HomeMy WebLinkAboutDrainage Reports - 09/12/1995RMINC.
Engineering Consultants
209 S. Meldrum
Fort Collins, Colorado 80521
303/482-5922
FAX:3031482-6368
Mr. Basil Harridan
City of Fort Collins
Utility Services, Stormwater
235 Matthews Street
.Fort Collins, CO 80522
Final Appfuved Report
August 18, 1995
RE: Addendum to Final Drainage and Erosion Control Study for the 1119w r ,$hopp
11RUTU�, Lot 3 (a.k.a. Pad 'B') Temporary Drainage Utilities.
Dear Basil:
This addendum is to follow up on our conversation on August 17, 1995 about the Tower
Shoppes, P.U.D. Erosion Control. The southwest lot of that site is requesting to develop
immediately, as opposed,to the,rest of the Tower Shoppes, which are to be constructed
later. This southwest lot, known as Lot 3 on the Plat, has been designated as Phase 1,
and the rest of the Tower Shoppes has been depicted as Phase 2.
In order for Phase 1 to be fully operational, a temporary drainage channel will need to be
installed to serve the site. The alignment of this channel has been laid out on the revised
Drainage and Erosion Control Plan, and a cross-section of it has been provided.
Accompanying this addendum are calculations for this temporary channel. The impervious
area added by Phase 1 is approximately 1.93 acres, including the off -lot access drives.
Per the rational method, the runoff from this added imperviousness is : Q ,o = 7.53 cfs and
Q100= 16.26 cfs. To contain the 100-year flow for the storm, a channel 1.0' deep needs to
be constructed from two points outletting Lot 3 (see Drainage Plan) to the existing channel
along the New Fort Collins High School property line. The High School Channel has
enough capacity as designed to contain this flow. Most importantly, the channel must be
constructed so as not to undermine the existing High School plaza. It shall be brought into
the existing channel no closer than 20' from the High School plaza.
In order to prevent excessive silt movement from the site, the sides of this temporary
channel shall be seeded per the requirements on the Drainage and Erosion Control Plan,
Denver3031458-5526
and a straw bale shall be placed at the downstream end of the channel. Also, to control
offsite siltation, a fabric erosion control fence shall be placed along the phase lines as
shown on the Drainage and Erosion Control Plan. The amount of escrow funds for these
Phase 1 erosion control devices shall be $2212.00. This total is for 350 L.F. of silt fence
(installed), 1 straw barrier, and the cost to seed the temporary channel, including 50%
contingency. I
A breakdown of this amount would be :
1 Straw Bale Barrier @ $150.00
1 Lump Sum cost to seed 700 L.F. of 8.0' wide channel @ $100..00
350 L.F. Silt Fence @ $3.50 per installed L.F. = $1225.00
The intention is to keep the erosion control separate for the two phases, so that the
erosion control bonds don't overlap in cost. Therefore, the Phase 2 erosion control plan
will be as follows.
Before overlot grading begins, a 462' silt fence shall be installed along the north property
line. The temporary channel and straw bale serving Phase 1 shall not be removed until
the large parking lot is set to be paved. The large permanent channel shared by the Tower
Shoppes and the High School shall have a straw bale place in its flowline. The two curb
inlets and one 15' culvert shall be protected with gravel silt filters per the detail in the Utility
Plans. Most importantly, after construction areas, such as the large parking lot area, the
access drives, and the parking in back of the anchortenent have been disturbed, they
shall be covered within 4 weeks with the gravel base course required for paving. This
base course will prevent the large, open area from kicking up silt during wind gusts. Once
the foundation is dug for the large anchor building and merchant shops, no silt will be able
to escape and run off site, since it will be the lowest point in the area, and the foundation
will act as a wind barrier.
The amount of escrow funds for these Phase 2 erosion control devices shall be $10640.00.
This total is for 350 L.F. of silt fence (installed), 1 straw barrier, and the cost to seed the
temporary channel, including 50% contingency.
A breakdown of this amount would be :
1 Straw Bale Barrier @ $150.00
3 Gravel Inlet Filters @ $150.00 = $450.00
Gravel Mulch @ 135 tons per acre (6.02 Acres) @ $4876.00.
