Archive for the ‘Stormwater Management’ Category

I’ve been looking into solutions for storing the rainwater we harvest from the roof, and there are all kinds of tanks out there.  The most cost-effective tanks seem to be polyethylene, which are not really the look I want so close to my house.  Other semi-expensive alternatives include: flat flexible ‘bladder’ tanks that fit under a deck or in the crawl space of a house; underground tanks that sit beneath a concrete slab or patio; and tall, flat ‘wall-like’ slim plastic tanks that can fit along a narrow walkway or other narrow space.

What I really like are colored-concrete and corrugated steel tanks.

 I finally found a website that got me excited about making a tank!  They are called Technicians for Sustainability, LLC  http://www.tfssolar.com/, consultants for green building solutions based in Phoenix, Az.  They make water tanks from large corrugated galvanized culvert pipe embedded in a concrete base. I emailed them, questioning how to really seal the intersection of the galvanized pipe and the concrete at the bottom of the tank.  They were wonderful, quickly emailing me back a description of how they make a watertight connection, along with product recommendations and encouragement. Here are some photos from their website.

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One rule of thumb I’ve come across is: for every 1,000 square feet of roof, one inch of rainfall can yield about 600 gallons of water.  Alternatively, this formula (which can be found on many websites) can be used:

Area of roof in sq. ft.  X inches of rainfall per year X 0.62  = gallons runoff per year

(Here’s a unit analysis for that formula: ft X ft X in/year X ft/12 inch X 7.48 gal/cu.ft.    So, 7.48/12 is where the 0.62 comes from.)

So for my ~1,200 square foot house, using an average of 29.7 inches of rain per year, I come up with roughly 22,000 gallons of harvestable water!  That’s a big tank!

1200 sq. ft. X 29.7 in./yr. X 0.62 = 22,096.8 gallons.

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In quantifying the stormwater at my site, I wanted to first figure out how much stormwater potentially runs off my property now, as it is before the remodel.   As a baseline I also wanted to know how much stormwater may have run off this property in its pre-development state, when it was riparian/oak woodland – a much better “sponge” that could have absorbed more rainwater than the current asphalt and concrete surfaces do.  This baseline would help in setting the post-remodel runoff target.

Why set this goal?  In all the reading I’ve done on municipal stormwater requirements, the most progressive target I’ve seen requires that for any given storm, the volume of runoff from a site after it’s developed shall be no greater than the amount of runoff that would have occurred before development.   I like this goal and want to see how close I can get to pre-development runoff conditions.

To determine runoff volumes from my site, I’m using the Small Storm Hydrology WQV (Water Quality Volume) Method, a straightforward model from one of my landscape architecture textbooks (Timesaver Standards for Landscape Architecture, 2nd ed. 1998, p. 330).  The model tells us how much water from a given storm we can expect to be available to run off of a property as opposed to soaking into the ground.   

To use this model you will need to know a few things about your property including the soil type, the size of each type of land cover on the property, and the size of the whole property.  Soil types can be found by looking for the Soil Conservation Service maps for your area, and these maps can usually be found at your public library.  They’ll tell you your general soil type, usually some combination of silt, clay and loam (for example, clay-loam, sandy-clay-loam, silty-clay, or some other combination).  You will also need the runoff coefficient charts for different land types, which I found in Timesaver Standards.

The formula for this model is:

 WQV (i.e. Water Volume) = (P)(Rv)  

Where P is the rainfall depth, and Rv is the weighted runoff coefficient (based on land cover type). 

The following link takes you to a walk-through of the model used to determine potential runoff from my property as it is now. Calcs-Runoff before remodel 

We find that for every one-inch storm, as my property is now, 3,817 gallons of  stormwater are generated that can run off my property, or an astounding 113,365 gallons of stormwater per year!

To compare, look at the calcs for what the runoff probably looked like before the property was ever developed, when it was more of a sponge and could absorb more stormwater.   calcs-natural-pre-development-state     In a pre-development state, the model tells us that every one-inch storm generated 1,061 gallons of runoff, with annual runoff of 31,512 gallons.  So, pre-development runoff rates were less than a third of what they are now.  Put differently, over two-thirds of what now runs off the property used to be absorbed back into the soil (and most likely the underground water aquifer) rather than resulting in downstream flooding.

Whatever we do with our remodel, the goal will be to try to match the pre-development runoff rate of 1,061 gallons per one-inch storm.  Now we have a numeric target!! 


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Being a watershed steward at the residential level can start by keeping impervious surfaces to a minimum, enabling more rainwater from storms to soak into the soil and less rainwater to flow off the property into storm drains and creeks. Keeping more rainwater on site helps reduce flooding and erosion of nearby creeks, and also reduces water pollution because runoff carries with it anything that’s on the ground (including garbage, oil and grease, fertilizers, and soil).
There’s great potential to improve the balance of soft to hard surfaces here at my property. As it is now, the driveway is oversized and asphalt, and a good portion of the area adjacent to the house and outbuildings is concrete.

 And since we’re going to tear down some of the outbuildings, the roof area will most likely decrease overall, even if we end up building a small guest house.

Although simply making improvements to the soft/hard balance is a respectable goal, I want to attempt to quantify changes in my property’s runoff from before to after the remodel. Using fairly simple storm water runoff models, it’s very possible to do this.

I also want to set a target of achieving pre-development runoff rates, which is something I’ve seen in some more progressive cities’ storm water regulations. I’d like to see if my property’s runoff could be no more than what it was before the property was ever developed, when this whole neighborhood was riparian oak woodland. I imagine that I’ll have to create some combination of swales, rain garden feature, and infiltration basin to offset the fact that at least most of my roofs will still be hard surfaces.

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