Surface Water Drainage and Slope
You create surface drainage by grading an area so that water collects and flows to a lower elevation away from the site. Regardless of surface characteristics, when it comes to surface drainage, slope is the most important issue to consider. For efficient drainage, paved surfaces should have a minimum 1-percent slope. Turf or landscaped areas should have a minimum slope of 2 percent.
- "Shoot" the grade. Because slope is so important for drainage, a transit or level is good to have on hand to "shoot" grades. It is also the best way to find the absolute lowest spot in a potential drainage area. Exceeding the 2-percent standard by too much can cause erosion problems. Slopes of more than 4 or 5 percent will seem very steep in most landscape situations.
- Calculate the slope. Each foot of elevation drop over a 100-foot length is 1 percent. Therefore, it takes 2 feet of elevation change over each 100-foot length of a swale to create a 2-percent slope. If the distance is 10 feet, you'll need a fall of 0.2 feet (roughly 2.5 inches) to create a 2-percent slope.
- Grade the area. Areas that will be paved, such as driveways or parking lots, are easy to surface drain by simply grading them so they slope away from structures and toward a lawn, storm sewer or street gutter. In open turf areas, you can create a swale by making a downward-sloping "crease" in the landscape where water will collect and flow to lower ground. If water flows to an area where it cannot completely exit the site, you can install a catch basin and pipe so the water will drain to its ultimate destination.
- Control erosion. Immediately after you create a swale, you should install stone or sod, sow seed or design other means of stabilizing the area. (See related article, "Temporary grasses stabilize soil".)