Run Off Capturing

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By: Zhi Chang Joo Hu

Water runoff is natural phenomena, but that was augmented by the growth of the city. As a result, capturing most of this excess runoff is not a complicated thing to do as it actually happens naturally. But first, a bit of history of how this water run off increased in Los Angeles, or in fact, in any big city with sprawl (expansion of urban developments around the city):

As the city of Los Angeles grew to its outskirts, more and more roads and walkways were needed in order to move people around. This not only caused the problem that everyone needed a car, but also caused the problem that most of the city was covered by an impermeable layer of either concrete or asphalt. As a result, water in the surface when it rains and given Los Angeles geography this water would eventually run into the ocean, dragging surface pollution along with it.

By contrast though, in a natural environment most of the water will normally be absorbed by the soil, while the rest would be runoff. Afterwards, the soil will act kind of like a “coffee filter” by filtering all the pollution of the water before reaching the underground aquifers. Once the water passes through the filtration process and reaches the aquifer, it can be consider to be potable.

However, it is not possible to simply change all the asphalt roads for ones of gravel, as it will increase the amount of dust in the surface -which could be harmful for people with asthma-, plus reducing the efficiency in cars. The solution is very simple and complex at the same time: to increase the permeable areas in the city of Los Angeles but by maintaining their actual functionality.

Grass Paving

Grass paving. Note that there are sections of grass that allow the water to flow through. Source: CUNY (http://macaulay.cuny.edu/eportfolios/greenparking/files/2012/10/Grass-Paver.jpeg)
Grass paving. Note that there are sections of grass that allow the water to flow through. Source: CUNY 

One of the many solutions for capturing runoff that is easily available but not known to the public is grass paving. Grass paving is technically the combination between grass and concrete in a way that combines the best of both: the resistance of the concrete and the permeability of the grass. In other words, as grass allows the water to permeate under the soil, the water is slowly filtered through the soil until reaching an underground aquifer. As many of you may be thinking, grass is not that resistant to tear and wear by cars, and thus the application of this technique is mainly limited to places with low flow of cars like maintenance access or overflow parking (“Performance FAQ”). In other words, this can be a viable solution to be implemented in LA, given its abundance of surface parking spaces. In a recent study about parking spaces in Los Angeles Historic Core made by Kimley-Horn and Associates Inc, they found out that there were 18,425 parking spots, of which about 6,930 were surface parking lots-or roughly 38% of the total- (4) (See end of this page for graph of the zone limits).

Surface Parking Lot. Note that in comparison to multistory car parks this one is built directly on the ground, allowing to capture runoff. Source: Archpaper.com
Surface Parking Lot. Note that in comparison to multistory car parks this one is built directly on the ground, allowing to capture runoff. Source: Archpaper.com

Given this information on the amounts of parking spots and the minimum size for the standard parking spot of 8 ft 6 inches of width (2.5908 mts) and 18 ft of depth (5.4864 mts) (“Parking Design” 1), it is then possible to calculate the area that would be capable of capturing runoff.

In meters: (2.5908 X 5.4864) = 14.21 mt^2 X 6,930 parking spots= 98,504.16 mt^2 or 1,060,290 sq ft.

Furthermore, as the average rainfall from 1981-2010 was 14.93 inches (.3792 mts), as given by the National Weather Service Forecast Office, it is possible to calculate the water that would be saved (considering that 100% of the water reach underground aquifers)

Area X rainfall: 98,504.16 mt^2 X .3792 mt= 37,352.78 m^3 = 37,352,777.47 liters of water or 9,867,560 gallons of water.

In other words, by just capturing the runoff in this area it is possible to save enough water for roughly 70 average single family residences, considering that the average single family residential water usage for the census tract of 2000-2010 was 387 gallons per day or 141,255 gallons per year (Hogues et all 3).

Some people may argue though, that this solution may be much more costly than a traditional parking lot made out of concrete and that there is no benefit for themselves. As a matter of fact, grasscrete -one of the many types of grass paving systems- is around 30% more expensive when compared to concrete (“Performance Faq”). However, it is possible to argue that these types of solutions are actually saving money as they “can eliminate or minimize the need for storm water collection systems” (“Performance Faq”). Furthermore, as grass plants are being planted in the place, it is also possible to argue that this system can help control soil erosion.

