HomeMy WebLinkAboutOLD TOWN NORTH - ODP - 28-99B - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTAPPENDIX II
Stuffer Envelope
Conceptual Drainage and Erosion Control Plan (1 Sheet)
le D.2. Subbasins for Which On -Site Detention is Required, and Their Allowable Release
Rates.
408
851
3.1
5.3
7.3
410
852
7.4
12.7
17.5
416
853
21.2
34.7
45.7
417
854
12.4
20.2
26.6
419
855
31.7
51.4
67.0
420
856
16.6
30.6
43.8
421
857
13.8
23.2
31.2
422
858
22.1
35.8
46.8
423
859
24.5
39.8
51.6
425
860
19.7
32.1
42.1
426
861
1.2
2.2
3.1
428'
862
84.6
87.6
90.2
4292
863
38.8
41.0
43.2
432
864
3.7
6.4
8.9
4333
865
61.6
63.8
65.8
434'
866
399.5
403.0
405.8
437
867
6.9
11.3
14.7
438
868
6.2
10.1
13.2
440
869
28.4
46.0
60.0
446
870
4.0
6.6
8.8
447
871
5.2
8.6
11.5
454
873
30.5
49.6
64.2
456
874
8.1
14.4
20.1
' Includes the 100-year runoff for the developed portion of the subbasin.
U:\ L A -A S S OCTLT\C 1TY\COFC9606.TD2
-.
Table 4.2. Summary of Subbasins for Which On -Site Detention is Required.
r,o.,PiPr� �nndition;Subbasiri ID Area (ac)':. Uni[ Release'=Rates`
401
14.2 (8.8)'
0.38b
406
56.9 (24.2)
0.45b
408
19.1
0.38
410
45.6
0.38
416
69.6
0.66
417
82.0
0.32
419
108.0
0.62
420
100.4
0.44
421
43.5
0.72
422
149.9
0.31
423
88.5
0.58
425
65.2
0.65
426
7.6
0.41
428a
35.7 (24.5)
0.39b
429a
16.3 (11.9)
0.54b
432
22.9
0.39
433'
24.9 (17.7)
0.40b
4342
85.6 (37.3)
0.32b
437
54.2
0.27
438
42.0
0.31
440
195.7
0.31
446
25.3
0.35
447
32.9
0.35
454
44.1
1.46
456
24.7
0.81
' Undeveloped area is given in ( ) for partially developed subbasins.
b For the undeveloped portion only.
Based on 10-year historical runoff rates.
27
As a result of this decision all new development within the Lower Dry Creek Basin would_
be required to provide detention such that in the developed condition, runoff during the 100-year
event would be reduced to the 10-year historical release rate for the site. Table 4.2 is provided
as a summary of subbasins for which on -site detention would be required and the allowable unit
runoff rate associated with the 10-year historical runoff for each of these subbasins.
The basin's response to developed conditions with the selected on -site detention alternative
is shown in Figures 4.1 and 4.2. Results of the developed condition with 10-year detention
analysis along the proposed channel alignment (and the existing channel upstream of the airport)
are graphically presented in Figure 4.1. Figure 4.2 shows the 100-year hydrographs along the
existing channel downstream of the airport. Full output for each of the return period events,
which includes hydrograph listings for all subbasins and conveyance elements, is provided on the
enclosed diskettes. ( t,_, rk�j <,_t«:
Table 4.1. Summary of 100-Year Peak Discharges for the Three On -Site Detention Alternatives.
100-Year Peak Discharge for the
Node
Description
Drainage Area (ac)
Given Detention Alternative (cfs)
2.
5-Yr
10-Yr
615
Total Flow at the Poudre River
3,107
1,480
1,620
1,730
614
At the Poudre River
620
1,030
1,030
1,030
(Existing Channel)
612
Mulberry Street
599
980
980
980
(Existing Channel)
611
At the Poudre River
2,488
1,000
1,120
1,220
(Proposed Channel)
610
Mulberry Street
2,469
1,010
1,130
1,230
(Proposed Channel)
603
Vine Drive
1,882
1,010
1,080
1,150
602
Lemay Avenue
79Q WO
850
860
870
600
College Avenue
155
560
570
570
The results presented in Table 4.1 indicate a relative insensitivity of the system to on -site
detention for the range of alternatives analyzed. For the three on -site detention alternatives, the
differences in 100-year discharges along the major flow paths are insignificant at many locations.
Only at Mulberry Street and the Poudre River confluence along the proposed Dry Creek Channel
do the 10-year detention discharges exceed the 2-year detention discharges by more than 20
percent.
4.3 Selection of the Preferred On -Site Detention Alternative
Based on the results presented in the previous section, the Stormwater Utility selected the
10-year detention scenario as the preferred on -site detention alternative. Optimization of the
basin's resources was the primary factor in making this selection. In choosing this alternative,
it was noted that the incremental increases in discharges between the 2- and 10-year detention
alternatives would not substantially affect the sizing of regional drainage facilities in the basin.
