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We're all aware of how unpredictable Mother Nature can be. One year certain parts of the country will receive an abundance of rain and snow, while other parts of the country received very little. The next year, the weather pattern may reverse itself.
December and January saw record rain and snowfall in parts of the U.S. Widespread flooding was reported in some states. It's been raining in Seattle from the end of December to press time (January 18th), almost thirty days of solid rain. We all watched on television the Tournament of Roses Parade in Pasadena, California only to find that it was raining. It was the first time in fifty seven years that it rained on the parade.
When we have this kind of deluge of water, we have to find ways to move it and keep it from damaging property. Many contractors have had to deal with drainage (or the lack thereof) on some of the properties they service. What is the best way to deal with too much water on a site? In this article we will look at some of the key factors to consider when designing and installing a drainage system.
Drainage problems can range from a simple nuisance to serious damage to the landscape and buildings. Poor drainage can cause unsightly areas and affect plant health. Wet areas can be difficult to maintain and mow. If you are dealing with sports turf then poor drainage can affect playability and limit full use of a site. Damage can occur to buildings, foundations, asphalt and can leak into buildings and damage equipment and belongings. A good drainage system pays for itself by correcting many problems including:
A properly designed drainage system will move excess water off site but leave beneficial water where it needs to be to maintain the landscape. Area and channel drains will remove excess surface water quickly and efficiently. If the problem is subsurface water than perforated or composite drain systems will keep the soil drained and usable. In many cases drainage systems are installed after the fact to correct problems. However, the best application is to design drainage into a site from the beginning.
Since drainage problems on many sites only occur only in periods of above average rainfall, many people neglect drainage and then pay the price when unplanned for heavy rainfall occurs. This has definitely been the case this year in many areas of the U.S., by then it is too late. Good drainage systems are out of sight and out of mind, but when it is needed, people are grateful for the foresight to installed drainage ahead of time.
So what is the best way to deal with too much water on a site? Let's look at some of the key factors that should be considered.
Why are we installing drainage in the first place? The main reasons are that we need a way to collect the excess water and to dispose of it correctly. What a drainage system does is it collects the water underground, channels it to a main location where it can flow into the sewer system or distributed in the ground. Poor design or installation of a drainage system is almost as bad as no drainage system at all.
The first basic rule of designing a drainage system is to understand grades and topography. You would be amazed at how many people forget that water runs downhill only. The first thing I would suggest you do is get a transit and learn how to use it, or hire someone to do this for you. Do not try to eyeball grades. I have seen some projects where the drainage slope was put in at too steep on an angle and when it got to the end, the pipe was 1-2 feet deeper than the discharge point.
Excess water can be collected through several means. We need to identify how we're going to solve the problem. Should we have surface or subsurface drainage? Surface water is normally collected through catch basins. These catch basins are connected below the surface to usually corrugated pipe that will channel the water from the catch basins or channel drains and have it flow to the collection point.
If it is decided to install surface drainage, we begin by designing for a specific volume amount. To begin with, we should consult design tables that will tell us the run-off coefficient for the type of surface or surfaces that we are dealing with on our site. For example water on asphalt will run-off at a faster rate than water on grass. Then we should calculate the worst case rainfall for a given location. This is done by consulting 100 year rainfall charts and maps (If you think that designing for 100 year worst case rainfall is overkill, you might consider consulting the residents of Southern California, the Northeast or New Orleans).
Before we begin to size our catch basins or channel drains, we have to determine the volume of water that might need to be moved. If we underestimate the heaviest amount of rainfall, the system will be inadequate to move all that water and the system will fail.
So before we go the sizing the catch basins, we need to do the calculations. For example; there is a property with 5,000 square feet of turf area. We calculate the run-off potential as such:
(.35 X 3.5 X 5000) / 96.3 = 6125 / 96.3 = 64 gpm
The .35 is the Run-Off coefficient for the surface we are dealing with (this is obtained from drainage design charts), in this case it is grass. The 3.5% is the maximum rainfall in one hour in one hundred years for the area in question. The 5000 is the square footage of the area and 96.3 is a constant used to convert the answer to GPM.
Therefore our run-off potential is 64 gallons per minute. We can now pick the proper catch basin or basins. We could choose one basin that can handle 64 GPM or we might decide to divide that by 4 and pick 4 catch basins that can handle 16 GPM each (better). These would be spread out on the property
The next step would be to size our catch basins or channel drains to support that level of rainfall. It is better to design several smaller catch basins than one big one. Your clients will be better served if smaller basins were place around the property than just one big one.
After sizing the catch basins, we have to size the pipe coming out of those basins to carry the estimated volume of water. We will feed the four basins to one main line. The pipes coming out of each basin will be sized for 16 gpm and the main pipe will be sized for 64 gpm. From manufacturer charts we decided to use 3 inch pipe coming out of the drains and 4 inch for the main pipe. Typically we size the pipe for about a 2 feet per second velocity. You should use specific drainage pipe since it is less expensive than pressure pipe.
Most subsurface systems collect water through slotted pipe or composite drainage product and the water is carried away in standard drain pipe that should be properly sized to carry the anticipated volume. The number of laterals and their length is determined by the size of the area, the type of soil and the severity of the drainage problem.
Some typical sub surface drainage layouts are shown in the following figures.
Subsurface drainage design is very similar to surface design. The difference being that there are a variety of products available. It is best to contract your local irrigation supply house or call a specific manufacturer for data on the composite material or corrugated pipe to be used. They can help you determine the volume of water that the product can receive and carry per foot.
Drainage installation requires as much skill as does the design. If the area drains are placed too high they will not work correctly. If the pipe is not installed at the correct slope it will not drain. If subsurface products are not installed with proper filtration they will clog and not function.
Proper drainage will help maintain a beautiful landscape or the playability of turf areas. Large project may require that one interacts with an engineering company for design assistance. Finally, most manufacturers are willing and able to assist you in your efforts to design and install quality drainage systems.
Editor's Note: Tim Wilson is a water management consultant with H2O Stewardship Solutions, LLC, a water management/training consulting firm. www.h2o-ss.com