[AT] OT Low water crossings

[Carl Gogol]

Much of our property lying lower than the house is prone to flooding from a 
major creek.  I took the same philosophy as your previous owner and where we 
have trails we have built "More weather" roads instead of "all weather". 
Constructing private trails using concrete over the pipe would be a bit 
pricy and I have looked for old heavier wall steel pipe that would easily 
take the loads without any compacted material overfill.  This way a very 
minimum road elevation is required for a driveway without any hump at the 
pipes.  Keeping the trail lower reduces the resistance to flow when the 
water raises 4' or so.

For your application only a few inches of concrete, enough to maintain 
physical integrity over time, would be required over such pipes.  For light 
wall corrugated or plastic pipes, the use of concrete would still only 
require minimal coverage.  There may be real engineering recommendations 
available, but since you are encasing the pipes in concrete you can almost 
treat them as forms and the thickness over the very top is again not going 
to have to be all that thick.  6-8" should be enough since the thickness of 
concrete is increasing as soon as you move away from the top center of the 
pipe.  The concrete is essentially forming an arch over the culvert.  If you 
wanted, it might be worth placing some short say 24 or even 48" pieces of 
rebar over the top of the culvert that are oriented perpendicular to the 
culvert's axis.  This would substantially increase the tensile strength of 
the concrete in that area.

I have built one bridge that is 18' long crossing a drainage creek that is 
about 6' deep that feeds into the main creek.  A culvert wouldn't work well 
here and after much hesitation the span material that was chosen was used 6" 
X 8" galvanized steel box beams that had seen service as guide rail along 
major roads.  Three of these are combined together into a plank by using a 
steel spreader assembly bolted to keep the beams about 3" apart.  The 
spreaders are placed at the 6' and 12' locations and are heavy enough to 
spread the load across the three beams.  Two of these assemblies form the 
bridge.  The total width can be changed by sliding one of the plank 
assemblies into a wider or narrower position.  This structure is very ridged 
and strong.  The major design challenge is not to make a bridge carry the 
vehicle load, it comes from water flowing over the top and uprooted trees 
floating down the stream in the spring runoff or a storm.  The use of the 
box beams gives the bridge a profile of only 6" for the water to push 
against.  Trees remain the real problem and cables to upstream anchors have 
so far (5 + years) worked sufficiently.
Carl Gogol
Manlius, NY

-----Original Message----- 
From: Don Bowen
Sent: Thursday, February 09, 2012 5:20 PM
To: Antique tractor email discussion group ; Little houses
Subject: [AT] OT Low water crossings

My driveway crosses two little creeks.  They are usually dry except
during storms when they can be as much as four feet deep.  After a storm
it may take days for them to come down.  The first crossing is gravel
and the previous owner had build a concrete and rock dam on the
downstream side to control washouts.  The result kept the water over a
foot deep for days after a small rain.  I cut a slot in the dam and now
it drains down to about 6 inches in a day.

The second crossing is a concrete low water bridge with two culverts.
usually within a few hours of a rain the water is under the concrete
flowing though the culverts.  The problem here is that the culverts plug
with gravel in heavy rains.  The reason is that who ever built the
crossing has the outlet end of the culverts higher than the inlet by
several inches.

I plan to take out the culverts and replace them with a single larger
culvert properly installed.  What I need to know is how thick the
concrete must be above a given size of pipe.  I am assuming that the
larger the culvert diameter the thicker the slab over it must me.

As for the gravel crossing I want to build a concrete crossing.  I need
to know what I need to do and references.  I have a good idea what is
needed but I would like comments from those with experience.

You can see in this picture the right culvert is plugged.  The water at
the inlet is almost at the top of the culvert.
http://www.braingarage.com/Dons/Travels/journal/May%2011/images/flowing.jpg

This is what it looks like after a heavy rain.
http://www.braingarage.com/Dons/Travels/journal/April%2011/images/flooded%20drive.jpg

And a heavier rain.
http://www.braingarage.com/Dons/Travels/journal/April%2011/images/flooded%20drive.jpg

The gravel crossing after a rain before I opened up the dam a
little\http://www.braingarage.com/Dons/Travels/journal/January%2011/images/summer.jpg


-- 
Don Bowen           KI6DIU
http://www.braingarage.com/Dons/Travels/journal/Journal.html

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