Alternative Spillways – Rock and Concrete

A few weeks ago, my sketches about building spillways for small ponds in small watersheds focused on using our most locally common construction material, which here in Johnson County is silty dirt (commonly called “clay” when wet and sticky). But sometimes the most available and durable construction material is rocks, especially in the younger glaciated landscapes to the north of us.

Today’s setting is a gravelly rocky landscape where we are building a chain of little wetlands stair stepping down a valley that has been over-deepened, exposing the water table. Berms cross the valley and heavy rain could cause overflow water to cascade down the valley, potentially eroding each berm along the way. But we built a pair of spillways to channel the overflow across each berm. Because rocks are abundant here, I opted to build the spillways from large rocks.

Building a stone spillway

The shape of these spillways is a chute with a stone floor tilted down across the berm. It has raised sides built only one additional stone high, which allows the side stones to be embedded in the sidewalls to help keep them anchored.

Our inspector is evaluating the little plants starting to grow in the cracks. Grasses and sedges are desirable, seedling trees are removed.

The floor stones of the spillways were closely fitted together, leaving only cracks between them. Sandy dirt was worked into the cracks with a stiff broom, producing a surface resembling an old-fashioned cobblestone road.

The reason I mention this type of spillway design, is that around here the aggressive pace of new construction is matched by an aggressive pace of demolition of the old. Broken concrete pieces from former sidewalks, parking lots, driveways, motel floors, etc., are hauled away for disposal or crushing into subgrade stone. These are potentially available for lining a little spillway with flat concrete pieces. Some of this older concrete contains no rebar, making it especially easy to recycle flat pieces.

Former motel in Coralville, now stripped down to its concrete slab.

Our Johnson County limestones occasionally weather to flat slabs, which can be used to line a spillway, overlapping like shingles on a roof. But the modern blasting process deliberately makes small and more equidimensional chunks so it takes some searching to find slabs in a local quarry.