SW8040916_COMPLIANCE_20040930

STORMWATER DIVISION CODING SHEET
POST -CONSTRUCTION PERMITS
PERMIT NO.
SW"
DOC TYPE
❑ CURRENT PERMIT
❑ APPROVED PLANS
❑ HISTORICAL FILE
COMPLIANCE EVALUATION INSPECTION
DOC DATE
�(?JJ�edQ3�l
YYYYMMDD

STORMWATER DIVISION CODING SHEET
PERMIT NO.
DOC TYPE
❑FINAL PERMIT
❑ MONITORING REPORTS
❑ APPLICATION
❑ COMPLIANCE
❑ OTHER
DOC DATE
❑
YYYYMMDD

STORMWATER CALCULATIONS
FOR
WILMINGTON POLICE HEADQUARTERS
FOR
NCDWQ
WILMINGTON
NORTH CAROLINA
RECEIVED
SEP 3 0 2004
Dwq
PROJ # 5W 60409 ( (O
ENGINEERS
WK DICKSON & CO., INC.
909 MARKET STREET
WILMINGTON, N.C. 28401
[phone]
September 29, 2004
SEAL
19742
q 7 *qo'll
r�

Stormwater Narrative
City of Wilmington Police Headquarters
WKD#: 00720.80.WL
September 29, 2004
GENERAL
The City of Wilmington plans to develop the property commercially with a new police
headquarters. The site is located at 615 Bess Street in Wilmington. The majority of the site
is moderately flat and drains by means of sheet flow towards the existing C.S.X. railroad
that runs along the eastern and western boundary of the proposed disturbed area. The
remaining portion of the existing site drains by sheet flow to an existing ditch that runs
along the southerly boundary of the proposed disturbed area.
PROPOSED IMPROVEMENTS
Proposed improvements will involve the construction of a 9.75 acre office development
and will include the construction of a wet detention basin. This basin will improve water
quality for post development runoff as well as ensure that our post -development flow
(13.94 cfs) is less that that our pre -development flow (14.48 cfs) for the 10-year event. All
of the surface water runoff from the developed impervious areas will be sent to a proposed
wet detention system.
DRAINAGE CONSIDERATIONS
Runoff will be conveyed to the detention system by means of piped drainage systems.
These piped systems will ensure positive drainage to the pond. The systems will be
designed to transport the runoff from the 10-year storm event to the pond. The grades
shown on the plans are final grade elevations. The plan currently proposed shows that the
imperviousness of the drainage area being conveyed to the pond will be 71.6%. During
construction silt fence, riprap aprons, inlet protection, and construction entrances will be
placed in critical areas as directed by the Erosion Control Plan. All design criteria is based
on City of Wilmington stormwater standards. Drainage areas were delineated for
determining the correct culvert size for each drainage segment.

WK DICKSON
Engineering 909 Market Street
Land Planning Wilmington, NC 28401
Surveying [phone]
WILMINGTON POLICE HEADQUARTERS
TOTAL AREA DRAINING TO POND = 349,309 SF = 8.01 AC
TOTAL PRE -DEVELOPMENT FLOW =(C)(I10-YR)(A)=(0.25)(7.23)(8.01) = 14.48 CFS
TOTAL PROPOSED IMPERVIOUS SURFACE AREA DRAINING TO POND:
BUILDINGS
= 54,000 sf
PARKING
= 173,594 sf
SIDEWALKS
= 15,506 sf
OTHER
= 6,900 sf — walls and mechanical yard
TOTAL = 250,000 SF
250,000/349,309 = 71.6%
REQUIRED STORAGE FOR 1-INCH OF RUNOFF FOR POND: (use Schueler Method)
= 0.05 + 0.009 (I)
= 0.05 + 0.009(71.6) = 0.694
= (1")(0.694)(1/12)(349,309)
= 20,202 CF (20,593 CF PROVIDED)
POND DEPTH SA/DA % POND SURFACE AREA (MIN.)
(FT) (SF) (c7bfU %75.�
7.5 2.38 8,314
IF POND DEPTH = 7.5 FT, MINIMUM SURFACE AREA = 8,314 SF
USED 90% TSS CHART TO SIZE POND
AREA @ ELEVATION 14.60 = 8,580 SF > 8,314 SF
REQUIRED STORAGE FOR FOREBAY: 20% OF TOTAL PERMANENT POOL VOLUME
= (0.2)(16,386 CF)<-- Z.
= 3,277 CF (3,455 CF PROVIDED --- 21.1%)
PROJECT: WILMINGTON POLICE HQ SUBJECT: POND CALCULAT ONS JOB NO.: 00720.80.WL
PREPARED BY: MNH DATE: Z ,01CHECKED BY: MNH DATE: Z' p PAGE NO_L Off'