350 L.F. Silt Fence @ $3.50 per installed L.F. = $1617.00
MR
I hope this addendum has been submitted to you with enough time to review. If you have
any questions regarding this design or this erosion control cost estimate; please contact
me.
Sincerely,
RBD, INC.
Project`C4gineer
cc: Bob Zachley, Stormwater Utility
Libby Glass, W.W. Reynolds Companies
tj
Q
r
T
ul
v •
r
.r
0
�.
i
to U
O
•
-
E- E�
~
m No
n1
J.-
UU
].
njn
u'
-
Q
c
w
t=
y(L°'
9
9
w --
w
a.
O o
9
9
I—
N
0
0
c
O
Q
s ; ::E
w^
a
7
>
.
F
O'er
w
--
J
Q�
z
�
z
_
tz �-
n�
-
'J
V
LL
C
z
o
a
Q
a
�
mQ
W QN
Q
o
RBD INC. ENGINEERING CONSULTANTS
CHANNEL RATING INFORMATION
TOWER SHOPPES
-�TovJ�t•= .�..J-4-1oPf'�S , ice..-.G � '1� I � LO-I" ��
STA ELEV
0.00 20.00
20.00 15.00
40.00 20.00
IN' VALUE SLOPE (ft/ft)
---------- -------------
0.030 0.0100
ELEVATION
AREA
VELOCITY
DISCHARGE
FROUDE
(feet)
---------
(sq ft)
-------
(fps)
--------
(cfs)
---------
NO.
------
15.50
1.0
1.9
1.93
0.68
16.00
4.0
3.1
12.27
0.76
16.50
9.0
4.0
36.17
0.82
17.00
16.0
4.9
77.88
0.86
17.50
25.0
5.6
141.18
0.89
18.00
.36.0
6.4
229.55
0.92
18.50
49.0
7.1
346.22
0.94
19.00
64.0
7.7
494.27
0.96
19.50
81.0
8.4
676.61
0.98
20.00
100.0
9.0
896.04
1.00
U
RAINFALL -PERFORMANCE STANDARD EVALUATION
PROJECT: STANDARD FORM A
COMPLETED BY: DATE: Oc3 1-7 9E
DEVELOPED
SUBBA§IN
ERODIBILITY
ZONE
Asb
(ac)
Lsb
(ft)
Ssb
M
Lb
(feet)
Sb
PS
PNA SB 1 \vl-r4Aiv3
E9PS l rJ 1 �.- PLht Y��'
1.70
Z . O
8C>. O
I
4,87
�40
1, �_._—
79.4
701
............
__._....
MARCH 1991 B•14 DESIGN CRITERIA
J11 . .
EFFECTIVENESS CALCULATIONS -
PROJECT:
COMPLETED BY: F.
STANDARD FORM B
DATE: 0-3 •I-7
Erosion Control'
C-Factor
P-Factor-
Method
-Value
Value Comment
-
P�Etn�
oto,
o� ,
MAJOR PS SUB AREA
.
BASIN (t) BASIN (Ac)
CALCULATIONS
i5O.3 PIIS, I I •70
.
=fir'•• L'
_.
I .70
1.Ob
\\ \
0.40
''K
(1, -7 C)-)
I 7p _ I Ol7
MARCH 1S91
8-15
DESIGN CRITERIA
EFFECTIVENESS CALCULATIONS
PROJECT: '7owEL� STANDARD FORM B
COMPLETED BY: DATE: 03 1-7,
Erosion Control' C-Factor P-Factor
Method -Value Value Comiaent
-
�1 L� � E �J:,E F...l��� 1 E >? 1 • O O r� - NcX%��.i, r �:�-1 Pv, s`r"':2
Pp.\�E�,�,81J�' � �U�LOIIJC-,i'+F'1tC'� cJ •G1 1 . O
MAJOR
PS
SUB
AREA
BASIN
(t)
BASIN
(Ac)
I
CALCULATIONS
9.5-7'
4.87 0 32
3. 7e (o. 6l
EFF
7'�I'��.� cal'•=IQ �.�
o-ter._
-To i7�11 `•.=Cl )
MARCH ISSI 8.15 DESIGN CRITERIA