What to Do When It’s Wet

Drainage and Erosion Options to Help Keep Work Moving

By Jennifer Howard

When doing a site walkthrough, you may cringe when you see pooling water on bare clay soil, or dry channels carved into steep slopes. Unfortunately, drainage problems can speed or slow rainwater movement, thereby dampening your work schedule. So, how can you keep clients happy and crews busy when a site is wet? Recently, horticulture experts from North Carolina State University shared some tips with us to help keep the water — and work — moving.

Dual Drainage Culprits

Most site-drainage problems in North Carolina are primarily a result of two colliding conditions: increasing weather intensity and rapidly changing land use.

“There has been a real change in the intensity and frequency of storm events, and the concentration and intensity of rainfall are no longer compatible with our native soil conditions,” said Julietta Sherk, NC State professor of landscape design. “As a result, although many past landscape designs may have been sound solutions in their time, they don’t necessarily work well in today’s climate conditions.”

“Especially in the piedmont, the state’s rolling topography makes our clay soils highly erodible,” said Lee Ivy, NC State senior lecturer in horticulture. “They just can’t absorb the increasing amounts of sudden rainfall we have seen in the past 20-30 years. Clay soils can only absorb and percolate 0.2-0.3 inches of rainfall per hour, but recently we’ve been seeing 0.5-3 inches per hour. They just can’t handle that.” In addition to the increased weather intensity, the onslaught of building development amplifies our water-infiltration problems. Quick-turn building projects often clear-cut sites, stripping away any water-absorbing soil organic matter and leaving acres of bare subsoil. And the ground that isn’t scraped bare — such as driveways, parking lots and buildings — is often paved or covered with impermeable surfaces, which are only capable of shedding volumes of water elsewhere. Together, those conditions create a perfect Photo courtesy of Denise Krebs on Flickr storm for localized soggy sites.

Sleuthing Out the Source

If you are tasked with remedying such a problem, experts suggest starting by finding the source. “The best way to identify drainage problems is to be on-site when it’s pouring,” Ivy said. “That’s when you can really understand how and where water moves.” Site scouting is also a productive use of time during weather conditions that limit many other activities. If visiting on a stormy day is not an option, however, you’ll need to do some detective work.

Barring an irrigation or water line leak, drainage problems almost always result from two sources (and sometimes both): impaired water infiltration and/or poor water conveyance. (Since water lines are generally the easiest fix, try to rule out those issues first.)

Water infiltration is the soil’s ability to move water downward through the soil profile — the initial capture and then slow release of rainfall into the ground’s natural aquifer system. “It’s ideal to handle water where it falls,” Sherk said. “You don’t want it to run off and have to deal with it again elsewhere.”

Compacted clay soils are notorious for slow water infiltration. If you suspect soil compaction, which is common in North Carolina, Ivy suggests taking a soil probe or shovel to dig down. “See if the wet and dry areas are where you expect,” he said. If water is landing but not being absorbed, compaction is a likely problem.

Water conveyance, on the other hand, directs water from its landing point to another area that is better suited to handle it. Gutter downspouts, for example, are building conveyances that can lead to drainage problems if runoff is not properly directed or dispersed. In the landscape, water conveyance is impacted by basic grading and natural topography features like swales and berms, or manmade features designed to move water to an alternate location.

Identifying the problems with your site’s water infiltration and/or conveyance will usually reveal your solution set. Armed with that information, the most comprehensive, lasting solutions are then usually rooted in a carefully considered design. However, when clients, timelines and budgets don’t allow for sweeping design changes, you can still prioritize solutions to achieve the best possible outcome.

Finding a Fix Through Triage

Once you’ve identified a site’s drainage problem source(s), you can triage by asking some general questions. For instance: • Is this an acute or chronic problem? (Ivy notes that acute or immediate problems are usually the warning bells of a larger, chronic problem.) • Is the drainage problem a minor thorn delaying a different task, or have you been called in to battle a known, recurring drainage problem? Regardless of the answers to those questions, drainage improvements likely will come down to three options: will you collect, direct or just accept the water volume?

Acute Issues (aka “I need to fix this to finish the job this week”)

If you find yourself facing an unworkable, wet site, short-term options are mostly install-related, as opposed to design-oriented. Although work shouldn’t be done while the soil is wet, improvements can be planned to promptly enhance infiltration and conveyance.

Infiltration (collect)

Adding organic matter or other soil amendments to help absorb and retain water can fix minor drainage problems, especially in turf applications. Core aerating and dethatching also can improve water movement down into lawn soil.

