June 20, 2026 · HVA Resilience Committee
Three Kinds of Flooding, Three Lines of Defense
Storm surge, sunny-day tides, and rainfall are three different flooding problems with three different fixes — and the space between the knee wall and the Battery Extension becomes stormwater storage. Here's how they work together.
How the Battery Extension, the knee wall, and the City’s drainage system fit together along Lockwood Drive
It’s easy to lump Charleston’s flood projects together, but they solve different problems. Harleston Village sits next to a stretch of the Peninsula that floods in three distinct ways — and each one calls for a different fix. Here’s how the pieces relate.
The three kinds of flooding
| Type of flooding | What causes it | Primary defense |
|---|---|---|
| Storm surge | Hurricanes pushing a wall of ocean water onto the Peninsula | The Battery Extension (10–12 ft perimeter wall) |
| Tidal / “sunny-day” | King tides that flood the road with no storm at all | The knee wall now; the Battery Extension later |
| Rainfall / stormwater | Heavy rain that can’t drain out to the river when high tides block the outfalls | Drainage, pumps & polders — the storage basin the knee wall (near term) and Battery Extension (long term) create, plus the Spring-Fishburne tunnel and Colonial & Alberta Sottile Long lakes |
The key insight: a wall keeps outside water from coming in — but on its own it does nothing for the rain that falls inside it, and that rain can’t drain out to the river when the tide is high. Build only a barrier and you’d turn the Peninsula into a bathtub. The fix is to make the wall do double duty: the space behind it becomes a storage basin that holds the rain until the tide drops and pumps can clear it. That’s why the City keeps stressing that the projects must be integrated — barrier, drainage, and storage working together.
How the pieces fit together
Picture a slice through the waterfront, moving from our neighborhood out to the Ashley River. The knee wall sits inboard, right along Lockwood Drive, stopping the everyday king-tide flooding that closes the road. The Battery Extension sits outboard, near where the marsh meets the river, tall enough to hold back a hurricane storm surge.
The space between becomes storage
This is the part that ties it all together. If the Battery Extension is built along the outer marsh-and-river line as envisioned, the land between it and the knee wall — much of it currently marsh — becomes a basin that can hold water: a “polder.” Rain that falls inside, and tidal water that needs somewhere to go, collects there until the tide drops and pumps can move it out. It’s the same job Colonial Lake and Alberta Sottile Long Lake already do today.
That storage role is why the knee wall isn’t just a temporary stopgap. City officials have noted that without it, the larger Battery Extension would eventually make the knee wall redundant — but as the inner edge of a polder, the knee wall keeps doing useful work even after the big wall is built. The near-term fix becomes a permanent part of the system.
In short
- Storm surge → Battery Extension
- Tidal / sunny-day flooding → knee wall now, Battery Extension later
- Rainfall → drainage, pumps, and the storage basin the two walls create between them
Sources: City of Charleston, Battery Extension Project Implementation Strategy (2026); The Post and Courier Editorial Board (Nov. 2024).