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Ineffective
flow areas often occur near roadway crossings when water levels exceed the
channel banks and when water cannot flow in the longitudinal direction
along the overbank areas due to roadway fill. When this occurs, flow must
contract to pass through the opening under the roadway adding additional
and often significant losses. However, if the roadway overtops,
flow becomes possible in the overbank areas as well as in the main
channel.
For
example, consider the above diagram. Channel link A-B approaches a
roadway crossing. Link B-C is the pipe under the roadway and link
C-D is a channel segment downstream of the roadway. The roadway has
a low spot at elevation 61' left of the pipe and elevation 62' right of
the pipe.
For discussion purposes, assume that the top of bank in
the channel immediately upstream of the roadway is at elevation 59'.
When water levels at node B are between elevations 59' and 61', the
overbank areas are inundated, but very little longitudinal flow can occur because
of the roadway fill. However, when the water level rises
above elevation 61', longitudinal flow can occur in the left overbank and
if the water level exceeds elevation 62', then flow can occur along both
overbank areas.
Ineffective
flow areas can be modeled in ICPR by using something called "auxiliary
cross sections" in the channel link data form. Auxiliary cross
sections allow you to change cross sections based on trigger elevations during the simulation without
stopping the model. The procedure for
doing this is described below.
Download
Example
ICPR
v3 Data File
1.
First, a cross section must be entered assuming fully effective
flow in the overbank areas. This section is called
"X-MAIN" in our example. Next, the
"encroachment" option is set to "yes" and the
stations corresponding to the ineffective flow areas are entered. In this
example, the left and right encroachment stations are set to
"-50" and "+50", respectively, which are the
left and right top of banks. This, in
essence, makes the left and right overbanks ineffective from a
conveyance standpoint.
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2.
An exact copy of cross section "X-MAIN" can be made by
clicking the "copy record" button on the tool bar, then
the append button (the green plus sign) and then the paste button.
If you're unsure which button to click, simply hover over one for a
second or two until you see a tool tip. Once a copy of the
original section has been made, change the name to
"X-AUX1" and set the left and right encroachment stations
to "-350" and "+50", respectively.
Anything to the right of station "+50" becomes ineffective.
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3.
The copy record-append-paste sequence is used again to create a
third cross section. Rename it to "X-AUX2" and set
the encroachment stations to -350 and +350. In this case,
the entire section is now effective. Another way to do this is
to simply set the "Encroachment" option to "No".
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4.
The last thing that needs to be done is to call out each of the
cross sections in the channel data form. These are set at the
downstream end of the link as indicated at the right. Note
that the first auxiliary section is used when the water level at
node B reaches elevation "61" and the second auxiliary
section begins at elevation 62'. ICPR uses the full cross
section (i.e., it ignores the encroachment) for continuity purposes,
but uses the reduced sections for conveyances purposes.
Note:
Channel link C-D can be handled in a similar fashion, except the
auxiliary sections would be used at the upstream end of that link
instead of at the downstream end. Flow expands instead of
contracts as it comes out of the pipe. |
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