Rainwater - Quantity or Quality
The Whole is Greater than the Sum of its Parts
For every 100 square feet of space on your property 600 gallons of water falls during a 1” rain storm. For reference, let’s focus on just the roof of a 2,000 square foot two-story home with a roof surface of 1,000 square feet. In an area with an average rainfall of 32” per year, that roof alone will shed 19,200 gallons of water per year!
Now, what can we do with that water?
We obviously are not going to store all of it. We can and should, however, store some in a rainwater catchment system but let’s discuss what to do with the rest.
Q the Rain!
After deciding the quantity of water to be stored in a rainwater catchment system, we’ll need to consider the quality of the water that will discharge off the site. The water leaving the site is often contaminated and whether it be a rural, urban, or suburban site, the discharge ultimately ends up in a natural waterway.
There Must Be Something in the Water
Common Contaminants in Rainwater Discharge
- Nitrogen, Potassium, and Phosphorus from bird excrement on your roof
- Excess fertilizer
- Hydrocarbons from vehicles parked on driveways
- Erosion (Soil)
- “First Flush” Bioremediation
- Discharge Delay
First Flush Bioremediation
Assuming we have our soil well secured with plants and mulch, we can focus on the other common contaminants. These contaminants are mostly concentrated in what we describe as “First Flush”.
First flush is approximately the first ¼” of rain that falls after a dry spell. This ¼” is the most heavily laden with toxins as it washes stagnant contaminants off hard surfaces like your roof and your driveway.
This is the water we want to direct into our landscapes for bioremediation and not flush directly off our properties and into our waterways.
[Bioremediation = using microorganisms to break down environmental pollutants and clear out contamination]
The good news is, short of hydrocarbons, the other common contaminants are actually food sources for plants! If we can get that nutrient rich water into the soil, we can sequester it in our garden beds and nourish our plants. Unfortunately, these same plant nutrients are devastating for our waterways and so we must delve further…
Historically, the number one contaminant in California streams has been soil.
Stream bank erosion from the over-concentration of storm water in our creeks is a real problem.
Our waterways were meant to slowly absorb water through the soil, like a sponge drinking in moisture, not by becoming engorged from pipes pouring water directly into the creek.
Since we are paving over more and more of our watersheds, it is absolutely necessary to implement strategies for percolating as much rainwater and runoff through the soil as possible.
In so doing we will:
- Clean the runoff
- Recharge the groundwater, and
- Protect the streams from erosion
In this Example we have directed the roof runoff through a series of swales and raingardens prior to it discharging to the road and ultimately to the creek.
Given the small size of landscape, we focused on the Quality of the discharge not the Quantity of storage.
Approximately 210,000 gallons of runoff was coming from the roofs and hardscape areas at the top of this two-acre site.
Standard civil engineering of the time would and did call for a ditch running straight down the property to discharge the water.
As you can see, we designed and installed a series of swales, sediment catches and waterways to slow down water, percolate it into the soil, and even create interesting landscape features and natural habitat.
(1) The most upper swales in the orchard system not only clean and percolate water, but combined with a healthy soil building program, will actually reduce the water needs of the orchard trees.
(2) The sediment trap catches the overflow from the orchard swales and the driveway runoff, and it has the aesthetic of a natural ephemeral pond. It’s thoughtfully located to be viewed from the master bath and also close enough to the driveway for easy machine access should it ever need to be cleaned out.
(3) After leaving the sediment trap, the water flows along the side of the house, directly down slope in a standard V ditch. This avoids concentrating any water near the structure.
(4) The water then flows through a meandering waterway planted with California grasses and flowering shrubs before it finally filters out in the meadow and off the site. (5)
These strategies have proven effective for over a decade and any have been adopted by municipalities throughout the U.S.
I hope these models have shown you how easy it is to clean your runoff water. Of course, should you have any questions, please feel free to call our office and we’d be happy to assist you.