Wednesday, January 06, 2016

After the rain 2

We're getting some much-needed rain.  Unfortunately, LA is (mostly) not built to make the most of the rain when it does come.  But, let's talk about what engineering for rain looks like.

First, read After the rain 1, which I wrote back in November 2010.

Remember this newly built and planted infiltration basin?
Freshly planted in November 2011
and the sign?
Is the sign accurate?
This is the way it looked today, after a downpour.  There is slight ponding.

Some ponding, but less than 12 inches deep.
Remember the notches in the parking lot to channel water from the asphalt to the infiltration basin?  Parking lots are notoriously dirty because cars are dirty for myriad reasons.  (A post about that to follow.)

Low point of lot.  Slight sheen on water indicates oil pollution.
A slight sheen on the standing water shows a light level of pollution.  The plants planted in the infiltration basin were selected for their ability to absorb the expected types of pollutants.

Plantings have filled in nicely.
Because LA depends so much on groundwater pumping, infiltration basins are the most efficient way to bank water for use in dryer times.  They take up less surface area, cost less, and look so much nicer than rain barrels.
The view from the uphill side shows more room to stow water while it seeps.
I hired a landscape designer to design a front yard with fruit trees and native flowering plants around the perimeter and a gravel infiltration area suitable for use as an outdoor lounge in the center.  But, my neighbor in our HOA objected to pulling out the grass; she says that she bought her house because the front yard looked so nice with the lawn.  Her vote counts as much as mine.  (Drat!)  So, my front yard is still half lawn.

This illustrates that hydrology is as much a social problem as an engineering problem.  Heck, I solved the engineering problem.  I just can't solve the social problem on my own.

I can console myself with the thought that the west side of my house does drain into a gravel infiltration area and my backyard (south side) also does a reasonable job of rain capture.


  1. Anonymous10:57

    I didn't know about infiltration pits. They seem really cool. I suppose they only work where the underlying soil is sandy/gravelly, so that soaking in happens fast? I worry that the pits seem optimally designed for mosquitos. Shallow nonflowing water that doesn't support a population of egg-eating fish. Is the idea that the water soaks into the ground so fast that there is no time for mosquitos to hatch? Do they spray to suppress mosquitos? Or is the over-all climate so dry that there is no stock of mosquitos to seed the process? Even here in arid Colorado, if you have an apartment complex where the maintenance people set the sprinklers on for too much/too frequent watering for the soil type, after a month or so that complex becomes Mosquito Central. Eric

    1. Infiltration pits and spreading grounds (larger-scale operations) are topped with gravel and have sand and rocks underneath. They are designed to allow the water to percolate through the ground to a stream or aquifer.

      In CO, where there is a clay barrier between the aquifer and the surface, you would need to punch through the clay layer to allow drainage. Alternatively, you can create a pathway of gravel (above or below ground) that can drain into a river or creek.

      My front yard plan allowed for a 4" layer of gravel with the excess soaking into the perimeter trees and the overflow going into the storm drain. Right now, all of it goes into a storm drain, so it would have been a big improvement.

  2. Mosquitoes can be a problem. But, infiltration pits are designed to NOT pool water at the surface for long enough to breed mosquitoes. It is a good idea to walk around to make sure they are working as designed, and make alterations as necessary.

    There are 3 infiltration pits in my neighborhood and I like to walk the hills for exercise. I scope them out while I am getting my cardio time.

  3. love your comment about the social problem. Here in the dry east bay I am seeing more and more homes change over from lawn to drought tolerant landscaping. Which is actually so much more interesting to look at. The lack of infrastructure to save rainwater runoff is disheartening - and I'm sure you know way more about it than I. love all your posts, about sewing, climate, whatever :)


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