Friday, November 20, 2009

Collecting Rainwater Through Grading

Grading is the process of moving and shaping dirt. It is usually done for aesthetics, but it can certainly be used to catch rainwater. The concept is actually pretty simple: You shape the earth so that runoff is captured and held in place by a berm long enough that it will soak in. This method works particularly well on a hillside. You can create a system of berms in a sort of fish scale pattern down the hill. Each area collects a certain amount of water and then spills over to the next basin below.

The methodology is actually pretty simple. The first and most important step is to observe your particular situation. Walk around and look at the grades. If you are on the top of a hill, this won't work as there is nowhere for water to come from. If you are in the middle of a large drainageway, it probably won't work as well because the large volume of water will wash away your berms or flood your plantings. It is particularly helpful to walk around during a rain. Sometimes visually inspecting slopes can be tricky, but flowing water never lies. Look for places where water collects, how far it comes from, and what sort of surface is picking up water. If you have a large area draining to your basin, it may affect how big you make it. Also beware of picking up water off of parking lots or other possible sources of pollution.

The next step is to build a berm (which is a mound of earth) in a line on the downhill side of your basin. This will hold the water in. Remember that water will always find its level, so the top of the berm needs to be level all the way around. If you mound it up on the downhill side and leave it low on the sides, the water will just flow around your berm.

There are also a few construction methods to use when building your berm. First of all, be sure to leave a low spot in your berm where you want the excess water to overflow and make sure that you protect that with some rock. You might be surprised how fast moving water can remove dirt. Secondly, you will want some degree of compaction in your dirt. Walking on the berm as you build it up works pretty well. Mechanical compactors work better. The compaction does several things for your berm. It reduces pores and keeps the water from flowing through your berm, it protects your berm from erosion and failure, and it ensures that as your berm compacts naturally over time, it maintains the elevation you built it at.

Now for the big question: how do you determine how tall to build your berm? Well, first we'll state the obvious. Your berm can't be taller than the spot where water is entering your basin, or the water will never get in. Other than that, the trick is to balance two factors: how much water you are getting, and how fast it will soak in. If you provide too little storage, it won't soak in before it runs off. If you provide too much storage, it will drown your plants.

To determine how much water you are getting, you will need to do a quick calculation. The hard part will be determining how big of an area is draining to your bed. Again, this comes down to observation. Do your best to determine the square footage of the area (A) draining to your basin. Rough numbers will do as this is far from an exact calculation, though better numbers are, well, better. Measuring is better than eyeballing. Next, look up your local monthly rainfall averages (R). The weather channel is a good place to look for this information. You will probably going to want to do this for each month as it will help you fill out your water budget. The last number you need is the runoff coefficient (C). This number explains how much of the water that fell from the sky actually ran off as opposed to what soaked in. Here are a couple of sample numbers:

Paved areas, roof areas, impermeable areas: 0.95

Bare ground: 0.25

Lawn area: 0.20

Suburban areas: 0.35

Steep terrain: 0.70

Obviously there are a lot more numbers to this table, but I don't want to overwhelm you. For more information, look up C values for use in the Rational Method.

Next you will calculate how much water (W) you are getting. To determine how much water you are getting in an average month, multiply the amount of rain that falls in that month (R) with the area it is running off of (A) and the runoff coefficient (C), or


Now take this number and divide it by the area of your basin. This will give you how many inches of water you are collecting in your basin for that month. Compare that with the numbers from your water budget.

To determine how quickly your water absorbs into the soil, you need to know a little about your soil. The bigger the particle size, the quicker it will absorb. Water flows through sand very quickly, while it flows through clay very slowly. The best way to test this is to do a perc (short for percolation) test. A basic perc test can be done pretty easily. Dig a hole at least 1' deep and as big around as you care to dig. Fill it with water and let it drain. Then fill it again and time how long it takes to drain. If it drains within a few minutes, you can make your basin as big as you want. If it takes a few hours, the basin should probably be a foot or two deep. If it takes over 24 hours, make a shallow basin, say 6" to a foot deep, unless you live in an arid environment.

Now for the tricky part: adjusting your system. Unless you hire an engineer to really calculate this exactly, or you are particularly good at this sort of calculation yourself, it is going to be difficult to size it exactly just by doing the math. Let's just say that I left out a lot of details to simplify the calculation. The fine tuning can be done pretty easily in the field. As I mentioned previously, you will want an overflow in your berm. By adjusting this overflow up or down, you can adjust how much water your basin will collect. If it is too soggy, lower your overflow. If it is too dry and too much water is running off, raise the overflow and/or the entire berm.

Finally, a little disclaimer: Always check with your local municipality before doing something like this. Different areas have different laws regulating this sort of activity. Also remember never to change the direction that water flows across your property. The place where water enters your property and leaves your property must remain unchanged throughout this process, or you may open yourself up to liability. Good luck!

1 comment:

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