How to Correct Drainage Problems in Your Landscape
Problems associated with poor drainage around the home can quickly lead to serious problems if corrective action is not taken. Aside from water entering the basement or foundation, poorly drained areas around the yard can destroy turf and expensive landscaping and become a harbor for disease carrying insects like mosquitoes. A basic understanding of the underlying cause of will allow you to redirect excess water to a municipal storm sewer, a drainage ditch, dry well or catch basin.
In established neighborhoods drainage problems are often a result of poor planning when homeowners add landscaping and trees that restrict the designed flow of water around your property. Repaving driveways or adding sidewalks is another common source or drainage issues. Other causes for poor drainage problems stem from existing soil conditions, settlement, or poor initial design of drainage features such as berms and swales.
Soils and Landscaping
Poor drainage is a common landscape problem and is often cited as the leading cause of plant death. While most people think that the slope or grade of the yard is the only determining factor, the space between soil particles, called pore space, controls soil drainage characteristics and in soils dominated by large pore space like sands and loamy sands, water moves rapidly. In poorly drained soils with small pore spaces whether they are compacted soils or soil with more than 20 percent clay allows water to move slowly causing root decline, root rots, and eventual death of shrubs and trees that are not wetland species.
Identifying Wet Areas
Typical symptoms associated with poor drainage include surface puddling or ponding, blue-gray soil colors, foul odors, soil mottles of grey, red, or orange spots within the top 24 inches of the soil surface, percolation rates less than 1 to 2 inches of water drop per hour, and impermeable crusts from rock formations like shale.
Left soil sample is normal upland soil; Right soil sample is wetland soil.
Gray anaerobic soil
Black layer of anaerobic soil in turf
Grading Near Foundations
To achieve proper drainage a typical slope to aim for when grading out from a foundation is about 5 percent, or approximately 6 inches for the first 10 feet. You may be able to get away with a lesser slope but you will need some other way of removing water from foundation such as a dry well, French drain, or swale.
Improving Soil Drainage
Poor surface drainage in many landscaped areas may be the result of excessive tilling, which most people think is the best way to control weeds and aerate the soil. Excessive tillage reduces drainage by destroying soil structure and pore spaces that help transport the necessary oxygen to a plant's roots. Correcting the problem involves reducing the number and frequency of tillage operations and incorporating a 1 to 2 inch layer of organic matter into the soil.
Soil amendments such as peat and compost aid in adding nutrients and improving soil structure temporarily as it decomposes with time and may cause settling. Usual recommendations warn not to add more than 20 percent by volume or 1 inch of peat per 5 inch root zone in any one growing season. Too much organic matter will lead to excessive settling, moisture and aeration problems.
Most organic matter will increase the water-holding capacity, so beware of the amount applied. Also beware of salt and ammonia problems if you use animal manures unless they are well composted or aged.
Adding inorganic amendments such as sand, expanded shale and calcined clay to poorly drained surface soils will increase soil aeration. If you use sand to improve aeration and drainage, it is crucial to know its particle size and its pH. Adding a small amount a high-pH limestone sand with fine particle sizes will damage or kill acid-loving plants. Silica sand would be a much better alternative to use than limestone sand because it will assume the pH of the soil it's mixed with.
Avoid using sand with lots of fine particles since the finer particles will occupy existing pore spaces producing a denser, less porous mixture. Not until sand constitutes 45 percent or more of the volume of soil will the soil begin to have some of the beneficial properties of sand. Hence, additions of 50 to 80 percent by volume are usually required to achieve any beneficial effects and that amount of sand can be very expensive for large areas.
Correcting Grading Problems
You may need to check local drainage requirements and ordinances to avoid potential lawsuits. If large scale modifications are necessary you will often need to apply for a permit and have a site plan approved by an engineer before you can proceed.
Visual observation of the landscape will usually indicate where surface drainage problems may occur. Many drainage problems exist simply because soil grading and leveling was inadequate. It is important to understand that drains are not a replacement of surface grading. Water can be removed from a site more rapidly by surface drainage.
How swales redirect water
For proper surface drainage on poorly drained clay soils, a minimum 2-percent slope is usually recommended, which requires a 2-foot vertical drop for every 100-foot horizontal distance or a 2 ½-inch drop over a 10-foot horizontal distance. By properly grading the site, surface water is directed to the lowest spot on the property, which will then empty into a drainage ditch, catch basin or dry well.
Turf or landscaped areas should have a minimum surface slope of 1 to 2 percent so water flows to a lower elevation away from the site. When adding or moving soil, always allow for about 15 to 20 percent settling on fine-textured soils and less for coarse soils. Surface grading, though sometimes costly and disruptive to an existing landscape, may be one of the best solutions for poor soil drainage problems and it may be the only practical and economical means of improving drainage for poorly drained or compacted clay soils.
For more open areas you can create a drainage swale by digging or grading a sloping ditch where surface water will be captured and flow to lower ground. An alternative would be to install French drains, which are trenches dug and filled with rock or gravel to the soil surface and crowned to prevent soil covering the drain. To be effective, French drains must also slope at least 1 to 3 percent and flow to a discharge outlet.
Correcting Isolated Wet Areas
Most isolated wet areas in yards are either low spots or have an underlying compacted soil. You can correct these areas by excavating the area and add material similar to the existing soil. This helps prevent water movement problems at the soil interface between two different soil types. This applies for raised beds or landscaping berms as you should not add finer-textured soils over coarser-textured soil or water pool on top of the denser soil and promote root stress, rot and plant death.
For small areas that are constantly wet you may consider installing plants that are tolerant to wet conditions such as native or wetland species. Trees such as river birch thrive in damp locations and will actually dry out the area as the trees absorb the moisture. Winterberry and Inkberry hollies, black chokeberry, pussy willows, willow trees, and red or yellow twig dogwood shrubs are other excellent choices for wet areas.
River Birch trees