East Mill Dam Removal Engineered Design
Approximately 1800 cubic yards will need to be excavated from the channel starting at the dam
and going upstream approximately 500 feet. The target slope will be 3.4%. This slope should
intersect and match the lower channel slope, so no excavation will be required below the dam
(see figure 5).
Figure 1 shows the extent of the HecRas model which includes about 270 feet of channel above
the dam, or starting at our first cross section, XS 474. If possible a reduced excavation would
occur upstream of XS 474, where the main objective would be to extend the target slope
upstream until it is within 1-2 feet of the existing surface elevation. The low flow channel could
be excavated here to approximately a six to eight foot wide channel, tapering to zero excavation
upstream.
Three surveyed cross sections are shown in figures 2-4, overlain with the design cross sections at
each location. These can be used to shape the channel with interpolation between the cross
sections. Pools and wood should be placed at outside of bends. The cross sections were
designed to meet the target thalweg elevation and to carry the 2-year flow and the 100-year flow
at approximately the same depth and width as the cross section 97 feet below the dam. Figure 5
shows the existing cross section 97 feet downstream of the dam. The sideslopes of this cross
section are approximately 1:1 and the water surface width of the 2-year flow is approximately 8
feet wide.
Excavated material can be used to lay against the existing steep banks on the left bank. Trees
and permanent vegetation should be planted above the 2-year water surface elevation shown on
the cross sections.
East Mill Mattole
Plan:
1) M3eastmill
4/19/2007
2) Oeastmill
4/19/2007
EastMill Dam
80
Legend
WS 2yr 58 - M3eastmill
WS 2yr 58 - Oeastmill
Ground
Ground
75
Elevation (ft)
70
65
60
55
50
0
100
200
300
400
500
Main Channel Distance (ft)
Figure 1 Estimated 2-year water surface elevation before and after the dam removal.
600
East Mill Mattole
Plan:
1) M3eastmill
River = EastMill Reach = Dam
2) Oeastmill
RS = 474
88
Legend
WS 2yr 58 - Oeastmill
WS 2yr 58 - M3eastmill
- Oeastmill
86
- Oeastmill
- Oeastmill
- Oeastmill
2.0 ft/s - Oeastmill
84
2.5 ft/s - Oeastmill
3.0 ft/s - Oeastmill
3.5 ft/s - Oeastmill
4.0 ft/s - Oeastmill
4.5 ft/s - Oeastmill
82
5.0 ft/s - Oeastmill
Ground - Oeastmill
Bank Sta - Oeastmill
- M3eastmill
80
Elevation (ft)
- M3eastmill
- M3eastmill
- M3eastmill
- M3eastmill
78
0 ft/s - M3eastmill
1 ft/s - M3eastmill
2 ft/s - M3eastmill
3 ft/s - M3eastmill
76
4 ft/s - M3eastmill
5 ft/s - M3eastmill
6 ft/s - M3eastmill
7 ft/s - M3eastmill
74
Ground - M3eastmill
Bank Sta - M3eastmill
72
70
0
10
20
30
40
50
60
70
Station (ft)
Figure 2 Existing and Design cross section at XS 474, 204 feet above the dam, a little upstream of well.
East Mill Mattole
Plan:
1) M3eastmill
River = EastMill Reach = Dam
2) Oeastmill
RS = 347
84
Legend
WS 2yr 58 - Oeastmill
WS 2yr 58 - M3eastmill
- Oeastmill
82
- Oeastmill
- Oeastmill
- Oeastmill
- Oeastmill
80
- Oeastmill
1.0 ft/s - Oeastmill
1.5 ft/s - Oeastmill
78
2.0 ft/s - Oeastmill
2.5 ft/s - Oeastmill
3.0 ft/s - Oeastmill
3.5 ft/s - Oeastmill
76
Ground - Oeastmill
Elevation (ft)
Bank Sta - Oeastmill
- M3eastmill
- M3eastmill
74
- M3eastmill
4 ft/s - M3eastmill
5 ft/s - M3eastmill
6 ft/s - M3eastmill
72
7 ft/s - M3eastmill
8 ft/s - M3eastmill
Ground - M3eastmill
70
Bank Sta - M3eastmill
68
66
64
0
10
20
30
40
Station (ft)
Figure 3 77 feet above the dam
50
60
70
East Mill Mattole
Plan:
1) M3eastmill
River = EastMill Reach = Dam
2) Oeastmill
RS = 280
85
Legend
WS 2yr 58 - Oeastmill
WS 2yr 58 - M3eastmill
- Oeastmill
- Oeastmill
- Oeastmill
- Oeastmill
- Oeastmill
80
- Oeastmill
0 ft/s - Oeastmill
1 ft/s - Oeastmill
2 ft/s - Oeastmill
3 ft/s - Oeastmill
4 ft/s - Oeastmill
5 ft/s - Oeastmill
75
Ground - Oeastmill
Bank Sta - Oeastmill
Elevation (ft)
- M3eastmill
- M3eastmill
- M3eastmill
4 ft/s - M3eastmill
5 ft/s - M3eastmill
6 ft/s - M3eastmill
70
7 ft/s - M3eastmill
8 ft/s - M3eastmill
Ground - M3eastmill
Bank Sta - M3eastmill
65
60
0
20
40
60
Station (ft)
Figure 4 10 feet above the dam
80
100
East Mill Mattole
Plan: M3EastMill
River = EastMill Reach = Dam
.06
4/19/2007
RS = 173
.04
.06
72
Legend
WS 2yr 58
0 ft/s
2 ft/s
4 ft/s
70
6 ft/s
8 ft/s
10 ft/s
Ground
Bank Sta
68
Elevation (ft)
66
64
62
60
58
0
5
10
15
20
Station (ft)
Figure 5 Existing cross section 97 feet downstream of dam.
25
30
35
XS 347 (77 feet
above the dam)
East Mill Creek Dam
XS 280 (10 feet
above the dam)
100.00
XS 474 (204 feet
above the dam)
90.00
Elevation (ft)
80.00
Elevation (ft)
Series2
Series3
Linear (Series3)
70.00
60.00
50.00
40.00
0
200
400
600
800
Station (ft)
Figure 6 Thalweg profile with target slope of -3.4%
1000
1200
1400
1600