of impermeable surface.

At present, Houston engineers don’t seem to have a good grasp of who floods and why, so we’re suggesting that all regional approval agencies (including all city, village, and township governments) agree to act as though the region is in a Special Flood Hazard Zone (SFHZ).  Truth is that if FEMA moves forward with their elevation based flood insurance rate mapping (FIRM), much of Houston and its extraterritorial jurisdiction will be placed in SFHZ’s by FEMA with commensurate flood insurance rate increases.  No one wants that.

Until the new flood district is operational, it’s useful to treat the area as if in an SFHZ for two reasons: 1) it serves as a dry run for what seems likely to happen; 2) It prevents the flooding from getting worse because properties within an SFHZ must mitigate 100% for concrete poured by adding a comparable amount of detention.  Furthermore, fill dirt cannot be added to any property.  Elevations can still be built by scalloping  on-site dirt or adding underground detention or by building elevated structures (pier and beam, for example). 

This puts all counties on equal footing – there’s no benefit to shifting development elsewhere.  It also gives local governments time to sort through their specific rules and regulations to harmonize them to work for the whole region.

Discussions with local developers indicate that “if the tide rises” they’ll do whatever is necessary to meet the requirements.  Key are 1) a level playing field – the same rules apply for everyone; 2) keep development streamlined. Governments can incentivize and/or penalize to encourage development rather than the current practice of granting variances.

The key to this approach is for the region to actively work to solve flooding.  Success is defined by removal from the pretend SFHZ and reductions in flooded homes.

Definitions of Terms

In order to better understand why elevation is important, a digression into some hydrological definitions would be helpful.  

  • A Base Flood is a flood that has a 1% chance of being equaled or exceeded in any given year.  
  • Base Flood Elevation (BFE) is the elevation above sea level that flood waters have been calculated to reach during the base flood at a specific location.  
  • A Special Flood Hazard Area (SFHA) is any area that is subject to a 1% chance of flooding in any given year and is defined by Federal Emergency Management Agency (FEMA) floodplain maps. If you’re in an SFHA, all projects that raise the BFE must be reviewed for impacts to other structures.  FEMA or one of their designated local flood plain administrators (e.g., Harris County Flood Control District (HCFCD) or City of Houston (COH)) defines the local flood plain maps.  Most of the areas flooded in 2015 (~55% in Houston and 65% in Harris County) are not in mapped flood plains (or SFHA’s) which typically are overflow from local rivers, streams, bayous and ditches.  Given the extent of FEMA costs associated with recent flooding, expect that to change.  

As an example of why the region should be included in an SFHA, consider the TIRZ 17 Regional Drainage Study (RDS) (4-30-14 RDS amendment)  which uses state-of-the-art software to predict with good accuracy where the 1% floodwaters will be within their study area,  It has been clear for several years that the area surrounding TIRZ 17 has flooding issues – no surprise – by the RDS clearly shows the extent of the problem.  With almost certainty, if surrounding areas had been included in their own Regional Drainage Study similar maps would show widespread flooding at significant distances from area bayous.  However, the point of this digression is to explain that elevating properties raises BFE’s, which expands the floodplains.  And that happens whether the area is officially in an SFHA or not!  


Chapter 19 of the City of Houston’s Code of Ordinances underpins FEMA’s Community Rating System (CRS) of 5 for Houston.  A rating of 5 gives Houstonians a 25% discount on business and homeowners insurance, not particularly helpful when costs were $400 or less, but incredibly important for those repeat flood victims who have seen flood insurance skyrocket.  Chapter 19 puts teeth into too often unenforced building requirements, but it also puts teeth in the RAF lawsuit.  Chapter 19 requires that the City act responsibly to prevent flooding your house.  There are many sections of Chapter 19 that support our argument, however, we’ve selected just two to show what we mean.  Section 19-4 admits that all areas that flood are not within a FEMA designated floodway (55% of Houston flooded homes are not) and provides protections for those areas.  Section 19-43 provides specific protections for those areas.  Please read the highlighted portions below:


From Chapter 19 of the City of Houston’s Code of Ordinances:

Sec. 19-4. – Use of other flood hazard data to supplement the effective firm.

(a)    From time to time elevation and flooding studies are undertaken by or under the auspices of the Federal Emergency Management Agency and local political subdivisions (ed. such as a TIRZ), such as the Harris County Flood Control District, that have the responsibility to abate flooding. 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 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.

(b)   Where the study data are being administered as provided in subsection (a), the following procedures shall apply:

(1)    To the extent of any inconsistencies between the study data and the effective FIRM, the more restrictive base flood elevations and special flood hazard areas shall be controlling, and in no instance may any determination or designation that is based on the effective FIRM be reduced by study data.

(2)    If alternative base flood elevations exist for the property because of the administration of supplemental data as provided in this section 19-4, the applicant shall provide two surveys, one of which shall be based in the effective FIRM and one of which shall be based on the study data.

