July’s Record Rainfall and Stormwater Policy

Last month was the rainiest July on record in Lakeland since records began around 1910, according to the National Weather Service. The gauge at the airport recorded 20.07 inches

The previous record—15.67 inches– was from 1960, which was considered the last “wet year” by those who contend declines in river and spring flow in recent years are the result of changing rainfall patterns caused by something called the Atlantic Multidecadal Oscillation.

For the record, the rainfall totals elsewhere in the area were less impressive. That is reflected in Tuesday’s flow of the Peace River at Bartow, which was 148 cubic feet per second. On the same date in 1960, the flow was 2,370 cfs, according to the U.S. Geologic Survey.

Following the month’s soggy finale, courtesy of Tropical Storm Emily, The Ledger reported flooding in a mobile home park in an older section of town west of downtown Lakeland.

Unsurprisingly, city officials said the flooding was likely caused by stormwater runoff downstream from new development that prevented the floodwater flowing through a ditch in the mobile home park from draining as quickly as it once did.

This problem has been building for years because of current and past development policies that required less stormwater storage than some thought was prudent.

In unincorporated Polk County, any development built before 1992, during a period when Polk officials were still avoiding the responsibility of updating their growth plan and development regulations as required by state law, the standard was that development need only be designed to handle a storm that had a 33 percent chance of happening in any year.

Newer developments have to design for storms that have 4 percent chance of occurring in any year unless they are in a closed basin—a place where stormwater flows in and stays until it evaporates or percolates—the design standard is a storm that has a 1 percent chance of happening, the so-called 100-year storm.

The general requirement in unincorporated Polk County and Lakeland is that land where a new development is located is not supposed to generate any more stormwater runoff after it is developed than it generated before it was developed.

Judging by the comments in the article, that isn’t exactly what’s happening.

There have been other problems caused by development downstream from Lakeland’s airport in the suburbs between Drane Field Road and State Road 60.

What is occurring, from what engineers say, is that the increased development has caused both the volume and velocity of the stormwater runoff to increase, sometimes causing scouring of canals and stream channels, potentially affecting bridges and occasionally causing flash floods in homes. Very little of this land is particularly well-drained in its natural state.

There has been some discussion in recent years about the effects of climate change on rainfall patterns, which could shift some of the discussion away from the threats of sea level rise in coastal areas and onto the effect on the levels of freshwater bodies in inland areas.

This week’s flood could be a quirky event or a warning of what we might come to expect in a few decades. This should be part of the discussion of design standards for new development in Florida in 21st century.

Posted in Group Conservation Issues.