Hydrology & Stream Stability Hydrologic Studies Unit Land and Water Management

Hydrology &
Stream Stability
Hydrologic Studies Unit
Land and Water Management
Why is that
Streambank
OR
Eroding?
Hydrologic Studies Unit
Land and Water Management
Causes of Streambank Erosion
• Natural river dynamics
• Sparse vegetative cover due to too much
animal or human traffic
• Concentrated runoff adjacent to the
streambank, i.e. gullies, seepage
• An infrequent event, such as an ice jam or
low probability flood
• Unusually large wave action
cont.
Causes of streambank erosion
• A significant change in the hydrology of the
watershed
• A change in the stream form impacting
adjacent portions of the stream, i.e.
dredging, channelization
Either of these two causes could
produce an unstable stream.
Assessing Stream Stability
A comprehensive assessment of potential causes
of erosion may be necessary so that the proposed
solutions will be permanent and do not move the
erosion problem to another location.
Gabion
baskets
damaged
by high
flows
Stream Stability is no net change in channel
shape and dimensions. Some sediment
movement and streambank erosion is natural.
Stream Instability causes excessive erosion at
many locations throughout a stream reach.
Stream Stability
A stream's channel morphology - its plan form,
dimensions, and profile - develops in response
to flood flows. Relatively modest flows,
because of their higher frequency, have more
effect on channel morphology than extreme
flood flows. Flows with a one to two year
recurrence interval are generally the dominant
channel-forming flows in stable streams.
Hydrologic changes that increase these flows
can cause the stream to become unstable.
ChannelForming
or
Effective
Discharge
The effective
discharge is the
product of the
transport rate of
individual storm
events and the
statistical
frequency of each
event.
For a stable stream,
the dominant
channel forming
flow is the
effective discharge
and occurs, on
average, about
every 1.5 years.
Changes within a
watershed can
increase peak flows,
increase total runoff
volume, and reduce
the lag time.
Note: Volume changes
are not indicated in this
illustration.
Stream Stability
Increasing discharge generally increases
both velocity and depth, and both increase
energy. Energy increases with the square
of velocity and linearly with depth.
If the velocity is doubled, water can carry
particles 64 times as large.
Discharge = Velocity x Depth x Width
Energy = V2/2g +p/g + h
Stability Indicators
 Field survey
evaluate extent of erosion
compare historical land use to current conditions
check for other causes (foot traffic, boat wakes)
anecdotal information
 Comparison of aerial photos
land use changes
stream channel movement
 Gage Analysis
 Hydrologic Study
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Discharge (cfs)
Gage Analysis
Muskegon River near Newago
12712
11712
Peak flows since 1930 that exceed the 67% (1.5 year) flow.
10712
9712
8712
7712
6712
5712
4712
Date
4712 cfs 67% (1.5-year), 6368 cfs 50% (2-year), 10397 cfs 10% (10-year)
Gage Analysis
1.5 Year Recurrence Flows
225%
200%
Per cent of 1.5 year flows expected
175%
150%
125%
Muskegon River,
Newago
River Raisin,
Adrian
River Raisin,
Monroe
Huron River,
Ann Arbor
AuSable River,
Mio
100%
75%
50%
25%
0%
1920's
1930's
1940's
1950's
1960's
Time (decade)
1970's
1980's
1990's
What is a Hydrologic Study?
1. Analysis of possible changes in the
parameters that determine the volume,
rate, and timing of surface runoff.
Estimate values for applicable parameters.
2. Calculate the impact of identified changes.
Modeling may be helpful.
3. Evaluate the meaning of the results.
Parameters That Affect Discharge
 Watershed size
(delineation)
 Ease of water
movement (Time
of concentration)
 Soils
 Land use
 Antecedent moisture
 Snow melt
 Frozen ground
 Spatial extent of
storm
Watershed Delineation
 Doesn’t usually change
 But ...
