- After a flood event we are often left
with a lot of questions such as,
how high did the water get,
what was the total discharge,
and how does this event compare to past flows?
To answer these questions the USGS
and it's hydrographers employ a variety
of techniques and methods to reconstruct the flow.
The Slope-Area Method is a common approach.
It allows hydrologists to recreate the peak
water-surface profile of a flood by using
the high-water marks left behind by the flood peak.
Because the slope-area computation relies
on the high-water marks left behind
from a flow event, the data it produces
is a single point showing stage and discharge
of the peak of the flood.
While this data is extremely valuable,
it doesn't give us a complete picture of a flow event.
The USGS has been working to develop a new method
to measure stream flow throughout the entire event.
Providing more robust data during
the rise and fall of flood waters.
This new method is the Continuous Slope-Area Method.
It's procedure is quite similar to the traditional
Slope-Area Method, and builds upon this foundation
by utilizing three or more continuously recording
pressure transducers within a stream reach.
These sensors are quite small and can be installed
at existing stream gage locations.
When these sensors are properly installed
they can more accurately document water surface profiles,
than surveys of high-water marks.
What sets this apart from the Slope-Area Method
is that the sensors measure the water surface
of the stream-reach at five minute intervals,
allowing for a complete computation of the flood hydrograph.
This method provides a safer alternative to
direct discharge measurements, while providing
discharge data at stream gages that are inaccessible
or have no provisions for making flood measurements.
When considering the location of a CSA reach,
there are a few guidelines to follow for best results.
You want to choose a straight reach, with a
stable channel, that doesn't undergo frequent scour or fill.
Sensors are installed at three or more cross sections
that are similar in shape and area for a given discharge.
These cross sections should be located at breaks
in the water surface slope, with a mild contraction
in the reach being preferable to expansion.
Lastly, there should be at least half a foot
of fall between cross sections.
Aerial photographs can be very useful
when deciding where to install a CSA reach.
They can help you identify straighter sections
of the channel, and get an understanding of
vegetation characteristics that may affect your data.
If available, you can also look at previous
slope-area measurements, which will give you
an idea of the cross section characteristics.
Additionally you will want to re-evaluate
cross section locations after flow events
to verify that there's enough fall between the
cross sections, and the cross sectional areas are similar.
The design of the installation will vary
depending on channel composition
and anticipated flow intensity.
All installations are designed for
low disturbance and survivability.
This limits debris pileup and draw-down
that can reduce the accuracy of the sensor readings.
Here we have several examples of installations.
One in a sand channel.
A bedrock installation.
And another in a cobble channel.
Within the housing is the data logger,
a non-vented pressure transducer that measures
the water stage in five-minute intervals.
These transducers have the capacity to record
five-minute data for up to a year.
And have an accuracy of three hundredths of a foot.
Since the stage sensors are non-vented, a barometer
is needed on site to compensate for atmospheric pressure.
One barometer at each site is sufficient.
After a flow event you will have to travel
to the site to download the data from the sensors.
To document that process we've developed a field form
with the necessary information you will need to record.
For instance, you will want to document the condition
of the sensor, whether it has been buried,
or has debris piled up around it, the condition
of the cross section, and any noticeable scour or fill.
Additionally, you should compare the clock time
reported by the sensor to the laptop time,
to check for any drift that could have occurred.
To account for changes in channel shape,
surveys are conducted upon installation
and after significant flow events.
You'll need to survey the CSA pin elevations,
cross section geometry, high-water marks,
and gage reference marks.
A total station or RTK GPS unit
can be used for these surveys.
While in the field you will also want to take photographs,
noting the location and including something for scale,
like someone holding a survey rod in the channel.
This will help the evaluation of vegetation
through time, so that the channel roughness
can be adjusted as it changes.
The CSA method brings a safer and accurate alternative
to the hydrographer's toolbox, allowing long term
streamflow data collection to occur
in even the most remote locations.
The stage data from the CSA gages can also be used
to calibrate one and two dimensional models.
This may be needed for low gradient channels
where the slope area method may produce
less accurate discharge data on
the rise and fall of the hydrograph.
However, the Slope-Area Method would still
be used to compute peak discharge.
The applications for this new method are promising.
And, by recording discharge data during
the rise and fall of a flood event,
we can now begin to apply that knowledge,
and improve current stage discharge ratings.
We believe the CSA method can become a valuable tool
to record hydrologic data, and we are
excited to be able to share it with you.
(calm piano music)