20220421 Ver 1_Report - Design - McCaskill - 2021-10-28_20220315

JOHN W. HARRIS, P.E. 5112 Bur Oak Circle
CONSULTING ENGINEER, Inc. Raleigh, N.C. 27612
www.harriseng.net [phone]

DESIGN REPORT AND CALCULATIONS

For

2800 Edgemont Rd

Wake County

Date: 09 April 2021

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Contents
Narrative: ................................................................................................................................................................ 3
Hydrology ............................................................................................................................................................... 5
Culverts ................................................................................................................................................................... 6
Driveway Pipe ..................................................................................................................................................... 6
Stream Crossing .................................................................................................................................................. 8

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Narrative:

The owner of this property wants to build his private residence on this 15.15-acre tract consisting of two
impervious structures totaling 8,770+/- with one driveway. The property is in the Falls Lake Watershed and is
zoned R-40W.

NCDEQ has determined two reaches to be subject to Neuse Riparian Buffers, which one of is
downstream of a dammed pond on the northern adjacent property. Wetlands have been determined and staked
on the property and are shown on the plans. The Site Plan design has the driveway crossing the reach in
between two wetlands. A Flood study was performed for the purpose of ensuring the overall drainage area for
this site does not impede downstream or off site locations

As this property has a significant stream on the southern side of the property the outfall of this area has a
significantly big drainage area. The development of the property to a private residence does not increase the
Peak Q of drainage of the perennial stream called out as stream “A (Rocky Branch)” in the Buffer
Determination Letter (DWR RRO #20-245) and according to FRIS it is Basin 10, Stream 6. The increase in
impervious is superfluous to the area that is in this drainage area and in the calculation for the rational method,
as well as the SCS method we find that the co-efficient and the curve number does not increase. This stream
does have a square mile of drainage along the southern property line and on sheet 4 and HEC-RAS (1) from
Fris, you will see the floodplain associated with this and though the limits of the FEMA floodway does
encroach into our access point, there is no concerns of impeding the water path, location, or elevation as the
elevation of the floodway for this stream does not get within 500’ of our access location. The location of the
limits of FEMA Zone X: 1% Future Conditions and the FEMA Zone AE: Floodway, 1% Annual was taken
from the shapefile in the FRIS Data Base.

The other concern of flooding as there is wetlands in the area of our access location and a cross pipe
under Edgemont Road immediately south of our driveway that has a contributing area of 21.76 acres draining
through this pipe but as the outlet of this existing pipe is 2.1’ lower and has 12” of freeboard from the 100 yr.
flood than our driveway pipe there is no interference with the property’s driveway grading. See sheet 5 for more
details and the HEC-RAS (2) Driveway Entrance.

The main concern is the crossing passing through the (2) 36” culverts designed for this driveway plan
and not have any water backing up onto the adjacent properties. See the enclose HEC-RAS (3) Crossing reports
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Existing and Proposed for the full detail. We have illustrated the findings on sheet 6 of the plans where the light
blue line denotes the existing HGL and the dark blue line denotes the proposed HGL. You will note that the
water elevation does increase slightly at the headwall but going through the pipes increases its velocity and
loses that raise in elevation. There are points on the profile that relates to the HEC-RAS where we confirm that
there was no raise in elevation (or decrease) at these locations.

As for the Nitrogen/Phosphorus level and treatment, they both are below the threshold for not needing to
treat any storm water, see snap tool. According to the old Wake County Hybrid Storm Tool, I used the woods
and lawn under residential and applied it to the SNAP tool where custom 1 is woods and have a TN of 1.47
mg/L and TP of 0.25 mg/L and custom 2 is lawn and have a TN of 2.24 mg/L and TP of 0.44 mg/L. Once that
was inputted then we see that all the results are sufficient to not need a SCM. Without using these numbers the
land, itself without an impervious indicates it will need a significant SCM to reduce the TP of the pervious grade
covering which did not make since.

We have added an erosion control sheet where it will consist of clean water diversion berms and silt
fence with silt fence outlets, each silt fence outlet has less than 0.25 acres of area draining to them.

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Hydrology

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Culverts
Driveway Pipe

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FHWA Disspator

Dissipator in defined ditched

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Stream Crossing

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Dissipator