On slopes, hardwood mulch, hydroseeding and sod installed over amended soil will also increase water retention. Carolyn Podger, owner of EcoPro Mulch and Erosion, suggests compost seeding: “Adding organic compost to seed mixtures adds moisture retention and nutrients. EcoPro installs compost seed by blower truck, which speeds application and works on slopes and hard-to-access areas.”

If the problem is more extreme, you can artificially improve infiltration by adding plumbed catch basins or dry wells to collect and slowly disperse large amounts of water.

Conveyance (direct)

If water accumulates around structures or impervious surfaces, start by redirecting gutter downspouts and raising the grade to change the direction of water flow. If water runoff is significant, consider managing water movement by adding a natural or artificial funnel, such as a french drain or even a low hardscape wall. And be sure not to underestimate the value of mulched planting beds or aggregate walkways to temporarily handle excess water.

Terraced landscape design interrupts erosion and makes yards more visually appealing.

Chronic Drainage Problems

If you’re coming in on the front end of a job, hopefully you have a longer horizon to impact change. Design enhancements generally take longer to accomplish but will also perform better in the long run, in terms of satisfying the client’s expectations.

Infiltration (collect)

Long-term infiltration improvements require extensive — and often invasive — soil work. “We can grow in clay soils, but they have to be ripped to a depth of 18 inches or more,” Sherk said. “Then you can till in topsoil or other amendments. Doing so properly will create a soil profile that acts as a sponge to filter stormwater and recharge the subsurface aquifers.”

Strategic grading, such as terraced slopes, also can significantly alter water movement. According to Sherk, planning these features into a cohesive design also opens the door to new planting areas and creates a more complex landscape that handles stormwater more effectively — and is actually often preferred by clients.

“There’s a perception that stormwater management features aren’t tidy, but research shows that people tend to prefer more visually diverse landscapes,” she said. “Terraced slopes planted with low-maintenance grasses, as well as trees in large landscapes, are some of our most valuable and effective stormwater-management structures.” The extensive, deep root zones of these plantings have enormous potential to slow, absorb and help evaporate rainfall.

However, if a large volume of water is pooling (especially if the problem starts off-property), your best bet is to catch it on the spot with a cistern or infiltration well. Both will handle a high volume of water for temporary storage, while remaining hidden in the landscape. These tools can even be engineered to function under impervious sites, especially in urban or commercial areas.

Conveyance (direct)

More dramatic water movement requires more invasive action. Grass swales and dry creeks provide movement and interest to the human eye, as well as catchment for rain. Both require thoughtful integration into the landscape and significant grading changes to function appropriately, but their on-demand water-handling service can be extremely valuable. Directing runoff into these graded channels can guide water into rain gardens or more substantial piping structures, or carry water off toward drains or ponds, helping to clean it before it runs into streams.

Reconfigure (accept)

“If you can’t beat it, join it,” Ivy said of chronically wet sites. There is a growing movement to use nature’s wetland example in our landscapes — and sometimes there are regulatory rewards for doing so. Rain gardens or other wetland installations allow adapted plants to

Catch basins can capture and direct water runoff.

sow and filter rainwater into the ground. “It’s a way to turn a problem into an opportunity that is dynamic and interesting, but also highly functional,” Sherk added.

The beauty of many rain garden or bog plants is that they are equally wet and drought tolerant. And there are numerous options to choose from, including native trees, reeds, shrubs and perennials, to keep the area of interest to people and pollinators all year round.

These intentionally designed wet spaces are a welcome solution in cases where invasive measures are unsuitable or undesired. As well, landscapers and clients are often delighted to find that municipalities will frequently cofund natural stormwater-catchment projects like cisterns and rain gardens, which prevent sediment from reaching natural water sources. Such reimbursement programs are another great incentive to consider the “less is more” approach to managing drainage.

Wet job sites can wreak havoc on structures and schedules, and understanding how to troubleshoot these locations is key to solving the root cause. Knowing whether to catch or redirect rainfall will allow you to advise clients in the most informed way — and finding a drainage solution that meets their needs (and yours) can be the ticket to successful jobs and future referrals.

Additional Resources:

Prevent Erosion on Your Sloped Property by USDA NRCS Dry Wells for Stormwater Management by American Geosciences Institute How to Build a Simple Dry Well by Ask This Old House Swale Terminology for Urban Stormwater Treatment by NC State Extension Rain Garden Site Selection and Design by NC Cooperative Extension Soak Up the Rain: Rain Gardens by the US Environmental Protection Agency

Rain gardens can be easily integrated into the landscape design.

Photo courtesy of NC State Cooperative Extension

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