At the same time, the 10-year detention scenario relieves on -site facilities of a significant burden,
as compared to the 2-year detention alternative.
c_
25
Basin will be diverted to the Dry Creek Diversion Channel and does not enter the Lower Basin;
and (b) the original Master Plan did not model the Lower Basin with the level of detail necessary
to accurately estimate the magnitude of runoff potential associated with fully developed conditions.
Table 3.2. Comparison of 100-Year Discharges for Developed Conditions as Reported in the
Original Master Plan [31 and as Estimated by the Current Hydrologic Model.
100-Year. Developed Condition Dtscharge (cfs) ;;
Location ;
Original Master Pian !
Revised Master Plan
Poudre River Confluenceb
2,700
4,410
Mulberry Street`
NRd
3,060 (1,770)
Vine Drive
NR
2,920
Lemay Avenue
NR
1,180
College Avenue
NR
700
FDownstream of the
2,900
0
Larimer &Weld Canal
a Current hydrologic model
b Combined discharge for the existing and proposed channels
Discharge along proposed channel (discharge along existing channel)
° NR = Not reported
,� 21
�3�F�ll
-04
2400
N
v
a1 1800
cc
L
U
0
1200
OWA@
V
_
Proposed Dry Creek at Poudre River
Proposed Dry Creek at Mulberry Street
_
— — Dry Creek at Vine Drive
1
--— Dry Creek at Lemay Avenue
/
1
_ Dry Creek at College Avenue
1
1
1
1
1
1
I
% j
II -—
I
1
I
II
1
1
II
I
I
`
-
I
`
I
�
II �
0 1 2 3 4 5 6 7 8 9
Time (hours)
Figure 3.1. Developed Condition 100-Year Hydros 'is along the Proposed Dry Creek Channel (Existing Dry creek
Upstream of the Airport).
Table 3.1. Summary of Results of the Developed Condition Hydrologic Analysis.
Drainage
Peak Discharge (efs) .
Node
Desenpuon :
2=Yr
5-Yr
10-Yr
50yr
100-Y,r,
Area (ac)
615
Total Flow at the
Poudre River
3,107
710
[1,440
2,000
3,590
4,410
614
At the Poudre River
(Existing Channel)
620
390
720
930
1,530
1,810
612
Mulberry Street
(Existing Channel)
599
380
710
920
1,500
1,770
611
At the Poudre River
(Proposed Channel)
2,488
450
980
1,370
2,470
3,040
610
Mulberry Street
(Proposed Channel)
2,469
460
980
1,380
2,490
3,060
603
Vine Drive
1,882
460
980
1,340
2,400
2,920
602
Lemay Avenue
790
240
460
600
990
1,180
600
College Avenue
155
160
290
380
590
700
Results of the developed condition analysis along the proposed channel alignment (and the
existing channel upstream of the airport) are graphically presented in Figure 3.1.
shows the 100-year hydrographs at major road crossings along the main channel
This figure
The attenuation
which was noted between Vine Drive and Mulberry Street in the existing condition analysis is no
longer present due to both the developed nature of the local watershed and the substantially more
efficient conveyance system. Figure 3.2 shows the 100-year hydrographs along the existing
channel. downstream of the airport.
A summary output file for the 100-year developed condition event, which includes a printout
of the main channel hydrographs at all major road crossings, is contained in Appendix C. Full
output for each of the return period events, which includes hydrograph listings for all subbasins and
conveyance elements, is provided on the enclosed diskettes.
Table 3.2 provides a comparison of 100-year discharges for the revised Master Plan
(current) model and original Master Plan model [3]. The table indicates significant differences
between the two models. This is primarily due to two factors: (a) the original Master Plan
assumed that approximately 2,900 cfs would be conveyed from the Upper Basin to the Lower
Basin during the 100-year event, while the current model assumes that all runoff from the Upper
18
1200
100C
80C
2) 600
9
w
b
400
200
—
Proposed Dry Creek at Poudre River
—
----• Proposed Dry Creek at Mulberry Street
—
----
— — Dry Creek at Vine Drive
--- Dry Creek at Lemay Avenue
—
---- Dry Creek at College Avenue
I
1 2 3 4 5 6 7
Time (hours)
Figure 2.1. Existing Condition 100-Year Hydrographs along the Proposed Dry Creek Channel
(Existing Dry Creek Upstream of the Airport).
8 9
•+ 7
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a •!I - park
Arrowheld?.. .
