Can We Stop Houston Flooding?

September 27, 2012

Overview – Chapter 9 of the City of Houston Infrastructure Design Manual, Stormwater Design Requirements, is up for comments. We have reviewed Chapter 9 and recommend that the following changes be made in order to prevent future flooding. Our central concern is the yearly rise in Base Flood Elevations and the increase in the size of the 100-year floodplain, which we believe is largely caused by the practice known as “Grandfathering.”

Our issues with Grandfathering are: unwarranted variances have been granted to builders, such as elevating property with no consideration of sheet flow obstruction or compensating detention; proper detention simply wasn’t installed; the land was developed prior to annexation by the City; or, for any number of other reasons, adequate detention and/or site drainage wasn’t installed. Once built, these properties are forever Grandfathered and new ones are added every year, so that the floodplain will continue growing until Grandfathering is removed from City Design Manuals.

Each year that the 100-year floodplain grows, Houston’s viability as a good location for new businesses is diminished. Moreover, the cost to residents increases in the form of diminishing home values, increased insurance rates, or actual flood damage to property. Our practice of using streets as drainage channels and neighborhoods as detention areas routinely brings the City to a standstill with untold property damage and work hours lost.

While water flow in roadways is certainly preferable to flooded homes, going forward with Rebuild Houston, we need to design to a higher level of service using realistic numbers. Progress has been made over the years; new subdivisions do a much better job at protecting structures than older neighborhoods, but developers and City engineers need to be cognizant that most people live in older homes that still need to be protected.

Temporarily waiving Rebuild Houston’s presently unimplemented Developer’s Fee can be used as a positive incentive for, for instance, adding more detention than necessary or building using low-impact design criteria. (editor’s note: the developer’s fee was passed by City Council Spring 2013 and will only add approximately $1.5M to Rebuild Houston.  Final deadline for comments on the revisions is May 24. It is located here.)


  • Change Section 9.02 F. to add the lines in italics. The most interesting comment about disallowing development in areas that have deficient drainage systems is that it would “bring development to a standstill.” Nothing could have proven the point better because that’s equivalent to saying, “Houston is knowingly developing in areas that negatively impacts flooding in adjacent properties and neighborhoods. If we continue to insist upon following this path, then it needs to be done in a more responsible way. A development so located must completely mitigate the effects of their project in-situ to a particular level of service (TBD). To incentivize Development despite the additional costs, the Rebuild Houston Developer’s Fee could be waived for a finite period.

Section 9.02 F. Development or Redevelopment in Areas that have Deficient Drainage Systems: Development or Redevelopment will not be allowed in areas that have deficient drainage systems until existing drainage infrastructure is improved to sufficient capacity to convey all existing and new run-off. Payment in-lieu of detention (Section 9.05 H.2.a.) is only possible if drainage infrastructure capacity is improved to carry all area run-off and runoff from the Development. Alternatively, Development may continue if project impacts are completely mitigated on-site. The City will consider joint project funding with a private entity for construction of drainage systems that improve existing drainage infrastructure. The City’s first priority will be to fund those projects included in the Capital Improvement Plan (CIP). Where feasible, City funding will be leveraged with other funding sources including private entities, civic organizations, and other public agencies (Harris County, HCFCD, Corps of Engineers, Housing and Community Development, and other funding sources). For drainage systems that have been identified as deficient and are not scheduled to receive funding in the current CIP, the City will consider authorizing improvements performed by the private entity that comply with the City’s objectives.


  • Amend Section 9.04.D to combine the definitions of “in-fill” and “redevelopment” to provide for two, instead of three types of development; i.e., development that is either (1) “new” because the storm drain infrastructure has not been constructed, or (2) “in-fill”/“redevelopment” because the storm drain infrastructure is already in place and takes advantage of the existing infrastructure in place as a drainage outlet. The only reason to bother to make the distinction is because redevelopment might take advantage of Grandfathering clauses and in-fill would not. Removing Grandfathering obviates this need.

