Historic Context of the Continuous Slope Area Method

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Detailed Description

The Arizona Water Science Center details the history and development of the Continuous Slope-Area Method. Learn about the people and events that began these new advances in the field of stream gaging. 

Music Artist: Glenn Jones, “Bergen County Farewell”. CC License. Music provided by www.FreeMusicArchive.com


Date Taken:

Length: 00:15:15

Location Taken: Tucson, AZ, US

Video Credits

Additional Footage shot by Chris Smith and other Arizona Water Science Center Staff


(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


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.