Mahony Bar – Floodplain Report
Marion County, Oregon
Prepared for:
Marion County Zoning and Planning Department
Courthouse Square
555 Court Street NE Room 2130
Salem OR 97301
Prepared By:
Glen C. Krogman
Lidstone and Associates, Inc.
4025 Automation Way Bldg. E
Fort Collins CO 80525-3448
May 2004
Table of Contents
1.0
INTRODUCTION ........................................................................................................ 2
2.0
BACKGROUND INFORMATION ............................................................................. 2
3.0
3.1
3.2
HYDRAULIC ANALYSIS .......................................................................................... 7
EXISTING CONDITIONS .................................................................................. 7
PROPOSED CONDITION ................................................................................. 9
4.0
RESTORATION PLAN ............................................................................................... 9
5.0
SUMMARY – HYDRAULIC ANALYSIS ............................................................... 11
6.0
PROJECT SCHEDULE ............................................................................................. 11
FIGURES
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
...................................................................................................................................... 3
...................................................................................................................................... 6
...................................................................................................................................... 8
...................................................................................................................................... 9
.................................................................................................................................... 10
TABLES
Table 1
Table 2
...................................................................................................................................... 4
...................................................................................................................................... 7
Mahony Bar – Floodplain Report
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1.0 INTRODUCTION
The purpose of this study is to evaluate the impacts of the Mahony Bar Enhancement Project.
Hydraulic analyses were conducted to ensure that the proposed enhancement plan for the Mahony Bar
would not increase the base flood level and/or adversely affect the stability of this reach of the river.
This investigation determined that the proposed fish ingress/egress channels and other construction,
including the anchoring of large woody debris (LWD) in the floodway, would result in “no net rise”
in the 100-year water surface elevation (base flood level).
The project is located on the Willamette River approximately five miles south of St. Paul, Oregon. It
is situated above Lower Lambert Bend at River Mile 65.8 on the right channel bank in SW ¼ Section
11, Township 5S, Range 3W, Marion County. Multiple landowners are involved. The project
location is known as Mahony Bar. Mahony Bar is situated on the right (east) channel bank of the
Willamette River and comprises 17 acres within the floodway of the Willamette River (Figure 1).
Natural elevations across the bar range from 86 to 95 feet. The elevations are based on a 2002 aerial
of the site. Hydrologic modeling indicates the two-year flood elevation at Mahony Bar is ~89 feet.
Historic extraction of sand and gravel has created seasonal ponds through the cumulative removal of
~150,000 yards and the creation of linear berms of soil removed from the extraction areas. These
artificial features are generally located parallel to the channel and flood patterns. A small berm
perpendicular to the flow pattern is proposed for removal.
2.0 BACKGROUND INFORMATION
The project proposes to extend an alcove and connect it to floodplain gravel ponds dug in the 196070s. The purpose is to provide a fish ingress/egress channel via a low-water connection designed to
prevent stranding of native fish and potentially to improve fish access to the floodplain. Mahony Bar
floods on a frequent basis. Flood frequency across the bar varies from annual to periodic flows.
Consequently, migratory fish may enter the ponds as they seek refuge during higher velocity events.
Monitoring at other floodplain mine ponds on the Willamette River suggest not all migrants leave
with the receding river. Fish remaining after the flood are stranded until the next flood event or
perish through predation or elevated water temperatures.
The plan is a series of integrated steps designed to be compatible with local flood hydrology and
geomorphic trends. These actions will promote fish passage and floodplain complexity at Mahony
Bar. Fish usage, water quality, geomorphic stability, and riparian plantings will be monitored for five
years.
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FIGURE 1
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The specific project objectives are:

Deepen ½ acre at the downstream end of Coleman pond to expose a minimum of six feet of
ground water.
Extend the natural alcove and connect it to the downstream end of the Coleman pond for a fish
egress channel.
Remove the soil berm blocking fish and wildlife movement between excavation areas on the
Coleman property.
Remove silt blocking the inlet to the natural alcove.
Realign the alcove inlet and bury LWD in the banks according to the design by ODFW Habitat
Biologist, Joe Sheahan. Design intent is to encourage localized scouring and limit silt deposition
and blinding off of the alcove inlet.
Bury LWD in the alcove extension and minimize reestablishment of reed canary grass by
aggressive tree and shrub plantings along alcove banks.
Evaluate the biological benefits and geomorphic stability of the alcove extension.