Section 9.04 D. Development – The term includes New Development, and In-fill/RedevelopmentDevelopment.

  1. In-fill/Redevelopment Development – Development of open tracts of land in areas where the storm drainage infrastructure is already in place and takes advantage of the existing infrastructure as a drainage outlet.
  2. New Development – Development of open tracts of land in areas where the storm drainage infrastructure has not been constructed and a drainage outlet must be extended to a channel under the jurisdiction of the HCFCD.


  • Find a more realistic way of defining rainfall events in Section 9.04 K.

In my relatively short time in Houston (35 years), my area has suffered no less than six 100-year events – something that is statistically improbable:

A rate of 6 events in 35 years is the same as 17 events in 100 years so:

Assuming independent stationary random events

PT= (PA)(PB)(PC)…(PN)

(PA) = (PB) = (PC) = (PN) = 0.01

PT ~1 in 1034

This very large number strongly indicates that hypothetical water level tables used by the City when defining the frequency of rainfall events (2-year, 10-year, 50-year, 100-year, etc.) are grossly inaccurate. Even if only six events occurred in one hundred years, the probability of such an occurrence would be one in a trillion – still indicating the tables are wrong. Given improved record keeping and technological improvements that can permit near real-time water-level monitoring, the Houston Metropolitan area needs to begin creating better definitions by analyzing actual long-term rainfall history.

Judging from the following comment, HCFCD’s grasp of probability is not much better. “In fact, about half of all flooding events in Harris County occur outside a mapped 1 percent (100-year) floodplain.” If there’s a 50/50 chance of getting flooded outside or inside of the 100-year floodplain, then the distinction is moot – we are all in the 100-year floodplain.

Section 9.04 K. Rainfall Frequency – Probability of a rainfall event of defined characteristics occurring in any given year at a given location. Information on Rainfall Frequency is published by the National Weather Service. For the purpose of storm drainage design, the following frequencies are applicable: (insert mathematically correct numbers here)



  • Section 9.05 B.1.a Rainfall Durations specified render the runoff coefficients in the Rational Method useless.

Section 9.05 B.1.a Rainfall Durations

  1. For design purposes, the rainfall duration for drainage areas less than 200 acres will be no less than 3 hours in duration.
  2. For design purposes, the rainfall duration for drainage areas more than 200 acres will be no less than 6 hours in duration.


  • Remove all calculations in Section 9.05 B.3.a.(1) for the runoff Coefficient for the rational method, particularly for areas designated as “urban.” Runoff coefficients are determined based upon the amount of impermeable cover, but it may be assumed that with increasing density, 100% impermeable cover will eventually occur.  Certainly everything designed for use inside the urban zone needs to assume that it will see maximum runoff, and hence, should use the maximum runoff coefficient. 
  • Why is the maximum runoff coefficient only 0.8? The rational Formula fails to account for storm duration and can provide low estimates of actual runoff.  If, for example, the intense duration of the storm exceeds the time that it takes water to flow to the watershed outlet from the most remote part of the awtershed, then the facto, C, should be 1.0.

  • TxDOT defines maximum runoff coefficients for downtown areas as 0.95. Using 0.95 for the runoff coefficient in urban areas would skew calculations toward more underground capacity. Clearly, application of 0.18 for the runoff coefficient for all parks ignores the dramatic permeability differences between pedestrian trampled parks like Hermann versus wooded portions of Memorial Park. I n reality runoff will also be dramatically different.
  • If we are truly interested in reducing the 100-year floodplain, then all development and redevelopment should be returned to runoffs equivalent to a pre-Columbian condition; i.e., prior to development, and every property would mitigate their runoff on-site. Adoption of better methods to compute runoff rates would likely improve drainage efficacy. There are valid arguments for defining all drainage requirements in terms of runoff volumes and peaks rather than impervious cover.

