PASSIVE APPROACHES FOR HARVESTING RAINWATER AND STORM WATER
THE GATHERING STORM
Many communities have tightened requirements for the handling of stormwater from commercial buildings. Impervious surfaces on roofs and large parking areas add to overloaded water treatment facilities. During heavy rains, the facilities can become overwhelmed resulting in raw sewage being dumped into open waterways. Not a good thing!
Water can be collected and processed with either a passive system or active system. Which is best for your building depends on a number of factors including the amount of rainfall received in your geography, the square footage of the building roof and parking areas, the value of collecting the water and using it in a directed way, etc. Water Harvesting Solutions may suggest a combination of passive and active systems depending on your unique situation and requirements.
A PASSIVE APPROACH TO HARVESTING
Building owners can help reduce this impact even if you do not plan to harvest storm waters. By “passive” we mean that there are no mechanical methods of collecting, cleaning and storing rainwater. The intent with passive rainwater management is to create areas to contain waters until they can naturally be absorbed into the land. Vegetative swales, wetland ponds, dry creek beds, green roofs and pervious concrete or pavers are some examples used to keep the water on the land longer and out of sewer or stormwater systems.
These methods are relatively simple and inexpensive and require only that building and landscape designers keep a “green eye” during the planning process. It is common to incorporate both passive and active approaches when handling rainwater and stormwater, optimizing the application of each method. The biggest disadvantage of detention ponds is that they utilize valuable real estate to hold stormwater – and may create problems with insects, odors and public safety. Read more about stormwater and harvesting here.
Cities like Chicago have begun to embrace planted “green” roofs to capture and use rain waters while delivering other benefits as well. In addition to adding beauty to the roofs of commercial buildings, green roofs deliver economic benefits by reducing heating and maintenance costs.
Limitations: Green roofs often must be planned and executed when a building is being constructed. They require an architectural design commitment to rainwater harvesting that may or may not fit with the design expectations of the building owner. Special considerations may be required to accommodate the weight of planters and soils, and the roof material should be compatible with the plans. Green roof access and maintenance are also considerations.
In drier climates, roof gardens may require additional watering using municipal water – somewhat defeating the purpose of the green roofs. And because the water is not being harvested for reuse, the benefit of using it to water ground level landscaping or to flush toilets is lost.
Benefits: Green roofs naturally capture and absorb normal rainfall quantities reducing the amount of stormwater discharged into municipal sewer systems or other property retention areas. The mass of soils and the natural shade of the garden and plantings help to insulate and cool the roof surface, significantly reducing loads on building air conditioning and heating systems. Protecting the roof membrane from sun and weather exposure with the natural green surface can extend the lifespan of the roofing material, reducing long term maintenance costs.
Vegetated swales (a.k.a. bioswales, dry swale, wet swales, rain gardens or biofilters) are constructed open-channel drainageways used to convey stormwater runoff. Vegetated swales are often used as an alternative to, or an enhancement of, traditional storm sewer pipes. They do not pond water for a long period of time and instead induce infiltration. Vegetated swales generally have a trapezoidal or parabolic shape with relatively flat side slopes. Individual vegetated swales generally treat small drainage areas (five acres or less).
Vegetated swales can be used as an environmentally sensitive alternative to conventional storm sewers for parking lot and grounds run-off in commercial buildings. They can offer a more attractive and environmentally superior alternative to stormwater retention ponds. Because they naturally slow waters that may otherwise end up in storm sewers, while naturally filtering out contaminants, swales help to recharge groundwater in local aquifers – benefiting the water community.
Limitations: There are very few limitations on the use of vegetated swales. They should not be used in steep slope areas and may be difficult to place in very urban settings due to space requirements. Otherwise, they can be adapted for use in most residential commercial and industrial land development projects.
Benefits: Vegetation in swales allows for filtering of pollutants, and infiltration of runoff into groundwater. Densely vegetated swales can be designed to add visual interest to a site or to screen unsightly views. Broad swales on flat slopes with dense vegetation are the most effective at reducing the volume of runoff and pollutant removal.
Cost: The cost of a vegetated swale depends on numerous factors including width, depth, length, slope, and plant material, to name a few. Substantial labor and material cost savings can be gained in areas where swales are used instead of traditional piping systems.
For more information and links on passive systems, see our Resources page.