HomeMy WebLinkAboutDrainage Reports - 05/11/19951
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VROPMTY OF tal2Aapg,
"�:M
FINAL STORM DRAINAGE REPORT
FOR
AMENDED SHADOWBROOK P.U.D.
FEBRUARY 27, 1995
STEWART & ASSOCIATES, INC.
103 SOUTH MELDRUM STREET
FORT COLLINS, COLORADO 80521
970/482-9331
' STEWART&kSSOCIATES
Consulting Engineers and Surveyors
' February 27, 1995
.Mr. Glen Schlueter.
t Stormwater Utility
City of Fort Collins
P.O. Box 580
' Fort Collins, CO 80522
Dear Glen:
tThe following is the final storm drainage report for the Amended Shadowbrook P. U.D..
Shadowbrook P.U.D. is an existing, partially completed, planned unit development on East Stuart
' Street at Brookhaven Circle. It is situate in the Northwest 1/4 of the Northeast 1/4 of Section 24,
Township 7 North, Range 69 West of the 6th P.M.. It is in the Spring Creek Drainage Basin.
The P.U.D. was started in 1979, and 36 units were constructed. All of the private streets, utilities
and open space were completed at that time. Four six -unit buildings were planned but not built at
that time, but their infrastructure was completed. Mr. Chris Allison is now proposing to complete
' the P.U.D. by constructing seven duplexes on the available building sites. The total number of
units will be reduced from 60 to 50. The area of hard surfacing for the approved 24 unbuilt units
was 36,288 square feet, and the area of hard surfacing of the proposed seven duplexes is 32,780
' square feet. The net decrease of impervious surface is 3,508 square feet. Therefore, the existing
storm drainage facilities will not be adversely affected.
' The duplexes will face Brookhaven Circle West and Brookhaven Court, which are private
streets. They are 28-foot wide streets which have a four -foot wide drain pan along both sides of a
20-foot asphalt driveway. There is a storm drain system which carries the storm water runoff
' from four existing area inlets located at sump points in the private driveways. The runoff is
discharged into the Southerly channel of Spring Creek undetained from the existing 18-inch
diameter storm drain.
' The Spring Creek Flood Way and Flood Plain lines are shown on the Grading and
Drainage Plan as per the Spring Creek Basin Study prepared by Engineering Professionals, Inc.
and dated March 1988. All proposed buildings are outside of the existing flood plain line as
delineated on the Spring Creek Basin Study and shown on this proposed grading and drainage
' plan.
The City Stormwater Utility has some concerns related to the buffer limits of Spring
' Creek. Buffer limits approach to Spring Creek are the same used in Section 4.3 of the Mail Creek
Stability Study submitted to the City of Fort Collins by Lidstone and Anderson dated January
tJames H. Stewart
and Associates, Inc.
103 S. Meldrum Street
' P.O. Box 429
Ft. Collins. CO 80522
303/482-9331
Fax 303/482-9382
I
' Final Storm Drainage Report
' Amended Shadowbrook P.U.D.
Page 2
' 1993 and included with this report. "Thus approach to the buffer limits is to use a minimum
setback for all structures that consists of25 feet for the potential lateral movement of the channel
' plus the horizontal distance associated with the 2:1 (H: V) failure plane of the existing channel
bank." This concept is shown on Sections for building "C" through "G" on sheet 3 of 5 of these
proposed utility plans.
Erosion Control
The existing ground cover on the proposed duplex sites is a good non -irrigated grass. The
ground cover will be disturbed as each foundation is constructed. The disturbance will be the
' same as for any residence in a fully developed subdivision. The foundation will be excavated, and
then immediately the foundation will be constructed which will deter wind erosion. The site will
remain disturbed until the final grading and seeding is done at the end of the construction
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sequence.
Rock filters will be placed at the two existing inlets on Brookhaven Circle West at the
beginning of construction of the first duplex and remain in place throughout the construction
period.
Silt fences will be constructed at the rear of Building "C" through "G" at the time each
building is under construction. All silt fences will remain in place until the grading and seeding is
completed for each duplex.
The Amended Shadowbrook P.U.D. is revising the approved site plan by reducing the
number of units in a fully developed P.U.D.. With the erosion control planned for, there should
be no adverse effect on Spring Creek or the surrounding neighborhood. The existing south bank
of Spring Creek is not to be disturbed.
