100 Year vs 500 Year Storm Event Design Standard

Tim Buscha, Chair WHA Flood Control and Drainage Committee

It is anticipated that the MAAPNext preliminary data and FIRM maps will be issued by FEMA in early 2024. Following that release there will be a formal response/comment window on that data and maps. This will be followed by a resolution period and ultimately a Letter of Final Determination will be issued dating the new FIRM maps. 

Upon release of the MAAPNext data Harris County, the City of Houston, and other regulatory entities will be updating drainage criteria and floodplain ordinances. As most of the smaller cities and entities will follow the lead of Harris County and the City of Houston, we believe it is paramount to maintain the 100-year (1% percent recurrence interval) storm event as the design standard for drainage. 

The choice of flood standard depends on various factors, including the level of acceptable risk, the consequences of flooding in a particular area, and the available resources for implementing flood mitigation measures. Designing infrastructure or flood mitigation measures to an overly high standard can be significantly more expensive and this financial burden can pose challenges, for example making housing less affordable. Moreover, we should not solely rely on a given standard for our flood protection; flood resiliency requires preparedness, risk management planning and education. The current 100-year floodplain data comes from the most recent county-wide floodplain update, published in 2007, but updated in some watersheds as recently as 2019.  

In 2018, two important milestones occurred that will impact our ability to better understand flood risk into the future. NOAA published Atlas 14 (volume 11) updating rainfall frequency data for the State of Texas, using more recent rainfall data (including the 2017 Harvey storm). And HCFCD, in cooperation with FEMA initiated MAAPNext, a county-wide floodplain mapping restudy, using current modeling technology and data analysis that have improved over the years, becoming increasingly accurate. The anticipated MAAPNext data is the next generation of information. 

In 2019 local regulations were modified to reflect the impacts of the change in rainfall defined by Atlas 14.  At that time entities adopted the Atlas 14 100-year rainfall as the minimum drainage criteria and floodplain ordinances adopted the 500-year floodplain models and maps as a proxy for the interim period until the MAAPNext data became official and the floodplain maps, flood insurance study and modeling were adopted by FEMA.  The local drainage standards, drainage criteria, and floodplain ordinances in place in Harris County since 2009 has resulted in very limited structural flooding of homes built since 2009, therefore maintaining the same standard (100-year) criteria would seem appropriate once the MAAPNext data is released. 

Designing to a standard higher than the 100-year, would require limiting construction or adopting specific building codes and regulations that would impact existing homeowners. Additionally, this could have unintended negative repercussions on urban planning, property development, and economic activities.  

As such we are of opinion that the WHA formal position and advocacy should be that upon issuance of the new Flood Insurance Rate Maps and Flood Insurance Study by FEMA, that local regulatory entities reaffirm and maintain the 100-year, 24-hour storm as the design standard for drainage related policies, ordinances, and criteria for residential and commercial development and for establishment of the finished floor elevation of habitable structures. We encourage you to support this position in your discussion with County and City leaders. Please reach out to the WHA Flood Control and Drainage Committee for specific details on this issue or if you have a question.