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Soft Engineering of Shorelines Based on a Binational Conference Sponsored by the Greater Detroit American Heritage River Initiative and Partners |
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Chapter 5 Constructing Islands for Habitat Rehabilitation in the Upper Mississippi River (Barry Johnson, U.S. Geological Survey and Jeff Janvrin, Wisconsin Department of Natural Resources) Introduction Loss of habitat diversity is a common problem in many
large rivers of the world (Ward and Stanford 1989). Islands can increase
habitat diversity within large rivers because they provide shallow-water
habitats, they shelter areas from wind and currents, and they trap sediments
and organic matter (Thorp 1992). They also provide terrestrial habitats
for birds, reptiles, and small mammals. In many large rivers, islands
are being lost due to channel modifications, erosion from wave action,
and inundation after impoundment (Figure 17A; Funk and Robinson 1974;
Sedell and Froggatt 1984). In addition, the natural processes that build
islands have been constrained in most large rivers due to dams, flood
control, and channelization. On the Upper Mississippi River, managers, agencies, and industry have worked together to restore habitat diversity by building islands, often from dredge spoils. In this case study, we relate our experience with three island construction projects that illustrate techniques and concepts that might be applied in other rivers. Two projects, the Channel-Border Islands project (also called the Pool 8 - Phase I Project; US Army Corps of Engineers 1989a) and the Stoddard Bay Backwater project (also called the Pool 8 - Phase II Project; US Army Corps of Engineers 1996), are near La Crosse, Wisconsin. The third project, the McCartney Lake project (US Army Corps of Engineers 1989b), is near Cassville, Wisconsin, just north of Dubuque, Iowa.
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Making Projects Happen: Cooperation, Partnerships, and
Regulations These island construction projects like most other
habitat rehabilitation projects on the Upper Mississippi River are a cooperative
effort among multiple stakeholders. These projects were conducted mainly
through the federally-funded Environmental Management Program (EMP), which
began in 1986 and is coordinated by a group of federal and state agencies.
The US Army Corps of Engineers is responsible for maintaining navigation
on the river, the US Fish and Wildlife Service has responsibility for
managing national trust species and for managing federal refuges (which
constitute much of the river's floodplain), the five border states are
responsible for managing other fish, wildlife, and lands, and the US Geological
Survey coordinates a Long-term Resource Monitoring Program as part of
the Environmental Management Program. These agencies work together to
plan, prioritize, and coordinate rehabilitation projects through the EMP
and to assure that projects meet all state and federal environmental regulations.
Industry and citizen groups are involved in project planning through agency
representatives or through membership on planning committees. In addition,
the general public is involved through public meetings, providing written
comments on planning documents, and participating in tours of rehabilitation
projects. With all these partners involved, planning for rehabilitation projects can be time consuming. However, in the end a consensus is reached among participants so that project implementation usually goes smoothly. Island Construction Techniques The basic techniques for building two types of islands used in the Upper Mississippi River projects are described below. The design features incorporated into these islands were derived from surveys of existing stable islands in the area and from analysis of hydraulic conditions at the project site. Thus far, all islands have been built in areas that are 3-4 feet deep during low water periods (usually summer and winter). Building islands in deeper areas may require changes in the techniques described here. Building Artificial Islands Construction of an artificial island begins by building
a sand base (Figure 18) . Sand is supplied by dredging nearby main-channel
or backwater sites. This technique often provides a useful method for
disposing of dredge material. Hydraulic dredging is typically used because
it is generally cheaper than mechanical dredging for large amounts of
sand. Final shaping and contouring of the sand is accomplished with the
use of bulldozers. For safe operation of heavy equipment, the top of the
sand base should be at least one foot above water level. The desired shoreline
slope of 20:1 is very difficult to build. Thus, a sacrificial berm, designed
to erode naturally to a 20:1 slope, is placed along the shoreline (Figure
18). On shorelines that may be subject to severe erosion, protective features
are often added. This can be limestone rip-rap applied around the upstream
head and tips of islands, but where possible, limestone groins (about
30 feet long) are used instead. Building groins are cheaper than armoring
the entire shoreline with rip-rap and the beach areas between groins provide
better access to the island for shorebirds, reptiles, and other animals.
After the sand base is complete, a cap of 1-4 feet
of fine soil is applied (Figure 18). The height of the island is typically
the elevation of a 10-year flood event at that site. Higher mounds can
be added to support plants that require dryer habitats. Finally, the island
is planted with willows along the sandy shoreline and a mix of grasses,
trees, and legumes in the fine soil. Using legumes in the mixture helps
to maintain adequate nitrogen in the soil.
