Even with strong code requirements to engineer properly, variances can override good practices.  As shown in the excerpts below, current law allows the City Engineer can grant variances for almost anything in the Infrastructure Design Manual (IDM).  Such power is corrupting, as evidenced by the lack of detention where clearly it should exist.

While there are many instances where “Grandfathering” is allowed, perhaps the most egregious is with respect to detention.  Existing impervious cover used to be completely allowed because the affect of the concrete was “already factored into the existing storm sewer system.”   This was simply ludicrous.

During the last revision of the code, equations were added (see below) that provided a modicum of additional detention for existing impervious cover, but it’s so paltry that it’s nearly impossible to detect compliance.

During the last year of Mayor Parker’s term, she bragged that she had eliminated Grandfathering.  A mad search of the IDM revealed nothing. What had happened was that the Chief Development Officer was granted the ability to determine whether an existing property requires detention.  It’s an unwritten policy –  a Cheshire Cat for bureaucrats.  How can anyone comply with or enforce an unwritten policy? Hopefully, this is not what is meant by government transparency – something that can’t be viewed must be transparent.




Obtain approval from the Office of the City Engineer (OCE) for exceptions or deviations from these requirements. Exceptions or deviations may be granted on a project-by-project basis.
(This is what we mean by a variance from standard practice. The Office of the City Engineer can grant a variance to any of these requirements.)

9.05.H. Stormwater Detention. (from the Houston PW&E Infrastructure Design Manual)

3. Calculation of Detention Volume.

a. Detention volume for Development areas is calculated on the basis of increases to the impervious cover associated with the project development and existing conditions at the site. Impervious cover includes all structures, foundations, driveways, parking areas, patios, walkways, etc. that exist or will exist on the property.
b. Single family residential (SFR) lots of 15,000 square feet in area or less: SFR Lots are exempt from detention if proposed Impervious Cover is less than or equal to 65 %. Detention volume of 0.20 acre feet per acre is required for Impervious Cover over 65%. 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 65%. If shared driveway is used, detention volume of 0.20 acre feet per acre is required. In other words, for projects that are platted to contain more than one lot and access to these individual lots is to be provided by a common or shared driveway, such as an access agreement, an access road, private alley or public alley, the detention requirements shall be calculated as follows:
(1) Detention Requirement = 0.2 acre feet per acre of increased impervious cover over 65% of the project area; ( This equates to a maximum of 7% detention for 100% impervious cover.)
(2) The area of the common or shared driveway, the access easement, access road, private alley or public alley must be included in the calculation of the project area.
c. Areas less than one acre and not subject to 9.05(H)(3)b: Detention volume will be required at 0.20 acre-feet per acre of increased impervious cover. Additionally, detention volume will be required to offset redevelopment of existing impervious areas.
(This is what we call “Grandfathering” of existing concrete.  This was an attempt to placate angry citizens flooded by newly developed properties that the City allowed to be both elevated by fill and Grandfathered, which citizens say strongly contributed to flooding their homes.)
Total Detention Volume required is calculated as follows:
VT= [43,560 x (0.20 x AII)] + (1815 x AEI)
VT= Total Detention Volume for the proposed project (Cubic Feet)
AII= Area of increased Impervious Cover (Acres)
AEI= Area of existing Impervious Cover (Acres)
Subdividing of larger tracts (greater than 1 acre) into smaller tracts of 1.0 acre or less to reduce stormwater detention requirements will not be permitted.
(Here’s an Excel spreadsheet that shows how little additional detention is required for existing impervious cover.  For 100% impervious cover on 10 acres, detention is 4.166%.  For 100% on 50 acres, it’s 7.5%. Compare that with the 50% required for untouched ground. )