Stormwater Runoff – What’s the Problem?
In an undeveloped forested site, storm events are moderated by the processes of infiltration into surfaces, evaporation from surfaces, and transpiration from vegetation. Very little stormwater, less than 1%, leaves the site in the form of runoff.
|Pre-development hydrology moderates rainfall. (Photo courtesy of AHBL)|
However as a site is developed, increases in impervious surface due to roads, rooftops, sidewalks, and parking areas cause significant changes in the ability of the site to handle stormwater. More water runs off the site – as much as 20-30% -- and far less water is infiltrated, evaporated or transpired. In addition to the increased volume of runoff, it is the speed with which it travels that causes concern.
If stormwater runoff is not addressed, it can cause dramatic changes in hydrological function and water quality of a watershed. As runoff travels overland, it can erode soils, change stream patterns, and cause flooding. It can also pick up pollutants along the way. Sediments, toxic metal particles, pesticides and fertilizers, oil and grease, pathogens, excess nutrients, and trash are common stormwater pollutants. Polluted runoff, if not diverted, ends up in waterways.
|Post-development hydrology produces erosion and pollution. (Photo courtesy of AHBL)|
In urbanized areas, stormwater is frequently diverted to combined sewer systems. This means rainwater is getting treated to same degree (and at the same cost) as raw sewage. In heavy rains, these systems can get maxed out, and raw sewage ends up in the receiving waters, untreated. Even in separated systems, increased volume of stormwater during heavy rains can result in high levels of pollutants ending up in waterways.
Since the primary cause of stormwater runoff is increased impervious surface, sustainable site design seeks to minimize impervious surface as a first step. The second step is to manage the stormwater so that as little of it leaves the site as possible.
This generally requires slowing the water down, so that it can be infiltrated, stored, and/or dispersed without harm.
|Photo Courtesy of CTLGroup|
How does Concrete Help?
Concrete has played a significant role in conventional surface water management by providing conveyance and treatment infrastructure that is durable and impermeable. With the movement toward managing water on site, concrete continues to play a major role, as it is frequently the material of choice to help construct flow control and treatment facilities on site.
With sustainable design, there is considerable interest in mimicking pre-development hydrologic functions as much as possible. This has resulted in a focus on using strategies that emphasize conservation and use of on-site natural features integrated with small-scale hydrologic controls. Combined these strategies are known as Low Impact Development or LID.
Four LID strategies use concrete:
In addition to providing a more naturalistic solution to stormwater management, LID can reduce the need for large (and frequently expensive) stormwater facilities such as detention ponds.
Pervious concrete can be used for light to medium duty applications. The composition of pervious concrete includes washed coarse aggregate (3/8 or 5/8 inch), hydraulic cement, optional admixtures, and water. It includes reduced or no fines (sand). The mixture yields a surface with “pores” through which water infiltrates to the underlying aggregate base and soil.
Pervious concrete allows as much as 8 gallons of water per minute to pass through each square foot of the material. By allowing rainwater to seep into the ground, pervious concrete helps recharge groundwater and reduce stormwater runoff.
(PCA No. 10103)
Pervious concrete is installed on an aggregate base. In some applications, the aggregate base will be fairly significant, acting as temporary storage. In this case, the stored water is then dispersed at a safe rate of speed.
For pervious concrete to provide good service as a “stormwater” facility, it is critical to follow design specifications, use qualified contractors, and control erosion and sediment.Permeable Pavers
Aggregate pavers include cast-in place or modular pre-cast blocks. The cast-in-
place systems are reinforced concrete made with reusable forms. Pre-cast concrete systems use high-strength Portland cement. Both systems have wide joints or openings that can be filled with soil and grass or gravel. Water infiltrates through the openings filled with grass or gravel.
Pavement performance is directly related to the quality of the soil and base material.
|Green roof at Chicago Center for Green Technology (Photo courtesy of City of Chicago)|
During rainstorms, green roofs act as a sponge, absorbing much of the water that would otherwise run off. Researchers estimate that three to five inches of soil or growing medium absorbs 75% of rain events that are one-half inch or less.
Green roofs also filter pollution from rainwater. This is achieved by the root systems' bacteria and fungi, which utilize the natural filtering processes of bioremediation and phytoremediation. As a result, pollutants such as nitrogen and phosphorus, are broken down and detoxified. This beneficial process increases over time as rooftop plants and root systems mature.
Rainwater Harvesting Systems
Typically, rainwater collection is used where environmental conditions limit the rainwater supply. In the context of Low Impact Development, however, rainwater harvesting performs two services, water conservation and the elimination or significant reduction of roof runoff. This practice is particularly applicable in medium to high-density development where roof surface is likely to be equal to or greater than the non-roof impervious surfaces. In such situations, achieving pre-development hydrology is probably not achievable unless rainwater harvesting is performed.
Rainwater harvesting technology is well developed and components readily available; however system design and construction is relatively complex and should be provided by a qualified engineer and experienced designer.
Rainwater can be used instead of potable water for landscaping irrigation, to flush toilets, and if properly filtered as drinking water.
Since precipitation rarely matches need, rainwater systems generally include cisterns to store the water until it is needed. Concrete makes an excellent and durable material for storage tanks.
LEED 2.2 SS Credit 6:
Stormwater Design: Quantity & Quality Control 1-2 points.
Intent is to limit disruption of natural water hydrology by reducing impervious cover, increasing on-site infiltration, reducing or eliminating pollution from stormwater runoff, and eliminating contaminants.
LEED 2.2 WE Credit 1:
Water Efficient Landscaping: No Potable Water Use 1 pt.
Intent is to eliminate the use of potable water or other natural surface or subsurface water resources for landscape irrigation. (Captured rainwater is an approved alternative)
LEED 2.2 WE Credit 3:
Water Use Reduction 1 pt.
Intent is to maximize water efficiency within buildings to reduce the burden on municipal water supply and wastewater systems. (Captured rainwater for non-potable applications such as toilet and urinal flushing and custodial areas is a recommended strategy).