The grading and drainage plan is enclosed as a part of this report. If you have any
questions concerning this project, please call.
' Sincerely,
Richard A.
Rutherford, P.E. & L.S.
President
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enclosures
J�V • �� Fo' ;9os,
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STEWARF&ASSOCIATES
Consulting Engineers and Surveyors
February 27, 1995
Mr. Glen D. Schlueter
Stormwater Utility
City of Fort Collins
P.O. Box 580
Fort Collins, CO 80522
Dear Glen:
The following is the erosion control cost estimate for the Amended Shadowbrook P.U.D.:
1. Silt Fence - 395 L.F. @ $2.50/L.F. $ 988.00
2. Rock Filters - 2 @ $75.00 150.00
3. Reseeding entire disturbed site - 51,000 S.F. @ $0.0294/S.F. 1499,00
Total $2637.00
The amount of collateral will then be 150% of $1499.00 = $2248.50
- If you have any questions concerning this estimate, please call.
Sincerely,
:�Q��ISrfgE,G '.OHO?
Richard A. Rutherford, P.E. & L.S. W
President t
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James H. Stewart
and Associates, Inc.
103 S. Meldrum Street
P.O. Box 429
Ft. Collins, CO 80522
303/482-9331
Fax 303/482-9382
5028 *
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Rainfall Performance Standard Evaluation
Project..Qweti�Fp jrta�oay zcor �1.r(.\ Standard Form A
By: Stewart & Associates Completed: k'4
103 S. Meldrum
Fort Collins, Colorado 80521 Date:
Developed
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TABLE 5.1
PAGE 23
Effectiveness Calculation
Project:y?t,k.-D, Standard Form B
By: Stewart & Associates Completed: pg-4-
103 S. Mcldrum
Fort Collins, Colorado 80521 Date: I
Erosion Control
Method
C— Factor
Value
P — Factor
Value
Comments:
F�)SD,�
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D 1, O
CxCb4v Na
S F-Itz
l.00
l7.
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UAlLS
HDI/SF—B:1989 m�A — b 0,1+ Ac = .
�IS (J2fi� P e:ca — +��:Ono4S = 0,I t{ —
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Effectiveness Calculation
Project: SFM�U J Li2o0t� l i V ) Standard Form B
By: wart&Associates Completed: At -IC
iS. Meldrum
Fort Collins, Colorado 80521 Date:
Erosion Control
Method
C— Factor
Value
P — Factor
Value
Comments:
Pam c
(.00
0,9 0
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STtp,,� gNtCS,
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1.37a
HDI/SF—B:1989
.: %%sTuvAe > k2ap.- — �cmea,, = D , O(v ft,
003 q %
0�
Effectiveness Calculation
Projcct: Shp,�Ri2 �j Lk Standard Form B
By: Stewart & Associates Completed: Ally_
103 S. Meldrum
Fort Collins, Colorado 80521 Date: 1 / 3 / 5
Erosion Control
Method
C— Factor
Value
P — Factor_
Value
Comments:
8AZ-c Satr
1.Oo
D,�it7
C7CCV�p��1� Fhov
4 r,VeL ttLz,
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. n * ' �SZuzvEp A4o,k —retz = C�,i7(ar'C.
PAGE 24
'
Table 5.2 C-.Factors and P-Factors for Evaluating EFF Values.
'
Treatment
C-Factor
P-Factor
BARE SOIL
Packed and smooth . . . . . . . .
. . . . 1.00
1.00
t
Freshly disked.
1.00
0.90
Rough irregular surface . . . .
1.00
0.90
'
SEDIMENT BASIN/TRAP. . . . . . . .
. . . . 1.00
0.50(1)
STRAW BALE BARRIER, GRAVEL FILTER, SAND
BAG. 1.00
0.80
'
SILT FENCE BARRIER . . . . . . . . .
. . . . 1.00
0.50
'
ASPHALT/CONCRETE PAVEMENT.
. . . . . 0 0.1
1.0.0
ESTABLISHED DRY LAND (NATIVE) GRASS.
. . See Figure
1.00
'
SOD GRASS. . . . . . . . . . . .
