Rainwater Catchment and Swale Design for Properties of Five Acres or More

The one number to remember: 0.623

One inch of rain falling on one square foot of surface is 0.623 gallons of water. Everything else in rainwater design follows from that. Multiply the collecting area in square feet by the rainfall in inches, then by 0.623, and you have the gallons that landed. Most extension services then apply an efficiency factor of about 0.9 for a hard roof, to allow for splash, evaporation and the first flush you divert.

The DC area and Northern Virginia get a little over 40 inches of rain a year, spread fairly evenly across the months. So a 3,000 square foot barn roof collects roughly 3,000 times 40 times 0.623 times 0.9, which is about 67,000 gallons a year. On the farmette we planned around about 30,000 gallons a year from the outbuilding roofs alone. A single house roof can supply more water than most gardens will ever use.

Size storage to use, not to rainfall

The most common catchment mistake on a large property is sizing a tank to hold everything the roof can collect. Because rain here arrives through the whole year, you rarely need to store months of it. You need enough to bridge the dry stretches between storms in the months you are irrigating, which in Virginia is usually July through September.

Demand is easy to estimate with the same number. Watering one inch a week works out to 0.623 gallons per square foot per week. A 2,000 square foot production garden that needs half an inch a week beyond what rain provides, over a twenty week season, uses about 12,500 gallons. A few thousand gallons of storage, refilled by summer storms, covers most of that. The rest of what the roofs collect is better sent to the landscape, which is where swales come in.

What a swale actually does

A swale is a shallow trench dug exactly on contour, with the spoil mounded on its downhill side into a berm. Because it is level along its length, water that runs into it stops moving and soaks in. It is not a ditch. A ditch moves water off a property; a swale parks it in the soil, where it recharges the ground beneath the trees planted on and below the berm.

On five acres or more, swales are how you turn the rain that falls on the land itself into irrigation. One inch on one acre is a little over 27,000 gallons, and on sloped pasture or lawn much of a heavy storm runs straight off. A series of swales across the slope catches that sheet flow in stages, and within a few seasons you can see the difference in the growth along each berm.

Sizing a swale without overdoing it

The capacity of a swale is its cross section times its length. A swale three feet across and one foot deep, with the usual rounded profile, holds about two square feet of water in section. A hundred feet of it holds around 200 cubic feet, or roughly 1,500 gallons, every time it fills.

The design questions are how much water arrives from the ground uphill in the storm you are designing for, how fast your soil drains, and where the overflow goes when a larger storm exceeds it. Every swale needs a level, stable spillway that returns excess water to the land gently, ideally into the next swale down. Clay soils need smaller swales spaced more closely. Sandy soils can take larger ones. Never build a swale above a septic field, above a foundation, or on steep, unstable slopes, and get an engineer involved on anything over a modest grade.

The permit lines in Virginia

Earthwork on a larger property can cross a regulatory threshold faster than people expect. Under Virginia's erosion and sediment control rules, disturbing 10,000 square feet or more generally requires an approved plan, and inside a Chesapeake Bay Preservation Area the line drops to 2,500 square feet. Localities can set their thresholds lower still. In Fairfax County every parcel lies in either a Resource Protection Area or a Resource Management Area, which together make up the county's Bay preservation areas, so the lower number is the one to plan around.

Resource Protection Areas, the buffers along streams and wetlands, carry extra restrictions: generally no land disturbance or vegetation removal without county approval. A swale system, a pond or a large tank pad can easily exceed 2,500 square feet of disturbance. None of this makes the work impossible. It means the permit belongs in the schedule from the start, not after a neighbor calls the county.

Put the catchment where gravity helps

The layout principle I rely on most is to collect water high and use it low. A tank placed uphill of the gardens delivers water without a pump, since every 2.3 feet of height gives about one pound per square inch of pressure. Roof water from a barn at the top of the slope can feed swales below it, which feed the orchard below them, which drains toward a rain garden or pond at the bottom.

That sequence also qualifies for help. Cisterns, rain gardens and conservation landscaping are all practices the Virginia Conservation Assistance Program reimburses, and in Fairfax County that means up to 75 percent of eligible costs if the district approves the design before work begins. A water plan drawn with those standards in mind costs no more to build and can come in well under budget.