Hi, my name is Scott Grzyb, and I am a hydrologist
at the Texas Water Science Center in Austin, Texas.
In the previous video, I gave a brief overview
of what a contracted opening indirect
discharge measurement is and discussed some things
to consider to ensure proper site selection.
In this video, I will give a more detailed explanation
of what information is needed to complete
the contracted opening computation.
For a more in-depth explanation of width contraction
or contracted opening indirect discharge measurements,
please refer to TWRI Book Three, Chapter A4.
For a contracted opening indirect,
there are many individual measurements
that contribute to the final computed discharge.
Some of the data are used to define channel
geometry while others are used to derive
an adjustment factor, or discharge coefficient,
from empirical charts created in a laboratory setting.
Like with any indirect measurement,
careful documentation is very important
for both the computation and subsequent review.
In an effort to keep the process efficient,
I will list the requirements in an order
that I find to be ideal when working in the field.
First, start with high water marks.
High water marks are always a good starting place
for any indirect measurement
because without reliable high-water marks,
a peak discharge measurement may not even be possible.
If available, begin your survey by tying
into a local datum so that recorded sensor readings
can be used as reference while you survey.
As discussed in the previous video
on contracted opening indirect measurements,
shoot some high-water marks on the upstream,
and downstream embankments to verify
that you have the 0.5 feet of fall required.
If the fall is 0.5 feet or greater,
continue shooting the rest of the high-water marks
on the downstream embankment.
If the high-water marks are poorly defined
on the downstream embankment, it may be necessary
to continue downstream until a reliable elevation
is determined for section three.
Finally, shoot the high-water marks
throughout the approach upstream of the contracted section.
It is important to remember, that the high-water marks
will be the sole representation of both the section one
and section three surface water elevations,
so take your time and survey plenty of marks.
Once you have shot the high-water marks
and determined they are sufficient
for a contracted opening measurement,
you may begin measuring the channel geometry.
It does not matter whether you start at the bridge,
or the approach section, as long they are tied
to the same reference datum.
The approach cross section is normally surveyed
one bridge width upstream and is used
to represent the natural, unobstructed flow in the channel.
One bridge width was adopted because it is
usually far enough upstream that you are no longer
in the drawdown zone.
While surveying, be sure to make a site sketch,
take pictures, and describe the roughness
throughout the section just as you would
with a slope-area indirect measurement.
After the approach section is finished,
you can move on to the contracted section.
The contracted section is located
at the smallest cross section between the abutments
on a line parallel to the contraction.
If the bridge is skewed to the flow,
Section Three is still parallel to the contraction
even though the minimum section may be
perpendicular to the abutments.
The most common location for the contracted section
is between the abutments at the downstream side
of the bridge, but it doesn't hurt to break out
a steel tape and double check.
For a contracted opening, since the bridge will be acting
as our measuring device, it is necessary to record
complete details of the bridge geometry
including all of the following where applicable.
Wingwall angles and lengths.
Measure the angle of the wingwalls using a protractor
and measure the lengths using a steel tape.
The easiest way to determine the angle is
to align the straight portion of the protractor
with the face of the bridge at the intersection
of the wingwall and the bridge.
You can now read the wingwall angle directly.
Lengths of the abutments.
Each bridge type defines a unique measurement
location for length based on
the type of bridge entrance you are working with.
The most common length measurement, denoted L,
is the distance from the intersection
of the section one elevation with the upstream embankment,
to the contracted section, or section three.
To make sure you are measuring the correct length,
refer to the TWRI manual based on your bridge type.
The positions of the embankments and abutments are important
for determining what additional adjustment factors
are necessary for your type of bridge.
It may also be important when starting computations
because additional information may be described
that otherwise wouldn't be.
An easy way to record this information is
to use your total station to shoot points
at both the top and bottom of the embankments and abutments.
Additionally, make sure to note any skew
or irregularities in the abutments.
Side slopes are needed to determine adjustment factors
for the discharge coefficient.
Unlike what you may remember from math class,
these slopes are measured as the ratio of Run over Rise.
You can measure these by using a level of known length
held horizontally out from the abutment, this is your run.
Next measure down with a folding rule
to get your rise elevation.
The ratio of the run over the rise, is your side slope.
If the side slopes are different from the inside
of the bridge to the embankment,
measure both so that later,
an average can be taken as the final side slope.
Radius of curvature.
This is the radius of curvature at the entrance
of the bridge abutments.
This measurement is only used for a type one bridge
with vertical embankments and abutments.
To measure, lay a small hand level
or another straight edge flat on one side of the abutment
and a folding ruler flat on the other side.
In between, there will be a gap
from the 90-degree corner created by the two tools
and the rounded corner of the abutment.
Then measure the distance on the folding ruler
from the hand level to where the abutment wall is
again parallel with the folding ruler.
Repeat this step by switching the positions
of the straight edge and the folding ruler.
The average of the two values is
the radius of the entrance rounding.
Elevation of the roadway.
This information is useful for ruling out
the possibility that the flood overtopped the roadway.
If it is determined that the flood caused flow
over the road, it will be necessary
to treat this measurement as a combination site
which is explained further in either the TWRI book three,
chapter A4, on page 33, or in the multiple components
video available in this series.
Top width of the embankment.
The top width of the embankment is necessary
for an accurate plan view and elevation view.
This can be measured either by a steel tape,
or in the event of heavy traffic,
can be shot using your total station
or differential leveling equipment.
Details of the piers or piles.
Complete information about the piers and piles
including the width, height, shape,
and quantity is necessary
so that you can accurately define the area submerged
in the cross section.
These occupied spaces will be subtracted
from the overall area when determining
the conveyance for the contracted
cross section, section three.
The pier and pile dimensions are also necessary
for determining the wetted perimeter
when computing the hydraulic radius, R.
Finally, elevations of the bottom of the girders
or beams spanning the contraction might need to be shot.
This information is useful for determining
whether the flood was touching or backing up,
against the girders or beams.
You can shoot these elevations using the reflectorless
setting on your total station, if so equipped.
If the bridge is low enough, you may be able to use
the inverted rod technique by flipping your digital rod
upside down and reading the negative elevation returned.
If all else fails, you can shoot the bridge seat
at both ends of the bridge and assume
a straight-line elevation between the two points.
If you think this was a possibility during the flood,
make sure to document the heights accordingly.
If you are having difficulty in the field,
or need additional help, don't hesitate
to contact your field office chief,
data chief, surface-water specialist,
or indirect measurement specialist.