20140333 Ver 1_RE: Holman Mill close-out work plan_20230202

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WILDLANDS
ENGINEERING
Technical Memorandum
Prepared for:
NC Interagency Review Team
Project Title:
Holman Mill Mitigation Site
Project No:
DMS: 96316, DWR: 14-0333, USACE: SAW-2015-00019
Subject:
Close-out repair
Date:
February 1, 2023
To:
NC I RT
From: Angela Allen, Wildlands Engineering, Project Manager
INTRODUCTION
The Holman Mill Mitigation Site Close-out site visit occurred on January 25, 2023. Individuals from
Wildlands Engineering, USACE, NC DMS, and NCDWR were present. During the site walk a headcut was
noted along UT2A near photo point 43 at approximately station 302+40. It is currently being held in
place by a large root system from a black willow livestake on the top of bank. The IRT requested to
stabilize the headcut as a part of the closeout process.
WORK PLAN
Wildlands inspected the feature during the site visit. We also examined up and downstream conditions,
and referred to design plans, as -built plans, and monitoring year photos (Appendix A) in our analysis and
development of a work plan.
UT2A has experienced input of large sediment pulses from an upstream, offsite source during the course
of the project. The majority of that plume has successfully pushed through the system to UT2 and down
to Pine Hill Branch. However, some deposition remains on channel banks and the floodplain. It appears
that the willow roots prevented some of the sediment plume in the channel from flushing through,
which increased the elevation of the two upstream riffles and caused a vertical drop over the willow
roots. The vertical drop formed a small scour pool at the bottom and caused erosion along the left bank
and likely lowered the grade of the head of riffle downstream of the drop. Upstream and downstream of
the drop, the channel appears stable. This vertical drop has remained in the same location since forming
and has not traveled upstream.
As the majority of the channel is stable, and the vegetation is mature in the area, Wildlands proposes to
do a minimally invasive approach. We will stabilize the vertical drop with rock underlain by geotextile
fabric to capture fines and prevent future piping under the root system. Rock will then be added to
stabilize the sideslopes of the scour pool as necessary to stabilize banks, specifically the left bank where
localized scour has occurred. Wildlands believes the scour will no longer be an issue on the left bank
once energy is dissipated properly in the scour pool by adding roughness. The left bank side slope will
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Technical Memorandum

transition into the point bar slope directly downstream. This may be done with hand/shovel work if
deemed appropriate by Wildlands during field installation of the repair. Wildlands believes this is the
best and least invasive approach to ensure the continued success of the project after closeout.
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Technical Memorandum

Appendix A:
Holman Mill Mitigation Site
Technical Memorandum

2022
2020
2018
Holman Mill Mitigation Site
Technical Memorandum
2021
2019
2017