Ryerson Creek
Initial Delineation
Final Delineation
~15% area increase
Time of Concentration
 Time for runoff (wave)
to travel from the
hydraulically most
distant point of the
watershed
 Decreases with
channelization,
addition of drains,
pavement
Soils
 Don’t usually change - possible exceptions:
clay caps, significant excavations, or fills
Land Use
 The most likely cause of hydrologic change.
 In 1954, SCS developed the runoff curve
number technique to evaluate surface runoff
based on land use and soils information. It
is the procedure most frequently used by
hydrologists nationwide to estimate surface
runoff from ungaged watersheds.
Selected Curve Numbers
Land Use
Condition A B
Meadow
C D
30 58 71 78
Woods
Fair
36 60 73 79
Row Crop, contoured
and terraced
Residential, 1/4 acre
Good
62 71 78 81
61 75 83 87
Row Crop, straight row Good
67 78 85 89
Commercial
89 92 94 95
Paved
98 98 98 98
Curve Numbers (cont.)
Curve numbers are not
a runoff percentage.
SRO = (P-0.2S)2/(P+0.8S)
S = (1000/CN) - 10
Internet address for more information.
Modeling
Purposes:
 To estimate changes in discharge volumes,
peaks, and timing due to changing hydrology
 To estimate the effectiveness or size of
added detention
 Cannot demonstrate river stability, although
may indicate instability
HMS Modeling - Data Needed
 Soils
 Land use: historical, current, future
 Energy slope of river reaches (can be
estimated)
 Detention storage-discharge relationship
Model, Detention Added
Sample of model results.
100-Year Storm at C&O,
No Detention
Sample of model results.
100-Year Storm at C&O,
No Detention compared to
2360 Acre-Feet of Detention
Examples
Sometimes the cause of the erosion is obvious.
No further analysis was needed in this case.
Pine River
This outlet of this detention pond did not detain water.
No hydrologic study was required since the detention
pond was sized for the development.
Schoolhouse Creek
Plaster Creek
A larger detention pond
was proposed for this
site. Field observation of
existing land use
indicated that land use in
the watershed had not
changed in the past 22
years. No further
hydrologic study was
required.
Sprong Lake Inlet
This stream flows through a
culvert under a road, makes a
right angle turn, and then flows
into the lake a few hundred feet
downstream. Homes could be
threatened by continued
erosion at this bend. Field
observations indicated stable
land use in the watershed.
Stabilization of this
streambank will protect nearby
homes. No further hydrologic
study was required.
Ryerson Creek
Land use comparison
Ryerson Creek, Holland Drain:
Projected Peak Flows (cfs)
1978 1997 Build-out
50% (2-Year) 26
35
84
10% (10-Year) 67
82
149
1% (100-Year) 143 164
250
Dramatic increases in peak flows are predicted for the
upper watershed unless appropriate BMP’s are utilized
to compensate for continuing development.
This is an unstable stream with extensive erosion all along
the banks, which was not caused by a low frequency flood.
The erosion is worse in some areas due to heavy foot traffic.
A hydrologic study, incorporating modeling was conducted to
help select the appropriate remediation techniques.
Hager Creek
Hager Creek
The detention areas due not
appear to detain the 50% flows.
Pine River Tributary
This stream may be impacted by
increased runoff from new
development along the edge of
a city, as well as loss of
floodplain due to filling.
Further hydrologic analysis
would be helpful to ascertain
this.
Bear Creek
The property owner stated that 30 feet
of stream bank has eroded. Anecdotal
observations can be valuable.
Bear Creek
This erosion may be caused by flow
diverting around debris or ice
periodically piling against the
former bridge supports. Nearby
streambanks are stable. Removal of
the former supports may eliminate
the cause of the erosion at this site.
This erosion is caused by the
diversion of approximately fourteen
miles of natural stream through three
miles of straight channel. A limited
hydrologic study was conducted.
East Branch AuGres River
Photo by John McColgan