(gentle guitar music)
- In 2001 we started this continuous slope area project.
Dan Evans and I were
discussing how we could
improve the rating or the high-flow
and the medium-flow accuracy of a new gaging station
on the lower Babocomari.
- [Chris] We have a crest-stage gages
on both sides of the channel.
This is the right bank looking over
to the left bank where Dan is.
- At that time, there was a new sensor that came out,
was a MiniTroll.
- [Chris] Dan's taking the MiniTroll out
of the crest-stage gage in Cross Section 1.
As you can see, he's pulling out the stick
and the MiniTroll is connected on the stick.
Dan, why don't you tell us pretty much how this
is set up.
- Well, here's the crest-stage gage stick.
And what we do is, of course, it operates like
a normal crest-stage gage.
If there's been a flood,
the cork will leave a line on the stick.
Of course measure up from the bottom.
Have the elevation on the bottom of the stick.
And get the peak.
The transducer also records
the peak.
It records the entire flow.
And we compare,
after we correct this to atmospheric pressure,
we compare this to the mark on the stick
and they've been lining up fairly well.
- The purpose of this was to improve the accuracy
of our gaging stations.
Some of our gaging stations we're not able to
go out and make direct measurements
where we go out on a bridge or cableway
and lower a current meter into the water
and measure the velocity and so,
we can't access some of these sites
because we can't reach them during the floods.
- In Arizona we have a lot of ephemeral streams
which means they only flow after significant rainfall.
These rainfall events produce very flashy flood events
that only last for a short time.
And a tool we can use to better the accuracy
of our stream gages is called
the continuous slope area sensors, or CSA sensors.
We use these in sites that we can't get to,
are far away, or are really flashy like I said.
And what they can do is, they can measure stage
at multiple places in the stream reach,
and then we can put those values into a
computer model which estimates discharge
for us over the entire hydrograph,
for the entire flow event.
So we can get multiple discharge measurements
throughout the flow while we're not there,
which makes things much easier
when these sites are so remote.
- I'm Steve Wiele, I'm a surface water specialist
in the Arizona Water Science Center.
I've been working with Chris Smith and many others
on the continuous slope area gages.
When computing the discharge with the CSA data,
the CSA gages work best for medium and high flows.
And you can see here in red, green and blue,
the slopes between the stage data.
Red is going from X2 to X3.
Blue from X1 to X2.
And the green is X1 to X3.
Just an average of the other two.
So early on in the flow the water surfaces are disconnected.
You can see a large spread between the stages here,
as the flow comes up, the slope converges
to this point here, then remains fairly continuous,
until the flow declines to about 600cfs.
So in a case like this we'd probably want to
make our calculations at and above around 600cfs.
- We knew these slope area reaches
were there at our gaging stations.
Because that's how we would rate the gages
after major floods.
We'd go out, survey the cross-sections,
survey the high-water marks left by the floods.
So we knew we knew these reaches existed at our gages.
So what we decided was to actually go out there
and instrument these reaches with pressure transducers.
- When we deployed the CSA sensors,
or put in a CSA gage, we put multiple
pressure transducers in small, low-profile
housings within the stream reach.
And inside those mounts we put small
pressure transducers.
Here's an example of a pressure transducer that
we would use.
It's about the size of my hand.
This has 40,000 data points
so we collect data every five minutes.
This can stay out at a site for four months
and continually be collecting
river stage data or eventually discharge data
while nobody is at the site.
- These sensors are deployed in many reaches,
both in Maricopa County and Cochise County, Arizona,
as an experiment to see how well this method
could work using these sensors.
And the results from these two studies
can be found in two reports.
These reports are important in this process because
they prove the accuracy and the limitations
of using sensors like this to calculate discharge.
- In the reports, Steve has described the different
phenomena that we ran into, and has shown that it
doesn't affect the quality of the continuous slope area.
- [Steve] Another consideration we've had
is just how critical is it to have the upstream
and downstream stage gages synchronized.
How much error can we tolerate.
Its turned out to be not much of an issue
because the clocks and the pressure transducers
have turned out to be extremely accurate,
showing little or no drift at all.
So as long as they're set up correctly,
there hasn't been a problem.
In the Babocomari data we were observing consistently
that the,
that when putting together rating curves based
on CSA data, that we were getting much tighter
much tighter curves using the upstream
stage
recorder rather than downstream stage recorders.
And here's an example at Centennial Wash.
Here's a rating curve using the upstream gage.
This is fairly tight and the rating curve in red,
using the downstream gage is showing a lot more scatter.
And it turns out that there's actually a reason for this.
It isn't just a coincidence.
And the reason for this is illustrated in
these two plots.
This is looking at the fluctuations and
the upstream pressure transducer.
If there's an upstream tick in the stage
the slope is going to increase,
which will increase the computed discharge,
so the stage moves up and the discharge moves back
towards the rating curve because the discharge is,
the computed discharge is increasing too.
Whereas, if there's an upward tick in the stage
in the downstream gage,
the stage goes up
but it's going to reduce the slope,
and consequently the computed discharge
is going to decrease and move away from the rating curve.