. . . . . 0.01
1.00
TEMPORARY VEGETATION/COVER CROPS . .
. . . . . 0.45(2)
1.00
HYDRAULIC MULCH @ 2 TONS/ACRE. . . .
. . . . . 0.10(3)
1.00
'
SOIL SEALANT . . . . . . . . . . . .
. . . 0.01-0.60(4)
1.00
EROSION CONTROL MATS/BLANKETS. . . .
. . . . . 0.10
1.00
'
c, V-A\JC,L
HAY OR STRAW DRY MULCH
O
After planting grass seed, apply
mulch at
a rate o 2 tons acre (minimum) and adequately
anchor, tack or crimp material into the soil.
Maximum
'
Slope Length
M (feet)
1 to 5 400 . . . . .
. . . . 0.06
1..00
6 to 10 200 . . . . .
. . . . 0.06
1.00
11 to 15 150
0.07
1.00
16 to 20 100 . . . . .
. . . . 0.11
1.00
'
21 to 25 75 . . . . .
25 to 33 50 ..
. 0.14
0.17
1.00
1.00
> 33 35
0.20
1.00
'
NOTE: Use of other C-Factor or P-Factor
values reported
in this
table must be substantiated by
documentation.
(1) Must be constructed as the first
step in overlot grading.
'
(2) Assumes planting by dates identified
in Table 7.4
thus dry
or hydraulic mulches are not required.
'
(3) Hydraulic mulches shall be used
May 15 unless irrigated.
only between March
15 and
(4) Value used must be substantiated
by documentation.
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PAGE 26
ESTABLISHED GRASS AND C . FACTORS
FORT COLLINS, COLORADO
10 20 30 40 50 60 70 80 90 100
ESTABLISHED GRASS GROUND COVER (7o)
FIGURE 5.1
' ' , /d � d 2211 710N SEQUENCE
�ROJECT: i/ncr! c a Dw /DD�
STANDARD FORM C
QUEt7CE. FOR 19 9r ONLY COMPLETED BY: DATE: .
kjdicate by use of a bar line or symbols when erosion control measures will be 'instal led.
or modifications•to an approved schedule may require submitting a new schedule for
approval by the City Engineer.
' YEAR
--- --MONTH-
IERLOT GRADING
1JND EROSION CONTROL
Soil Roughing
Perimeter Barrier — —
Additional Barriers
Vegetative Methods
Soil Sealant
Other
IINFALL EROSION CONTROL
'STRUCTURAL:
Sediment Trap/Basin
Inlet Filters
Straw Barriers
Silt Fence Barriers
Sand Bags
Bare Soil Preparation
Contour Furrows
Terracing
Asphalt/Concrete Paving
. Other
'VEGETATIVE:
Permanent Seed Planting
Mulching/Sealant
'Temporary Seed Planting
Sod Installation
Nettings/Mats/Blankets
' Other
-------------------------
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SiUCTURES: INSTALLED BY C. MAINTAINED BY
V ETATION/MULCHING CONTRACTOR - h[VA, •
E SUBMITTED Z- `j5'
I[tSF-C:1989
APPROVED BY CITY OF FORT COLLINS Oil
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STEWART&JkSSOCIATES PH 482-93 1 F X F4882 9 COLLINS, CO 80521
Consulting Engineers and Surveyors \
By: _ �1lnQ� Date: 12- 2 1-14 Client: Sheet No. of i
Project:
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STEWART&O SSOCIATES 103 S. PH. 482-93 1 F X FORT
COLLINS, CO 80521
Consulting Engineers and Surveyors
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STEWART&O SSOCIATES103 S. PH 82-93 1 FAX FORT
COLLINS, CO 80521
Consulting Engineers and Surveyors
By: Date: 12-W-W Client: hLUSDa) Sheet No. of
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Consulting Engineers and Surveyors
By: AV-2, Date: 12� - SO Client: CziS Au.o,,... isSheet No. of f
Project: k"i�DU JP,aAt--)['
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1
STEWARTMSSOCIATES PH. 482-93311RUMF FAX, FORT
482-93 103 S. COLLINS, CO 80521
Consulting Engineers and Surveyors
By: _� Date: - IZ-30-5q Client: 0- kU <Y?) C Sheet No. � of �
l
Project: SHA1niJ�uRoo� S�i�� 1J2w��x EylSiiI,- \
i
Subject:_-- AVJAL is x 7p oacC� "'tf Ccp wl ' -M EGSi I0��tsT�w`
- - - -- -- -�-- ,, -�t 40, -4
-
i iI III
L f
1 Z4
_I -_-I S = � 219G;9, ! -'� i.,l� __�_ �_-�-_ _�-- ! j � ! � !- I i I � I ► � , I
-I5, L�F
REPORT OF STORM SEWER SYSTEM DESIGN
USING UDSEWER-MODEL 10-19-1992
DEVELOPED
BY
JAMES C.Y. GUO ,PHD, PE
DEPARTMENT OF CIVIL ENGINEERING, UNIVERSITY OF COLORADO AT DENVER
' IN COOPERATION WITH
URBAN DRAINAGE AND FLOOD CONTROL DISTRICT
DENVER, COLORADO
'** EXECUTED BY STEWART AND ASSOCIATES (FT
OLLINS-COLORADO)............................
