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School
University of Iowa *
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Course
2410
Subject
Geography
Date
Dec 6, 2023
Type
Pages
4
Uploaded by kekoukele123
To:
Applied Environmental Geosciences Inc
From: Ben
Re: Raven Creek Assessment
We have been awarded a contract with the Michigan Department of Natural Resources to evaluate
potential risks to Raven Creek, a high-quality trout stream near Cadillac, MI.
A section of the stream,
flowing near an old carp pond, has been eroding for years, and the macroinvertebrate productivity and
diversity have been decreasing.
Additionally, the farmer who owns the pond wanted to convert it to an
aquatic plant farm, so he recently dumped a heavy dose of piscicide into the water to eradicate the carp.
We were hired to investigate the relationship between channel flow and size, estimate groundwater-
surface water interactions, and to determine if there will be any major impacts to the stream from the
fish poison.
The DNR has provided channel cross section topography, well locations and water table information,
and other important data, and I have provided historical flow data from the gage just upstream of the
site.
Please use these data to answer the following questions using the provided Excel Spreadsheets.
The piscicide is moving towards the stream…please get your evaluation back to me by Thursday, Nov 26.
A memo is not required, but the spreadsheet is.
Groundwater Flow
The DNR has installed piezometers throughout the area down-gradient of the fish pond.
I have never
seen so much water table data for one site (so don’t get used to it)!
Please produce a piezometric
surface map with 40ft head contours, and with 5 flow lines, from the provided data and use the
information to answer the following questions.
1.
What is the groundwater discharge to the stream over an area demarcated by 200 ft of stream
length and 30 ft of stream bed if we assume the following:
a.
The soil is homogenous and isotropic with a porosity of 12%
b.
The hydraulic conductivity is 0.00055 m/s
2.
What is the Darcy Flux and the Velocity of the groundwater?
3.
If the fish poison is completely conservative, how long will it take to reach the stream?
4.
The concentration of the CarpBeGone is 256 mg/L in the pond and nearby groundwater.
Toxic
affects in trout begin appearing at 50 mg/L and death occurs at 100 mg/L.
a.
At current discharge rates, how much (mass) poison will enter the stream each day?
b.
At current flow rates, what will the concentration of the poison be in the stream at the
downstream end of the plume (none is in the water upstream)?
You’ll need to calculate
discharge of the stream before you can complete this calculation.
c.
At what upstream discharge do we start to impact fish?
Stream Discharge
The DNR also collected cross-section elevations and velocities using a GPS and Marsh-McBirney Velocity
Meters.
1.
Plot the XS data and mark where the current water level is?
This elevation is also the current
water table elevation.
2.
Please use the data in the spreadsheet to estimate discharge at the time of the measurements
(assume they are current) and a manning’s roughness value.
The slope is 0.0016 ft/ft.
3.
Please use the cross-section data and your manning’s n to estimate bankfull discharge (mark the
bankfull elevation water surface on the XS).
Bankfull slope is 0.00175.
Peak Stream Discharge
I have downloaded the peak discharge data from the USGS gage website and provided it in the Excel
Spreadsheet.
The gage actually has data covering the past 90 years, which is abnormal for most sites,
which may only have flow data covering the last few decades. You can estimate the probability of an
annual peak flow occurrence by sorting the flows from high to low and then ranking them.
The
probability is estimated by dividing each flow’s Rank by the the total number of years of measurement
plus one (P = M/(N+1)).
The recurrence interval is the inverse of P (1/P).
1.
Please plot the peak discharge data to observe any trends in annual peak flow events over the
time the gage has been active.
2.
What are the average, median, range, and standard deviation of the annual peak flows for all
the data, from 1931 to 1979, and from 1980-2019?
3.
What are the 2-yr, 10-yr, and 50-yr flow over those 3 periods?
4.
How do these flow statistics compare to each other?
How have peak flows changed over this
time period?
Does these types of changes matter when you are using peak flow data for current
and future resource management or restoration or infrastructure design?
5.
Do your existing bankfull numbers make sense compared to your peak flow analysis?
6.
If annual peak flows are increasing by 10% every two decades, as indicated by the trend line,
what will bankfull discharge be in 2040 based off our existing channel?
Flow Competence
The gage data indicate annual peak flows are getting larger, with a larger range.
In fact, they never
exceeded 190 cfs prior to 1980, and since 1980, floods have gone above that value 9 times,
including 5
times in the last decade.
The streambed along the channel in this reach comprises sand and gravel.
Pebble counts indicate the
median grain size (D
50
) is around 1 cm, with a D
16
of sand (0.5 mm) and a D
84
of 2.5cm.
The stream is
incised (see XS data), so it is unclear how stable the bed is.
1.
If
stream competence is represented by the sediment grain size diameter a stream can move at
bankfull flow
, based on the D
50
, please use the provided information to estimate whether the
bed is stable, and will continue to be at current bankfull flow, bankfull flow in 2040, and floods
topping 200 cfs (ie, calculate D
crit
)?
2.
If the stream is incising, what might this change mean for the groundwater flow to the stream
and the water table?
Assume a critical shear threshold (
τ
crit
) of 0.042, densities of 2650 and 1000 for sediment and water,
respectively, and 9.81m/s2 is gravity.
τ
* =
τ
/((
ρ
sed
-
ρ
wat
)*g*D
50
)
, where if
τ
* >
τ
crit
then transport occurs
D
crit
=
τ
/((
ρ
sed
-
ρ
wat
)*g*
τ
crit
)
Where,
ρ
sed
– density of sediment
ρ
wat
– density of water
G - gravity
τ -
shear stress =
γ
wat
* Depth * slope
γ
wat
- unit weight of water = 9810
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