Local View:
Here’s why cities flood so much.
The flooding in Houston from Hurricane Harvey makes us wonder why the flooding is so bad in urban Texas. Experts say it is due to record amounts of rainfall, but also from so many impervious surfaces like pavement, concrete and rooftops that do not allow water to soak into the ground. As a veteran of several groundwater and surface-water state task forces and administrative hearings in Florida, I think we should understand why our cities flood so much.
I am not familiar with Texas regulations, but Florida has been nationally known as a model for land-use regulations since the 1980s. Florida’s protection of rural development and antisprawl regulations were some of the best in the country until the Great Recession, when many of the regulations were eliminated to encourage growth and stimulate the economy. Most of the Orlando area’s development and storm water is managed by the St. Johns River Water Management District and its governing board.
Basically, unimproved land soaks up some rainfall and stores it underground, while the surplus flows into wetlands, which hold the runoff and slowly release it into lakes, rivers and streams; this minimizes flooding.
As we develop property, we destroy wetlands while adding more impervious surfaces and creating more runoff. In general, rural areas have the least runoff; residential areas have more; and commercial, industrial and urban areas have the most. The more dense the development, the larger the quantity and the more polluted the runoff from intense rainstorms like tropical storms and hurricanes.
As new development occurs, each project is required to build stormwater ponds to hold rainfall runoff on-site. These ponds and culverts are sized to prevent more water leaving the site than before development in order to protect close neighbors from flooding. However, these ponds are designed to manage runoff from rainfalls that occur only once every 25 years over three days, or about the first 11 inches of rainfall for Orlando.
Because hurricanes frequently drop 10 inches in a day, the excess rainfall simply overflows and floods low-lying areas. Harvey dropped 50 inches in some areas.
Higher temperatures cause stronger storms and more rainfall as our climate changes, necessitating the recalculation of the 25-year storm in 2018. The old number is an average of about 50 years of data before 1996. The Earth’s surface temperature has increased about 2 degrees Fahrenheit since the late 19th century, and most of that warming occurred in the past 35 years. In fact, 16 of the 17 warmest years on record have occurred since 2001. The recent rainfall data need to be incorporated into designing stormwater ponds for future development.
State rules ignoring excess floodwaters places the uninsured taxpayer at huge risk. Add in frequent development of the 100-year flood plain, and you have a recipe for disaster. In Houston, for example, experts estimate only 15 percent to 20 percent of homeowners have flood insurance, and about half the residential and commercial properties outside of flood zones are at high risk of flooding. Even if covered, the Federal Emergency Management Agency doesn’t cover all the actual flood damage.
Climate-change discussions focus on coastal flooding but should also consider urban flooding from strong storms like hurricanes Harvey (2017), Katrina (2005) and Charlie (2004). Unless regulatory agencies consider plans for future catastrophic flood conveyance systems as our cities grow, this type of flooding will worsen in low-lying areas like Florida and end up costing taxpayers more in damages than if they were built right the first time.