Culverts:  Not just something to pass over (part 7)

Culverts: Not just something to pass over (part 7)

Culverts – most people drive right over without even
knowing they’re there or how important they are. A well-designed, well built, and
well maintained culvert protects the road from flooding and washouts and at
the same time protects important natural qualities of
the stream. But there’s a lot more to putting in a culvert than simply
punching a pipe through a bank. It takes a basic understanding of the problems
associated with culverts along with various design solutions. It requires making good decisions right
from the start and it means doing the job right. These things will make the difference
between creating a culvert that will work well add last a long time or letting
a lot of time, money, and effort go down the tube. Roadways and flowing water. Often a culvert is the
best solution for places where the two intersect but any culvert must be
designed with some basic goals in mind Or there could be serious and costly consequences.
What are the basic design criteria? First and foremost, a culvert must conduct
water freely and inefficiently year-round. A road grade that crosses a wetland or
stream is basically an earth dam and unless there’s a way for the water to
drain through, especially during wet transitional seasons, the road will be in
constant danger of flooding or washing out. Consequences can range from
inconvenient to dangerous and the road can remain
impassable until the water recedes or until major repairs are made. Bridges do the
best job of letting water flow under a road without restriction but bridges
are too expensive to be used at every crossing. A culvert can be a good
economical alternative but it must be sized large enough to handle the flow
particularly during storms or in the springtime when streams are swollen with
runoff. A larger culvert may be slightly more expensive to buy and install but it
will pay off with the savings of time, money, and effort over the long haul. A
second important design feature is the ability to resist erosion. Without a rock armoring or a covering of binding
vegetation flowing water is free to eat away at the banks around the culvert,
eventually causing them to slop or fail. In addition, without proper compaction water will sepe between the culvert pipe
and the surrounding fill Causing a form of erosion known as piping which
can eventually cause the culvert to wash out. Erosion wreaks havoc with water
quality. It’s a huge source of pollution. Spended sediments carried downstream
not only cloud the wate,r they eventually settle out making water shallower and
muddier – a threat to fish habitat, swimming, and other recreational
activities. By using the right materials, following good installation techniques,
and designing erosion controls into the project both the culvert and the
surrounding environment will be better able to stand the test of time. Finally, a culvert must be designed so that it isn’t a barrier to fish or other types of wildlife. Fish often migrate upstream to spawn and many
desirable aquatic animals including fish need freedom of movement to forage. A culvert with
too much water velocity, water that is too shallow, or with an end raised above the
waterline, even for part of the year, will limit or block this movement as well as
the ability of these creatures to survive. The loss of fish and wildlife will
dramatically impact the health, enjoyment, and any recreational aspects or revenues
of this environment. The solution is a culvert design with these things in mind.
One that will permit fish and other critters free passage and protect the ecological
balance of the stream. Achieving these goals and realizing
these benefits is not difficult but it does require some focused thought and
action right from the very beginning. Proper planning is essential. Smart planning begins before a problem or a
need ever arises. A written inventory of existing culverts, noting their
locations and conditions, will save time and money by revealing potentially
costly problems before they develop. Problem culverts should be re- inspected
after each major storm. Once a need is identified it’s still important to
consider the bigger picture: what’s causing the problem or what
caused the previous culvert to fail? What kind of design will cure the
problem and how will this affect the surrounding countryside including any
private property up and down stream? Are there any alternatives such as a
bridge or even abandoning the road would be
better, more cost-effective options in the long run? Addressing questions like these will
help identify a good solution. One that will reduce problems in the future. It will
also help with prioritizing work. There often isn’t enough money or time to
tackle all the projects that need to be done. The trick is to identify the worst
problem areas, fix those properly and completely, and then continue working
down the list as time and resources permit. There are also some project
specific questions that need to be considered up-front. These include the
diameter, shape, length, and material of culvert pipe to be used. The answers will
depend on two things: the site and allocating enough money for the project.
When it comes to money it’s generally best to wait until there are enough
funds to do the job right. Partial fixes such as dumping loads of
gravel after every storm are only temporary, they’re hard on the environment, and
expensive as the cost of repeated fixes quickly add up. We’ve seen how a culvert must be sized
to handle the maximum flow at the site. Generally, the diameter of a replacement
culvert should be based on the performance of the previous design along
with any changes in the watershed that may increase flow. Again, check into the
site’s history, investigate why the old culvert needed replacement, and base the
new one on what you learn. Size is also a factor if beavers are an issue. A larger culvert
will be harder for these animals to block. For these situations, one big pipe rather than several smaller ones is the better choice. A decision will also have to be made
regarding shape. Generally, a pipe with an oval or arch cross-section is often a
better choice for several reasons. First, the lower part of the culvert will carry
more water than a round design. Secondly, flatter cross-sections require less fill
above them which means the road won’t have to be as elevated. If the stream intersects the road at right angles the length of the pipe is fairly easy to determine. The ends must extend far enough out so the slope of the banks
will not exceed a two-to-one ratio. If the stream crosses the road at an angle,
however, the pipe should angle along with it and a longer pipe will be required. It is important to work with the stream. The temptation to save money by
using a shorter pipe and forcing the stream to dogleg under the road will often
end up costing more in the future. Any bends where water enters or leaves
the pipe are potential trouble spots, highly prone to erosion. Pipe materials range from PVC plastic to
corrugated metal to concrete. Because plastic is affordable and relatively
easy to work with it is a popular option for culverts less
than 24 inches in diameter. For larger projects metal and concrete
are competitively priced. The concrete will be more labor intensive to install. In some situations it may be possible to
put a liner in a culvert to avoid total replacement. One thing to consider with a
steel pipe is corrosion. If the water or soil is acidic, a corrosion resistant
