HomeMy WebLinkAboutDesign Narrative for Mitigation Improvements South Lagoon 1981DESIGN NARRATIVE
FOR
MITIGATION IMPROVEMENTS
SOUTH LAGOON
PREPARED BY
CENTURY/QUADRA J.V.
JULY 1981
The proposed Industrial Marine Park for the City of Seward
located in the Fourth of July Creek valley will alter the
lagoon and stream configuration of Spring Creek. It has been
observed that pink and chum salmon use the Spring Creek system
for spawning. As a requirement of the permit to construct
the industrial park, a method of mitigating the impacts of
the project on Spring Creek is required. It was decided to
develop a spawning channel and a holding pond, using groundwater
as a source, in a wetlands area south of the project.
The wetlands area consists of a lagoon behind a cobble beach
berm and a couple of small incoming streams that are fed by
groundwater and surface water. The lagoon bottom is covered
with a soft, fine-grained material and contains grasses and
many dead trees. There is no outlet channel through the berm
and all inflow into the lagoon exists as seepage through the
berm. Also, the beach berm appears to be frequently
overtopped by waves and the berm is frequently rearranged by
storms.
The mitigation project consists of constructing a spawning
channel suitable for pink and chum salmon, collecting
groundwater and surtace water in this channel, deepening the
existing lagoon to prevent winter freeze-out and constructing
an outlet channel that will remain open against the longshore
transport of sediments.
R REMENT
The Alaska Department of Fish and Game (ADF&G) has issued a
permit (Permit No. FG-81-11-392) approving construction of
the Industrial Marine Park with six stipulations. Of the six
stipulations, three are applicable to the South Lagoon
Fisheries Mitigation:
1. Sufficient habitat to maintain 700 adult pink and chum
salmon shall be provided in the relocated Fourth of July
Creek and Southern Lagoon system.
2. The protective berms for all fisheries mitigation will be
designed and maintained to withstand a 50-year flood.
3. Prior to any construction, detailed plans and specifications,
a design narrative including schedules and hydrologic data
shall be submitted to the Habitat Division of the Alaska
Departinent of Fish and Game for review and approval.
SUGGESTED DESIGN CRITERIA
Along with the stipulations ADF&G provided design criteria.
Provide 9 square yards of spawning area per pair of adult
Spawners,.
Provide a minimum of two acres of holding pond at a low
flow depth of 6-feet.
All spawning riffles shall have a velocity range of 0.5
to 2.0 feet per second and depth range of 12 to 36-inches.
Winter velocity at the stream bed shall be 0.5 feet per
second.
5. The substrate in the spawning riffles shall be between
0.5 and 4.0 inches in diameter with less than 10% fines
and not more than 2% of the material with diameter greater
than 2.5 inches (Fines have a diameter 0.03 inches or less).
6. Substrate thickness shall be 2.5 feet.
SPAWNING CHANNEL
As stipulated in the permit, spawning area for 700 pink or
chum salmon must be provided. “The 700 fish would be 350
pairs and at 9 square yards per pair would mean 28,350
square feet of spawning channel. The two spawning channels
h
provided in the relocation of Fourth of July Creek contains
20,660 square feet of spawning area leaving 7,690 square feet
that must be provided for in the South Lagoon area. This
area along with the 2 acre holding pond is to fit in the
area between the relocation South Levee and the base of the
mountains.
The hydrology of the south lagoon area is not known in
quantitative terms. It is not known what flows can be expected
when a collection ditch is excavated to the groundwater table.
Also the yearly fluctuation of the groundwater table elevation
is not known at this time. Therefore the design of the spawning
channel consisted of studying several channel sizes and slopes
to determine a channel size and slope that will meet the
velocity and depth criteria over a wide range of flows and
provide the necessary spawning area in the space available.
A trapezoidal channel shape with side slopes of one vertical
on two horizontal was studies for flows of 2 to 20 cfs. This
initial study indicated a channel with a bottom width of 6 or
8-feet could meet the criteria. The 6 and 8 foot channels
were studied on several slopes and flows and velocities were
determined for the depth range given in the design criteria.