Section 9.05 B.3.a.(1) Determination of runoff

Use 0.95 for the runoff coefficient C values in urban areas when using the rational method formula.



  • If Houston wants to stop increasing the 100-year floodplain, runoff from New Development must be mitigated on-site, otherwise downstream areas will be subjected to greater runoff than experienced before the New Development. Change Section 9.05 C. 1.a. and Section 9.05 C. 1.b. to add the lines in italics. This is one of the Grandfathering clauses that need to be changed.

Section 9.05 C. 1.Design Frequencies

a. New Development: The Design Storm Event for sizing storm sewers in newly developed areas will be a 2-year rainfall. In addition, on-site detention must be installed for 100% mitigation of New Development runoff.

b. Redevelopment or In-fill Development: The existing storm drain (sewer, ditch) will be evaluated using a 2-year design storm, assuming no development takes place. The storm drain will then be evaluated for the 2-year design event with the Development in place.

(1) If the proposed Redevelopment has a lower or equal impervious cover, and the existing storm drain (sewer, ditch) meets 2-year level of service, then no modifications to the existing storm drain are required; otherwise, the property will need to be brought up to this code.

(2) If the proposed Development results in the hydraulic gradient of the existing storm drain below the gutter line, no improvements to the existing storm drain are required. Detention shall comply with Paragraph 9.05.H. Flow discharged to the storm drain shall be in compliance with Paragraph 9.05.H.4.b.

(3) If the analysis of the existing conditions finds that the existing storm drain is deficient (i.e. the hydraulic grade line is above the gutter line), the applicant should check with the City to see if a CIP project is proposed that will require a capital contribution. If a CIP project is not proposed for the subject system, then on-site detention will be required in accordance with Paragraph 9.05.H. Flow discharged to the storm



  • The City has long touted Regional Detention Basins as the answer to regional flooding, but the obvious problem with this approach is that drainage systems under Houston’s roadways were only designed to carry water from a 2-year event. They were not designed to also carry all the water from a large Development. Doing so clearly exacerbates regional flooding. If drainage capacity from the new Development to the Regional Detention Basin is insufficient, then the Development should not be allowed to forego on-site detention. This entire section needs to be rewritten, but the salient points are there.

Section 9.05 H.2.a.

The use of on-site detention is required for all Developments within the City and for new or expanding utility districts within the City’s ETJ. Detention will not be required if the City has developed detention capacity for a drainage watershed, and/or infrastructure improvements, to serve the drainage watershed in compliance with the requirements of this Chapter. Drainage capacity from the Development to the City’s detention capacity must be capable of carrying not only the water from the Development, but it must also do so while maintaining 2-year level of service for all neighborhoods and Developments already located in the drainage watershed. If these criteria cannot be met, on-site detention must be installed. In lieu of this, either the City or Development may elect to improve area drainage capacity to insure 2-year level of service. Under these conditions, the City will consider a funding contribution in lieu of on-site detention volume constructed by the owner.


  • We recommend that Section 9.05 H.2.d be replaced. This is the primary Grandfathering clause responsible for much of Houston’s increasingly bad floods. First, the old verbiage, then recommended changes.

Section 9.05 H.2.d.If Redevelopment occurs without increasing the overall impervious character of the site, then no detention will be required by the City.

Section 9.05 H.2.d. Redevelopment of preexisting sites must be brought up to current code for detention requirements. If the site cannot easily be altered and there are adequate existing storm sewers to convey site runoff, detention may be installed within 0.5mile of the site. If adequate storm sewers do not exist, developer may petition PW&E for upgrades.

  • Section 9.05H.2.e.1. needs to be modified to make it clear that the conveyance channels must be large enough between the Development and the regional facilities to not increase the risk of flooding to adjacent regions.

Section 9.05 H.2.e.1.

