Little Wekiva River Flooding Letter from City Manager

Over the past couple of years, many residents have believed that a retention pond was the solution to the flooding. This is likely because few people understand the complexities of hydrology. The flooding occurs due to three key factors: the amount of rainfall we receive, the fact that 98% of the Little Wekiva River is located north of us and needs to drain before the 2% in our area can, and because the river is a flowing system, not a lake, meaning there is inflow from the south as well.

These are the systemic reasons, but there are also practical reasons why a retention pond is not a viable solution. Spring Oaks lies in a floodplain, which is a natural low point that has flooded for millions of years. The area is also very close to the water table, so digging down would only lead to more waterlogged soil. There simply isn’t any dry ground for percolation within a floodplain.

Now, let's hypothetically explore the idea of collecting or diverting the water. At its peak, the Little Wekiva River flowed at a rate of 784 cubic feet per second. This amounts to 67.7 million cubic feet of water per day, or 506.7 million gallons, weighing 4.23 billion pounds. To hold that amount of water, a retention pond would need to cover the area of 1,176 football fields, each with 12 inches of water—or cover about 30.5% of Altamonte Springs with 12 inches of water. And remember, this is just one day of peak flow.

The weight of that one day’s water is staggering—around 4.23 billion pounds. Any structure meant to hold that water would need to be massive and incredibly strong. For some context, a 10-story building with a floor space of 250,000 square feet weighs about 20 million pounds. Since digging into a floodplain is not an option, an above-ground tank would be necessary. To hold just one day's worth of water, you’d need a tank the size of a football field but standing 1,176 feet tall—about 116 stories. Imagine such a structure at Merrill Park and consider the enormous cost involved.

To break it down further, the walls of this tank would need to withstand a pressure of 2,812,224 pounds per square foot. At a cost of $150 per cubic yard of concrete, constructing such a tank would require about 6.8 million cubic yards of concrete, which would cost roughly $1.024 billion—and that’s just for the materials to hold one day’s flow. Furthermore, we haven't even discussed the size and power of the pumps needed to lift water 1,176 feet into the air, nor the energy required to keep up with the flow. And, of course, there remains the critical question: What would we do with the water on the second day of rain?

If it feels like we’ve discussed this before, it's because we have. We’ve done the math in order to respond to these questions. There are some federal programs that residents could explore. While may received individual assistance, no one applied for federal mitigation funding for their homes in 2022, when Hurricane Ian flooded the areas.

We understand that some residents—on Facebook and elsewhere—have criticized the City for not building walls or retention ponds. We empathize with their frustrations, especially those whose homes have been flooded during Hurricane Milton and previous storms. However, this part of Spring Oaks is located in a floodplain, where flooding is a natural occurrence. Most of the homes were built over 50 years ago, long before FEMA regulations existed. Since the late 1970s, when FEMA established floodplains, homes in these areas have been required to carry flood insurance for mortgages.

Hurricanes are natural disasters, and the power they bring is beyond what we can engineer against on a citywide scale. Our annual budget is $136 million. Building a tank with just one day’s worth of capacity would cost ten times that amount, not including the cost of pumps, additional land, and liability insurance.

The reality is that the Little Wekiva River will continue to flood during hurricanes and major rain events. However, the good news is that the flooding is receding.

I appreciate your question, and I hope this information helps explain why a retention pond is not a feasible solution.

Best regards,

Frank