HomeMy WebLinkAboutSP201600003 Study 2016-01-19FLOOD STUDY INFORMATION BOOKLET
FOR
WEST GLEN SUBDIVISION
EXTENSION OF CLING LANE
January 19, 2016
LOCATION:
TM 55C -3-A, 56 -115,55C -3 -Al, 56A9-9-25, 56A9 -9-26A
White Hall District, Albemarle County, Virginia
PREPARED BY:
Dominion Engineering & Design, LLC
172 South Pantops Drive
Charlottesville, VA 22911
P: 434.979.8121
F: 434.979.1681
INDEX
Tax Map
Flood Insurance Rate Map (FIRMETTE)
Cover Letter to Glenn Brooks, P.E., dated January 18, 2016
FEMA Summary Information for Discharges, n factors
FEMA Floodway data and Flood Profiles
Existing Floodplain Calculations Plan View and Summary Table
Proposed Floodplain Calculations Plan View and Summary Table
CH L F. P.1YFFS j
No. 3302.3
.� ! Dominion 172 South Pantops Drive
Engineering Charlottesville, VA 22911
January 18, 2016
Glenn Brooks, P.E.
County Engineer
Albemarle County Community Development
401 McIntire Road
Charlottesville, Virginia 22902
RE: West Glen Subdivision - TM 55G3 -A, 56 -115,55C -3 -Al, 56A1-1-25, 56A1 -1-26A -Flood
Study/Mitigation Narrative
Dear Glenn,
434.979.8121 (p)
434.979.1681 (f)
DominionEng.com
This letter is to provide background information on the flood study/conceptual mitigation plan for the referenced
Special Use Permit Application to provide fill in the floodway and floodplain fringe for a proposed stream crossing to
connect Cling Lane to Orchard Drive in accordance Z.O.30.3.11, Stream Crossings and Grading Activities, more
specifically:
30.3.11 -Special Use Permit in the floodway and floodway fringe for Bridges, ferries and culverts not serving
single-family dwellings"
The applicant is proposing a stream crossing of Cling Lane Extended over Powell's Creek from approximate station
17+00 to 18+00. There will be a maximum fill of approximately 12'. A double 12'W x 8'H Box Culvert at
approximate station 17+50 of is proposed to convey the base flood without overtopping the road and with no net
rise in the base flood elevation.
30.3.11- Special Use Permit for fill in the floodway fringe far "Grading activities, including cut or fill, in
compliance with the Water Protection Ordinance, but for which the floodplain administrator determines
will or may cause the base flood elevation to rise or the horizontal limits of the floodplain to expand."
Cling Lane Extended and associated grading encroaches into the floodway fringe from approximately station 23+40
to 25+60. The proposed fill causes no rise in the base flood elevation predicated on the attached HEC -RAS model.
HEC -RAS NARRATIVE
A HEC -RAS model was used to establish the existing and proposed 100 -year base flood elevations for the project
area, which spanned from Jarmans Gap Road to past the subject property (FIS sections F through L). Albemarle
County GIS Topographic mapping was used for the model. The 100 -year flows and n factor were taken from the FIS.
As a result of the study, the proposed fill caused the base flood elevation to fluctuate but there is no rise at Section L
and no offsite properties are impacted. Pertinent sheets from the HEC -RAS model are included in the flood study
notebook and a CD has been included with the submission.
The applicant is proposing a number of mitigation measures to offset potential impacts on environmental features
as a result of the road construction within the stream buffer. These measures are described below:
Page 1 of 2
Dominion
172 South Pantops Drive
r Engineering
Charlottesville, VA 22911
434.979.8121 (p)
•�,,� �N ;_
434.979.1681 (f)
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s
DominionEng.com
1. Removal of Existing Dam in PoweIl's Creek. The applicant is proposing to remove an approximate 5' -high
concrete dam that impounds water in Powell's Creek as shown on the plans. Upon removal of the dam, the
applicant proposes to restore the native stream flow patterns. This will serve to gradually restore the
riparian habitat that has been altered due to the dam construction.
2. Provide armoring stabilization of the streambank with gabion baskets, cutting back of the slope, or other
approved means to reduce the further displacement of soils in areas of extreme erosion along Powell's
Creek. Final design to be indicated on the WPO plans for the project
3. Reforestation in areas along Powell's Creek where tree cover is sparse in locations as shown on the plan.
4. Provision of structural (Filterras, Jellyfish) and non-structural (rain gardens, dry/wet swales) BMP facilities
downstream of new impervious areas to reduce the non -point solution. The proposed measures will be
provide additional water quality above and beyond the minimum required.
We thank you very much for your review of this project and look forward to your thoughtful review on this project
Please feel free to call me with any questions or if you require additional information.
Best Regards,
MicjilaeI Myers, P.E., C.F.M.