By data collection, add to body of knowledge regarding creation of fish access channels to mine
ponds. Collect data on fish assemblages, fish populations, and life stages. Evaluate project
benefits to other wildlife species.
Monitor dissolved oxygen and temperature pre- and post-construction





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A table is presented in this section which lists the amount of material proposed for removal. It also
details the amount of material to be placed in disposal areas within the floodway and outside of the
floodway. All disposal sites within the floodway will be below the natural elevations of the
floodplain and inside of the boundaries of the old mine site. The only exception to this is the silts
excavated at the entrance to the natural alcove. It would require a substantial amount of tree removal
and road building to access the 25 yards with haul trucks. Consequently, this material will be leveled
near the alcove. The depth of fill will average less than two feet. See Table 1.
Excavation and Placement Volumes
Removal*
(in yards)
Silt
Sand & Gravel
Silt
Placement
(in yards)
Alcove entrance
25
0
251
Berm removal
50
0
502
6,666
17,600
0
0
2,600
2,0662
0
3,600
3,600
0
0
1,000
Alcove extension
Coleman Pond (upland mine pond)
Silt disposal trench
(100’ x 30’ x 12’ deep)
Hop field (upland)
*All sand and gravel will be removed from the site.
1
Revegetate in place above 2-year flood elevation.
2
Place adjacent to upland mine pond where soils have been excavated and removed from project site.
Table 1
The proposed actions at individual locations within the project are described below along with a
discussion of where the excavated material will be placed. A total of 24,800 cubic yards will be
Mahony Bar – Floodplain Report
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removed from the floodway and 6,741 yards will be relocated and placed below grade within the
floodway (see Figure 2).
These improvements will enhance and create fish egress out of man-made ponds (that act as fish
barriers) when the river stage drops during the summer months. No alternative site or project designs
are possible. The need for this project is based on the degradation by simplification of the Willamette
River and its floodplain over the last 150 years. This project will enhance connectivity between the
Willamette River and its floodplain. Currently, only limited information is available on the utility of
constructed fish channels. Many are now proposed to meet ESA requirements for commercial
floodplain gravel operations. The primary objective of this project is to monitor fish usage of the
constructed channel to improve the current body of knowledge for fish passage between floodplain
ponds and rivers and then disseminate that information.
For fisheries enhancement, dependant upon cost and availability 10 to 30 logs with rootwads will be
buried in the channel banks of the fish egress channel and at the inlet to the natural alcove. The LWD
will be anchored by burial of a minimum of one third of the stem (15 to 30 foot logs) during
placement to ensure the LWD will remain on the project site as habitat features.
After these construction activities are finished, the site will be revegetated with native riparian
species. A five year fish monitoring program will be conducted to evaluate fisheries benefits.
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Figure 2
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3.0 HYDRAULIC ANALYSIS
The hydraulic analysis was conducted to determine the impacts of the fisheries enhancement work on
the 100-year flood level. Hydraulic analyses were conducted for the existing and proposed
conditions, as listed below.
1. The Existing Condition model represents current conditions. River cross sections surveyed by
Bernert River Towing; (May 2002) and 2002 aerial photography; and two FEMA cross sections
were used to develop an existing condition model.
2. The Proposed Condition model incorporates the fish ingress/egress channel and all other aspects
of the enhancement plan including placement of excavated silts in a disposal cell, placement of
silt to flatten pond banks and the placement and grading of the silt excavated from the natural
alcove inlet.
3. The lower limit of the study reach is river section 63.23. The hydraulic model includes 8 cross
sections through the study reach, encompassing approximately 2.5 miles of the river. Cross
section locations are shown on Figure 3.
The site is located within the 100-year floodplain and floodway of the Willamette River, as
determined by the Federal Emergency Management Agency (FEMA).
3.1 EXISTING CONDITIONS
The United States Army Corps of Engineers (USACOE) HEC-RAS computer program Version 2.1,
was used to perform the hydraulic analysis for this study. FEMA cross sections 63.23 through 68.98
were copied from the original FEMA HEC-2 model and incorporated into an Existing Condition
model. Cross section 65.95 was generated using information from upstream and downstream FEMA
cross sections and available USGS quadrangle map contours. The 100-year flood flows, channel
roughness characteristics, and starting water surface elevations were obtained from the FEMA HEC-2
model. The flow values for the study reach are shown in Table 2.
