How to Design a Drainage System that Works Lessons from the Flood of 2001 Back to Drainage Plans

Overview relating to the June 2001 flood. With all the rain that we have had recently, I would expect that there are contractors coming "out of the wood work" to sell you a drainage system. Before you say "yes" let's do some homework. The rain we had in June was not a 100 year flood. At best, it was between the 25 year design storm and the 50 year design storm. In the Houston area within a 24 hour period "x" much rain must fall to be considered a "x" design storm. 4.5-5.5" in a 24 hour period =  6.5-7.5" in a 24 hour period =  8-9" in a 24 hour period =  9-10" in a 24 hour period =  10-12" in a 24 hour period =  12"+ in a 24 hour period =     2 year design storm    5 year design storm   10 year design storm   25 year design storm   50 year design storm  100 year design storm I have already heard several times that "this is the worst I have ever seen the water rise." That is probably true - but take into account that Houston is a rapidly growing city. There are more houses and roads than when Alicia was here in the 80's. There is more water being sent into Simms, White Oak and Buffalo Bayous at a faster rate than ever before. These flood ways may not be able to handle the "load" as evidenced by the water rising so quickly in the front yards. This does not mean the water quit draining in these areas - there was just too much water at an increased pace for the waterways to be able to drain it as quickly as they have drained it in the past. The Corps of Engineers is working to increase the drainage capacities for the amount of runoff water at various design storm calcuations. Draining a back yard, the non-engineering way. In this section we are going to discuss how to get the water out of the back yard and to the street. The most critical elements are elevation above the street and the volume of water you want to remove. When designing a proper drainage system the first step is to look at the watershed. The watershed is the area of your property that needs to be drained. This includes the pervious and impervious area. Pervious area is area where water can be absorbed into the ground. (i.e.:grass, pool, pavestone) Impervious area is the area where water does not absorb into the ground. (i.e.: roof, house, and concrete) Water runs off concrete and roofs faster than it does over grass. The amount of water and the rate at which it needs to be drained is called the "load". You need to size the pipe based on the load calculation. Without going into the engineering side or the Mannings equation for drainage calculations, there is a good rule of thumb that can be used for light drainage situations. The more detailed engineering information is in the next section. 4" SDR 35 pipe from downspouts to the catch basin (or drain), 6" SDR 35 pipe from the catch basin to the street. Why? Because most downspouts are 4" pipes and two or more downspouts will overload a single 4" pipe going from the house to the street. A single 4" pipe can not handle the load in a 25 year design storm when it is trying to drain a medium sized back yard. Especially if you are draining your neighbor's water, too. A special note: 4" black ADS pipe is not legal for drainage systems. It degrades over a period of years and crushes under marginal weight. Yes, ADS is cheap (about $1/lf) compared to doing it with SDR 35 or PVC schedule 40 . But you will be paying twice to get it right. To be self cleaning a 4" pipe needs to slope at a minimum of 1%. This means that in order for the pipe to drain debris, water, and mud; it needs to drop one foot for every one hundred feet of distance. Most homes will have that kind of fall. However, if you don't have a house that sits at least two feet above the centerline of the street, you will need a larger pipe that needs less slope to be self cleaning. A 6" pipe will be self cleaning at a minimum of .75% slope. It should drop 8" for every 100 linear feet of distance. A 6" pipe can usually handle the load of a 25 year design storm when sloped to be self cleaning. Average cost for installation is $20.00 per lineal foot. An 8" pipe will be self cleaning at a minimum of .52% slope. It needs to drop about 6.25" for every 100 linear feet of distance. An 8" pipe will handle the 50-100 year design storm. Average cost for installation is $25-35.00 per lineal foot. A quick note on drains or catch basins. Do not use plastic ones. Use concrete catch basins with metal grate and frame. The grates should have 1/2" minimum openings. Plastic is much cheaper - but you may be very unhappy with their performance. Let's look at your yard. #1 Measure the elevation the entrance to your house sits above the center line of the street. Let's say that the elevation of the street is 100.00' and your house ff is 102.00'. This means that you have 2' of elevation difference between the centerline of the street and your finished floor elevation. Or you have 2 feet of "fall". #2 Subtract the elevation distance from the top of the catch basin to the entry of the pipe. (Usually 4-6"). 2'-6" = 1'6"'. #3 Subtract the pipe diameter from the amount of fall you have - because water drains from the inside of the bottom of the pipe (not the top of the pipe). ie: 1'6"-6"=1'. You have 1 foot of fall for the 100' of distance or 1%. It would seem that a 4" pipe can do the trick - but decide how much water will be going into the 4" pipe. IF your backyard is larger than 30x60 with a 2000 sq ft house - there is too much water for a single 4" pipe to drain effectively. The pipe will not be able to handle the load and water will "back up" while waiting to get into the pipe. This is exactly what you are trying to avoid. Sloping any pipe at more than the minimum will make the water drain faster. So there are ways to use smaller pipe to achieve better drainage - but that depends on the amount of fall you have. Just because someone puts drain pipes into the yard does not mean that you are protected. The pipes need to be cleaned monthly if they do not slope enough to be self cleaning. The openings of the catch basins need to be large enough that debris does not get trapped on top and keep water from entering the system. The catch basins should have a trap at the bottom to catch debris to keep it from getting lodged in the drain pipe and this needs to be cleaned out every month. Standing water in the catch basin is also a great place to breed mosquitos. If all this seems to be too much of a headache - consider digging drainage trenches. The only cost is labor. They do not need a permit whereas a drainage system does require a City of Houston permit. But, the drainage swales will need to be maintained. Portfolio for Drainage