To help illustrate the process of
analyzing the culvert for inlet control conditions
I want to do a quick example. So let's say this is our roadway,
and the brown line shown there is our embankment, and the blue line is
a flow line going underneath it, and so we need the size the covert
that will allow that water to pass into the roadway without overtopping.
So we cut a section,
and show what our roadway section looks like at the crossing.
There's the crown of our road at the top,
the pavement edge on the
the line underneath and the crown of the roadway is about 871.65 feet,
and the invert of the channel right now is about 861.82 feet.
So from our hydrology analysis our flow rate is a 162 cubic feet per second.
And then our orifice flow equation has
Q but we already know cue so we need to solve for
H in this case. So we'll solve for H and get
this expression. The other challenge to
our design of our culvert is that we also don't want to interfere with the road
subgrade grade, so there's another
foot perhaps underneath the roadway edge
that can not be touched
for fear of destroying the integrity the roadway above.
So there's our pipe selection. I'm going to assume
we have a headwall. So we have a grooved en pipe
with the headwall which gets us a coefficient of 0.2.
minor loss entrance coefficient of .2 and this is
an 84 inch diameter pipe so one of those pipes is
38.5 square feet. There's only one pipe so that means the total
culvert area is 38.50 square feet. I plug those values into my expression that i
solve for H, and
I get H equal to 6.88 feet
now the first design great year they can easily see if maybe this is specified or
not but we'll just
check it is if the headed the headwater death is less than 1.5 times a day after
the pipe which in this case it is
so that's good but we define what is the water surface elevation
and in this case the water surface elevation is the inverted the pipes plus
6.88 which is the headwater death
plus the pipe diameter divided by two
another word seven feet divided by two fairly simple calculation wouldn't get
that in yourselves
I get 870 2.32
army and that is bigger than the top of our roadway and water is
spilling over that's no good so we need to do some revising here
one other easiest thing to do just use multiple barrels that are smaller
where you can take 84 shrink it down a little bit
use multiple barrels I suppose you could pick a bigger pipe diameter instead of
an eighty four ish maybe 296 are 108 but
when you start getting that high now you're interfering with the roadway so
great
so it put in multiple barrels
I think 54 inch diameter pipes three of them
gives me a total call for area forty 7.7 square feet
that means is a you change that
unit what that means is that we have a three by 54 ish
RCP and that the area is already bigger than our 184
so using their calculation again we've
soul H to be 4.48 feet
which is less than 1.5 D so we're not finding excessively
what is the water surface elevation 860 8.55
were less than that of the roadways were not overtopping
which is very good so done right all week
is the multiple barrels now to why
well how do we know with the with this will
as you might suspect the with is at least three times the diameter heat for
these pipes
so d times three
right but there's also wall thickness ago worry about in space with concrete5
relatively sick
so here in this little expression I develop into the number a very roles and
he is the wall thickness
so for 54 inch pie wall thicknesses five-and-a-half
inches in the with
love that area is a hundred ninety five inches or 16 feet 3 inches doesn't work
will that be too y8 will
this is where you now need to transport this into your space into your
surveyed construction alignment determine if whether or not there is
enough space for
that many berles in your waterway
one thing is hopeful the check is what is the with the natural channel are you
installing something that is wider than the natural channel because if you are
chances are a you know he you doing too much here that's not gonna fit properly
you don't want to excavate out in order to fit water through covert necessarily
so here's some other examples so we have an existing covert
that the four by four RCB so that means it's a reinforced concrete blocks
and there's three OVA so this is three by four by four RCV concrete box
groove didn't with the headwall
job is find the discharge
where the hi-eight is less than or equal to 1.5
planes that be for the words I've given you essentially
area you need to look at the sea value
have given you H as being 1.5t
boots QB today just need to calculate you fairly simple
example here's another one let's say we have a temporary creek crossing at a
construction site
the flow rate is 78 cubic feet per second
in the contractors installer once since dole one
66 inch RCP it's got a protruding into
grooved and so the plane and by
a from there find what is the embankment elevation above the inverter
you gonna give me back at how many feet above the inverter elevation
is the the pike there
so try out those two examples give them a try and
then post your answers as part of your lecture activities this week