Proposed Revisions to Harris County Flood Control District Regulations
In response to the NOAA Atlas 14 rainfall which was recently published, the Harris County Flood Control District (HCFCD) has proposed interim guidelines and criteria for projects affecting HCFCD right-of-way. This includes but is not limited to higher rainfall amounts based on Atlas 14 data, higher stormwater detention rates, and the extension of the “No Adverse Impact” requirement to the 500-year flood event. The regulations that will have the greatest impact on development are summarized below:
Minimum Detention Rate
The minimum detention rates for gravity drained facilities is proposed to be increased by 0.1 ac-ft/ac, from the existing 0.55 ac-ft/ac to 0.65. In addition, the threshold at which a drainage impact analysis and report is required has been reduced from 50 acres to 20 acres. The detention rate for pumped systems will remain at 0.75 ac-ft/ac.
Increased Rainfall Amount
The 24-hour rainfall depths are proposed to be updated based on the Atlas 14 rainfall data for the 2-year, 10-year, 100-year, and 500-year storm events. The county is currently divided into three (3) hydrologic regions. The changes to the 100-year rainfall depth for reach region is shown in the table below:
1% Annual Chance (100-Year) Rainfall Summary
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.
These interim guidelines and criteria are scheduled to become effective on June 25, 2019. However, the HCFCD is currently conducting updated floodplain modeling and mapping studies, incorporating new methods of hydrologic and hydraulic analysis that may affect the current HCFCD criteria. Therefore, it should be understood that these interim guidelines and criteria are subject to change as additional information is obtained and further analysis developed.
For a complete understanding of the impact these new criteria and regulations have on your project, please reach out to WGA Consulting Engineers at firstname.lastname@example.org