Berms and Swales

Digging the original swales at the
 VegHeadz garden in 2018

Preparation for the planting of a small blueberry patch is under way at the VegHeadz garden. In preparing a site to plant the blueberry bushes, the question of berms and swales came up. One of the Florida Friendly practices recommended by UF/IFAS is to use rain gardens, swales, and berms to catch and filter stormwater runoff.   When the garden was redesigned about five years ago, a series of berms and swales were created perpendicular to the slope of the garden to mitigate erosion occurring during heavy rains. There are a number of reasons to dig swales, which are just shallow ditches, and to back them with berms, which are just earthworks that are created from the soil removed from the swale area.The first reason, of course, is to control the flow of water on the surface of an area. A swale is cheap and easy, requiring only a shovel and your labor. The swale is dug along the contour line of the land so that the water is distributed evenly throughout the length of the swale and is not allowed to erode or accumulate at one spot. This slows the water down and allows it to percolate into the soil along the length of the swale for storage. Planting trees and shrubs along the swale is important to this process, because the trees help to absorb the water that is being stored, and the roots stabilize the berm and the area around the swale.Additional benefits of a berm and swale system are to treat water quality using soil, vegetation and microbes; reducing the total volume of stormwater runoff; increasing infiltration and groundwater recharge; and they can be a an attractive part of the landscape and improve biodiversity. This is a simple and low tech method to irrigate an area. As the water is absorbed into the soil, it is stored there until it is used by area vegetation. This also captures nutrients and organic matter that flow down the slope, and keeps these materials from flowing into area water bodies to increase algae blooms and create pollution.  The berm is a convenient place to put the dirt that is removed from the swale, it allows the swale to hold more water in the event of a heavy downpour, and it provides a place to plant things that needed a more arid growing environment. Over a period of time organic matter accumulates in the swale, and it may need to be dug again, either completely or partially. This is what brought up the subject in the VegHeadz garden. Once the blueberries are planted, it would not be a good idea to re-dig the swale, because blueberry roots could be damaged in doing so.  We are digging and cleaning out the swale now, before planting the blueberries. The soil that is dug from the swale is rich and full of nutrients which have accumulated there as water has percolated into the soil, and from organic matter that has decomposed there.  It can be added to the existing berm or deposited in other areas of the garden.The contour of the property is easily determined with the use of a simple A-frame level which can be built of long straight sticks, scrap lumber, bamboo, or purchased 1x2s.

All that is needed are two long pieces of wood or bamboo approximately the same length, and a third shorter piece.  Line up the bottom of the two long pieces, so they are flush and then secure them together at the top with a screw or twine or some other method. Measure the same distance up from the bottom on both legs and place a mark.  Secure the third piece of wood across the two legs at the mark on each leg, creating a crossbar.   

There are two options for finishing the A-frame level. One way is to secure a bubble, spirit, or carpenter’s level to the crossbar. The second is to suspend a weighted string or plumb bob from the top of the “A,” and locate the level center point of the crossbar.  These two methods are explained in detail here and here.

Once you have created your level, you are ready to find the contour of your property.  Place the A-frame upright on the ground where you want your swale and mark the foot of one leg with a flag. We’ll call that the first leg. Rotate the other leg around until the frame reads level either with the bubble in the carpenter’s level or with the weighted string at the center mark on the crossbar, depending on which method you have chosen. Place a flag at this point, the second leg. Keeping the second leg in place by the flag, now rotate the first leg around to find the next level point and mark that. Keep going as far in each direction as you like, marking each point, and connecting them with a smooth curve.   For multiple berms and swales down a slope, measure each location individually as the variations in elevation will differ from one level to the next.   This is your level contour where you will dig your swale. Dig the swale the depth and width that you prefer, keeping the bottom of the swale as level as possible. The swale should be closed at each end unless it is being used to direct water to another area. The final step is to run water into the swale to make sure it flows evenly along the entire length of the swale and to make any adjustments necessary if it does not.   

Photo from Santa Cruz Permaculture.com