Constructing Seed Islands A second type of island, called a seed island, has also been used in the Upper Mississippi River. A seed island is a linear pile of rip-rap placed perpendicular to the current in areas of high sediment transport and uses the river's natural processes to build a new island. As water is deflected around the seed island, sediments are deposited in the area of slower-moving water, which forms behind the rock pile. In addition, increased current velocity around the sides of the seed island scours the river bottom and creates deeper areas. These processes occur primarily during floods because high flows and current velocities are needed to mobilize and transport the sand substrate that is deposited to form islands. The dimensions of seed islands, especially their elevation, should be based on an analysis of hydraulic conditions and sediment transport occurring at the project site during flooding. Upper Mississippi River Island Projects The projects described below were selected to illustrate specific features of different types of island construction. The Channel-Border Island Project In this project, conducted within an impounded area
of the Upper Mississippi River (Figure 17), existing islands were protected
from erosion, while new barrier islands were built along the border of
the main channel. The objective was to shelter off-channel areas from
currents and wakes to create better conditions for plant growth in shallow
water (US Army Corps of Engineers 1989a). The project began in 1990 by
installing rip-rap on shorelines of existing islands to prevent further
erosion. Then, downstream of the existing islands, approximately 13,000
linear feet of barrier islands were built (totaling 30 acres) using 320,000
cubic yards of dredge spoil. Groins were installed to protect shorelines
of the new islands, which were completed in 1993. Seed islands were also constructed near the Channel-Border
islands (Figure 17A). Most seed islands were about 200 feet long, 30 feet
wide at the base, and the tops 3-5 feet above the water level at low flows.
Two seed islands near the Channel-Border project have been in place since
1995, with six additional islands built in 1998. Substrate monitoring
around these islands has shown that scouring and deposition are occurring
as expected. The Channel-Border Islands Project benefited an estimated 1,000 acres of aquatic habitat. The islands have weathered waves, wakes, and annual flooding, including the Great Flood of 1993, with very little loss of material. Lush vegetation has become established on the islands from the original seeding. In the shallow protected areas behind the islands, water transparency has increased as has the abundance of aquatic plants (Figure 17B), which is a key component for increasing fish abundance (Johnson and Jennings 1998). The Stoddard Bay Islands Project Stoddard Bay is a backwater area along the town of
Stoddard, Wisconsin (Figure 17) that was previously protected by a line
of barrier islands extending about 3,000 feet from the east shore of the
Mississippi River and curving downstream about 6,000 feet (US Army Corps
of Engineers 1996). These islands enclosed an area of about 600 acres.
By the 1970s, these islands had been lost to erosion and the site had
become a relatively homogenous open-water basin. The purpose of this project,
begun in 1997, was to rebuild those barrier islands. These islands would
reduce wind fetch and improve habitat conditions within the enclosed area,
including over-wintering habitat for fishes. Sand for island construction
came from dredging 15 acres of nearby backwater habitat. To improve flow conditions within Stoddard Bay, low
rock sills were incorporated into the barrier islands (Figure 17). Sills
were constructed of limestone rip-rap and incorporated a fine-mesh barrier
fabric to help reduce water seepage through the sill. These low sills
are over-topped during floods, which allows high flows to flush the bay.
However, to allow some inflow during low-water periods, a notch was placed
into the upstream sill . Sill heights were determined based on concerns for
winter habitat. Good over-winter habitat for backwater fishes should have
current velocities less than 1 cm/sec (Knights et al. 1995). Under normal
winter conditions, inflow to the bay occurs only through the notch in
the sill, which produces acceptable current velocities. However, if the
sills are overtopped, current velocities increase to undesirable levels.
To reduce this possibility, we analyzed winter water elevations at the
site and built the sills to an elevation that produced only a 10% chance
of being overtopped for more than five consecutive days during winter.
That sill height also corresponded to a two-year spring flood elevation.
To help design the Stoddard Bay island complex, computer
modeling was used (J. Hendrickson, US Army Corps of Engineers, St. Paul)
to predict current velocities within the bay under alternative island
configurations and various flow conditions (high flow, low flow, and low
flow with two feet of ice cover). The design objective was to produce
a variety of current velocities within the bay, with low velocities always
available somewhere and high velocities occurring in some areas during
high water to scour the substrate. Modeling helped determine the number
of the sills to incorporate into the barrier islands and the number and
location of interior islands. For the Stoddard Bay Project, 27 acres of islands were constructed, which provided benefits to at least 600 acres of aquatic habitat. Since this project was completed in 1999, there has been little time for evaluation. However, during summer 1999, water transparency in the bay was much greater than outside the bay and aquatic plants were much more abundant than in previous years (Figure 17B). Angling success was high within the backwater and along the exterior of the barrier islands, causing the area to quickly gain a reputation as a great fishing spot. In addition, the project protects much of the shoreline along the town of Stoddard from excessive wave action. The McCartney Lake Project McCartney Lake is an extensive backwater complex that
has experienced considerable sedimentation, with subsequent loss of deep
water areas. Rehabilitation of the lake was begun in 1989 and included
stabilizing an inlet channel to reduce sediment inflow to the system and
dredging 8,200 feet of connected channels, about 10 feet deep, within
the lake (Figure 19; US Army Corps of Engineers 1989b). The resulting
400,000 cubic yards of dredge material were used to construct a single
22-acre island at the downstream end of the lake to reduce wind-generated
waves on McCartney Lake (Figure 19). McCartney Island was constructed using techniques
similar to those described above, but the design was different. Rather
than a barrier island, this was a large island designed to provide a variety
of aquatic and terrestrial habitats. Thus, a 10-acre wetland was built
on one end of the island and upland habitat at the other end (Figure 19).
