Getting water off a sloping Piedmont lot

Why the water is there

It is arithmetic rather than a soil defect. The USDA series description for Cecil, the reference soil across the Georgia Piedmont, calls the soil well drained with moderately high saturated conductivity and records surface runoff as medium. The Metro North Georgia Water Planning District puts the working figure for clay absorption at about half an inch of water per hour. Atlanta summer storms are convective and short, and they routinely deliver more than that in fifteen or twenty minutes.

So the water is not sitting because the soil is bad. It is sitting because it arrived faster than any soil could take it, and then had nowhere to go. On a lot that falls from a street at the top to a creek at the bottom, that surplus runs the whole length of the property, gathering volume from the driveway, the roof, the neighbouring lot and every compacted area on the way down.

Telling the two problems apart

The test is time, not depth. Water that stands after a storm and is gone within a day is surface runoff that had nowhere to go. That wants grading and conveyance: a swale, a regrade, a collection point, a pipe with capacity. Ground that stays soft for days after the rain has stopped is saturated, and that wants subsurface interception: a properly built French drain, or a change to what is planted there.

A great many Sandy Springs properties have both, in different places, and the mistake is to buy a French drain for a problem that is entirely on the surface. A four-inch perforated pipe moves a modest flow. It cannot absorb the runoff from a driveway during an Atlanta thunderstorm, and installing it in that role guarantees an unhappy homeowner.

What each fix does

• Regrading. The cheapest and most under-used option. Changing the shape of the ground so water leaves instead of collecting fixes more yards than any pipe does.
• Swales and dry creek beds. A shallow shaped channel across or along the slope, vegetated or stone-lined, that carries large volumes slowly. On a wooded lot a dry creek bed does the hydraulic work and reads as part of the garden the rest of the year.
• Catch basins and solid pipe. Where volume is genuinely high, a grated inlet at the low point and a solid pipe to a discharge point. Clean-outs at direction changes mean the run can be jetted rather than dug up.
• French drains. A graded trench, washed stone, a perforated pipe, and a filter fabric wrap so clay fines do not migrate into the aggregate. Without the wrap it is a slow-motion failure.
• Downspout collection. Roof water is the largest single input on most lots and the easiest to move. Taking it well away from the foundation in solid pipe is cheap and disproportionately effective.
• Slope stabilisation. Matting, deep-rooted planting and coarse mulch on any bare grade. Bare Piedmont clay sheds soil in the first storm after it is exposed.

Where the water is allowed to go

This is the part of a Sandy Springs drainage job that has no equivalent in most metros. Designated streams and water bodies here carry a 75-foot stream buffer, with the inner 50 feet undisturbed and the outer 25 feet an impervious setback where limited vegetation removal and grading may be allowed, and construction inside those areas generally requires an approved variance. Properties within 2,000 feet of the Chattahoochee sit inside the state River Corridor as well, with a 50-foot undisturbed buffer from the bank and a 150-foot impervious surface setback.

The city publishes a Community Development Map with a stream buffer layer that shows whether either applies to a given address. Checking it is step one, not step five. There is more detail in the buffers and corridor guide.

Permits

Sandy Springs generally requires a land disturbance permit once 1,000 square feet or more of ground will be disturbed, and states directly that a permit may still be needed below that where the work requires erosion control measures or changes topography or stormwater drainage patterns. A drainage project does both by definition. Residential fees are a $25 administrative fee, a $50 certificate fee, a $100 review fee and $200 per lot, plus a technology fee. Work must comply with the Georgia Stormwater Manual and the Manual for Erosion and Sedimentation Control in Georgia, a pre-construction meeting with a site inspector happens before any disturbance, and the final inspection comes only after landscape stabilisation.

Separately, Georgia's Erosion and Sedimentation Act establishes a 25-foot buffer along the banks of state waters measured from the point where vegetation has been wrested by normal stream flow, and 50 feet along waters classified as trout streams. Single-family residential work under an acre is exempt from the state land-disturbing activity permit, but the buffer requirements still apply.

What not to do

• Do not send it next door. A regrade that solves your low corner by directing flow onto a neighbour is exactly what the city's review of grading and drainage plans exists to catch.
• Do not pipe into a buffer without checking. Discharging into a protected stream corridor is a variance conversation at best.
• Do not fill a stone trench without fabric. Clay fines will find it.
• Do not build over the flow path. A patio, a wall or a new bed placed across an uncontrolled drainage route is a repair with a date on it.

Where the grade itself is doing the damage, the answer is usually structural rather than hydraulic. See the wall types guide, and what a drainage scope covers.

Sources

USDA NRCS Official Series Description for the Cecil series. City of Sandy Springs Land Disturbance Permits, Natural Resource Protection and River Corridor pages. Georgia EPD buffer guidance under the Erosion and Sedimentation Act, O.C.G.A. 12-7. Metro North Georgia Water Planning District irrigation guidance.