Cc: Charlie Armstrong
Page 2 of 2
TABLE 3 - SUMMARY OF DISCHARGES - continued
FLOODING SOURCE
DRAINAGE AREA
PEAK DISCHARGES
(efs)
AND LOCATION
(s4. miles)
10 -YEAR 50 -YEAR
100 -YEAR 500 -YEAR
NORTH FORK MEADOW
CREEK
At confluence with
Meadow Creek
0.9
450
800
1,100
1,700
NORTH FORK
RIVANNA RIVER
At confluence with
Rivanna River
178
17,700
31,900
39,700
62,700
At mouth
176
14,300
29,900
36,000
73,400
Upstream of Preddy
Creek
122
14,100
24,700
30,000
47,200
POWELL CREEK
At mouth
3.8
1,070
1,880
2,340
3,740
POWELLS CREEK
At confluence with
Lickinghoie Creek
3.88
920
1,480
1,630
2,300
Upstream study Iimits
3.08
740
1,170
1,350
1,980
RIVANNA RIVER
Downstream study limits
568
35,900
71,900
93,000
157,400
Upstream of Buck Island
Creek
530
35,100
67,500
86,500
146,000
Upstream of Moores Creek
472
32,800
62,200
80,000
134,000
ROCK CREEK
At confluence with Moores
Creek
1.9
1,200
2,000
2,600
4,100
At confluence of 5t'
Street Creek
0.9
650
1,100
1,500
2,200
SLABTOWN BRANCH
At confluence with
Lickinghole Creek
1.21
600
1,060
1,240
1,820
Upstream study limits
0.11
70
120
150
210
SOUTH FORK
RIVANNA RIVER
At mouth
268
19,700
42,200
53,400
106,000
15
TABLE 4 - MANNING'S "n" VALUES - continued
Stream
Powells Creek
Rivanna River
Rock Creek
Slabtown Branch
South Fork Rivanna River
Channel "n" Overbank "n"
0.070 - 0.100 0.060
0.060 - 0.120 0.045 - 0.060
0.045 -0.085 0.025 - 0.120
0.070-0.100 0.035 - 0.055
0.080 - 0.120 0.040 - 0.060
All elevations are referenced to the North American Vertical Datum of 1988
(NAVD 88). Qualifying bench marks within a given jurisdiction that are
cataloged by the National Geodetic Survey (NGS) and entered into the National
Spatial Reference System (NSRS) as First or Second Order Vertical and have a
vertical stability classification of A, B, or C are shown and labeled on the FIRM
with their 6 -character NSRS Permanent Identifier.
Bench marks cataloged by the NGS and entered into the NSRS vary widely in
vertical stability classification. NSRS, vertical stability classifications are as
follows:
a Stability A: Monuments of the most reliable nature, expected to hold
position/elevation well (e.g., mounted in bedrock)
a Stability B: Monuments which generally hold their position/elevation well
(e.g., concrete bridge abutment)
a Stability C: Monuments which may be affected by surface ground
movements (e.g., concrete monument below frost line)
a Stability D: Mark of questionable or unknown vertical stability (e.g.,
concrete monument above frost line, or steel witness post)
In addition to NSRS bench_ marks, the FIRM may also show vertical control
monuments established by a local jurisdiction; these monuments will be shown on
the FIRM with the appropriate designations. Local monuments will only be
placed on the FIRM if the community has requested that they be included, and if
the monuments meet the aforementioned inclusion criteria.
To obtain current elevation, description, and/or location information for bench
marks shown on the FIRM for this jurisdiction, please contact the Information
Services Branch of the NGS at (301) 713-3242, or visit their Web site at
www.ngs.noaa.gov.
19
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FEDERAL EMERGENCY MANAGEMENT AGENCY
FLOOD PROFILES
co
ALBEMARLE COUNTY, VIRGINIA
PQWELLS CREEK
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AND INCORPORATED AREAS
AND THE INDEPENDENT CITY OF CHARLOTTESVILLE
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FEDERAL EMERGENCY MANAGEMENT AGENCY FLOOD PROFILES
rn ALBEMARLE COUNTY, VIRGINIA
a AND INCORPORATED AREAS POWELLS CREEK
AND THE INDEPENDENT CITY OF CHARLOTTESVILLE
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FEDERAL EMERGENCY MANAGEMENT AGENCY FLOOD PROFILES
rn ALBEMARLE COUNTY, VIRGINIA
a AND INCORPORATED AREAS POWELLS CREEK
AND THE INDEPENDENT CITY OF CHARLOTTESVILLE
a x I w /."A G
HEC -RAS Plan: Plan 02 RWar. Powells Creek Reade Main Reach Profile: PF 1
W,5
kk =
IM
[ielpll -'
1830.00
749.00
751.10
751.10
75230
0.0411767
9.51
243.80
92.50
0.79
1040.00
737.00
741.99
�-
-...74233
0.009739
4.49
418.35
189.70
0.36
1930.00
732.30
739.30
736.70
0.02"34
832
34M59
243.96
0.59
-
1530.00
714.00
71859
718.58
719.61
0.0"708
9.89
208.07
100.06
0.80
1930.00
701.00
707.60
746.49
708.34
0.0190097.40
248.82
74.59
4.54
1&10.00
897.00
891.64
991.90
89228
0.031498
7.28
294.58
221.72
0.87
1830.00
873.00
878.40
97729
878.89
0.018188
_ 6.42
802.58
108.90
0.49
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POWELLS CREEK EX Plan- Plan 02 111MO16
NEC-RAB Alan: Plan 02 RAar Powells Creek Reale M.I. Ruch PmHle' PF 1
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757.10
751.10
752.20
0.041787
9.51
203.80
92.50
0.79
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737.00
742.12
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742.59
0.008101
420
413.98
192.98
0.95
1630.00
--_ 732.30
736.14
_
798.14
736.68
0.035000
7.39
299.79
241.21
0.70
- Itaw
1030.00
714.00
719.11
_
719.74
0.022989
6.97
266.50
117.35
0.59
Refak.
4' ` a;:
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t630.00
710.30
_
719.39
714.91
719.48
0.001106
2.42
785.58
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0.15
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712.97
714.22
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701.00
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708.96
705.08
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8.97
_194.79
203.48
85.99
0.69
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1830.00
685.00
898.98
89923
0.013431
4.42
422.94
24126
0.45
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892.00
690.14M.14
690.02
0.033003
7.03
328.34
302.04
0.07
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867.00
891.84
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69226
0.031498
7.28
294.58
221.72
0.87
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1890.00
673.00
678.401
677.26
876.86
0.015166
6.02
902.56
105.80
0.49