An article by our principal, Richard “Rick” Quinn, was featured in the most recent issue of the Landscape Industry Council of Hawai‘i’s (LICH) Landscape Hawai‘i Magazine (July/August 2014). Here’s a look!
Drywells – A Micro Scale Storm Water Management Tool
By: Richard Quinn, ASLA
It’s such a simple and old idea that it can be easily forgotten. Almost any home with a rain gutter can incorporate a small drywell to help mitigate storm water run-off in urban areas. Landscape designers should look for opportunities to use them more often in the typical residential or small commercial projects, as in their own small way a dry well can do a lot to help our environment and reduce storm water pollution. It is also great for the low spots in lawns that seem to always remain soggy after a rain or from irrigation. Small dry wells in urban landscapes are a simple and practical way to incorporate a Low Impact Development (LID) tool that can work with nature to help manage stormwater as close to its source as possible.
Benefits of drywells can include:
- Reduces puddling and keep lawns from getting soggy
- Helps re-charge ground water
- Reduces pollutants to streams and oceans
- Provides deep water to plants and trees
- Reduces the potential for soil erosion
In terms of helping the environment, a drywell could be considered as a simple “first flush” device to help remove pollutants (such as oils, fertilizers, and other chemicals) from paving, lawns, and roof run-off that would otherwise go into our streams and ocean. A drywell can also help to recharge our aquifers and provide sustained moisture to trees and plants in our garden that better mimicks natural processes. Diverting roof runoff can make for a less slippery and soggy lawn in low spots or areas without adequate slope to drain well. Drywells can reduce the need for irrigation for trees by creating a deep watering system that saturates the sub-soil resulting in a persistent moisture source for plants.
The management of storm water is typically the realm of the civil engineer, as well it should be, as calculating and managing large volumns of water can be a very real public safety issue. But often times, on a micro scale in residential or small commercial settings, the use of small landscaped storm water solutions can be an opportunity to enhance the sustainability of a landscape and can complement the larger scale engineered solutions. Small efforts can add up and make a big difference over the long run.
Locations for drywells can vary, but in general they should be located at least 10ft away from building foundations. Roof top gutters can be extended into drywells, rather than daylighting at the base of buildings. Gravel borders along building foundations can have perforated pipe that leads to drywells. Or a low spot in a lawn or groundcover area can be a good location for a drywell. Use a drywell to capture water from impermeable surfaces. Impermeable surfaces are not just paving and roof tops, but can also be dense lawn or compacted soils that have limited permeability. Consider incorporating a bio-swale or rain garden in with a drywell to further enhance storm water management.
The design of drywells can be simple and affordable. They are basically gravel filled holes, wrapped in fabric, with a connection to drain lines or surface drains. They can be buried under soil with grass or groundcovers on top so that they are completely invisible.
A river stone swale or groundcover is a form of surface drywell and can be connected to a sub-surface drywell for greater capacity. If you have gravel strips adjacent to house foundations, but be sure to connect them to drywells or to daylight to lower areas in the landscape, to insure that the gravel strips don’t become moats for water saturation rather than water elimination.
If you expect a lot of debris that would clog a simple gravel drywell or a slow percolation rate for the soil, or only have a limited area for a drywell, than a pre-fabricated drywell (such as NDS Flo-Well TM ) may be the way to go, as it would facilitate a larger cavity and water capacity in the same size drywell area.
Here are some guidelines to using drywells in small landscapes:
- Be careful that the drywell is not on or near a slope, or adjacent to a retaining wall, as under conditions of soil saturation a dry well can lead to failure of the slope, resulting in a mud slide or slippage of the hillside.
- Incorporate a drain outlet over the drywell, to allow for water and air to escape without water backing up into downspout piping.
- Wrap drywell in drainage fabric to prevent infill.
- If silting is anticipated, use a settling box or other means to allow the occasional clean out of silt and debris that would otherwise clog a drywell over time.
- Don’t expect drywells to manage high volumes of storm water, but consider them as secondary resources that can have positive effects on low water storm and rainfall events (the most common kind!). Drywells typically cannot take the place of a properly engineered storm water management system required to handle major storm events and to prevent flooding.
- If practical, keep them at least 10 feet away from building foundations.
- Do a percolation test to insure that water will drain from a dry well. If needed, punch deeper holes into the sub-soil below a dry well to improve drainage.
Along with his role as a principal at HHF Planners, Rick Quinn is a landscape architect with over 30 years of design experience. Rick is well known for his extensive knowledge of tropical and subtropical landscape plants. He specializes in incorporating native Hawaiian plants into his design work, in the application of native ecosystem restoration and the utilization of an ecosystem approach to the use of native Hawaiian plants in urban areas.
Here is more of Rick’s work.
Check out the Landscape Industry Council of Hawaii website.