Drainage System Components
There are several drainage components that are fairly common in most
drainage systems. This section introduces some basic components that are
part of the drainage systems described in the following section - Drainage
Systems. Successful construction involves selecting the appropriate
materials for the collection, conveyance, and discharge requirements of your
system. Attention to proper capacity and durability of each drainage
material is critical. The performance of your system will be improved by
using good construction techniques and by performing routine periodic
Installation Considerations |
Care and Maintenance
The quality and availability of drainage products vary greatly from area
to area. However, with a couple of weeks of pre-planning, any material can
be delivered to your site at a reasonable cost. Consequently, do not settle
for poor quality materials when you can have any item you need with just a
Drainage pipe is available in rigid wall and flexible wall lengths. On
individual lots most applications require fairly small diameter pipe (4
inches to 12 inches). Generally, plastic pipe is used by contractors based
on cost, ease of installation, and availability. The walls of pipe vary
from thin and corrugated to thick and solid. Each pipe type has some
degree of flexibility over the length which has many advantages in slope
applications and some disadvantages. The biggest disadvantage is that the
installer must check the grade of the pipe to confirm there are no reverse
slopes or low points along the length of pipe which may reduce the
performance of the drainage system.
A variety of solid and perforated
pipes shown with a strip drain.
The least expensive pipe selection is usually the thin corrugated pipe
which is available at most home centers and discount stores. The pipe is
very flexible and is sold in coils of one hundred feet or greater. This
pipe may be satisfactory for very shallow installations without
significant backfill or backfill compaction, but it can fail easily by
being crushed during installation with heavy equipment and by the weight
and compaction of soil backfill. It may also be susceptible to degradation
by ultraviolet light. Finding watertight connections (couplings) can be a
problem with thin corrugated piping. Duct tape is not a long-term
connection solution. Using corrugated pipe with thicker walls and
watertight connections can help avoid the crushing and leaking problems
common with thin-walled pipe installations.
Other plastic pipe is available including corrugated pipe with smooth
interior walls, rigid pipe, light duty and heavy duty pipe, and perforated
and non-perforated pipe. Aluminum culverts and concrete cylinder pipe are
available for surface water routing under road surfaces and down slopes.
Culvert pipes are usually purchased in diameters of 12 inches or greater.
Plastic drainage pipe are produced from different materials but are
commonly made with polyvinyl chloride (PVC), polyethylene (PE), and high
density polyethylene (HDPE) material.
Rigid pipe is sold in short lengths of approximately 10 to 20 feet and
requires good connections at each end. Pipe sold in coils requires fewer
connections which is a benefit but may have a low strength for wall
crushing or buckling depending on the pipe manufacturer's specifications.
Ask your pipe supplier about pipe details.
Perforated pipe allows water to enter or exit through small openings
along a length of a pipe. The openings can be circular or slots. The more
opening space per foot of pipe, the greater the capacity of the pipe to
collect and move water. Slotted pipes have an advantage over pipes with
small holes because they tend to reduce the amount of fine soil particles
that get pulled into a drainage system and discharged downstream. Solid
pipe has no openings in the walls and is commonly referred to as tightline
pipe in drainage applications.
A geotextile is a permeable fabric material made from synthetic
polymers. Geotextiles are used in many engineering applications. The
primary functions of a geotextile in drainage applications are filtration
and drainage. Simply, the geotextile retains the soil while water passes
through the fabric and into the drainage collection system. Geotextiles
used in drainage applications can be woven or nonwoven fabrics. The woven
geotextiles have a weave pattern while the nonwovens are formed from a
random pattern of fibers bonded together.
Typical drainage applications include interceptor drain construction,
drainage blanket installation and geotextile wraps for pipes. For
interceptor drains, the geotextile is used to line the trench prior to
backfilling with gravel drain rock to limit the migration of the adjacent
soil into the gravel backfill. For drainage blankets, the geotextile is
used in a similar manner, except it is more commonly placed on a slope or
a more horizontal location. Sometimes, the geotextile is wrapped directly
around pipes for ease of construction at difficult locations.
Geotextiles can be obtained from local distributors. Each distributor
represents multiple manufacturers of geotextiles. The geotextiles can be
picked up directly or shipped from the distributor to the buyer. The
geotextiles are commonly sold by the roll, with few distributors willing
to sell smaller quantities cut from a roll. The common roll width is 12.5
feet with a typical total square yardage per roll ranging from 500 to 600
square yards (sy). The cost to buy quantities less than a full roll varies
depending on distributor.