Porous Asphalt

Another great place to capture a big amount of water runoff are the streets, as the streets normally collect all the water before reaching the storm drains. In order to do this, it would be ideal if there was a type of asphalt that supports cars as normal asphalt does, but also allowed the water to flow through it. Luckily though, there is this type of asphalt: porous asphalt. Porous asphalt is just normal asphalt on which particles have been added in order to increase the spaces between asphalt particles and allow water to flow through it. In order to implement this idea though, it is necessary to aggregate a sub base of more dense material in order to act as both storage and structural support (“When It Pours, it’s Porous”).

Water being collected by storm drain. Note how all the water is collected by the streets , as they are in a lower position than walkways. Source: Athensclarkecounty.com
Water being collected by storm drain. Note how all the water is collected by the streets , as they are in a lower position than walkways. Source: Athensclarkecounty.com

The advantages of porous asphalt over normal asphalt are various. For instance, as the rainwater will naturally filter down through the asphalt to the rockbed, there will not be need for a storm water collection system. Furthermore, there will be a significant reduction in standing water -as water will be naturally filtered to an underground deposit instead of accumulating in the surface-, which will significantly increase the security for both pedestrians and cars. For the pedestrians, there will be fewer possibilities of water being splashed to them, while drivers will benefit from fewer possibilities of aquaplaning. Aquaplaning -as the word suggest- is the effect that occurs when too much water gets between the tyres and the road, which causes sudden loss of traction and thus control of the car. Consequently, the implementation of porous asphalt would not only help to save water, but could also save many lives over its life period. A representation of this idea can be seen in the picture below.

Static water from run off. Note the amount of water there is between the wheels of the car and the tyre, water that can lead to aquaplaning. Source  Xsorb.com
Static water from run off. Note the amount of water there is between the wheels of the car and the tyre, water that can lead to aquaplaning. Source: Xsorb.com
Comparison between normal asphalt and porous asphalt. Source: http://www.wolfpaving.com/Portals/98698/images/porous-parking-lot.jpg
Comparison between normal asphalt and porous asphalt. Source: Wolf Paving

Unfortunately, not everything about porous asphalt is positive. One of the problems that can arise with the installation of porous asphalt is soil pollution. As water filters down through the porous asphalt some of the pollutants that are attached to it may also reach the soil, a problem that if it is recurrent may cause the soil to be toxic. Luckily though, porous asphalt seems to be a very effective way of filtering the pollutants from runoff. In a recent study conducted by Dr. Brian A. Dempsey, a professor of Environmental Engineering in Penn State University,  it was revealed that porous asphalt can remove many of the pollutants that are found in runoff water (Adams). In fact, porous asphalt was shown to remove up to 91% of total suspended solids and 90% of the metals, among others(the rest can be found in the table below) (Adams). Although the removal rate of pollutants is high, it is still recommended to not build porous asphalt in places where the threat of “spills and groundwater contamination is quite real” (Adams).

Pollutants filtration by porous asphalt. Source: Adams, Michele
Pollutants filtration by porous asphalt. Source: Adams, Michele

As in the case of grass paving though, porous asphalt is more expensive than regular asphalt, but this cost is normally offset by the elimination to build water collection systems.  Furthermore, given the scenario that many of the streets normally need fixing and repaving, porous asphalt can be more easy to implement than grass paving, and thus it is an alternative worth considering. Another point to consider, as previously mentioned, is that porous asphalt reduces the amount of standing water, which improves road safety. Last but not least, as asphalt surfaces are one of the most abundant, they could also lead to the highest amount of run off water collection.

Long term view on the problem

Some of you may be thinking that runoff capture is actually not enough to solve the problem. And you are mostly right, runoff will not solve the problem by itself. With the low average rainfall  in Los Angeles, added to the high variability of the rain, it is highly improbable that rainwater will be enough. However, it should help reducing the amount of water imported. In the long term though, it is necessary to reduce and reuse as much water as possible, being a great solution of this greywater, as they are more of a really expensive investment rather than local solutions. You can read more in the tab conservation at home.

Appendix:

Area on which the parking study was made. Note that this area is more densely occupied than other areas, and thus there are fewer surface parking spots. Source: Kimley-Horn and Associates, Inc (http://lahd.lacity.org/lahdinternet/Portals/0/Policy/parking.pdf)
Area on which the parking study was made. Note that this area is more densely occupied than other areas, and thus there are fewer surface parking spots. Source: Kimley-Horn and Associates, Inc
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