Figure 1.2. Vicinity Map for the Lower Dry Creek Basin
I. INTRODUCTION
1.1 General
Dry Creek, located in north Fort Collins, Colorado, is a major tributary to the Cache la
Poudre River. The creek's confluence with the river is located in the City of Fort Collins south
of Mulberry Street and west of Summit View Drive. The watershed encompasses roughly 75
square miles, with about 62 square miles actually contributing runoff [1]. It is approximately 22.5
miles long and 5 miles wide, with the long axis having a north -south orientation. The northern
terminus of the Dry Creek watershed is located approximately nine miles southeast of the Town
of Virginia Dale, Colorado. The watershed is roughly bounded on the north by Bobcat Mountain
and Boxelder Creek, on the west by a ridge which is located directly east of U.S. Highway 287,
and on the east by Boxelder Creek and County Road 15. The elevation of the watershed ranges
from 7,100 to 4,900 ft, msl. A location map of the entire Dry Creek drainage area is provided
in Figure 1.1.
For the purposes of this study, the City of Fort Collins has divided the Dry Creek
watershed into two separate and distinct areas, an Upper and Lower Basin, with the dividing line
being defined (except as described below) as the Larimer & Weld Canal. This canal transects the
watershed in a roughly west -east direction approximately two miles north of Mulberry Street. The
Lower Dry Creek Basin encompasses approximately 4.9 square miles. The distinction between
the Upper and Lower Basins has been made in order to reflect the impact of the proposed Dry
Creek Diversion Channel. As proposed by the City of Fort Collins, this channel would begin at
the Larimer & Weld Canal and divert all of the.upper basin flows (up to the 100-year event) to
the Cache la Poudre River in an alignment to be located along the west side of the Union Pacific
Railroad. Consequently, the diversion channel would serve to hydrologically separate the Upper
and Lower Basins.
The Lower Dry Creek Basin includes in its entirety, the Evergreen/Greenbriar Basin. The
Evergreen/Greenbriar Basin is, in and of itself, a separate major drainage basin as defined by the
City of Fort Collins. The Evergreen/Greenbriar Basin encompasses approximately 1.2 square
miles and is bounded by College Avenue on the west, Dry Creek on the south, Lemay Avenue
on the east, and (except for the Crestview Subbasin) Country Club Road on the north. Reference
is made to the Evergreen/Greenbriar Master Plan for the definition of all subbasins and hydrologic
modeling efforts associated with that basin [2]. The northern boundary of the Evergreen/
Greenbriar Basin defines the northern limit of the Lower Dry Creek Basin between College and
Lemay Avenues. It is only in this area that the Lower Dry Creek Basin extends north of the
Latimer & Weld Canal. A vicinity map of the Lower Dry Creek Basin is included in Figure 1.2.
FINAL REPORT
HYDROLOGIC MODEL UPDATE
FOR THE
LOWER DRY CREEK BASIN
MASTER DRAINAGE PLAN
PREPARED FOR:
City of Fort Collins
Stormwater Utility
235 Mathews Street
Fort Collins, CO 80524
PREPARED BY:
Lidstone & Anderson, Inc.
760 Whalers Way, Suite B200
Fort Collins, CO 80525
(LA Project No. COFC96.06)
September 10, 1997
APPENDIX II
Portions of the Final Report Hydrological Model Update for the Lower Dry Creek Basin
Master Drainage Plan; Prepared by Lidstone and Anderson; Dated September 10, 1997;
Project No. COFC96.06
Job No. 1646-01-98 Shear Engineering Corporation
Designer: meo Old Town North
12/1/2000
8:02 AM
LOWER ORIFICE
UPPER ORIFICE
PRIMARY WEIR
HEAD
HEAD
HEAD
SUB
ELEV.
OVER
LOWER
OVER
UPPER
OVER
WEIR
TOTAL
LOWER
ORIFICE
UPPER
ORIFICE
WEIR
FLOW
FLOW
ORIFICE
FLOW
ORIFICE
FLOW
ft
ft
cfs
ft
cfs
ft
cfs
cfs
58.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
59.00
0.31
4.37
0.00
0.00
0.00
0.00
4.37
60.00
1.31
9.04
0.00
0.00
0.00
0.00
9.04
61.00
2.31
12.02
0.00
0.00
0.00
0.00
12.02
62.00
3.31
14.39
0.00
0.00
0.00
0.00
14.39
61.95
1 3.26
14.29
0.00
0.00
0.00
0.00
14.29
OVERFLOW
SPILLWAY
HEAD
ELEV.