(3)    Any applicant for a plat, permit or other approval that is denied because of the application of the study data may appeal the denial of the permit, plat or other approval based on the validity of the study data as applied to the applicant’s property or project. The appeal shall be considered in the same manner as a variance application under article II of the chapter. In any such appeal, the city engineer shall provide the documentation for the study data; however, the burden of demonstrating that the study data are incorrect as applied to the applicant’s property shall rest upon the applicant and must be supported by the agency then responsible for the study data. Any appeal pursuant to this section shall not result in the change in any of the study data. In addition, if the study data being used has been published by the Federal Emergency Management Administration for comment as a draft or preliminary FIRM:

  1. The appeal process shall be limited to the application of the study data by the city to the specific application that is the subject of the appeal;
  2. The appeal process shall not be regarded as an appeal under part 67, or a request for map amendment under part 69, of Title 44 of the Code of Federal Regulations;
  3. Any outcome of the appeal to the city is in no way binding on the Federal Emergency Management Administration, nor will it affect or limit any action the agency may take; and
  4. Any challenge to the use of the study data as the basis for a FIRM should be separately addressed to the Federal Emergency Management Administration under the applicable federal rules.

(c)    For any special flood hazard area for which a floodway has not been designated, the applicant may submit an engineering analysis by a registered professional engineer licensed in the State of Texas that defines the floodway with respect to the site for which a development permit is sought.

Sec. 1943. – Floodways.
(a) Except as may be otherwise provided in this chapter, no permit shall hereafter be issued for a development to be located in any floodway, or any special flood hazard area for which a floodway has not been designated, if that development provides for:
(1) Encroachment by the deposition of fill, or other similar construction, within the floodway, or the special flood hazard area if no floodway has been designated; or
(2) New construction, additions to existing structures, or substantial improvement of any structure within the floodway, or the special flood hazard area if no floodway has been designated.
The general restriction stated herein shall not apply to a repair or renovation that is not a substantial improvement.

___________ Example___________

TIRZ 17 extended the Tropical Storm Allison Recovery Project (TSARP) Flood Plain maps for W140, which were used to extend the FEMA Flood Insurance Rate Maps (FIRM) for Harris County.   They have already performed the function defined above in 19.4.(a) of Houston’s Code of Ordinances when LAN, the TIRZ hydrologists, defined the flood plains for Briar Branch Creek.  Their flood plain definitions were shown to be erroneous by both the RDS and by the Klotz analysis of W140.  Had they correctly defined the Level of Service (LOS) for W140, none of the commercial properties would have been allowed to elevate their properties because that would have raised Water Surface Elevation’s (WSE’s).  The point being, TIRZ 17 has been allowed to designate a floodplain before using rather crude analysis.  Done using a much more sophisticated model, the RDS now defines the WSEL throughout the area around TIRZ 17.  That should mean that 19-4 b(1) and 19-43(a) apply.  Recently the Lipex properties (Metro National) at the NW corner of I-10 and Gessner were elevated 1.5 feet in an area that the RDS shows has severe flooding.  By permitting this, the City of Houston has violated their ordinance. 

Chapter 9 of the Infrastructure Design Manual must comply with Chapter 19, as shown in the highlighted from the excerpt 9.02.D below.  PW&E Permitting must follow Chapter 9 when issuing building permits and there are many sections of Chapter 9 that define how WSE must be managed in a flood prone area.  Clearly, the COH is ignoring their own requirements.


From Chapter 9 of the City of Houston Infrastructure Design Manual

9.02.C. Proposed New Development or Redevelopment greater than 1 acre shall not alter existing overland flow patterns and shall not increase or redirect existing sheet flow to adjacent private or public property. Sheet flow from the developed property shall discharge only to the abutting public R.O.W. Where the existing sheet flow pattern is blocked by construction (i.e. raising the site elevation) of the Development, the sheet flow shall be re-routed within the developed property to return flow to original configuration or to the public R.O.W. Except under special circumstances dictated by natural drainage patterns, no sheet flow from the developed property will be allowed to drain onto adjacent private property.

9.02.D. The City is a participant in the National Flood Insurance Program (NFIP). The flood insurance program makes insurance available at low cost where the municipal entity implements measures that reduce the likelihood of structural flooding. The design criteria in this chapter are provided to support the NFIP. All development located within the City limits shall comply with Chapter 19, FLOODPLAIN, of the Code of Ordinances.

9.02.D. The City is a participant in the National Flood Insurance Program (NFIP). The flood insurance program makes insurance available at low cost where the municipal entity implements measures that reduce the likelihood of structural flooding. The design criteria in this chapter are provided to support the NFIP. All development located within the City limits shall comply with Chapter 19, FLOODPLAIN, of the Code of Ordinances.

9.05.D.3.c. For Development or Redevelopment by private entities, the post- project maximum WSE shall be no higher than the pre- project maximum WSE in surrounding areas, and proposed finished slab elevation shall be above the post-project maximum WSE. The Maximum Ponding Elevation is determined from the physical characteristics of an area, and may change as a result of the proposed Development. Where existing topographic conditions, project location within a special flood hazard area, and/or other site conditions preclude achieving this objective, the City will consider waiver of this requirement upon submittal of documentation and analysis prepared, signed, and sealed by a professional engineer, registered in the State of Texas. Analysis shall demonstrate that structural flooding will not occur and will identify the rainfall frequency event that will be conveyed within the R.O.W. The limiting parameter will depend on project-specific conditions, and the most restrictive condition (the lowest ponded water elevation) shall govern.