Project overview
In one illustration, the typical justification for a rain garden begins with stormwater management issues, but extends to taking advantage of resources. The issues to be addressed:
A rain garden at work. This temporary pond has just received over 4,500 gallons of water in less than three hours, briefly innundating large ferns.
• Roof runoff from half of a structure was falling on the upslope side of the building. Flow alternatives impacted the foundation and crawl space beneath the structure, or it could be routed toward an existing septic field.
• A long, straight grassy area near the building faded undramatically into the leading edge of a planned stroll garden, which continues upslope away from the structure. Site preparation for that project meant a light backhoe would be conveniently on site for excavation.
• Because the site is a dry ridgetop, natural water resources are limited and artificial irrigation from an on-site well is costly and can draw away from the usual home requirements, especially in periods of drought.
In this illustration, a suitable location to route roof runoff was followed by defining limitations and resolving general design concerns. In heavy rainfall, enormous amounts of water (as much as 4,000 gallons) had to go into a relatively small space - about 25 by 25 feet; this implied the swale would have to be deeper than the usual six to eight inches. In the end, the primary bowl became about 18 feet in diameter and 20 inches deep; the surface of the bowl was 24 inches below the bottom of the downspout.
A boring long corridor would be interrupted and the path turned toward a planned stroll garden entry arbor, but that also meant the path would have to cross over the route of water coming from the roof and to the garden, about 30 feet from the corner of the building.
A deep rain garden suggested that surrounding areas - some landscaped, some not, would have to be graded to create a natural-looking swale. Finally, from a design standpoint, it was determined that the rain garden would ultimately be modelled on a low thicket habitat that would also serve to screen the stroll garden for the sake of its ambiance.
Review: some basic guidelines
The site should be a minimum of 15 feet away from foundations and downslope from buildings.
Never be built a rain garden over a septic field; it will counter the drainage purpose of the field.
The garden should completely drain within 72 hours - the length of time it takes mosquito larvae to hatch.
Be very attentive to the location of any underground utility, water or sewer lines and steer clear of them. Utility services are always willing to mark their lines.
Plan the garden to integrate and enhance the overall landscape. This merits time and care.
Rain garden designs
General rain garden designs are widely published and range from elemental suggestions to highly detailed bioretention basins engineering manuals. Those unfamiliar with concepts can obtain them from many websites, as well as county extension services and many conservation organizations.
Almost inevitably, however, a specific design must be developed for a specific site. On gentle grades, a rain garden can be a very large shallow pan dug by hand, with the excavated material bermed on the downslope side. On steeper slopes where substantial depth is a concern, consider a series of smaller basins connected by rills to handle overflow from each.
Process suggestions
Identify drainage problems and the flow of runoff.
Define a landscaping objective, perhaps using natural habitats like fens, seeps, wet meadows, seasonal creeks and ponds, palustrine woodlands or backswamps as models. Bogs, swamps and permanent ponds are water features, not rain gardens.
Determine soil capabilities. Rain gardens should drain quickly, which means sandy soils are helpful and high-clay soils a challenge. A simple test is to dig a hole a foot deep, fill it with water, and determine how long it takes to drain. Anything more than 72 hours will invite mosquitoes.
Consider the consequences of design, which may include assets and liabilities as new gardens impact surrounding landscapes, alter foot traffic flow, and will require interior maintenance.
Test the design for a reasonable period. Construction marking stakes in place over the course of a preceding season permit sufficient design consideration periods and identify concerns to be adjusted before construction begins.
Project conclusion
A long, boring corridor is now interrupted with the interest of an emerging thicket. Eventually a serious stormwater challenge will evolve into a useful woodland garden habitat and part of a visual welcome to a stroll garden beyond.