OVER
WEIR
Total
WEIR
FLOW
Flow
ft
ft
cfs
cfs
58.00
0.00
0.00
0.00
59.00
0.00
0.00
4.37
60.00
0.00
0.00
9.04
61.00
0.00
0.00
12.02
62.00
0.10
16.44
30.83
Storm
sewer
From
Irrigation Ditch
Slope 0.0040 ft/ft
From
To
Pipe
Invert
Slope
Invert
in
out
Length
in
out
ft
ft
ft/ft
ft
Ditch
MH
1
16.00
4552.90
0.0040
MH 1
MH
2
187.00
4553.16
0.0040
4552.96
MH 2
MH
3
373.50
4554.11
0.0040
4553.91
MH 3
MH
4
548.58
4555.81
0.0040
4555.61
MH 4
HW
32.34
0.0040
4558.00
cr,. co..,A,- rrnesinn with Rxiatina Water and Sewer Lines
Crossing is downstream from Proposed MH 2
Invert Dist to Ex. Pipe Storm Top of Ex. Pipe Ex. Pipe Top
in crossing Invert Storm Inv Diam Ex. Pipe
ft ft ft ft ft
4554.11 77.61 S/L 4554.42 4557.21 4549.53 1.25 4550.82
4554.11 95.86 W/L 4554.50 4557.29 4549.00 1.00 4550.04
Storm Crosses Sewer 105.61 DS from MH 11
Inv out MH 11 4549.73 ft
Inv in MH12 4549.19 ft
Dist between manholes 286.96 ft/ft
Slope between manholes 0.0019 ft/ft
Assume W/L same depth as S/L since it has to go under the ditch
Ex. Pipes assumed to have 0.04 foot wall thickness
POND CUHP.xls
Outlet Page 5 of 5
Job No. 1646-01-98 Shear Engineering Corporation
Designer: meo Old Town North
DETERMINE ORIFICE SIZE
Q= CA(2gH)'0.5
A = Q/(C(2gH)A0.5
* (ft) = 3.88
C 0.65
Q (cfs)= 14.29
A (sf) = 1.39
R (ft) = 0.666
D (ft) = 1.331
INPUT FOR OUTLET STRUCTURE
Design Release Required
Storm Rate WSEL
yr cfs ft
100 14.29 61.88
Use 1.39 ft diameter orifice = 16.68 inches
LOWER ORIFICE DIAM. (ft)
LOWER ORIFICE INV. (ft)
LOWER ORIFICE COEF. (Cl)
UPPER ORIFICE DIAM.(ft)
UPPER ORIFICE INV. (ft)
UPPER ORIFICE COEF. (Cu)
Pipe Diameter (ft)
PIPE SLOPE (ft/ft)
MANNINGS n
1.39 WEIR LENGTH (ft) 0.00
58.00 WEIR INVERT (ft) 0.00
0.65
WEIR
COEF.
(Cw)
0.00
0.00
TOP OF BERM
(ft)
62.00
0.00
INIT.
DELTA
(ft)
1.00
0.00
DELTA HEAD
(ft)
1.00
OVER
FLOW
yes
2.50
WEIR
LENGTH
(ft)
200.00
0.0040
WEIR
INVERT
(ft)
61.90
0.012
WEIR
COEF.
(Cwo)
2.60
*OUTPUT*
Pipe Area (sf) 4.91 P 7.85 ft
HYD. RAD (FT) = 0.625 ft R�2/3 = 0.7309
Outlet Pipe Capacity = 28.10 cfs
LOWER ORIFICE AREA (A) = 1.5175 sf
UPPER ORIFICE AREA (A) = 0.0000 sf
ORIFICE EQtN: CA(2gH)"1/2
✓iEIR FLOW EQTN. CwLH-3/2
NOTE: HEAD OVER ORIFICE = HEAD OVER CENTER OF ORIFICE
POND CUHP.xls
Outlet Page 4 of 5
12/1/2000
8:02 AM
Job No. 1646-01-98
Designer: meo
Shear Engineering Corporation
Old Town North
Average End Area Method for determining Pond Volumes
nvert
58.00 ft
First Even Contour
59.00 ft
ncrement
1.00 ft
Top Elevation
62.00 ft
Stage E ev Area Volume Cum. Cum. Req'
(A2) (V1) Volume Volume Volume
ft ft sq ft cf cf ac-ft cf
0.00
58.00
0
0
0
0.00
328403
1.00
59.00
17120
8560
8560
0.20
328403
2.00
60.00
70172
43646
52206
1.20
328403
3.00
61.00
160798
115485
167691
3.85
328403
4.00
62.00
F205742
183270
350961
8.06
328403
Design
Required
Required
Required
Release
Storm
Volume
Volume
WSEL
Rate
year
cf
AF
ft
cfs
100
328403
7.54
61.88
14.29
Stage Storage
400000
350000
300000
250000
U
E 200000
0
> 150000
100000
50000
0
58.00 59.00 60.00 61.00 62.00
Elevation (ft)
-.-Volume Provided -&-Required Volume
Elev
East
West
Area
Area
59.00
0
17120
60.00
0
70172
61.00
28896
131902
62.00
53119
152623
12/1/2000
8:02 AM
POND CUHP.xls
Stage Storage Page 3 of 5
Job No. 1646-01-98 Shear Engineering Corporation
Designer: meo Old Town North
12/1/2000
8:02 AM
Inflow Inflow Release Storage Storage
Time Time Intensity Rate Volume Volume Volume Volume
min sec iph cfs cf cf cf ac-ft
5
300
9.95
323.93
97180
4288
92893
2.13
10
600
7.72
251.33
150800
8575
142225
3.27
15
900
6.52
212.27
191040
12863
178177
4.09
20
1200
5.60
182.31
218777
17150
201627
4.63
25
1500
4.98
162.13
243194
21438
221757
5.09
30
1800
4.52
147.15
264877
25725
239152
5.49
35
2100
4.08
132.83
278941
30013
248928
5.71
40
2400
3.74
121.76
292224
34300
257924
5.92
45
2700
3.46
112.64
304140
38588