Next the area that these two channels could provide was
compared to the space available. Finally it was decided that
a trapezoidal channel with one vertical on two horizontal
side slopes and a slope of a 0.02% would meet the depth and
velocity criteria for flows from 7 to 55 cfs. The channel
would have to be 1,000 feet in length requiring that it
meander back and forth across the project area to fit within
the available space. This channel would flow into the
holding pond or lagoon, so the backwater effects of the
lagoon had to be considered. A backwater curve with the
lagoon water surface elevation at +4.0 and channel invert
elevation at the lagoon, of +3.0 was calculated and found to
produce lower velocities. The backwater calculations were
continued for several slopes and it was found that the
channel slope could be steepened to 0.06% and still meet the
depth and velocity criteria for flows of 5 to 20 cfs. Flows
above 20 cfs will be above the 2.0 fps velocity criteria if
the lagoon water surface elevation remains at +4.0. However,
when the flows increase the lagoon water surface will
increase and the channel velocities will not become excessive.
In conclusion, the channel selected is 1,000 feet in length,
with an 8 foot bottom width, one vertical on two horizontal
side slopes and an invert slope of 0.06%. This channel will
meet the velocity and depth criteria for a range of flows
from 5 to 20 cfs providing the lagoon water surface is at
elevation +4.0 for a flow of 20 cfs. The channel will
accommodate higher flows which result in velocities beyond
the stated criteria but this will not be excessive due to the
backwater effect of the lagoon that occur with the higher
flows.
The elevation at the end of the channel will be approximately
+3.5. Based on observations of nearby borings and test pits,
the groundwater table is assumed to range from Elevation +3.0
to +5.0 under similar seasonal conditions. Since access to
the area is difficult, field work has not been performed in
the area where the upper reaches of the channel will be and
this assumption has not been verified. Visual observations
of surface conditions indicate that a shallow aquifer exists
and the lagoon maintains a water depth of several feet in
its deepest locations throughout the year. Because of these
observations, the general assumption that the excavated
spawning channel will intercept groundwater appears to be
valid.
Within the channel, two ponds will be provided. These ponds
will be 1.5 to 2.0 feet deeper than the channel. These ponds
are provided as suggested by the Habitat Division of ADF&G
for chum salmon spawning habitat. The channel and the two
ponds will contain 2.5 feet of substrate as suggested in
the permit criteria.
The existing lagoon has no outlet. The flow into the existing
lagoon presently seeps through the cobble beach berm. In
order to provide the proper velocity and depth in the spawning
channel the lagoon water surface elevation will have to be
controlled by some type of outlet. In order to do this, the
flow through the berm will have to be stopped. This will be
discussed later. It is desirable to maintain the lagoon
water surface as high as possible to minimize excavation and
cost and to provide access to the lagoon for the fish over a
wide range of tide levels. Also a high lagoon elevation will
reduce the instrusion of salt water. Two schemes were
considered, a culvert outlet and an open channel outlet.
A conversation with Mr. McHenry in Seward indicated that a
culvert outlet would work because it works in the Seward
Lagoon. The culverts at the Seward Lagoon are set at about
the half tide level and Mr. McHenry said fish negotiate these
culverts at all tide levels above half tide. Mr. McHenry
suggested a similar arrangement for the South Lagoon area with
the culverts set at the half tide level (approx. EL 0.0 on
the datum used for this project). A 2.0 foot culvert was
looked at and determined inadequate because the lagoon water
surface would fall below the +4.0 elevation for flows below
15 cfs and for tides greater than +2.0 salt water would flow
into the lagoon. Next a baffled 4.0 foot culvert was
considered. The baffles would allow the lagoon water surface
to remain high enough during low flows to allow acceptable
performance of the spawning channel and keep out salt water
at all tides except the higher tides. The flow over the
lower baffles will flood out the higher baffles so the fish
will not have to jump over the baffles. The four baffles in
the culvert range in height from 0.5 feet to 2.0 feet in
half-foot intervals with 25-feet between baffles. The
culvert slope is 0.25% and the culvert length is 120 feet so
the top elevation of the first baffle is +2.3. The lagoon
water surface elevation will not be below elevation +2.3.
