20201004 Ver 1_Restoration Summary_20201110

Jennings Environmental PLLC is licensed with the North Carolina Board of Examiners for Engineers and Surveyors and is authori zed to
practice engineering under the provisions of Chapter 89C and 55B of the General Statutes of North Carolina. License Number P-1932

7 Samuel Ashe Drive, Asheville, NC 28805
[e-mail]
919-600-4790

October 31, 2020

STREAM RESTORATION DESIGN ANALYSIS
Slusher – Sapphire Farm, Thompson River, Transylvania County, NC

Owner: Mr. Shannon Slusher
3 Stansbury Dr., Asheville NC, 28803
[e-mail]
215-888-0994

Engineer: Greg Jennings, Ph.D., P.E., President
Jennings Environmental PLLC

This document summarizes stream restoration design analysis performed by me to support the
proposed restoration of 333 linear feet of Thompson River on the owner’s property. The attached
spreadsheet lists morphological and hydraulic parameters for the existing, reference, and design
conditions. Parameters are explained below:

• From USGS StreamStats, the watershed drainage area for the restoration reach is 0.8 sq mi and
the estimated 2-YR Discharge from regional regression of gage data is 94 cfs.

• The reference reach for this project is a 200-ft reach of Thompson River located upstream 100 ft
of design reach. Reference reach morphology and calculated hydraulic information is presented
below.

• Target bankfull design Parameters were obtained from Regional Curves (Blue Ridge Ecoregion)
and the Reference Reach. Design targets for Bank Height Ratio of 1.0 and Entrenchment Ratio of
> 3 are based on optimizing floodplain connection for minimizing applied shear stress during
overbank flow conditions.

• Restoration reach existing condition top of bank morphology and calculated hydraulic
parameters are listed to document departure from morphological equilibrium. Key indicators of
morphological disequilibrium are Bank Height Ratio of 2.3 and Entrenchment Ratio of 1.2,
resulting in a Rosgen Stream Type classification of G4/1 with evidence of active widening
through bank erosion. The existing channel will carry greater than 500 cfs (approximate 100-
year discharge) at an average velocity of 11 ft/s and applied shear stress of more than 3 psf.
These hydraulic conditions will mobilize substrate with a diameter of approximately 14 inches,
based on Shields Curve. Photos below show existing conditions of incision and bank erosion in
restoration reach.

Jennings Environmental PLLC www.jenningsenv.com 2

• Design bankfull morphology and calculated hydraulic parameters are listed for the optimal bank
height ratio of 1.0. In order to achieve this floodplain connection condition, the streambed will
be lifted using a constructed boulder/cobble/gravel cascade as shown on attached plan sheets.
The proposed design Rosgen Stream Type classification is C4b, matching the upstream reference
reach. The design bankfull channel will carry 82 cfs (less than the 2-year discharge and
approximately matching the Regional Curve) at an average velocity of 5.1 ft/s s and applied
shear stress of more than 1.2 psf. These hydraulic conditions will mobilize substrate with a
diameter of approximately 7 inches, based on Shields Curve.

• In order to size rocks to be used in building the proposed cascade, the maximum competency
for the bankfull flow condition was multipled by a Factor of Safety of 3 to determine the
minimum Threshold Substrate Size of 21 inches. Boulders to be used in this construction will be
specified greater than this size. The existing stream bed in the channel section where the
boulders will be installed is gravel overlaying bedrock. The boulder arches providing the
backbone of the cascade will be installed in shallow notches in the bedrock and integrated into
the bank to resist mobilization during extreme flow conditions.

• In summary, benefits of the proposed stream restoration include (1) improved floodplain
connection with elevated baseflow and reduce shear stress during overbank flows; (2) reduced
bank erosion; and (3) enhanced riffle-pool bedform conditions.

Respectfully Submitted,

Gregory D. Jennings, Ph.D., P.E.
President, Jennings Environmental PLLC

Hydraulic Calculations Thompson River, Slusher Property, Sapphire, NC
Blue Ridge Ecoregion 65 Greg Jennings, PhD, PE 10/30/20
Watershed Drainage Area, DA (sq mi)0.8
USGS StreamStats 2-YR Discharge, Q2 (cfs)94
Reference Reach
Bankfull Design Parameter Regional Curve Reference Reach
Watershed Drainage Area, DA (sq mi)0.8 0.7
Discharge, Q (cfs)78 70
Riffle Cross-Section Area, A (sq ft)15.7 14.0
Riffle Width, W (ft)15.0 14.0
Riffle Mean Depth, d (ft)1.0 1.0
Bank Height Ratio, BHR (ft/ft)1.0 1.0
Entrenchment Ratio, ER (ft/ft)> 3 2.5
Parameter (Top of Bank)Existing Channel Design Bankfull
Reach Slope, S (ft/ft)0.022 0.022
Manning Roughness Coefficient, n 0.035 0.040
Width, W (ft)15.2 16.0
Cross-Section Area, A (sq ft)49.8 16.0
Bank Height Ratio, BHR (ft/ft)2.3 1.0
Entrenchment Ratio, ER (ft/ft)1.2 2.5
Stream Type G4/1 widening C4b
Mean Depth, d (ft)3.3 1.0
Width-to-Depth Ratio, WDR (ft/ft)4.6 16.0
Hydraulic Radius, R (ft)2.3 0.9
Average Velocity, V (ft/s)11.0 5.1
Discharge (in channel), Q (cfs)546 82
Applied Shear Stress, t (lb/sq ft))3.14 1.22
Unit Stream Power, w (lb/ft/s)34.48 6.23
Maximum Competency, Dmax (mm)353 176
Threshold Substrate Factor of Safety, FoS 3.0
Threshold Substrate Size, D (inches)21
Thompson River upstream 100 ft of design reach
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