alternative such as aluminum or coated metal will hold up better over time. Simple pH testing will show whether or
not steel is a good long-term solution for the site. Sometimes planning will
require getting outside help. The County Highway Department or DNR can
offer up basic advice. For more complicated situations, for example if
water is topping the road, an engineer should be brought in for some expert
design assistance. If you do decide to call in a professional expert or
consultant make sure the person has experience with these types of projects.
Don’t be afraid to get advice from others who have used consultants in your area. The final stage in the planning process
involves scheduling the work. Obviously times when spawning is occurring or when
higher water levels are expected should be avoided. As you put together a
schedule be sure to allow enough time to complete the project. Doing a culvert job right means doing it a to z. Beware of over scheduling – taking on too many jobs in one season invites half-finished or rushed work.
Remember, part of good planning is prioritizing. Finally, it’s important to check into
and secure any permits required for doing this type of work. Once a culvert project has been identified, planned, and scheduled the next step is the work
itself. These jobs are often complicated by the fact that the best way to install
a culvert is to put it in drive, but by nature the work site will have water
running through it. Again, scheduling the work during the
low-water months is important. In addition, coffering the sight, creating a dam or a temporary culvert nearby are ways of redirecting water away from the
work site while the project is under construction. In any case, it’s important
to consider the impacts these or other actions will have on the surroundings
and plan accordingly. This includes using temporary erosion control such as mulch, matting or silt fences while care soil is
exposed or disturbed. With very few exceptions putting in a culvert is going
to take some digging. It’s essential to set the pipe deep enough – approximately one-sixth
the diameter of the culvert should be installed below the level of the stream bed.
This will help protect the culvert from being undermined by erosion and it
allows the culvert to fill with enough water to allow fish to pass through, even in dry seasons. Similarly, the grade or slope of the pipe should
drain water but not so quickly that fish and other aquatic critters will have trouble
moving upstream against the flow. Firm, uniform bedding beneath the pipe is
extremely important for supporting and distributing a load of the overhead
fill. Corrugated steel and PVC pipes should be placed on a bedding of four to
six inches of compacted soil. Soil must also be compacted underneath the curves
of the pipe along either side up to at least half the diameter of the pipe. These
flexible or semi flexible pipes should not be set directly on bedrock or
concrete piling although such surfaces can be an appropriate footing for a concrete pipe. Properly backfilling the trench is a
critical step that boils down to two things: choosing the right materials and
applying them correctly. Fill should be a good mixture of soil
including fine material that will help bind the larger bits of gravel. Compaction is also essential for strength,
support, and resistance against corrosion. One of the most common reasons that culverts fail is because the loose fill was not properly compacted as it was
put in. Good compaction requires backfilling in
layers, laying down six to eight inches of fill, then tamping it thoroughly before the
next layer is applied. Special attention should be given to the
inlet and outlet ends of the culvert. A flared intake opening will help direct water
into the pipe especially during times of turbulent flow. Flaring the outlet end
will help slow the velocity and disperse water across a broader area minimizing any scouring of the stream
bed. In fact, the installation isn’t finished
until erosion control has been thoroughly addressed. This includes
armoring the banks with rock riprap and planting native vegetation that will
bind and shield the soil against running water. Temporary erosion control such as mulch,
hay bales, matting or seeding with oats or rye should be used until the plants become
well established. Finally, the road itself must be groomed to
prevent erosion. water trapped or puddled along the surface will quickly eat away at
the road, causing ruts and carrying soil into the drainage. For this reason,
the road must be properly crown, ditched, and regularly graded to shed water and channel it away from the road bed. “A few years ago we had a major storm. We had several inches of rain one evening and as a
result of that there were sixteen to eighteen culverts that washed out. Our township was declared a disaster area along with other townships in the county. Federal money was available to us to repair the damages. Because we were using federal money the DNR had specifications for these culverts to be put in properly. It was required that we put a culvert end on each end of the culvert fabric around the culvert ends and riprap
covering the fabric. Now this last spring we had a similar storm – six or seven
inches of rain overnight – and all of those culverts that we installed three
years ago they all held. I’m sure that most towns are in the same financial
circumstances that we are in. Where money is tight all the time and we don’t want
to spend any more money on projects than we absolutely have to but we have found
out by installing culverts the proper way, I believe in the long run it has
saved the taxpayers a tremendous amount of money by not having to do the project
over and over again.” Once construction is finished it’s
important not to forget about a culvert. Some regular, easy maintenance will be
required to extend the life and value of the project. As we’ve discussed, regular inspections
can catch many problems early on while they’re still relatively cheap and easy to fix.
Things to look for include damage or deterioration, cracks, corrosion, or
physical changes to the inlet or outlet ends that must be repaired. Also check the road
surface for any unusual settling or movement. Check inside the pipe – any debris plugging the culvert will need to be cleared. If beavers are intent upon blocking up the
culvert their barriers will have to be removed and the animals themselves may
need to be controlled. Monitoring and controlling erosion is
also part of good preventive maintenance. If any slumping or in cutting near or
around the pipe is occurring that can be stabilized before it undermines the
entire culvert. And of course regular grading – grading done properly so that
there are no berms along the sides of the road to trap or channel water will be necessary to ensure good drainage of the road itself. Doing the job right – we’ve stressed this
over and over again and it’s really the key to success with culverts. The bottom line
is this: culverts require an investment in effort and financial resources. The cost
of skimping or cutting corners will be much much higher down the road. A
successful culvert project – one that drains water rather than money and time –
can be done at a reasonable cost and with long lasting results. But it
requires following through at every step. Proper planning careful and complete job execution and some simple follow-up maintenance. When it comes to culverts, these are not
things to simply pass over.

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