20190145 Ver 1_Springside Pond_PCN_Narrative_20190130

Springside Pond
PCN Project Narrative
Village of Clemmons
january 2019
��EWELL
� ENGINEERING CONSULTANTS, PC
311 S. Main Street, Suite A
Kernersville, NC 27284
NC PE Firm # C-1842

Springside Pond - PCN Project Narrative
Village of Clemmons, NC
The location of the proposed culvert replacement is on Springside Drive, which is a dead-end street
within a residential area. On the upstream side is a privately-owned pond. The original outlet for the
pond failed long ago and the pond has for many years been drained by 4-18" CMP's with inverts roughly
2- % ft below the top of the road. Approximately 183 acres of watershed is drained to Springside pond,
including 90 acres north of I-40. Due to the hydraulic inadequacy of the existing outlets, the road may
be overtopped multiple times within a year. The downstream side of the roadway embankment is very
steeply sloped and has been subjected to substantial erosion over time, requiring the Village to heavily
reinforce the slopes in order to protect the roadway. Geotechnical investigation indicated that the
existing roadway embankment construction was not done in a manner adequate to function as a dam
and would require significant improvements if the Village wanted to maintain the pond. JEWELL
recommended that the Village permanently drain the pond before installing new culverts in order to
avoid liability concerns with management of the roadway as a dam structure, and the pond owner
agreed to this arrangement. Figure 1 shows the area of the Springside Drive culvert crossing and the
pond outlet.
The proposed replacement of the culvert crossing presents significant design and construction
challenges. First, there is approximately a ten-foot drop from the top of the sediment on the upstream
side in the pond to the channel thalweg on the downstream side of the embankment. Based on the
geotechnical investigation, we expect about 8 feet of this is sediment accumulation in the pond and 2
feet appears to be due to downcutting of the bed in the channel on the downstream side. There has
been heavy accumulation of sediments in the pond, probably from construction of I-40 and other
development projects constructed in the watershed over the lifetime of the pond. During construction,
the contractor will use either a coffer dam or a slide rail system to dewater and remove sediment from
an adequately-sized area of the pond to facilitate construction of the inlet structure and gabion wall. A
concrete structure at the inlet, with wingwalls up to the approximate level of the top of sediment and a
gabion wall at the upstream end (with filter stone on the pond side). The gabion wall is intended to
create a temporary dam to prevent mass migration of the sediments downstream, while also facilitating
dewatering of the pond bottom. The gabion wall will be constructed in 1.5 to 3-foot rows of gabions
such that one row at a time can be removed by Village crews as the sediments become dried out and
vegetated and a channel forms through the pond bottom. We expect a channel will form in the upper
portion of the pond, but that the lower area may develop into more of a wetland without a very well-
defined channel.
The second major challenge for the project construction is the need to maintain access to four homes
located on the side of the crossing where Springside Drive dead-ends. Although strong consideration
was given and investigations made into possible alternative access options or a temporary drive around
the construction site, the conclusion was that one-lane access adequate to accommodate emergency
�� Springside Pond - Village of Clemmons, NC
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vehicles will need to be maintained within the Springside Drive right-of-way during the culvert
construction. This makes the culvert installation more difficult and expensive for the contractor. We
requested input from a contractor on probable construction costs and their estimate was a half-million
dollars. They noted that the need to keep the road open was a huge factor in the project cost.
Because of these construction challenges, the project is a very expensive one for the Village relative to
the citizen benefit. However, the project is a high priority for the Village due to the liability associated
with having a roadway embankment at risk of failure. The project has been planned to be done as
economically as possible within the constraints. The Village investigated options and costs for sediment
removal and stream restoration, which would have maximized the long-term ecological benefit by
providing a restored natural channel, aquatic habitat, and fish passage, but found that those measures
would have likely doubled the project costs.
JEWELL contracted with Piedmont Environmental to do site investigation and check for the presence of
possible wetlands in the project area. They concluded that there are no wetlands and that the survey
information adequately defines the open water limits and the stream channel location. The staging area
for the project is proposed to be on the paved cul-de-sac shown in Figure 1.
Open water impacts, within the normal water surface elevation of the existing pond, are estimated at
0.014 acres of permanent impacts, inclusive of up to a 10-foot wide maintenance access to be
established adjacent to the concrete inlet structure for long-term use, and total of 0.08 acres of
temporary impacts. The temporary impacts include 0.05 acres for proposed placement of excavated
soils in the corner of the intersection of Edgewater Drive with Springside Drive to the extent the area is
suitably stable without heavy accumulation of sediments. The extension of the culvert will permanently
impact up to 20 feet of jurisdictional stream, if the existing stream is judged to begin at the end of
existing 18" culverts, high up on the roadway embankment. The riprap stilling basin and grading back of
stream banks will impact up to 54 linear feet of existing stream on the downstream side of Springside
Drive.
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Springside Pond - Village of Clemmons, NC
Summary of Potential Breach Impacts
� EWELL
�
EN::i\-F� IIVG
CONSLJLTANTS, PC
Page 3