% ` WK DICKSON
Engineering 909 Market Street
Land Planning Wilmington, NC 28401
Surveying [phone]
Pond Volume
Contour
Incremental
Cumulative
Elevation
Area
Volume
Volume
(ft)
(sf)
(cf)
(cf)
Bottom Elevation 7.10
198
0
0
8.00
478
304
304
9.00
827
653
957
10.00
1,270
1049
2005
11.00
1,827
1549
3554
12.00
2,621
2224
5778
13.00
3,528
3075
8852
14.00
4,893
4211
13063
`(D
Permanent Pool 14.60
6,183
3323
16386 <-
2
AyQ
Forebay Volume
Contour
Incremental
Cumulative
Elevation
Area
Volume
Volume
(ft)
(sf)
(cf)
(cf)
Bottom Elevation 10.60
55
0
0
11.00
118
35
35
12.00
432
275
310
13.00
952
692
1002
14.00
1,662
1307
2309
Permanent Pool 14.60
2,158
1146
3455
Forebay Volume is
21.1%
PROJECT: WILMINGTON POLICE HQ SUBJECT: POND CALCULATIONS JOB NO.: 00720.80.WL
PREPARED BY: MNH DATE:` _ c CHECKED BY: MNH DATE: PAGE NO2 OF 3

WK DICKSON
Engineering
Land Planning
Surveying
Temporary 1" Storage Volume
Contour
Incremental
Cumulative
Elevation
Area
Volume
Volume
(ft)
(sf)
(cf)
(cf)
Norm Pool Elev. 14.60
8,580
0
0
15.00
9,264
3,569
3,569
16.00
10,980
10,122
13,691
I" Storage Elev. 16.60
12,028
6,902
20,593
RECEIVED
SEP 3 0 2004
owe
PROD # �LV�U ULf 1
909 Market Street
Wilmington, NC 28401
[phone]
M
PROJECT: WILMINGTON POLICE Q SUBJECT: POND CALCUL TONS JOB NO.: 00720.80.WL
PREPARED BY: MNH DATE: O'tHECKED BY: MNH DATE: I'PAGE NO / OF

WK DICKSON
Engineering
Land Planning
Surveying
909 Market Street
Wilmington, NC 28401
[phone]
POND RELEASE
TIME FOR CONTROL PIPE: POND
ELEV.
EFF. STORAGE
VOL. RELEASED
OUTFLOW RATE
RELEASE TIME
(FT)
(CF)
(CF)
(CFS)
(HOURS)
16.60
20,593
6,897
0.149
12.9
16.00
13,696
10,127
0.124
22.6
15.00
3,569
3,569
0.066
14.9
14.60
0
TOTAL HOURS
50.5
DAYS
2.1
CONTROL PIPE:
2
INCH ORIFICE HOLE
USED ORIFICE EQUATION TO DETERMINE
OUTFLOW RATE
Q = Cd`A`(2gh)^112
PROJECT: WILMINGTON POLICE HQ SUBJECT. POND CALCULATIONS JOB NO.: 00720.80.WL
PREPARED BY: MINH DATE 2 ' �~� CHECKED BY: MNH DATE: tr x�" I PACE NOS OF