The following table summarizes the depth of water and
velocities over each baffle for 5 and 20 cfs. The culvert
will accommodate flows greater than 20 cfs but the velocities
will retard fish passage.
2 cfs 20 cfs
Baffle Baffle Depth Velocity Lagoon Depth Velocity Lagoon
Number Height _ft _fps WS, EL ft, fps W.S, El
a 2.0 0.53 2.38 +2 .83 1.49 3.74 3.79
2 5) 0.52 2.43 eee 4.02
3 1.0 0.55 2.45 1522 4.25
4 0.5 0.63 2.48 Ld2)” Ses
From Baffle 4 to 0.95 2.19 2.04 30
end (Normal
Flow)
The velocities listed in the table are only over the baffles so
the velocities between the baffles will be less. Also as the
tide rises above the downstream invert elevation of 0.0 the
velocities will decrease.
elevation of the lagoon, flow will be from the bay into the
lagoon and salt water will flow into the lagoon but this flow
When the tide elevation exceeds the
will be limited due to the size of the culvert.
A similar analysis was made for an open channel outlet resulting
in a trapezoidal channel with an 8-foot bottom width, one
vertical on 1.5 horizontal side slopes and invert slope of 1%.
Baffles were used with the open channel for the same reasons
as the culvert outlet. The following table summarizes and flow
depth and velocities over the baffles for flow of 5 and 20 cfs.
The channel will handle flows greater than 20 cfs.
Baffle
3
4.
20 cfs
E
5 cfs
Baffle Depth Velocity Lagoon
Height Fr fps Ww
2 0.22 1.62 3.42
Lid 0.24 1.67
1.0 0.26 1.75
0.5 0.28 1.88
0.26 2.29 From Baffle 4 to
end (Normal
Flow)
Depth Velocity Lagoon
ft
0.56
0.60
0.64
0.67
0.59
2.55
2.67
2.84
3.14
3.82
S,. E
3.76
Velocities between the baffles will be less and velocities will
decrease as tide rises. When the tide elevation exceeds the
elevation of the lagoon salt water will flow into the lagoon.
The amount of salt water inflow will be greater for the open
channel than the culvert because the flow area increases with
depth for the open channel allowing for more flow.
In conclusion, the culvert outlet was selected for the following
reasons:
It works at the Seward Lagoon.
The higher velocities will attract fish better and will
help keep sediment deposition from accumulating.
One structure will be easier to construct and hold in
place than four separate structures.
A minimal amount of riprap will be required. (The total
length of the open channel would require riprap
protection.)
The baffles will not be subject to damage from wave
action.
Lower amount of salt water intrusion at high tides due to
a smaller flow area.
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7. ‘Trash will not be a problem because the small drainage
area and groundwater source does not contain a lot of
trash.
The outlet was located at the opposite end of the lagoon from
the spawning channel to provide flow through the entire length
of the lagoon.
The permit criteria required a two acre pond with a minimum
depth of 6-feet during the low flow months (winter) to prevent
freeze out of the young coho. Subsequent conversations with
ADF&G in Seward and Habitat Division in Anchorage indicates
the average depth should be 6-feet. Habitat Division would
prefer to see variable depths, some greater than 6-feet and
some less than 6-feet. If the minimum elevation of the
lagoon is +2.3 than a 6-foot depth would be an elevation of
-3.7. Using "Gravel Removal Guidelines Manual", U.S. Fish
and Wildlife Service, as a guide a lagoon configuration was
derived that provides an average depth of 6-feet over the two
acres with a portion of the lagoon having a depth of 8-feet.