265552
6.10
50
3000
3.23
105.16
315469
42875
272594
6.26
55
3300
3.03
98.65
325529
47163
278366
6.39
60
3600
2.86
93.11
335198
51450
283748
6.51
65
3900
2.72
88.55
345356
55738
289618
6.65
70
4200
2.59
84.32
354146
60026
294120
6.75
75
4500
2.48
80.74
363327
64313
299014
6.86
80
4800
2.38
77.48
371922
68601
303321
6.96
85
5100
2.29
74.55
380223
72888
307335
7.06
90
5400
2.21
71.95
388525
77176
311349
7.15
95
5700
2.13
69.34
395264
81463
313801
7.20
100
6000
2.06
67.07
402394
85751
316643
7.27
105
6300
2.00
65.11
410208
90038
320169
7.35
110
6600
1.94
63.16
416849
94326
322523
7.40
115
6900
1.89
61.53
424565
98613
325951
7.48
120
7200
1.84
59.90
431304
102901
328403
7.54
Detention Volume Require = 3284 33 7-
Detention Volume Required = 7.54 ac-ft
POND CUHP.xls
Prelim Cuhp Page 2 of 5
Job No. 1646-01-98 Shear Engineering Corporation
Designer: meo Old Town North
12/1/2000
8:02 AM
Detention Volume Required Foi 100 -Yr Storm for
Cumulative Runoff Method for Sizing Detention Ponds
Release Rate =
0.38
c s acre from hydrologic model update by L & A
Lan use
Assumptions
SFH
Roads
Open
MFH
100%
0.69 - C2
%
%
%
%
C2
Percentage
C2*Percentage
0.65
0.95
0.20
0.70
40%
25%
10%
25%
0.26
0.24
0.02
0.18
Design Storm
Release Rate
100
year
cfs
Runo Coe icient
Area
0.87
- C100
acres
14.29
37.61
Initia Time 5.00 minutes
Time Increment 5.00 minutes
Contributing Areas
West 5.88 ac
East 31.73 ac
Total 37.61 ac
Inflow vs. Outflow
500000
450000
400000
350000
u
3 300000
0
LL
0 250000
W
200000
O
> 150000
100000
50000
0
h �h ti� �y ah 5h 6y A<3 0h 9h soh ^^5
Time (min)
�— Release Volume (cf) --MI-- Inflow Volume (cf)
POND CUHP.Xls
Prelim Cuhp Page 1 of 5
APPENDIX I
Preliminary Drainage Calculations
Page 11
Conceptual Drainage and Erosion Control Report
Old Town North; Ft. Collins, Colorado
IX. REFERENCES
City of Fort Collins "Storm Drainage Design Criteria and Construction
Standards"; May 1984, revised May 1997
2. City of Fort Collins "Erosion Control Reference Manual"; January 1991
3. Major Drainageway Planning Dry Creek; Prepared for City of Fort
Collins, Larimer County and Colorado Water Conservation Board;
Prepared by Gingery and Associates; Dated April 1980
4. Final Report - Hydrological Model Update for the Lower Dry Creek
Basin Master Drainage Plan; Dated September 10, 1997; Prepared by
Lidstone and Anderson, Inc; Project Number COFC96.06
5. FIRM map - Community Panel number 080102-0004-C; dated March
18, 1996.
6. Final Utility Plans prepared for Evergreen Park by VTN Colorado Inc;
Plan approval Date by the City Engineer 03/27/74; VTN Project No:
172242.
Page 10
Conceptual Drainage and Erosion Control Report
Old Town North; Ft. Collins, Colorado
VII. EROSION CONTROL:
A. General Concept
1. Erosion control measures will be specified on the Final Drainage and
Erosion Control Plan. These measure will consist of the following;
a. Silt fence along the property lines
b. Gravel inlet filter at all storm sewer inlets
c. Haybale barriers in all swales
d. Wheel cleaning station at all construction entrances
e. Haybale barriers around any inlets
f. Buried riprap at all storm sewer outfalls
g. Any other measure deemed appropriate at the.time of final design
2. Maintenance of erosion control devices will remain the responsibility of
the contractor and the owner until the project is complete.
VIII. CONCLUSIONS
A. Compliance with Standards
1. The conceptual grading and drainage design for Old Town North complies
with the City of Fort Collins storm drainage design criteria and the
recommendations of the Final Report — Hydrological Model Update for
the Lower Dry Creek Basin Master Drainage Plan.
2. The erosion control measures shown on the erosion control plan comply
with the City of Fort Collins standards and generally accepted erosion
control practices.