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STORMWATER SUBMITTAL REQUIREMENTS
DETENTION POND
A. Site Plan
i/ 1.
a Develop ment/Project Name
✓ 2.
Designers seal, signature, name and firm.
✓ 3.
Legend
✓ 4.
North Arrow
5.
Location Map with direction, distance to named streets, NCSR numbers.
6.
Scale
7.
Date
L 8.
Revision number and date.
9.
Original contours, proposed contours, spot elevations, finished floor elevations.
10.
Existing drainage, including off -site. Provide a map of any offsite areas.
✓ 11.
Property/Project boundary lines, dimensioned.
ti A 12.
Mean High Water Line, if project is adjacent surface waters.
✓ 13.
'.Drainage easements, catch basins, pipe sizes, inverts.
14.
:; Wetlands delineated, or a note on the plans that none exist. Ponds may not be
located in a wetland.
15.
' Details of the road, parking, cul-de-sacs, including sidewalks, curb and gutter..
0IA 16.
Drainage area to each pond delineated.
17.
Dimensions of the permanent pool.
IDYo 18
'Vegetated filter detail if using 85% TSS. This is NOT a connector ditch.
19.
Dimensions of the road, parking cul-de-sacs, sidewalks, and curb and gutter.
4_4 20.
Unit footprint with dimensions and concrete wood deck areas noted.for
apartments or condo development. c r BvC (d�„� dimenS icvls.
21.
Forebay at each inlet. (Prefer one inlet)
22.
Inlets located such that entire pond is used for treatment and no short-
circuiting.
23.
Pondlforebay section detail with. -
Bottom elevation
Li Permanent pool elevation PP
Storage pool elevation SP
Slopes: 3:1 above permancnt pool
Vegetated shelf located below permanent pool, 6:1 slope, with begin
and end elevations around main pond. (Not required inside forebay)
Forebay bottom elevation, 3:1 slopes,
Forebay weir elevation between PP and DP.
24.
Outlet structure detail with trash rack and orifice size specified. Maximum trash
4A 25.
rack opening size is 4".
Note that drains be
on plans all roof must directed so as to be picked up by
the pond. — 21t dr2,:,is InEv j72rkilI lot
c✓ 26.
A 3:1 L:W ratio has been provided for the pond.
27.
Maximum average pond depth of 6'. (Parts of pond may be deeper -up to 73)
✓ 28.
Vegetation requirements for pond slopes and wetland species forthe 6:1 shelf
are listed. Weeping Love grass is not suitable for use in North Carolina.

STORMWATER SUBMITTAL REQUIREMENTS
DETENTION POND (cont)
B. Calculations
1.
- 85% or 90% TSS Chart noted.
2.
..-Pond depth.
3.
SAIDA ratio calculation.
✓
4.
:' Required surface area at permanent pool.
✓
5.
- Provided surface area at permanent pool (Based on pond dimensions)
6.
Provided surface area at temporary design pool.
7.
Required volume calculation.
8.
Provided volume calculation.
9.
Orifice size based on required 1" volume, using average head to design pool
elevation.
10.
Designer=s seal.
t✓
11.
Built -upon area to each pond.
t✓
12.
Forebay designed to hold 20% of the permanent pool volume. (Range of 18-
13.
22 OK.)
Non -erosive flow for 10 yr. storm in the vegetated filter, if using 85% TSS.
C. Supporting
Documentation
1. ;-Completed permit application and supplement.
a. Is the person who signed the landowner or a lessee? Must be
landowner.
v b. All sections and blanks completely filled in.
�✓ C. Check signature per. 1003 (e). Make sure title is provided.
till d. If agent signs, signatory authorization from owner.
v e. Wet detention supplement sheet provided.
f. Mailing address & phone of applicant- no CIO.
✓ g. Verify water body classification.
u h. Justifications for not meeting a design requirement.
uj i. If a modification, include Designer=s Certification.
j. Hand initial design requirements.
2. Proposed deed restrictions