PZ Item 10 - Kimley Horn_Detention MemoDecember 20, 2019
kimley-horn.com 13455 Noel Drive Two Galleria Tower Ste 700 Dallas, TX 75240 972-770-1300
MEMORANDUM
To: Chris Paulus—Cooper and Company
From: Steven Galloway, P.E.—Kimley-Horn and Associates, Inc.
Danny Torres, E.I.T.—Kimley-Horn and Associates, Inc.
Cc: Frank Bliss—Cooper and Company
Date: December 20, 2019
Subject: Southlake Town Square—Block 11 and McPherson Pond Assumptions
P R E L I M I N A R Y
FOR REVIEW ONLY
Not for construction or permit purposes.
Engineer Steven D. Galloway
P.E. No. 94899 Date 12-20-2019
Kimley-Horn (“KH”) has reviewed the proposed improvements to Block 11 of Southlake Town Square
(“STS”) to determine if McPherson Pond may accommodate its runoff without detention. KH has
reviewed construction plans within the watershed and evaluated the capacity McPherson Pond based
on current drainage criteria. KH has determined that:
• Block 11 does not require onsite detention;
• Block 11 should send at least 6.30 acres to McPherson Pond to be consistent with the STS
Master Plan;
• Block 11 may send up to 7.50 acres to McPherson Pond without modifying the pond’s geometry
or outfall structure; and
• Block 11 may bypass the remainder of its runoff un-detained.
Block 11 is within the watershed of a 3-5’x4’ culvert across SH 114 (“TxDOT Culvert B”). KH obtained
the following data for the watershed:
• Exhibit A - March 2005 construction plans for Southlake Town Center (“the STS master plan”);
• Exhibit B - May 2008 construction plans for Kirkwood Boulevard, Grace Lane, & Blessed Way
(downstream of STS, across SH 114) (“the Gateway plans”);
• Exhibit C - November 2012 construction plans for Patrizio’s (now il Calabreze, downstream of
STS, at the southwest corner of State Street and SH 114);
• Exhibit D – the effective crest of McPherson Pond per LiDAR topography for the area; and
• Exhibit E - an Architectural concept plan for Block 11.
December 20, 2019
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k:\dal_hydro\064538800 - southlake town square\docs\20191220 memo.docx
S T S MASTER PLAN
Exhibit A shows that 36.43 acres of STS property are within the watershed of TxDOT Culvert B:
• Basin C-A, shown on the drainage area map, is a 25.70-acre drainage basin proposed to drain
to McPherson Pond.
• Basin C-B, shown on the drainage area map, is a 10.73-acre drainage basin proposed to
bypass McPherson Pond and drain un-detained to TxDOT Culvert B.
Exhibits B and C consider that 25.70 acres of STS will be detained in McPherson Pond, and 10.73
acres the STS site will bypass McPherson Pond un-detained.
BLOCK 11 D EVELOPMENT
Block 11 is an area bounded by Division Street, State Street, N. Carrol Avenue, and Federal Way. In
Exhibit A, this was a 6.30-acre area intended to drain to McPherson pond. However, the area is
approximately 7.90 acres today because Division Street was built north of its proposed alignment in
Exhibit A.
Exhibit A proposes McPherson Pond’s bank was be graded 4:1 from top-of-bank (634’) to toe-of-bank
(628’) with an outfall invert at approximately elevation 627’. LiDAR topography shows that the pond
was built as planned, but that the actual top-of-bank is an effective crest at elevation 634.80’ north of
the outlet structure.
The City of Southlake Code of Ordinances § 9.5-61 adopts the City of Fort Worth Drainage criteria for
detention calculations, which requires detention ponds to contain the ultimate 100-year storm with one
foot of freeboard and recommends SCS hydrologic modeling for detention design.
KH first modeled the watershed at pre-development conditions, with the 36.43-acre basin from STS at
pre-developed conditions. The pre-development runoff from the STS property was 225.90 cfs.
Next, KH modeled the watershed at fully developed conditions with the drainage areas from Exhibit A.
The proposed runoff from the STS property is 104.5 cfs—significantly less than pre-development
conditions—and the 100-year water surface elevation in McPherson Pond is 633.60’. In this condition,
6.30 acres of Block 11 is routed to McPherson Pond, and 1.60 acres bypasses the pond.
KH then evaluated how much additional area could be detained within McPherson Pond while
maintaining one foot of freeboard. With 26.90 acres to McPherson Pond and 9.53 acres bypassing the
pond un-detained, the runoff from the STS property is 94.5 cfs, and the 100-year water surface
elevation in McPherson Pond is 633.80. In this condition, 7.50 acres of Block 11 is routed to McPherson
Pond and 0.40 acres bypasses the pond.
December 20, 2019
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kimley-horn.com 13455 Noel Drive Two Galleria Tower Ste 700 Dallas, TX 75240 972 770 1300
k:\dal_hydro\064538800 - southlake town square\docs\20191220 memo.docx
CONCLUSION
Predeveloped
Conditions
STS
Master Plan
Maximum
Detention
Area to be Detained from STS (AC.) N/A 25.70 26.90
Area to be Detained from Block 11 (AC.) N/A 6.30 7.50
Runoff (cfs) 225.90 104.50 94.50
McPherson Pond Freeboard (ft) N/A 1.20 1.00
The STS master plan considered that Block 11 would be a 6.30-acre area entirely detained within
McPherson Pond, but as-built conditions indicate that Block 11 is approximately 7.90 acres. McPherson
Pond satisfies current drainage criteria when detaining up to 7.50 acres from Block 11. Therefore:
• Block 11 does not require onsite detention;
• Block 11 should send at least 6.30 acres to McPherson Pond to be consistent with the STS
Master Plan;
• Block 11 may send up to 7.50 acres to McPherson Pond without modifying the pond’s geometry
or outfall structure; and
• Block 11 may bypass the remainder of its runoff un-detained.
December 20, 2019
kimley-horn.com 13455 Noel Drive Two Galleria Tower Ste 700 Dallas, TX 75240 972-770-1300
EXHIBIT A – STS Master Plan
December 20, 2019
kimley-horn.com 13455 Noel Drive Two Galleria Tower Ste 700 Dallas, TX 75240 972-770-1300
EXHIBIT B – Gateway Church Phase 1 Roadway Plans
December 20, 2019
kimley-horn.com 13455 Noel Drive Two Galleria Tower Ste 700 Dallas, TX 75240 972-770-1300
EXHIBIT C – Patrizio’s Plans
December 20, 2019
kimley-horn.com 13455 Noel Drive Two Galleria Tower Ste 700 Dallas, TX 75240 972-770-1300
EXHIBIT D – McPherson Pond Top-of-Pond
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LEGEND
EXISTING STORM SEWER
1-FT EXISTING CONTOURS
EXH
December 20, 2019
kimley-horn.com 13455 Noel Drive Two Galleria Tower Ste 700 Dallas, TX 75240 972-770-1300
EXHIBIT E – Architectural Concept