Tailwater & Tidal Impacts on Flooding

Tailwater & Tidal Impacts on Flooding

In coastal and low-lying areas, tailwater conditions and tidal influences can significantly impact how stormwater drains, often leading to flooding even without direct rainfall. This occurs when the receiving waterbody, such as a river, pond, or ocean, is at a higher level than the stormwater drainage system, preventing stormwater from efficiently flowing out.

Stormwater systems are typically designed to use gravity to drain excess water from roads, yards, and developed areas. However, their efficiency depends on the difference in water levels—known as the head difference—between the storm drain system and the receiving waterbody.

• During low tide, the head difference is large, meaning water can flow freely from storm drains into nearby waterbodies. Stormwater moves quickly through outfall pipes, reducing standing water and minimizing flood risks.
• During high tide, the head difference is small or nonexistent, significantly slowing down drainage. In extreme cases, high water levels can cause backflow, where water from the receiving body enters the storm drain system rather than flowing out. This can result in flooding, even on sunny days without rain.

These tidal influences can create temporary closed basins, where stormwater has nowhere to drain. This leads to prolonged standing water in streets, yards, and sometimes even homes, increasing the risk of property damage and infrastructure strain. Sea level rise, extreme weather events, and king tides exacerbate this issue.