Harris County and the Harris County Flood Control District (HCFCD) recently updated the rainfall rates used in their regulations following the release of the National Oceanic and Atmospheric Administration (NOAA) Atlas 14 study. This study is the first comprehensive rainfall rate study released by NOAA for the state of Texas in over 60 years.
The Atlas 14 study was an ongoing study analyzing historic rainfall data in certain areas of the United States, and the results for Harris County were staggering. A key takeaway from these findings is a rainstorm that once was believed by the NOAA to have a 1 in 100 chance of being exceeded in a year is now reported to have a 1 in 25 chance of being exceeded. NOAA also indicated the updated 100-year rainfall frequency is nearly equal to the previous 500-year rainfall rate, which is the main factor for Harris and surrounding county’s regulation overhaul.
HCFCD Increases Mitigation Detention Rates
HCFCD took note of the findings in Atlas 14 and made significant changes to their regulated stormwater detention rates. New developments must now provide a minimum of 0.65-acre-foot per acre of stormwater detention, a 20 percent increase from previous standards.
HCFCD and Harris County Floodplain Fill Mitigation Changes
Mitigation for fill within a regulatory floodplain, also known as zero net fill, is the compensation for any added fill with equal amounts of excavation. rather than mitigating to the 100-year flood plain, zero net fill will be required up to the 500-year floodplain, requiring future developments to adhere to a higher detention standard.
Harris County Infrastructure Changes
The release of Atlas 14 caused Harris County to implement new regulations for development within unincorporated areas of Harris County. Following HCFCD, the Harris County increased stormwater detention rates for developments outfalling into county storm sewer systems from 0.65 acre-foot per acre to 0.75 acre-foot per acre.
Takeaway from New Regulatory Changes
Changes stemming from the findings in Atlas 14 have the potential to significantly increase the cost of future developments. There are methods, however, that can reduce this impact.
One cost reduction option for new developments is the use of pier and beam foundation systems. This construction method requires less mitigation for detention and allows the structure to sit 18-36 inches off the ground, which can also further the structure’s risk of flood damage. An improved focus on master planning in the beginning stages of development and collaboration between developers and regulatory authorities can also prove beneficial in reducing development costs.
The Harris County and HCFCD regulations will continue to evolve. In fact, these new regulations will likely only be in effect until the current remapping efforts within Harris County are completed. It is currently anticipated that Harris County and HCFCD will have preliminary floodplain maps utilizing the new rainfall data in late 2021. Harris County will utilize these preliminary maps as best available data until the Federal Emergency Management Agency (FEMA) formally adopts them for use in the National Flood Insurance Program (NFIP).
Contact Jones|Carter to speak with an H&H engineer about how these updates could impact your project.