B. Drainage Concept
1. The conceptual drainage design for Old Town North will be effective for
the control of storm runoff with a considerable reduction in potential
downstream effects.
Page 9
Conceptual Drainage and Erosion Control Report
Old Town North; Ft. Collins, Colorado
a. Due to the project location within a currently defined floodplain, and
due to the design constraints of existing drainage and irrigation
facilities, the detention pond will be formed by berming along the
north side of the Lake Canal. The elevation of the top of the berm will
require that units adjacent to the pond be elevated to meet flood
protection elevations.
b. Release will occur through a headwall with an orifice plate to the Lake
Canal. Release to the Lake Canal will be restricted 0.38 cfs / acre.
Approximately 37 acres of the site will contribute to the pond. The
remainder of the site will contribute to the proposed Dry Creek
Channel located in Phase 2 of the project. The stormwater will be
conveyed underground to the existing irrigation structure located in the
Dry Creek Drainage.
c. Preliminary pond grading has provided approximately 8.0 acre-feet
VI. WATER QUALITY
A. General Concept
1. Water quality measures will be specified on the Drainage and Erosion
Control Plan. Water quality measures will be provided in accordance with
the latest details from the Urban Drainage and Flood Control District and
the City of Fort Collins Water Quality Criteria.
2. Maintenance of water quality measures will be the responsibility of the
contractor and the owner until the project is complete. The Homeowners
Association will be responsible upon completion of the construction.
Page 8
Conceptual Drainage and Erosion Control Report
Old Town North; Ft. Collins, Colorado
V. DRAINAGE FACILITY DESIGN
A. General Concept
1. The grading of the site will be done to convey as much of the stormwater
runoff to the proposed detention pond. If any portion of the site cannot be
graded to drain to a detention pond, the release rate will have to be
reduced to compensate for the undetained flows or the flows considered in
the design of the Dry Creek channel.
2. The detention pond will be located along the southern property line of the
site.
3. Any swales that have slopes less than 2.0 percent will have 3' valley pans
installed at the flow line of the swale.
4. The detention pond will be designed using the following criteria;
a. Bottom slopes of 1 % or greater
b. Provide valley pans for trickle flows and areas with a slope of less than
2%
c. All side slopes will be 4:1 maximum
d. Provide one (l') foot of freeboard for the 100-year storm event.
e. Finished floors or lowest opening elevation of any building adjacent to
a detention pond shall be set a minimum of 1.5 feet above the
maximum Water Surface Elevation (WSEL).
f. Release rates from the site will be in accordance with the
recommendations of the Final Report - Hydrological Model Update for
the Lower Dry Creek Basin Master Drainage Plan.
B. Specific Details - Detention
There are approximately 37 acres of the site that will contribute
stormwater to the detention pond. Based on the allowable 0.38 cfs / acre
release rate noted for this area in the Final Report - Hydrological Model
Update for the Lower Dry Creek Basin Master Drainage Plan, we have
estimated a volume requirement of 7.54 acre-feet.
.f�
Page 7
Conceptual Drainage and Erosion Control Report
Old Town North; Ft. Collins, Colorado
s. Detention will have to be provided in accordance with the
.recommendations of the Final Report - Hydrological Model Update for the
Lower Dry Creek Basin Master Drainage Plan.
9. Improvements within the area known as the "Product 6 Corridor" as
defined with the Poudre River floodplain study, are not to increase the
100-year water surface elevation by greater than 0.10'
a. The portion of the proposed southern access, between Vine Drive
(existing) and the south side of the Lake Canal, lies within the
"Product 6 Corridor". The design of this access will be provided so
that there will be no rise in the Poudre River 100-year water surface
elevation exceeding 0.10'.
C. Hydrologic Criteria
t. Runoff calculations at various design points are based on the "Rational"
method. The 2, 10, and 100-year storms will be analyzed with the Final
Drainage and Erosion Control Report. All runoff calculations will be
performed using the current rainfall IDF curves dated March 16, 1999.
2. Detention is proposed with this subdivision to accommodate the
recommendations of the Final Report - Hydrological Model Update for the
Lower Dry Creek Basin Master Drainage Plan. Detention pond release
rates are defined in the report as 0.38 cfs / acre. This is taken from Table
4.2 in the Final Report. A copy of this table is included in Appendix II.
D. Hydraulic Criteria
i. Storm sewer inlet design will be based on the inlet curves provided in the
City of Fort Collins Drainage Criteria Manual.
2. Storm sewer design will be based on Manning's Equation with Manning's
coefficients as suggested in the City of Fort Collins Drainage Criteria
Manual.
Page 6
Conceptual Drainage and Erosion Control Report
Old Town North; Ft. Collins, Colorado
B. Development Criteria Reference and Constraints
1. The alignment of Dry Creek in this area will ultimately be changed with
the proposed re -alignment of Vine Drive. A grass -lined channel will
ultimately be constructed along the south side of the future Vine Drive
alignment. The channel section will parallel the south ROW line. Box
culverts will be provided where Dry Creek crosses the future Vine Drive
alignment and Redwood Street. Site grading along the channel will have to
match the required channel section for the estimated design flows.