Development is located in an area determined by the City to not need detention due to the geographic location in the watershed, the Development’s proximity to regional facilities, or the capacity of the receiving outfall facilities and there are adequate existing storm sewers to convey site runoff while maintaining a 2-year level of service for all neighborhoods and Developments already located in the drainage watershed. Such conclusion by the City shall be supported by submittal of a Hydraulic Report as described in Paragraph 9.05.H.2.e(2).


  • Neighborhoods have requested that the Hydraulic Report in Section 9.05 H.2.e.(2) be accessible on-line. Too often totally fabricated analyses have been permitted, which have resulted in neighborhoods being flooded. City Permitters and the Professional Engineer who signs his name on the report would know that their work can be seen and reviewed. While there may still be honest mistakes made, fraud would be dramatically reduced. Transparency is miraculous for preventing corruption.

Section 9.05 H.2.e.(2) Hydraulic Report: Submit a hydraulic analysis prepared, signed, and sealed by a professional engineer, registered in the state of Texas, to demonstrate compliance with the conditions stated in this Chapter. The hydraulic analysis shall consider (1) the current developed condition of the watershed of the stormwater conveyance system, and (2) the fully developed condition of the watershed. The probable land use for the fully developed condition will be determined by the design engineer for review and approval by the City. The hydraulic analysis shall demonstrate no negative impact to upstream or downstream conditions and shall demonstrate that a positive impact will be achieved (reduced flood crest due to the exemption. The City of Houston will post Hydraulic Reports in a public database indexed by actual property address.


  • Once again, if Houston wants to stop increasing the 100-year floodplain, runoff from Development must be mitigated on-site, otherwise downstream areas will be subjected to greater runoff than experienced before the New Development. Section 9.05.H. 3. should be modified accordingly.

Section 9.05H. 3. Calculation of Detention Volume

a. Detention volume for Development areas is calculated on the basis of the amount of area of increased impervious cover. Impervious cover includes all structures, driveways, patios, sidewalks, etc.

b. Single family residential (SFR) lots of 15,000 square feet in area or less: If an adequate area stormwater conveyance system is available that meets 2-year minimum criteria, then SFR Lots are exempt from detention if proposed impervious cover is less than or equal to 75.0 %. Detention volume of 0.20 acre feet per acre required for impervious cover over 75%.

Existing SFR lots of 15,000 square feet or less may be further subdivided and exempt from detention provided the proposed impervious cover remains less than or equal to 75.0%. If an adequate area stormwater conveyance system is not available that meets 2-year minimum criteria, then 100% of runoff must be mitigated on-site.

c. Areas less than 1 acre: If an adequate area stormwater conveyance system is available that meets 2-year minimum criteria, detention will be required at a rate of 0.20 acre feet per acre of increased impervious cover. The subdividing of larger tracts into smaller tracts of 1.0 acre and less will require the detention volume of 0.5 acre-feet per acre of increased impervious cover. If an adequate area stormwater conveyance system is not available that meets 2-year minimum criteria, then 100% of runoff must be mitigated on-site.

d. Areas between 1 acre and 50 acres: 100% of runoff must be mitigated on-site.


  • Missing piece 1: Drainage systems that were designed years ago had different requirements than those existing today and often slab foundation’s base elevations are lower than those required now. As a result, these older neighborhoods are more prone to flooding. Yet, when redevelopment begins in a region, City code has no provision to check surrounding locales to make sure that increased flooding does not occur. This common-sense check should be mandatory. Tools exist to study the cumulative local, nearby, and downstream impacts – impacts that should be mitigated whenever possible and disclosed when not. This impact review should be part of the initial platting review process and should not wait until after a plat has been approved.


  • Missing piece 2: Public and private detention ponds can be breeding grounds for mosquitos. Either these ponds must be graded so that water drains toward the exit channel, reducing the possibility of standing water, or redesigned as permanent wet-bottomed detention ponds containing mosquito fish (Gambusia).


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