Agricultural Market Applications
French & Farm Landscape Subsurface Drains
In gravity flow drain pipe systems the grade of the pipe greatly determines the water flow velocity as the included Hydraulics Graph and Chart indicate. Another important hydraulic factor to consider is the opportunity that water may or may not have for entering into a pipe system.
Inlet Drains and Catch Basins are structure devices for improving the opportunity of water to enter a pipe from above ground. In slow-soak, leach filtering subsurface gravity drain pipes, the rate of water entry is to a small degree determined by the Inlet Area sizing of perforations (slots or round holes) in the pipe.
The most important water inlet control factor for subsurface drain pipes however is the Percolation Rate (PERC Rate) at which water filters, oozes or trickles through a particular soil type profile. Water will not filter or perc through a heavy, stiff clay soil as fast as it will through a sandy or gravel soil type.
Local Farm Officers or Soil Conservation Service Engineers can supply general soil perc rates. At sites with several different soil types, soil perc rates can be complex to discern and even unknowable. To speed up a soil’s perc rate in a stiff or complex soil, rock or sand is often placed around the perforated underdrain pipe. This technique is also used in house basement perimeter subsurface drains.
A filter between the rock and the native site soil will prevent the native site soils from migrating with a subsurface water flow into the rock, and thus over time plugging up the rock rendering the rock-pipe system useless. If sand is used as a perc rate enhancer, a filter is generally applied directly to the pipe to prevent sand fines from entering the pipe.
These underground rock or sand pipe systems are sometimes called “French Drains”, and do not require the maintenance of trash removal from metal grates that a Catch Basin inlet system does. However, in very slow perc rate soils, some type of ground level, surface flow Catch Basin or Inlet Flow Drain structure may be required for improving the opportunity of water to enter a pipe system. Any good drainage system of any size will generally require a balance of both surface and subsurface drainage capability in order to handle both flash-flood rain events, and also long term soaking rain events.
ASTM-F-449 Installation Standard for Corrugated Plastic Pipes may also be referenced for CPP Farm Drain Pipe applications, and this specifications is recognized by USDA-SCS.
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