Properties within a Closed Basin

Properties within a Closed Basin

A closed basin is an area where water has no natural outflow, meaning that any rainfall or stormwater that collects within it must either evaporate, be absorbed into the ground, or be artificially pumped out. Since there is no direct drainage connection to a river, lake, or ocean, water in a closed basin can accumulate quickly during heavy rainfall, leading to localized flooding. When the ground becomes saturated and can no longer absorb additional water, flooding can persist for an extended period, particularly if the basin is located in a low-lying area. Properties within closed basins are more vulnerable to flooding because water has nowhere to drain naturally, making them highly dependent on efficient stormwater management systems to control excess water.

Image 1: Normal Conditions

Under normal conditions, the closed basin appears as a low-lying area with a pond or depression. The elevation contours surrounding it indicate that water naturally collects in this location. While small amounts of rainfall can typically percolate into the soil or evaporate, the basin remains stable during dry weather.

Image 2: Moderate Rainfall Event (Hurricane Irma – 6 inches in 24 hours)

With six inches of rainfall over 24 hours, stormwater begins to accumulate in the basin. Because there is no natural outflow, water levels rise, and temporary flooding occurs. While this amount of rainfall may not immediately impact homes and roads, it demonstrates how even moderate rainfall events can overwhelm closed basins, leading to standing water that takes time to dissipate.

Extreme Rainfall Event (Hurricane Milton – 18 inches in 24 hours)

In an extreme weather event, such as Hurricane Milton, where 18 inches of rain falls within 24 hours, the closed basin reaches critical flooding levels. The floodwater spreads beyond the natural pond area, inundating streets, parking lots, and homes. Since the ground is already saturated and cannot absorb additional water, flooding persists for an extended period. The adjacent aerial image shows the real-life consequences of such an event, where homes and businesses become submerged.