Letter: Minto West Could Increase Flooding

Editor’s note: The following letter about the Minto West project is addressed to County Commissioner Jess Santamaria. It was also sent to the other members of the Palm Beach County Commission, the Indian Trail Improvement District and Minto. A copy was sent to the Town-Crier for publication.

Dear Commissioner:

I am contacting you regarding Minto West with concerns that the development could increase flooding in Loxahatchee. My concern is the accuracy of the drainage runoff calculations for Minto West during the rainy season.

The Minto West site received between 15 and 20 inches of rainfall in 72 hours relating to Tropical Storm Isaac in 2012. For 3,200 acres with rainfall of 20 inches is 1.7397 billion gallons of water. That’s a lot of water to discharge. Any error in the runoff calculation due to soil saturation will be compounded by the size of the project.

My concern is that one of the key assumptions in drainage runoff calculations might be incorrect. I performed drainage runoff calculations as a project engineer in the mid 1990s for subdivisions in Broward County. A key assumption in rainfall drainage runoff calculations is that some of the rain will be absorbed into the soil or other surface area. During hurricane season, the soil in Loxahatchee often becomes saturated with rainwater, so very little of the new rainfall is absorbed into the soil during a major storm. As a result of the soil saturation, a substantial portion of rainfall becomes “runoff.”

For instance, a drainage runoff calculation may assume that only 20 percent of the rainfall on grassy area will become runoff because 80 percent is absorbed into the ground. However, if the soil is already saturated with rainfall because of the rainy season, the calculation will grossly under estimate the amount of runoff during a major storm. The excess rainfall runoff could cause flooding, as it did with Isaac.

I saw houses in the neighborhood flood after Isaac in August 2012. The road in front of my house was under several inches of water. Allowing Minto West to increase the amount of impervious area (roofs and paved areas) will increase runoff. When the soil is saturated during the rainy season, the runoff will be greater on pervious areas (lawns, etc.). The water must go somewhere; most likely it will flood the older houses in the neighborhood that were built at a lower elevation than the newer houses. Water seeks the lowest level and it has to go somewhere.

Water retention ponds are used to mitigate runoff and improve water quality. I understand Minto West plans include 400 acres of lakes (13 percent of the 3,200-acre project size). During the rainy season, water retention ponds can become full before a major storm, so the ponds do not have much excess capacity when a large rainfall event comes during the rainy season.

The major problem is getting rid of the potential floodwaters during the rainy season without increasing the risk of drought. It seems to be it is either a drought or a flood in South Florida. Use of the major canals during peak rainfall events can be a problem; during flooding from Isaac, Wellington received priority over Loxahatchee to discharge flood waters into the main drainage canal.

A possible solution to mitigate flood waters would be to require Minto to provide high-capacity deep wells injection to pump the excess rainwater into deep underground aquifers. These wells would automatically turn on and off based on the water level in the retention ponds, have backup power, etc. The deep injection wells would go into a different aquifer than the drinking water aquifer and deter salt intrusion into the drinking water aquifer. The deep injection wells might be costly to construct, maintain and operate, but Minto’s financial strength and the size of the project might make this feasible.

Thank you for your prompt attention to the matter.

Mark Young, The Acreage