Revised_Attachment_3_Project_Description.docx

Swannanoa Creek Stream Enhancement Project 2019 – Project Description Brady Dodd (NFsNC Hydrologist), May 15, 2017, edited December 3, 2018/May 28, 2019

Project Background:

Swannanoa Creek (Figure 1), in the Catawba River Basin, is classified by the state of NC as Class C, Trout. The watershed has a history of timber harvest with a concentration of activity, including roads and farming, in the valley bottom. Currently there is private land occupied by small buildings and a grassy field on the north bank and a Forest Service picnic area on the south (Photo 1) within this reach. The property boundary follows the centerline of the channel.

In the past, this reach has been impacted by channelization and therefore is entrenched without normal access to its broader valley bottom floodplain. Much of the channel scour has occurred in the past leaving exposed bed rock in the bed and banks. Because of rocky banks and numerous large live trees on the bank edge, banks are moderately stable (Photo 2). Exceptions occur at the most upstream section of the reach where the channel bends at a steep eroded side slope and downstream along the left bank where erosion is occurring annually due to loose, unconsolidated cobbley soils and a lack of deep rooted vegetation. Several sections of channel have bankfull benches that promote floodplain interaction, but the bankfull channel is largely within the entrenched valley bottom defined by high and low terraces. Over the reach, aquatic habitat diversity is low; pools are largely lacking and of low quality.

Figure 1. Site location of Swannanoa Creek Stream Enhancement Project.

Photo 1. Photo of Swannanoa Creek, looking at eroded bank and FS picnic area, May 11, 2017.

Photo 2. Swannanoa Creek Current Condition looking downstream at project reach.

Project Description:

Within this ~1,200 feet stream reach of Swannanoa Creek work is proposed to stabilize erosion and sources of sedimentation and improve aquatic habitat diversity. Under a Wyden Agreement with the neighboring landowner, this will be accomplished by the following:

1. From about 200 feet upstream from the Old US Highway 70 bridge crossing on Swannanoa Creek, the proposed reach extends upstream for about 1,200 feet, construct stream structures using imported boulders and trees from the surrounding area. These structures are designed to protect stream banks from erosion and reestablish natural riffle-pool bedform features, and thus increase habitat quality and diversity. Nine structures including boulder vanes, log vanes and toe-wood revetments are planned (Table 1 and Figures 2 & 3) in this reach to establish and improve habitat diversity as determined from the upstream reference reach data and Regional Curves (Table 2). In conjunction with existing riffle-pool habitat the proposed design should resemble the reference bedform.

a. In-stream structures and channel substrate alteration will be designed to achieve natural channel design objectives and support aquatic habitat. See Appendix A for construction details.

i. The boulder and log vanes are intended to hold grade while creating flow convergence to promote pool scour and turning the flow away from the bank.

ii. Toe-wood revetments are used to enhance bank stability while providing a simulated undercut bank for aquatic organisms.

2. Work would be accomplished using a medium-sized trackhoe to excavate the channel bed and banks to install these structures.

3. Erosion control measures would be taken during and after construction to avoid adverse effects (see Appendix A & B).

Table 1. Proposed stream enhancement sites, disturbance type and length of channel disturbed.

Figure 2. Plan view of Swannanoa Creek with proposed location and type of structures, Page 1 of 2.

Figure 3. Plan view of Swannanoa Creek with proposed location and type of structures, Page 2 of 2.

Table 2. Swannanoa Creek bedform design parameters from reference reach and regional curves, Appendix C.

Drainage Area = 2.6 miles2

Bankfull riffle cross-section area

38.8

Bankfull discharge (cfs)

199.6

Bankfull mean depth - riffle

1.47

1 Blue Ridge Ecoregion 66, Regional Curve Data

APPENDIX C – BLUE RIDGE ECOREGION 66, REGIONAL CURVES

Blue Ridge Ecoregion 66: Bankfull Riffle Mean Depth

North Carolina + Tennessee Regression: NC + TN y = 1.10x0.301 R² = 0.93

0.28000000000000003 0.37 0.42 0.42 0.69 0.81 1.29 1.31 1.6 1.94 2.33 3.16 8.9600000000000009 9.48 10.1 17.399999999999999 17.7 19.5 31.3 61.1 106 2 5 5.46 11.7 13.8 14.5 27.6 40.4 46.9 51.5 66.7 67.900000000000006 92.1 126 0.1 0.18146699999999999 0.262548 0.59073300000000006 0.60231600000000007 1.942083 2.6447849999999997 2.969109 4.2857099999999999 4.918914 5.4401489999999999 6.1930439999999995 8.9613810000000012 16.069482000000001 0.7 1 0.9 0.9 1.3 1.1000000000000001 1 1.2 1.2 1.3 1.1000000000000001 1.4 1.9 2 2.2000000000000002 2.1 2.6 2.9 3 3.6 5.3 1.4 1.7 1.7 2.5 2 2.5 3.4 3.6 3.2 4.2 4.5 6.6 4.0999999999999996 3.7 0.82 0.4264 0.82 0.82 1.0167999999999999 1.3120000000000001 1.476 1.4431999999999998 1.804 1.804 1.7712000000000001 1.7712000000000001 1.9351999999999998 2.2303999999999999 South Carolina Watershed Drainage Area (mi2)

Bankfull Riffle Mean Depth (ft)

Blue Ridge Ecoregion 66: Estimated Bankfull Discharge

North Carolina + Tennessee Regression: NC + TN y = 96.1x0.765 R² = 0.94

0.28000000000000003 0.37 0.42 0.42 0.69 0.81 1.29 1.31 1.6 1.94 2.33 3.16 8.9600000000000009 9.48 10.1 17.399999999999999 17.7 19.5 31.3 61.1 106 2 5 5.46 11.7 13.8 14.5 27.6 40.4 46.9 51.5 66.7 67.900000000000006 92.1 126 0.1 0.18146699999999999 0.262548 0.59073300000000006 0.60231600000000007 1.942083 2.6447849999999997 2.969109 4.2857099999999999 4.918914 5.4401489999999999 6.1930439999999995 8.9613810000000012 16.069482000000001 25 73 23 74 102 84 73 101 112 167 181 315 465 384 495 429 1377 1337 2018 1231 4450 242 352 231.5 470 560 855.7 2433 1457 1320 3450 2263 3226 3492 2763 14.240230619066519 66.716191288403209 83.91320377119591 94.70769365562704 198.85467338165748 141.79896693641729 194.58537517760297 331.5979810980412 315.72539893983611 421.58620107197197 315.02611077037949 483.12068789991906 629.25345142021808 South Carolina Watershed Drainage Area (mi2)

Estimated Bankfull Discharge (cfs)

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