So that's the likely explanation for why we're getting
smoother rating curves using the upstream gage.
- We had a project in Maricopa County
that was for measuring channel change.
And that was ideal with this continuous slope area,
because when you go out in the field and install these
sensors in a slope area reach,
you go ahead and do the surveying of the channel.
And then after a flow event occurs you come out again
and you survey the same channel again.
And so you're recording channel change through time.
- My name's Claire Bunch.
I'm a Hydrologist with the U.S. Geological Survey
in Tucson, Arizona.
When determining the best location for a CSA gage,
we look for a stable reach.
Stable cross-sections are important at CSA gages
to verify that the cross-sectional area
that is surveyed after the event
is representative of the area during the flow event.
In order to document any potential channel change
of the CSA gage, we make sure that we document the
condition of the reach upon installation.
So we will take numerous photos and videos.
We also survey the cross-sections to document
the channel geometry at the time of installation.
After flow events, we go back and we re-survey
the cross-sections and also take additional photos
and video to document the conditions.
We're looking to see if any scour, or fill
has occurred since the installation,
or since the previous survey.
By documenting whether or not channel change
has occurred at our CSA gages, we'll be able
to determine the magnitude of an event
that causes any measurable channel change.
Nine of our CSA gage locations are part of
a Maricopa County channel-change study.
These gages have been in operation for almost 15 years
and at these locations we have not detected any
significant channel change.
One of the gages, Vekol Wash, experienced a 25 year flood
during the period of record that we've been monitoring,
and did not experience significant channel change
from that event.
- [Steve] Another consideration is whether or not we're
capturing the peak discharge.
We've been using typically a five minute interval
and we're concerned that we might be,
especially with the flashy flows we experience
around here, we might be missing the absolute peak.
But in looking at the data from the nine gages
in Maricopa County, we're consistently seeing that
even in very flashy flows, the peak discharge
is not all that sharp.
You can see the stage on the vertical axis.
And even though this is a very flashy flow the,
there is quite a good coverage of
the peak
here with a five minute interval.
So we're pretty confident that,
even with a five minute interval,
we're not missing out much on the peak discharges.
In this plot we can see a sample calculation
of the error that we do introduce by using a
steady flow equation for unsteady flow.
And it turns out that, in addition to the rate
at which the flow goes up and down,
the reach slope is also a factor.
And so you can see an example here
for a simple trapezoidal channel,
using three different slopes:
point zero one;
point zero zero one;
and point zero zero zero one.
For the steepest slope in blue, you can see
that the error on the vertical axis is very small,
even with very rapid changes in velocity.
Whereas for the very low slope, zero zero zero one,
the error can be very, start to get very large.
Even for mild changes in velocity.
In Arizona we're typically in this region here,
fairly steep slopes so it's not been an issue for us,
but if you're working in very low slopes you'd
want to pay attention to this.
Here's an example from the Babocamari.
Our initial installation.
This was a very significant flow there,
reached almost 10,000cfs,
and falling very, very rapidly on the first peak.
And then a secondary peak that's a bit milder.
And you can see in red the error that we calculated
over this hydrograph.
For most of the hydrograph the error is very, very
small and reached a peak of plus or minus three percent,
which is something we can live with.
Here's a histogram of the errors for that same event.
Most of the errors are very, very small.
Just a half a percent, and then it ranges up to just a
very small portion of the hydrograph,
it's up about around three percent.
- The other thing that we're doing now is
we're using the continuous slope area reaches
to actually compute Manning's n value.
And what we're doing is we're,
using the slope area reach, measuring the stage,
but we're also measuring direct measurements from
either a bridge or a cableway in the same area.
So we're actually measuring the discharge directly
with our current meters and then what we're going
to do is compute n instead of discharge.
And so this will be the first time that we can
actually measure the n value
and a flow
and a runoff event, and we can actually evaluate it
for different stages, and so we can have it for the
high-flow, medium-flow, and low-flow,
and we'll see if the n value changes through the stage.
One other development that I'd really like to see occur
is the particle tracking.
If we can go ahead and do particle tracking
at one of the cross-sections
of the continuous slope area reach
particle tracking will give us the surface velocity
in that cross-section.
And then that sensor will actually give us the area,
and with that we'll be able to compute discharge.
So with this particle tracking, what you do is you use
a video camera that's installed on the bank
and then you shoot footage of the actual flow
of the water at different stages.
Other people that have been working on this
continuous slope area to develop it is
Wes Heaton.
He's in our Tempe office.
Wes has done an incredible job over the years.
He's an expert in slope area measurements and
we're very fortunate to have somebody like him
involved with the development of this method.
The other person that's been really important in
this process was Jeff Cordova.
- We're out on the Hot Springs Wash,
trying to find a location for the possible
continuous slope area.
Right now we're just checking cross-sections.
Right now I'm looking downstream right here.
- [Chris] And Jeff was instrumental in installing a lot
of these reaches.
Running the equipment and processing the data.
So the future is to continue to
develop the n verification,
process channel change
and improve the accuracy of our gaging stations.
And I think that with the people we have working
on this now I think we'll be able to do that.
We really appreciate all their work and help.