Thursday, April 29, 2010

Rainwater Harvesting

Last week I had the pleasure of attending a talk by Brad Lancaster, a rainwater harvesting expert. Brad has a background in permaculture, which is the practice of designing human systems to mimic natural systems with the goal of increasing efficiency, with the ultimate goal of making our practices fully sustainable. Brad, who also has a blog, makes the point that we, as a society, are pretty thoroughly water-phobic and do our best to shed water away from our structures and properties as quickly as possible. He encourages us to treat water as a precious resource and harvest it with our landscapes. By doing this himself, he was able to transform his own lot in Tucson, Arizona, USA from a dry, dead landscape with a few of the toughest desert plants to an urban oasis with lush vegetation and abundant fruit, all with little or no additional water. He did this by collecting as much water as he could, mostly by grading the dirt to retain water and by covering the dirt with organic material, which helps the dirt act like a sponge, soaking up rainwater. He also collected rainwater from his roof and the street* in front of his house. All of this means that the runoff from his property is very minimal, as is the water he uses from his tap to water his plants.

As a civil engineer, I think his observation that we are water-phobic is a little extreme, but not too far off base. As engineers, we sometimes get sued over our designs. 90% of the lawsuits against civil engineers are because of drainage or traffic. Water can be very damaging and must be handled carefully. However, our caution of the damage it can do locally has created other, wider problems, opening the door to more damage by water. In a natural system, dirt absorbs water and ground cover, like plants, slow down the flow of water. During rainstorms in natural environment, the water levels in creeks and streams rise slowly to a peak flow and then subside back to normal flow. In urbanized areas, surfaces don’t absorb and are designed to get water out of the way as quickly as possible. That means that for the same amount of rainfall, more water runs off and it runs off more quickly. So the flood stage in the local streams occurs quicker than with a natural system and the water level is higher. Municipalities have begun trying to alleviate this problem with detention and retention basins (detention basins detain water while retention basins retain water). By collecting and slowing the water, we can help to restore the water to a more natural runoff rate. However, I believe that Brad’s way is better yet. By treating water as a resource, he takes extra steps to allow the water to seep into the ground, replenishing aquifers and decreasing our reliance on irrigation. It also increases plant cover, especially for arid climates, which improves our air quality.

So I really agree with Brad, and as an engineer, I think we can do better. But a lot of times we have to convince others. I can help a little with that. When faced with an intractable city engineer, put it in terms they can understand. Tell them that you are exceeding the requirements for retention on your site.

* As a civil engineer, collecting water from the street makes me a bit nervous, for two reasons. The first is that the curbs and ditches in front of your house usually belong to the city and modifying them can get you in trouble. Secondly, a street is an engineered system. Just cutting holes in the curbs modifies the engineering. It would be like cracking open your computer and soldering on a few more wires. 90% of the time it would be fine, but the other 10% of the time you are potentially opening yourself up for flooding or other problems. Talk to your local city or county engineer before attempting this. Again, use arguments that explain how you will be increasing retention.


  1. Your blog got some nice info about the the rain water harvesting. It will surely give a basic idea of how it works.i have installed the similar system to store rain water in water tanks.

  2. Can you give the basic idea that in how much bucks i can get some water harvesting system get installed in my house and is it affordable enough to fit in everyone budget and what will be a maintainace cost of such a system.
    Rainwater filters

  3. David, since you left a link, I am guessing you are spam. However, in the off chance that you aren't, mine is a blog of how. If you want details on how much, that is what the rest of the internet is for. Happy searching!