2. The design flow for the channel is 600 cfs. This design flow is based on
discussions with the Stormwater Utility staff and on the assumption that
the Upper Dry Creek Diversion Channel is constructed to divert flows
around the Lower Dry Creek Basin. This design flow will have to be
verified.
3. Redwood Street is currently platted. It was designed with the Evergreen
Park development to the north. The actual street does not currently extend
to Vine Drive. Street improvements currently terminate at the south end of
the Meadows at Redwood. However, utilities have been installed in
Redwood Street to Vine Drive. This results in a major design constraint
issue for the conveyance of Dry Creek floodwater across Redwood.
a. The existing approved design for Redwood Street obtained from plans
prepared for Evergreen Park by VTN Colorado Inc; Plan approval
Date by the City Engineer 03/27/74; VTN Project No: 172242.
4. The Josh Ames Ditch is currently utilized as a drainage conveyance for
properties to the west, including some properties on the west side of North
College Avenue.
a. The Josh Ames Ditch release flows into the proposed detention pond.
b. An overflow weir will have to be built into the detention pond weir
that can safely pass the offsite flows brought in via the Josh Ames
Ditch.
5. All proposed grading along the south property line will have to match the
existing topography along the north side of the Lake Canal.
6. All proposed grading along the west property line will have to match the
existing topography at the commercial sites.
7. All proposed grading along the east property line will have to match the
proposed grading for the extension of Redwood Street along with the
grading required to install the box culverts under Redwood Street for the
Dry Creek realignment.
Page 5
Conceptual Drainage and Erosion Control Report
Old Town North; Ft. Collins, Colorado
b. Development within the Poudre River floodplain is not allowed.
B. Sub -Basin Description
1. The site slopes from the northwest corner of the site in a southerly manner
to a low point located near the center of the south property line.
2. There is approximately seven (7') feet of fall across the site.
C. HISTORIC CONDITIONS
1. Historically runoff from the Old Town North site flows overland towards
the Lake Canal. Until recently, stormwater was intercepted by the Josh
Ames Ditch and conveyed over the Lake canal via a pipe. This no longer
occurs as the conveyance pipe has been removed. The main existing
concrete structure remains in place but is heavily damaged.
2. Currently, all storm runoff upstream of the Lake Canal is conveyed
directly to the Lake Canal. Refer to the discussion under "Major Basin
Description" concerning the intersection of the Lake Canal with the main
Dry Creek channel.
IV. DRAINAGE DESIGN CRITERIA
A. Regulations
1. This conceptual report and the Conceptual Grading, Drainage and Erosion
Control Plan for Old Town North were prepared in accordance with the
requirements of the current City of Fort Collins Storm Drainage Design
Criteria and Erosion Control Criteria and the recommendations of the
Final Report - Hydrological Model Update for the Lower Dry Creek Basin
Master Drainage Plan.
2. Approvals from the Lake Canal Company will be required for any storm
drainage improvement proposed that affects the Lake Canal.
a. The Lake Canal Company has requested that the stormwater from the
detention pond be conveyed east underground and released into the
canal at the existing concrete structure located in the Dry Creek
channel rather than directly into the canal at the center of the pond.
b. A preliminary profile has been designed that verifies that it is possible
to convey the stormwater as requested.
Page 4
Conceptual Drainage and Erosion Control Report
Old Town North; Ft. Collins, Colorado
2. Old Town North has a platted area of approximately 45.01 acres. The site
is currently vacant and is covered with native vegetation.
3. The Lake Canal runs along the south property line of the site.
4. The Josh Ames Ditch is located on the property and currently conveys
stormwater from the west side of College Avenue across the property into
the Lake Canal.
III. DRAINAGE BASINS AND SUB -BASINS
A. Major Basin Description
t. The entire site is located in the Lower Dry Creek Basin as delineated on
the City of Fort Collins Stormwater Basin Map.
a. The limits of the 100-year floodplain for Dry Creek were taken from
FIRM map — Community Panel number 080102-0004-C; dated March
18, 1996. The limits of the 100-year floodplain for Dry Creek was also
verified with the maps taken from the report prepared by Gingery and
Associates titled Major Drainageway Planning - Dry Creek; Prepared
for City of Fort Collins, Larimer County and Colorado Water
Conservation Board; Prepared by Gingery and Associates; Dated April
1980.
b. Development within the Dry Creek floodplain will require that all
finished floors be elevated a minimum of 1.50 feet above the 100-year
water surface elevation. Finished grades are shown for all the lots.
2. This portion of the Lower Dry Creek Basin is developed with a
combination of residential, commercial and retail uses. There is some
remaining open space at this time north of the site that will be developed
in time.
