20191250 Ver 1_2019_06_11_75%_Drainage_Design_Narrative_20190919

Kimley»>Horn
MEMORANDUM
To: Bergen Watterson, PE- Town of Chapel Hill
From: Chad Beck, PE
Kimley-Horn and Associates, Inc.
Date: June 11, 2018
Subject: Elliott Road Extension- 75% Storm Water Design Narrative
Attached you will find the 75% construction drawings for the Elliott Road Extension project along with
the hydraulic calculations package.
75% STORM WATER DESIGN
In general, the 75% storm water design follows the previously prepared functional design. Some of
the design features, and coordination items are discussed in detail below:
• The existing culvert under the existing portion of Elliott Road will be extended on the northern
side with 14'-11" x 9'-1" triple elliptical steel arch culvert to allow for the desired road
widening. The proposed extension will be 41'-0". A no -rise certification has been approved
for this extension.
• The existing culvert under Fordham Boulevard will be extended on the eastern side using 11'-
0" x 1 V-0" triple reinforced concrete box culverts to allow for the desired typical section of a
12' multi -use path and guardrail. The proposed extension will be 22'-8". A culvert survey
report has been developed for this extension and reviewed by NCDOT. Included with this
submittal is the revised CSR based on NCDOT comments for approval.
• There are two proposed culvert cross pipes included in the new location of Elliott Road:
• The existing 48" pipe will be intercepted with proposed junction box 0602 just north of the
extension, and transitioned to a 72" RCP pipe that will go under the proposed Elliott Road
extension before daylighting at the proposed downstream invert of the culvert extension
under Fordham Blvd. Based on previous discussions with NCDOT, upsized portion of
the culvert was design to accommodate the 50-yr storm should the upstream system ever
be removed and converted to an open ditch conveyance system.
• The existing V- ditch is proposed to be accommodated by a proposed 42" RCP pipe that
will cross under the Elliott Road extension before daylighting into a portion of the existing
V-ditch that is to remain. This pipe will also act as the bypass system outlet for the portion
of the storm water coming from the proposed site and roadway improvements that will not
be treated due to existing topography, and the inability to daylight this system at the
proposed storm water treatment facilities located within the site.

Kimley»>Horn
Page 2
• There are multiple junction points listed below where a proposed onsite storm water system will
tie to the proposed Elliott Road storm water system, and their drainage areas have been
incorporated into the offsite system hydraulic calculations appropriately.
• At CB 0720 a proposed system that will run along the proposed site drive will carry onsite
water to tie into the Elliott Road system.
• CB 0716 will have two proposed pipes coming from onsite. One will be a 15" RCP carrying
storm water from the proposed onsite court yard and amenity area, and one will be an 8"
HDPE roof drain.
• CB 0714, CB 0715, CB 0716, CB 0717, and 0802 will have site water added coming from an
8" HDPE roof drain.
• CB 0713 will also have site storm water coming outside the ROW that will enter into the
proposed system along Elliott.
• The proposed storm water system west of the site drives along Elliott will be the bypass system
that will not be treated onsite and will outlet directly into the existing stream at outlet 0707.
• The proposed storm water system east of the site drives along Elliott will outlet at node 0721 into
an onsite catch basin that is part of the storm water system that will be treated onsite.
• The proposed storm water system along Ephesus Church Road will tie to the proposed system
that will enter the site at the northern site drive at CB 0802