In response to the National Oceanic and Atmospheric Administration’s (NOAA) Atlas 14 rainfall data which was recently published, the Harris County Engineering Department has proposed a number of revisions to design criteria and regulations regarding infrastructure design and floodplain management.  These revisions apply to development of areas within unincorporated Harris County, or where drainage outfall is directed to a Harris County drainage feature such as roadside ditch or storm sewer.

The regulations that will have the greatest impact on development are summarized below:

Minimum Detention Rate

The minimum required detention rate for gravity-drained facilities to a storm sewer is proposed to be increased by 0.1 ac-ft/ac from 0.65 to 0.75 ac-ft/ac.   Facilities draining to a roadside ditch are still required to provide detention volume at a minimum of 1.0 ac-ft/ac. These rates can be lowered through demonstration of detailed hydrologic and hydraulic analysis as defined in the most recently amended Harris County Flood Control District Design Criteria Manual.

For pumped/gravity combination outfalls, the minimum required detention rates are 1.0 ac-ft/ac to roadside ditch, and 0.75 ac-ft/ac to storm sewer (no consideration given for detailed hydrologic and hydraulic analysis).

Revised Intensity-Duration-Frequency Curves

The Intensity-Duration-Frequency (IDF) curves which are used to size stormwater conveyance features such as storm sewer and drainage ditches will be updated to reflect the higher rainfall amounts. The new IDF curves are anticipated to have an impact on how large stormwater conveyance features need to be sized.

Floodplain Mitigation Criteria

Under the current criteria, projects are required to mitigate any fill placed within the 100-year (1% annual chance) floodplain. The proposed regulations will require that projects mitigate for any fill placed in the 500-year (0.2% annual chance) floodplain.

As part of the transition to the new regulations, Harris County will allow for vested rights/grandfathering projects meeting the eligibility requirements.

Links:

http://www.eng.hctx.net/Portals/33/Publications/Public%20Notice/FPMReg_PCD190603.pdf

http://www.eng.hctx.net/Portals/33/Publications/Public%20Notice/InfraReg_PCD190603.pdf

For a complete understanding of the impact these new criteria and regulations have on your project, please reach out to WGA Consulting Engineers at wgaregs@wga-llp.com