100 YR Water Surface Elevation Comparison
Cross
Section ID
63.23
64.63
65.58
65.95*
66.32
67.45
68.12
68.98
Q Total
(cfs)
253,000
253,000
253,000
253,000
253,000
253,000
253,000
253,000
100 Year Water Suface Elevation
Existing
Proposed
Condition
Condition
(ft)
(ft)
105.48
105.94
106.47
106.82
107.06
107.79
108.07
108.45
105.48
105.94
106.47
106.81
107.06
107.78
108.06
108.44
Table 2
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Figure 3
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3.2 PROPOSED CONDITION
A hydraulic analysis was also conducted for the Proposed Condition. The Proposed Condition model
incorporates the floodplain alterations that will be constructed as part of this fisheries enhancement
plan. This includes the following:






Construct a fish egress channel and connect it to a natural alcove
Enhance constructed fish egress channel with LWD and native riparian plantings
Remove silts from alcove entrance and burial of LWD to keep the entrance open
Construct a disposal trench for excess silt overburden
Deepen the Coleman pond
Relocate a silt berm adjacent into the Coleman pond to flatten banks
The locations of the removal and fill activities are shown on Figure 2. Figure 4 is a typical detail of
the proposed fish ingress/egress channel. A brief description of the proposed activities is presented in
the following section. The proposed construction activities are located in the floodway. Figure 5 is a
cross-section which illustrates all proposed activities in the floodway.
Figure 4
Cross sections 66.32 and 65.95were modified to reflect the proposed enhancement plan. These
modifications were used to evaluate the potential impact to the 100-year water surface elevation. The
HEC-RAS modeling analysis indicated that there would be no net rise in the 100-year water surface
elevation for the proposed fisheries enhancement. Table 2 presents the water surface profiles
generated by the HEC-RAS modeling for the existing and proposed conditions.
4.0 RESTORATION PLAN
Following is a brief summary of the proposed floodway enhancement activities including estimated
cut and fill volumes.
Berm Removal
This area contains angle of repose soil piles perpendicular to the movement of flood water. The berm
is a fish barrier between the pond segments. It is vegetated with annual and perennial grasses and
small patches of blackberries. A total of 50 cubic yards will be removed to accommodate fish
passage.
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Figure 5
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Alcove Extension
This area is 900 feet long by 200 feet wide at elevation 87 feet. All of the vegetation in this area is
reed canary grass. The dimensions of the proposed extension will be 4:1 (H:V) cutslopes in silts and
2:1 (H:V) cutslopes in sand and gravel with a bottom width of 20 feet. Approximately 6,666 cubic
yards of silt will be excavated; 3,600 cubic yards will be hauled and placed at or below grade in the
scalped area within the upland mine disturbance; and 1,000 cubic yards will be removed from the
floodway and stockpiled for use on the hops field located on an upper terrace. The remaining silt
(~2,066) will be placed within the upland mine disturbance. Approximately 17,600 cubic yards of
sand and gravel will be excavated from the alcove extension and removed from the site.
Alcove #1-Inlet Reshaping
This is a natural alcove that is a barrier to fish passage during the summer months. A maximum of
25 cubic yards of silts will be removed from the inlet. The excavated material will be placed and
leveled in an open area downstream of the inlet and above ordinary high-water elevation.
Pond Deepening and Silt Disposal
To provide year-round ponding of ground water, a half-acre area of the Coleman pond will be
deepened by an additional 10 feet to provide a minimum of 6 feet of water during low-flow
conditions. In the deepening process, all pond banks steeper than 3:1 (H:V) will be flattened with an
excavator. The excavated material will be sand and gravels. The volume proposed for removal from
the floodway is 2,600 yards.
Within the mine area, where silts have been scalped, a disposal trench 100’ by 30’ wide by 15’ deep
will be excavated and filled with silts. All silts will be placed below grade of the FEMA floodway
elevation of 86 feet.
Care will be taken to minimize the disturbance to surrounding areas during construction of the
proposed features. DOGAMI will be on site to manage the construction activities.
5.0 SUMMARY – HYDRAULIC ANALYSIS
The hydraulic analysis indicated that the Mahony Bar enhancement project will result in no net rise in
the 100-year water surface elevation. Also, the hydraulic modeling evaluation indicates that there
will be no adverse impact to adjacent structures or landowners. This is a rural area and there are no
structures in or near the floodway for several miles.
6.0 PROJECT SCHEDULE
All construction work will be conducted during September 2004. Revegetation will take place during
November 2004.
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