It is important to differentiate between landscape fabric available at
local hardware and garden centers and appropriate geotextiles for drainage
applications. Typically, landscape fabric is not durable and will
eventually clog which can lead to serious slope stability issues. See the
links section for geotextile material sources.
Catchbasins and Manholes
Drainage systems need a method to collect and concentrate water flow at
a location. Catchbasins and manholes allow pipes coming from different
directions and elevations to converge at specific locations. They can
convert surface flow to subsurface pipe flow. Catchbasins and manholes can
trap larger sediment and debris allowing only drainage with fine sediments
to enter pipes. Also, they can provide a drop in elevation down a slope
and dissipate the energy of pipe flows. Catchbasin and manhole structures
are commonly constructed of concrete or polyethylene and have a number of
lid options ranging from open grates to watertight construction.
A pipe coupling connects one length of pipe to another without leaking.
They are sold as rigid or flexible connections. Improper or poorly
retrofitted connections on drainage pipes are very common failure
locations. It is important to use a good connection system or couplings
specifically manufactured for your pipe. The couplings should have a
watertight gasket seal. When pipe is placed above ground on a slope and is
not anchored correctly or has too much flow resistance, you will likely
see a coupling failure in the future. By planning ahead, you can minimize
the number of couplings by ordering longer lengths of pipe.
Pipe anchors should be used for pipes on steep slopes. Pipe bends
should be adequately supported. Water flowing downhill can place
tremendous forces on pipe connections and bends. Do not underestimate the
need to resist these forces by anchoring and supporting the pipe. Pipe
anchors are not purchased but are built in place. Pipes should be anchored
above grade as well as below grade. Figure 10 (below) illustrates a number
of anchoring systems.
Drainage gravel should be rounded rock ranging in size from 3/4 inches
to 11/2 inches in diameter. The gravel provides a uniform bedding for
drain pipes to create a consistent pipe slope and provide a free draining
material adjacent to perforated pipes. Water moves through the spaces
between the gravel before entering a pipe or other means of conveyance.
This material is usually delivered to your property by a gravel supply
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Always mark the locations of utilities before you begin construction. You
can call the Utilities Underground Location Center "1-call" at
1-800-424-5555 for a free service that will locate the utilities that
service your property. Utilities can include power, telephone, water, gas,
cable, and others. There may be some utilities and underground features that
only you know about (power to an out-building, drainage lines, buried
tanks). Be sure to also locate these utilities prior to construction.
Drainage system installation should be done during dry weather periods
except in emergency situations. Excavations can quickly be flooded with
water making proper construction difficult and dangerous. If there is a lot
of water present while excavating soil, you will have sediments accumulating
in the gravel openings, on the geotextile, in pipes and catchbasins, and in
marine waters where the drainage will eventually discharge.
Avoid construction when the ground is wet. Your system will perform
better with dry weather construction and not cause sedimentation in other
Figure 10. Anchoring Systems
Before you complete the installation of your system check to make sure
pipes have not been crushed by heavy equipment. Make sure each connection is
solid and not leaking. Check the slope of pipe runs. It is a common practice
to water test your drainage system before covering it with soil
Water must move downhill so double check to see that water moves to your
planned locations. If you do not do the installation yourself at least
observe your construction so you can help troubleshoot any future problems.
Make a photo record of the work if you have a camera handy. During
construction you should take your plan drawing and sketch onto the plan what
actually was installed. Important items such as locations, depths, sizes,
and problems will help you improve or expand your system later. A system
"as-built" will also help you or a future property owner avoid damaging the
system during other site improvement work.
Care and Maintenance
Every drainage system needs some periodic inspection to see that the
system performs properly. Surface features like yard drains, roof drain
catchbasins, manholes, swales, above ground pipes and couplings, pipe
anchors, and discharge areas can be quickly checked. Catchbasins and
manholes are usually designed to capture debris and heavier sediments and
will require the removal of a few buckets of material from time to time to
prevent pipe clogging and discharge of material into water bodies.
Below ground drainage features like pipes, strip drains, couplings, and
overall system performance should be checked regularly for signs of failure
during rainfall events. Overflows, leaks, wet areas, flow bypassing your
system, and discharge interferences can be noted and immediately repaired if
you detect the problems early.
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