An analysis of the freezing indix predicts the worse case
ice depth of 4.16 feet. This ignores the effects of warm
sea water nearby and the constant heat source from inflowing
=] 1=
water. Very conservatively, it can be assumed that there
will always be 4-feet of free water in the deeper portions of
the Lagoon beneath the ice cover.
BEACH BERM
The present beach berm consists of sands, gravel and cobbles
with a maximum top elevation of +8 with an area in the center
at elevation +5.0. It is obvious that the berm is frequently
overtopped and beach material deposited in the lagoon. The
beach berm should be built up to minimize overtopping and
filling of the lagoon with beach material. From the
Technical Appendices of the Environmental Assessment for the
Marine Facility, a 50-year combined flooding event is made up
of a 50-year storm runup and a 10-year still water level
which would be an elevation of +15.0. To build the beach
berm to this elevation would not be economically practical.
So the beach berm will be designed to be overtopped with
acceptable damage to the berm itself and no filling of the
lagoon due to the overtopping. A 50-year storm and a tide of
+4.8 (MHHW) will place the wave runup at elevation +11.0 if
the slope is one vertical on two horizontal. If the beach
berm is at elevation +11.0 the tide would have to be
higher than the MHHW level to be overtopped. The U.S. Army
Corps of Engineers, Shore Protection Manual requires a
uniform stone size of 1100 pounds for a breakwater to resist
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the 50-year storm wave of 7.8 feet. This size stone will be
costly to make, haul and place and will require underlayers
of smaller stone. To do this would require regrading the
existing berm. A smaller stone size was sought that could be
placed on the existing berm with minimal alteration of the
existing berm. The State specification for Class III riprap
will provide adequate protection with acceptable damage for
the 50-year storm. This riprap has a 50% stone size of 700
pounds. Using the Shore Protection Manual as a guide, it is
estimated that 15 to 20% damage can occur for the 25 year
storm event and 20 to 25% damage can occur for the 50-year
storm event. This amount of damage is acceptable for this
application. The riprap will be carried down the lagoon
side of the beach berm to protect the lagoon from overtopping
waves. The beach berm will have slopes and a top width that
approximates the existing berm, that is one vertical on four
horizontal slope on the bay side and a 20-foot top width.
ERV. E
Seepage through the rock in the beach berm and coarse gravel
materials immediately beneath the berm must be controlled
in order to have a relatively constant water level in the
lagoon. To accomplish this an impermeable aquaculture grade
reinforce chlorinated polyethylene (CPER) liner will be placed
on the lagoon side of the beach berm from Elevation +6.0 to
=13=
-6.0. The material will be laid on the same slope as the
berm and covered with gravel and riprap.
An analysis of water loss with this configuration was made
using seepage theory beneath dams. The boundary conditions
for the analysis were low tide at -9.0 feet, water surface
in the lagoon at +4.0 feet and an impermeable zone at -40.0
feet. The low tide at -9.0 feet is a worse case situation
and assuming an impervious zone at -40.0 is also conservative
since the water table with artesian pressures is known to
exist as high as Elevation +3.0 (depending on seasonal
conditions).
The coefficient of permeability was determined by the
formula:
K = 100 D (10)
Where D10 is the grain size, centimeters, representing 10
percent of the gradation. Two soil samples taken at
Elevation -10 and -20 feet within 200-feet of the Lagoon were
used. The permeability coefficient was found to be 0.16
cm/sec.
Using the above described boundary, conditions a graphical
solution of the flow net was obtained and the estimated
seepage below the membrane -.liner was calculated. The
=14=
estimated loss is 8.5 cubic ft/sec which is within the estimated
range of inflow.
It should be noted that the seepage analysis involves a
number of very complex issues and the exercise performed
for this design gives only a rough order of magnitude. Since
the general assumptions were conservative and the results
are within the range of inflow it is felt that the design is
reasonable and a controlled water surface in the Lagoon is
possible.
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