Video summary
This video introduces the fundamental concepts and procedures of stream flow analysis, specifically focusing on creating a stage-discharge relationship for a given stream bed. The primary objective of this exercise is to establish a mathematical connection between the water depth, known as the stage, at specific gauge locations and the total volume of water flowing through the stream, or discharge. By understanding this relationship, engineers can accurately estimate the flow rate simply by measuring the water depth at any point along the stream, provided they know the characteristics of the stream bed. The analysis takes place in a natural setting where various factors influencing water velocity are considered, such as the roughness of the riverbed caused by vegetation and roots, which slow down the flow compared to smooth surfaces like concrete.
To determine the slope of the stream, a critical component of the discharge calculation, the team sets up two green stakes in the middle of the stream bed, one upstream and one downstream. A string is stretched horizontally between these stakes to serve as a consistent reference level, ensuring that depth measurements taken from this line are comparable. Using a plumb bob since a standard level is unavailable, they verify that the stakes are vertical and then measure the distance from the horizontal string down to the stream bed at both locations. The difference in these depths represents the "rise" of the slope equation, while the horizontal distance between the stakes, measured with a wheel along the center of the stream, provides the "run." This process allows for the calculation of the stream's gradient, which significantly impacts how fast water moves through the channel.
The measurement process extends to capturing the detailed profile of the stream bed across its width, as the depth is rarely uniform and often resembles a trapezoidal shape rather than a simple rectangle. To achieve this, pairs of red stakes are placed at four evenly spaced locations between the green stakes, creating specific gauge points for data collection. At each location, a level string is stretched across the stream, and the bed is divided into smaller intervals to measure depth at regular marks using plumb bobs and measuring tapes. Consistency is paramount in this procedure; the team maintains a strict left-to-right measurement direction when facing downstream to ensure that all recorded data aligns correctly. While higher accuracy could be achieved by measuring every centimeter, the video highlights the engineer's role in balancing precision with practical effort, selecting an interval size that provides sufficient data without unnecessary labor.
Finally, the collected field data is intended to be used in a classroom setting to construct the stage-discharge relationship curve for the stream. This analysis incorporates Manning's number, an empirical value representing the roughness of the riverbed, which is estimated based on visual observations of the stream bed conditions such as vegetation density and substrate type. Once the slope, cross-sectional profile, and roughness coefficient are determined, the resulting equation allows hydrologists to predict discharge rates efficiently. Ultimately, this comprehensive approach transforms simple depth measurements into valuable insights about water flow, enabling effective monitoring and management of stream systems without the need for complex continuous instrumentation at every moment.
Read the full video transcript
This video is an introduction to stream
[music] flow analysis.
The goal of this exercise is to create a
stage discharge relationship for a
stream bed.
Allowing us to determine the flow or
discharge
>> [music]
>> if we know the depth or stage of the
stream at the gauge location.
We are at a park [music] in the south of
Durham in September 2020.
Where we are taking measurements to
[music] create the stage discharge
relationship for this stream.
Notice the vegetation in the stream
[music] bed.
One of the factors that affects the
discharge or the flow of water
is the surface on which it [music]
flows.
If the stream bed is a smooth concrete,
the flow will be faster than if the
stream [music] bed is full of roots and
vegetation that disrupts and slows down
the flow of water through it.
Another factor that [music] affects the
discharge is the slope of the stream.
So, the first thing we [music] do is
measure the slope of the stream.
We put two green stakes in the middle of
the stream bed.
One upstream [music] from the other.
We tie a piece of string to one [music]
stake
and run the string to the second stake.
We measure the depth of the river bed
from the string. [music]
But in order to be consistent in
measurements to get slope,
we need to make sure [music] that the
string is level and the stakes are
vertical.
We use a level against the stake [music]
to make sure they are both straight up.
We don't have the right level today, so
we are using a plumb to make [music] it
as vertical as possible.
We hang a level on the string and adjust
the strings on the stake so that the
bubble is in the center of the level.
