S. Gregory L. Ashkenas
D. Oetter K. Wildman
Longitudinal Patterns - Revetments
Introduction
The Willamette River has been straightened, channelized, and armored
along its length by federal and state agencies, municipalities, and private
citizens (see Revetments, pages 32-33). The more urban and industrialized
area near Portland tends to have 40-60% of the river banks in revetments and
bank control structures. Other cities along the river also tend to restrict
movement of the river by bank armoring and other structures designed to
control the path of the river. Upper reaches of the Willamette River between
Albany and Eugene are more geomorphically complex. These areas contain
intermediate amounts of channel control structure, largely associated with
attempts to prevent erosion of agricultural lands in the complex floodplains
(Fig. 44, p. 32). The lands along the river between the Portland metropolitan
area and Salem and between Salem and Albany have some of the lowest
amounts of bank protection.
Figure 187. Bank revetment used to repair a slump that
occurred on the river bank of the Willamette River in
Corvallis during the flood of February 1996.
Revetments and banks that have been armored present both negative
and positive implications for restoration of large rivers. From a river management perspective, the areas with low amounts of bank revetments are
important resources to protect and opportunities to resist the trend to simplify the river and reduce its natural dynamic processes. They allow the river
to function more naturally and, as a result, maintain ecological functions
through natural processes of channel formation. These reaches are important
for conservation and restoration, particularly when considered in comparison
to the heavily armored sections of the river, which offer reduced ecological
functions.
From a restoration perspective, areas with high amounts of revetments
but less intensive human development offer potential opportunities for
modification or removal of revetments. In this illustration of approaches for
Figure 188. Blockage of historical side channel and armoring with
riprap in the upper Willamette River.
prioritization of river restoration, we will consider revetments to be opportunities for restoration, though we acknowledge that the revetments lower
the current ecological condition of the river. River reaches that once were
dynamic and complex can be restored to higher levels of ecological function
by reconnecting existing but isolated side channels. Riprap and pilings can
be removed completely or modified to maintain some stability but still
allow higher flows to reconnect with the side channels. Efforts to reconnect
historically complex river channels are greatly affected by hydrologic
regimes. High flows are necessary to maintain open flow through side
channels. Efforts to lower these high flows with flood control reservoirs
reduce the ability of rivers to maintain off channel habitats. Restoration of
channel complexity through modification of bank control structures will be
increased if flood management strategies also allow for more natural flood
regimes. Careful attention to concerns of private citizens and local communities is essential for constructive discussions of restoration options related
to restoring the dynamic processes of the Willamette River and its floodplain.
Figure 189. Bank protection and drainage for new
apartment buildings in the riparian area of the
Willamette River in Salem.
Figure 190. Development of docks and bank armoring with riprap
on a steep bank near the Portland metropolitan area.
138
Figure 191. Fringe of native riparian vegetation with riprap to stabilize
the lower bank in Corvallis.
PNW Ecosystem Research Consortium
RIVER RESTORATION
Revetments
Scale: 1:1000000
Portland Metro
1995 Urban Growth Boundaries (UGBs)
0 mi
10 mi
20 mi
Slices which intersect UGBs
0 km
10 km
20 km
30 km
S
N
Salem/Keizer
Eugene/
Springfield
Albany
Corvallis
N
S
5
kilometers
0.0
0.01 - 0.08
0.09 - 0.25
0.26 - 0.46
0.47 - 0.70
0.71 - 1.08
1.09 - 1.59
1.60 - 6.63
4
3
2
Figure 192.
Length of revetments
in 1995.
225
215
220
210
200
205
195
180
185
190
170
175
155
160
165
150
125
130
135
140
145
120
110
115
85
90
95
100
105
70
75
80
55
60
65
25
30
35
40
45
50
5
10
15
20
0
1
0
80
Percent
0.0
1.1 - 2.9
3.0 - 7.9
8.0 - 13.9
14.0 - 22.9
23.0 - 29.9
30.0 - 38.8
38.9 - 70.1
60
40
Figure 193.
Percentage of river
banks with revetments in 1995.
20
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
115
120
125
130
135
140
145
150
155
160
165
170
175
180
185
190
195
200
205
210
215
220
225
0
Portland Metro
Salem/Keizer
Albany Corvallis
Eugene/Springfield
Note: 1 kilometer equals .62 mile
Willamette River Basin Atlas
2nd Edition
139