Elevating properties and adding slopes or large underground drainage pipes definitely changes the rate that water flows off a property and that affects adjacent property owners, which state law doesn’t allow, but which we ignore.  It’s should also be obvious to local residents that much of Houston is built on fine clay that’s not particularly porous.  Once the ground is saturated, such as after a week of rain, it can be considered impervious too and will have nearly 100% runoff.

  • All Houston has been designated as an urban area.  Therefore, remove all calculations in Section 9.05 B.3.a.(1) of the Infrastructure Design Manual (IDM) for the Runoff Coefficient for the rational method.  Runoff coefficients are determined based upon the amount of impermeable cover, but it may be assumed that with increasing urban density, 100% impermeable cover will eventually occur.  Certainly everything designed inside an 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 in the IDM? 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 watershed, then the factor, 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. In 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.