control factors

rench drains can require a lot the voids and subsequently push over of effort with only marginal into the pipe. Once again, there must effectiveness for many reasons. be enough fall to allow the water to FMost descriptions of a french drain drain out of the pipe; otherwise you are involve a slotted or perforated pipe, allowing the water to sit underground a , fabric and rather than on top of the ground. backfill. To operate properly, the drain Sometimes this is the only way to work must have adequate slope and a place to with a drainage problem, but it is not the open onto the ground, into a drain pipe best way because it does not transmit the or somewhere else where water can exit. water away from foundations, structures, Unfortunately, this fact oftentimes pools, patios, etc. is overlooked. Properly constructed, A further issue with the structure french drains will work well to transmit of a traditional french drain is what Russ Adsit, FASLA water from the pipe to the exit of the is called “blinding” of the geotextile pipe; however, they are not very efficient fabric. This is the process by which in dewatering poorly drained . small particles of clay and silt are Adequate fall means at least 2 ft lower stopped by the fabric and, over time, The Fallacy of per 100 ft of pipe or 0.25 in. per foot of the fabric becomes stopped up and slope. In many cases, inadequate slope will no longer allow passage of water. French Drains to drain the area is the original problem When enough sand, clay or silt gets that usage of a french drain aims to solve. past the fabric, the gravel spaces fill Potential drawbacks, Once a 4-in. drain pipe is placed and the effectiveness of the french underground, it should be covered with drain ends. This is why most experts and alternative 2 in. of and attempts should be state that a french drain is not a long- drainage solutions made to have ample slope to drain the term solution to a drainage problem: pipe. If these conditions are not met, You have to dig it up and reinstall it it would be better to allow the water every eight to 10 years. to drain across the surface. For these What to do when you have a drainage reasons alone, a french drain can require problem? The best solution is to drain a lot of effort with minimal effectiveness. the water across the surface using a Another reason why a french drain is minimum of a 1.5% to 2% slope. If the not always an effective solution relates water of concern is standing water after a to the basic properties of water. Water rain event, you may be able to use one of “sticks to itself” by a process called the newer products such as a sand-filled cohesiveness. A simple science class trench with a slotted pipe conveyance. experiment in which a penny is placed Regardless, you still need to empty the on top of a glass of water makes the pipe somewhere. penny float. It does not really float, but With storm water or surface water rather rests on the surface tension of from rainfall, runoff or other sources, the water. Once a drop of soap is placed you will need to use surface runoff into the water, the penny sinks because (best practice), underground pipes with the surface tension gives way to the adequate fall and discharge points or weight of the penny. a type of containment and retention In much the same manner, rainfall or sump system. Without one of these sticks to itself and to the surface of soil, solutions, you run the risk of overtaxing gravel, sand or whatever other surface the system and backing up water where with which it comes into contact. Water you do not want or need it. SWS WEBresources  would rather stick to something than Related search terms from simply fill a void. Thus, water usually Russ Adsit, FASLA, is executive director www.waterinfolink.com: sticks to the gravel in a french drain of the International Erosion Control Assn. french drain, drainage, runoff rather than filling the voids. Adsit can be reached at 800.455.4322 or In this manner, the only time that the by e-mail at [email protected]. For more information related to this drain pipe fills with water is when there article, visit www.estormwater.com/ For more information, write in 5002 on lm.cfm/st061002 is enough water to stick to the surface of the gravel and be heavy enough to fill this issue’s Reader Service Card.

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