In addition, because of the large size of the island no shoreline protection
was used. The McCartney Lake project was completed in 1991 and
since then, the island has remained stable. Immediately upon completion,
dissolved oxygen levels and water depth improved within the dredged areas.
The island was used almost immediately by waterfowl, shorebirds, turtles,
amphibians, and small mammals. However, increases in adult fish populations
were not evident until six years after project completion due to time
lags in fish reproduction and growth.
Funding for habitat rehabilitation projects on the
Upper Mississippi River typically comes from a variety of sources. Funding
for construction costs come mostly from the Environmental Management Program
and occasionally from operation and maintenance funds of the US Army,
Corps of Engineers, Fish and Wildlife Service, or state agencies. Costs
for project planning and evaluation are typically shared among agencies
with funds coming from the Environmental Management Program (including
the Long-term Resource Monitoring Program) and from in-kind contributions
of labor, equipment, and supplies from partner agencies. The islands constructed in the projects described above have a life expectancy of 50 years. For all three projects the costs of island construction were similar. Costs averaged about $75,000 per acre (1995 US dollars). This does not include costs for planning or evaluation of the projects. Evaluation Techniques and Learning as You Go Evaluation of each island project has produced more
effective island designs and more efficient construction techniques. The
evaluation process has typically involved pre- and post-construction assessment
of various features of islands and habitats including physical (flow patterns,
current velocity, wave activity, water depth, substrate erosion/deposition),
water quality (temperature, dissolved oxygen, transparency), and biological
(aquatic and terrestrial vegetation, invertebrates, fish, birds) components;
relative abundance and diversity of habitats; and stability of island
shorelines. Changes in physical and chemical variables are relatively
easy to measure and often show improvement immediately after construction.
However, many biological changes are slower and less obvious. Quantitative monitoring has shown that islands can be very affective at modifying currents, wind, and waves, which in turn affects water transparency, plant growth, and sedimentation rates. In fact, the Channel-Border Island Project changed flow patterns enough that the design of new island projects downstream had to be modified. Qualitative observation and photographs of islands over time, especially in relation to on-site reference points, have provided useful evaluation of how shorelines and vegetation are fairing. Detecting changes in invertebrates and fishes usually requires more intensive on-site sampling. In addition, long-term quantitative monitoring will be needed to determine if projects have actually increased biological productivity in the system as a whole.
References Funk, J.L., and J. E. Robinson. 1974. Changes in the channel of the lower Missouri River and effects on fish and wildlife. Aquatic Series 11, Missouri Department of Conservation, Jefferson City, Missouri. Janvrin, Jeff. August/September 1998. Mississippi River Rehab. Wisconsin Natural Resources Magazine, Wisconsin. http://www.wnrmag.com/stories/1998/aug98/missrv.htm Johnson, B.L. and C.A. Jennings. 1998. Habitat associations of small fishes around islands in the Upper Mississippi River. North American Journal of Fisheries Management 18:327-336. Knights, B.C., B.L. Johnson, and M.B. Sandheinrich. 1995. Responses of bluegills and black crappies to dissolved oxygen, temperature, and current in backwater lakes of the Upper Mississippi River during winter. North American Journal of Fisheries Management 15:390-399. Sedell, J.R., and J.L. Froggatt. 1984. Importance of stream-side vegetation to large rivers: the isolation of the Willamette River, Oregon, U.S.A., from its floodplain by snagging and stream-side forest removal. International Vereiningung fuer Theoretishche und Angewandte Limnologie Verhandlungen 22:1828-1834. Thorp, J.H. 1992. Linkage between islands and benthos in the Ohio River, with implications for riverine management. Canadian Journal of Fisheries and Aquatic Sciences 49:1873-1882. US Army Corps of Engineers. 1989a. Definitive project report/environmental assessment (SP-4), Pool 8 island construction - phase I. Upper Mississippi River, Vernon County, Wisconsin. Environmental Management Program, US Army Corps of Engineers, St. Paul District. June 1989. US Army Corps of Engineers. 1989b. Definitive project report with integrated environmental assessment (R-3), Bertom and McCartney lakes rehabilitation and enhancement. Pool 11, Upper Mississippi River, Grant County, Wisconsin. Environmental Management Program, US Army Corps of Engineers, Rock Island District. March 1989. US Army Corps of Engineers. 1996. Definitive project report/environmental assessment (SP-20), Pool 8 islands phase II. Upper Mississippi River, Vernon County, Wisconsin. Environmental Management Program, US Army Corps of Engineers, St. Paul District. St. Paul, Minnesota. Ward, J.V., And J.A. Stanford. 1989. Riverine ecosystems: the influence of man on catchment dynamics and fish ecology. Pages 55-64 in D. P. Dodge, editor, Proceedings of the international large rivers symposium. Canadian Special Publication of Fisheries and Aquatic Sciences 106.
Contact Persons Barry Johnson Jeff Janvrin Jon Hendrickson Jim Nissen |
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Last updated:
July 9, 2008
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