Every Stream in This City Ends Up in the Chattahoochee

If there is a creek at the back of the property, it is not scenery. It is the destination for everything that leaves the yard, and that makes it the first thing a drainage design has to account for.

The named watersheds

The City of Sandy Springs publishes watershed improvement studies for its major streams, and the names appear on its own stormwater page: Nancy Creek, with an executive summary, chapters and appendices plus a consolidated watershed-based plan; Long Island Creek, with the same set of documents; and Marsh Creek, which also gives its name to the Marsh Creek Rain Garden, a city park where a rain garden functions as a natural stormwater treatment system filtering runoff through plant life and engineered landscape features.

Alongside those the city publishes an Impaired Waters Phase II Plan and a fecal coliform study. This is a city that has looked hard at its own water quality.

Nancy Creek is the most studied of them. The US Geological Survey maintains gauging on Nancy Creek through this part of the metro area, which is unusual for a suburban stream and reflects how much of the built-up north side drains into it.

The city’s own framing is the useful one to hold: all city streams are tributaries to the Chattahoochee River. Whatever leaves a yard here ends up in the river, by way of a creek that is probably at the bottom of somebody’s garden.

What a suburban creek does in a storm

It rises fast and it falls fast. A creek draining a built-up catchment gets its water in minutes rather than hours, because roofs, drives and roads deliver almost everything that lands on them straight into the channel. The visible consequence on a private lot is the bank: water at that speed cuts into the outside of every bend and works at any point where flow arrives concentrated rather than spread.

That is why the last stretch of a drainage design matters more than the middle of it. A pipe that ends in mid-air over a soft bank undercuts it, and the ground lost that way does not come back. Discharging into washed stone, onto a stabilised slope planted through matting, or into a shaped dry creek bed all give the water somewhere to slow down before it reaches the channel.

Knowing what your own lot does

Two things are worth establishing before a design starts. The first is where water crosses the property, which is best learnt by walking it during a storm rather than reasoning about it afterwards. Fifteen minutes in the rain settles arguments that a plan cannot.

The second is whether the low ground floods. The Georgia Department of Natural Resources publishes an interactive flood map searchable by address, and it is worth a look on any lot near a creek. Ground that takes water in a serious storm is not ground to put a patio or a shed on, and it is very good ground for planting.

What it changes about a landscape

Hard surfaces move uphill. Every square foot of paving near a creek adds runoff to the part of the property least able to take it. The flat ground by the water is usually the tempting place to build and the worst one.

Discharge points need a destination that can take the volume. The most common late surprise on a drainage job is finding that the intended outfall has nowhere to release without cutting something.

Planting does the structural work. Bank ground is not dead space. It is where a woodland or riparian planting earns its place, because deep-rooted species hold soil against moving water in a way that turf and mulch never will.

Storm flow gets slowed, not just moved. A dry creek bed, a broad swale and a stone-lined outfall all buy time, and time is what stops a bank being cut.

Why the creeks get this much attention

Because the ground sheds water fast. The Cecil-type soils across the Georgia Piedmont are recorded as well drained with medium surface runoff, and the Metro North Georgia Water Planning District puts practical clay absorption at around half an inch of water per hour. NOAA’s 1991 to 2020 normals for the Atlanta area record just under 50 inches of precipitation a year across about 113 days with measurable rain, arriving in short convective storms.

Every square foot of new hard surface in a watershed like that adds to what the creek receives in the first fifteen minutes of a storm. Keeping the last hundred feet before the water arrives planted and rough is the cheapest way a single property can take some of that back out.

More detail in the drainage guide and on holding a slope in the red clay guide.

Sources: City of Sandy Springs Stormwater and Streams and Creeks pages. Georgia Department of Natural Resources flood map. USDA NRCS Official Series Description for the Cecil series. NOAA 1991 to 2020 climate normals for Atlanta Hartsfield-Jackson.