3. The site is defined as sub -basin 410 in the Final Report Hydrological
Model Update for the Lower Dry Creek Basin Master Drainage Plan.
4. There is a major irrigation structure placed in the Lake Canal in the Dry
Creek channel. This crossing is located near the northeast corner of the
proposed Old Town North development. The Dry Creek channel was
essentially blocked with the installation of the structure.
5. Portions of the site are located within the limits of the 100-year
floodplains of the Cache la Poudre River and Dry Creek.
a. The limits of the 100-year floodplain for the Cache La Poudre River
were taken from FIRM map - Community Panel number 080102-
0004-C; dated March 18, 1996.
Page 3
Conceptual Drainage and Erosion Control Report
Old Town North; Ft. Collins, Colorado .
I. INTRODUCTION
1. This report presents the conceptual drainage and erosion control design for
the proposed Old Town North site.
II. GENERAL LOCATION AND DESCRIPTION
A. Property Location
1. Old Town North is located in the south '/z of the southwest ''/< of Section 1,
Township 7 North, Range 68 West of the 6th P.M., Larimer County, Ft.
Collins, Colorado.
2. More specifically, it is located on the north side of the Lake Canal, east of
North College Avenue and north of Vine Drive and south of Conifer
Street.
3. The site is bounded on the north by the future re -alignment of the Vine
Drive ROW, on the east by Redwood Street, on the south by the Lake
Canal and on the west by several commercial sites. The Cache La Poudre
River is located south and east of the site.
a. The proposed Vine Drive alignment is represented in the most current
City of Fort Collins Master Street Plan. It is more specifically defined
in the City of Fort Collins North College Avenue Access Management
Plan.
B. Description of the Property
Old Town North is a proposed commercial / residential subdivision in the
City of Fort Collins, Colorado. The project will be developed in 3 phases.
Phases 1 and 3 will consist of the residential portion of the development.
Phase 2 will consist of commercial development in the future. Phase 3 will
occur after the delineation of the Dry Creek floodplain is updated.
a. Phase I is within the currently defined Dry Creek floodplain. Phase 1
is outside the limits of the currently defined Dry Creek floodway. This
will be the limit of development on this site until the Dry Creek
floodplain is reviewed and . amended to account for improvements
made in the basin since the current floodplain was mapped.
Page 2
Conceptual Drainage and Erosion Control Report
Old Town North; Ft. Collins, Colorado
TABLE OF CONTENTS
I. INTRODUCTION................................................................................................. 3
II. GENERAL LOCATION AND DESCRIPTION ................................................ 3
A.
PROPERTY LOCATION..............................................................................................
3
B.
DESCRIPTION OF THE PROPERTY..............................................................................
3
III.
DRAINAGE BASINS AND SUB-BASINS......................................................... 4
A.
MAJOR BASIN DESCRIPTION....................................................................................
4
B.
SUB -BASIN DESCRIPTION.........................................................................................
5
IV.
DRAINAGE DESIGN CRITERIA..................................................................... 5
A.
REGULATIONS..........................................................................................................5
B.
DEVELOPMENT CRITERIA REFERENCE AND CONSTRAINTS ......................................
6
C.
HYDROLOGIC CRITERIA...........................................................................................
7
D.
HYDRAULIC CRITERIA.............................................................................................
7
V.
DRAINAGE FACILITY DESIGN......................................................................
8
A.
GENERAL CONCEPT.................................................................................................
8
B.
SPECIFIC DETAILS (DETENTION)..............................................................................
8
VI.
WATER QUALITY..............................................................................................
9
A.
GENERAL CONCEPT.................................................................................................
9
VII.
EROSION CONTROL: ......................................................................................
10
A.
GENERAL CONCEPT...............................................................................................
10
VIII.
CONCLUSIONS.................................................................................................10
A.
COMPLIANCE WITH STANDARDS............................................................................
10
B.
DRAINAGE CONCEPT..............................................................................................
10
IX.
REFERENCES....................................................................................................11
Appendix I Preliminary Drainage Calculations
Appendix II Portions of the Final Report Hydrological Model Update for the Lower
Dry Creek Basin Master Drainage Plan; Prepared by Lidstone
and
Anderson; Dated September 10, 1997; Project No. COFC96.06
Appendix III Stuffer Envelope - Conceptual Drainage and Erosion Control Plan
(1
Sheet)
CONCEPTUAL DRAINAGE AND EROSION
CONTROL REPORT
For
OLD TOWN NORTH
Ft. Collins, Colorado
Prepared for:
PALLADIAN CONSTRUCTION CO.
PO Box 270053
Ft. Collins, CO 80527-0053
SHEAR
PRINT DATE
Prepared By: DEC 9 2000
SHEAR ENGINEERING CORP
SHEAR ENGINEERING CORPORATION
Project No: 1646-01-98
Date: December, 2000
4836 S. College, Suite 12 Ft. Collins, CO 80525 (970) 226.5334 Fax (970) 282-0311 www.sliearengineering.com