Once we have the string at the same
elevation for both [music] stakes,
we hang the plumb from the string.
The plumb should just [music] touch the
bottom of the stream bed.
Then we measure the length of [music]
this string with the plumb.
This string length gives us the depth of
the stream bed from a set point [music]
on the stake.
We repeat this process for the stake
downstream.
Since we are measuring the depth of the
stream [music] from the same height at
both stakes,
which is the horizontal string between
[music] the two green stakes,
when we get back to the classroom,
we will manipulate [music] this data to
get the depth of the stream at each end.
This will be [music] the rise in our
slope equation.
To get the run of our slope equation,
[music]
we measure the length of the riverbed
between the two stakes.
We use this [music] measuring wheel. To
start, we make sure that the measuring
wheel is zeroed [music] out.
Then we place the measuring wheel at one
stake and roll down the center of the
stream bed, maintaining contact [music]
and keeping the wheel in the center of
the stream bed until we reach [music]
the second green stake.
This measurement will give us the length
that the water travels in the stream
and is the [music] run of our slope
equation.
Another factor that affects the flow of
the water is how deep [music] the water
is.
For this analysis, we measure the depth
of the water at several locations along
the [music] river.
To do this, we put a pair of stakes on
either side of the stream at each
measurement location. [music]
We want to space the pair of stakes
evenly between the green stakes.
We are choosing four locations [music]
to do measurements.
If you want more accuracy, you can do
more measurements. It just takes [music]
more work.
These four locations become our gauge
locations.
Once we complete [music] the analysis,
we'll be able to estimate the discharge
at the green stake downstream by
measuring the depth of the water at any
one [music] of these four locations.
The riverbed is not the same depth
[music] across the entire bottom.
Typically, it has has its shape more
like a trapezoid.
So, rather than measuring just one depth
[music] at each stake location, we
measure the profile of the stream at
each of these four locations.
To do that, we [music] start by putting
a string across one pair of red stakes
as a reference point.
Again, we want [music] to make sure that
the string is level, so we attach the
level and adjust the strings until the
bubble is in the middle [music] of the
level. We measure the distance between
the two stakes, which we can see [music]
is 36 in.
To get the profile of the stream bed at
this stream cross-section,
we measure the [music] depth from the
string to the bottom of the stream bed
and then manipulate this data in the
classroom [music] to get the actual
profile.
Since we want the profile and not just
the depth [music] in the middle of the
string,
we will divide the length of the string
into equal parts.
Here we have chosen [music] 6 in as our
distance between measurements.
The smaller the distance between
measurements [music] means more
accuracy, but it's also more work.
As an engineer, [music] it's your job to
determine what accuracy is needed.
We could be here all day if we measured
every centimeter, but is that level of
accuracy needed?
At each [music] mark on the string, we
hang the plumb so that it just touches
the bottom of the riverbed.
Then we use a measuring tape to measure
the length of the string [music] and
record it.
We continue to do that for each mark.
It is very [music] important to note the
direction of the stream.
Right now, we are looking downstream
[music] and measuring from left to right
across the stream bed.
Once we finish with measuring the depth
at each 6 in along [music] the string,
we move to the next set of red stakes
and repeat the process.
It's important [music] to be consistent
in the way we measure and record the
data.
Since we started from left to [music]
right looking downstream,
we measure from left to right facing
downstream at all of the red stakes.
Once we have all of the data,
>> [music]
>> we use this data to create the
stage-discharge relationship for this
stream bed.
You will [music] be given a set of data
that a previous class collected to do
this analysis.
When the analysis [music] is complete,
you will be able to estimate how much
water is flowing at the downstream stake
by measuring the depth [music] of water
at any of the locations where the red
stakes were located.
As mentioned at the [music] beginning,
one of the factors that influences the
flow is the roughness of the riverbed.
This is a value called Manning's number
that we [music] estimate based on
empirical data.
We will look at tables to do this back
in the classroom.
But you need to remember what the stream
bed looks like in order to be able to
estimate this number.
>> Mhm.