Low impact development (LID) includes means to reduce the effects of increased stormwater runoff due to urbanization. Rain gardens are an important tool for this.
The increase in impervious area—buildings, driveways, and streets—results in more stormwater being discharged rather than infiltrating into the ground.
Urbanization in all its forms, including dense urban centers, suburban sprawl, and even the move to smaller exurban centers, is responsible.
The current trend is to find ways to mitigate this increase in runoff. Many cities have regulations that require developers to control the peak flow, but not the total amount of stormwater that runs off.
Low Impact Development (LID) Attempts to Mimic Natural Processes
When a natural or agricultural field is covered with something impervious, rainwater or snowmelt that used to sink in now runs off the land, into nearby streams, later into rivers and all the way to the ocean. This increase in runoff has negative impacts on the natural environment.
- Higher water levels affect stream bank stability.
- Bank-dwelling animals (small mammals, insects, some birds) find their habitat changed, and must relocate or drown.
- Reduced sunlight reaching native plants results in other species taking over.
- In addition to greater volume, stormwater also carries pollutants washed off the man-made environment.
Low impact development (LID) is a planning and design technique that utilizes a series of best management practices to reduce this runoff.
The goal is to develop with different materials (such as permeable pavement) so that stormwater runoff sinks in as it always did when the field was covered only with plants. Some of these techniques are man-made systems, such as rainwater cisterns, water barrels, small ponds, etc.
However, natural systems are also used in LID, and are preferred whenever they can be implemented. The rain garden is one such natural technique, along with bioswales and bioretention systems.
How a Rain Garden Helps to Reduce Runoff
A natural field, even in the flattest terrain, includes minor ups and downs that do at least three things:
- Slow down the speed of water as it runs off
- Minor ponding of water, increasing infiltration and reducing runoff
- Depending on the size of the depression, support vegetation that thrives in standing water
A rain garden mimics that. In its simplest form, a rain garden is “a special kind of stormwater garden designed to collect and absorb runoff….” [Rain Gardens, at Rain Gardens of West Michigan]
A rain garden consists of a depression where runoff can collect and stand for some time. The top level soils will be a mixture designed to both allow water to infiltrate and provide plants with structure.
With special soils and with plants that can survive a couple of days of standing water and still do well when it’s dry, a rain garden, though small, can have a significant positive impact on the natural environment.
Rain Gardens are a Low-Cost Solution to Increased Runoff
Rain gardens are suitable for small drainage basins, not large ones. In fact, an individual house lot is an ideal place to locate a rain garden.
The small size of a rain garden makes it an economical way a homeowner can have a positive impact on the environment. A small rain garden for a family home is relatively inexpensive because:
- Some minimum research and planning is necessary; however this can be done with essentially zero expense.
- Most of the labor requires no special tools. While mechanical equipment to dig the depression is nice, it is not necessary. Use a shovel and get some exercise in the process. Adjacent homeowners can assist each other.
- The main expense will be purchasing plants; however, a little bit of research and careful selection will result in using low-cost plants that are native to the region. Exotic, high-cost plants are not necessary.
- Maintenance costs are low. Some weeding will be necessary. If the rain garden is begun in a dry time of year, some watering will be needed to establish the plants in place. Eventually, as plants begin to mature, thinning and pruning will be required. But this will be no different than for any other type of garden.
Rain gardens are an excellent LID tool to reduce the impact of urbanization on the downstream environment. The small size, simple construction, and ease of maintenance make them ideal for a wide range of sites, from an individual home to an apartment complex to a subdivision right-of-way.