ON DATA 12-30-1994 AT TIME 14:51:53
'** PROJECT TITLE :
Shadowbrook P.U.D. Existing Storm Drainage System Analysis
*** RETURN PERIOD OF FLOOD IS 2 YEARS
1
'** SUMMARY OF HYDRAULICS AT MANHOLES
---------------------------------------------------------------------------
MANHOLE
CNTRBTING
RAINFALL
RAINFALL
DESIGN
GROUND
WATER
COMMENTS
ID
NUMBER
AREA * C
DURATION
INTENSITY
PEAK FLOW
ELEVATION
ELEVATION
'---------------------
MINUTES
INCH
1.00
0.00
0.00
0.00
---
7.70
--------------------
59.00
54.92
OK
2.00
0.00
0.00
0.00
7.70
58.88
55.99
OK
'
21.00
0.00
0.00
0.00
2.80
58.88
58.57
OK
3.00
0.00
0.00
0.00
4.90
58.88
56.70
OK
31.00
0.00
0.00
0.00
2.00
58.88
58.80
OK
'
4.00
0.00
0.00
0.00
2.90
62.97
59.98
OK
41.00
0.00
0.00
0.00
1.20
62.97
61.65
OK
5.00
0.00
0.00
0.00
1.70
62.97
60.48
OK
51.00
0.00
0.00
0.00
1.70
62.97
61.83
OK
MEANS WATER ELEVATION
IS LOWER THAN GROUND ELEVATION
1
1
SHADBROK.DAT
** SUMMARY
OF SEWER HYDRAULICS
NOTE:
THE GIVEN FLOW DEPTH -TO -SEWER SIZE RATIO= .9
SEWER
MANHOLE NUMBER
SEWER
REQUIRED
SUGGESTED EXISTING
ID NUMBER
UPSTREAM DNSTREAM
SHAPE
DIA(HIGH)
DIA(HIGH)
DIA(HIGH)'
WIDTH
'
------------------------------------
ID NO. ID NO
(IN) (FT)
----------------------------------
(IN) (FT)
(IN) (FT)
(FT)
100.00
2.00 1.00
ROUND
12.80
18.00
18.00
0.00
101.00
200.00
21.00 2.00
3.00 2.00
ROUND
ROUND
11.27
10.80
18.00
18.00
18.00
18.00
0.00
0.00
201.00
31.00 3.00
ROUND
9.94
18.00
15.00
0.00
300.00
4.00 3.00
ROUND
9.32
18.00
15.00
0.00
'
301.00
41.00 4.00
ROUND
8.21
18.00
15.00
0.00
400.00
5.00 4.00
ROUND
7.63
18.00
15.00
0.00
401.00
51.00 5.00
ROUND
9.35
18.00
15.00
0.00
1IMENSION
UNITS
FOR ROUND AND ARCH SEWER
ARE IN INCHES
IMENSION
UNITS
FOR BOX SEWER ARE IN FEET
EQUIRED DIAMETER WAS DETERMINED
BY SEWER
HYDRAULIC CAPACITY.
UGGESTED
DIAMETER WAS DETERMINED BY COMMERCIALLY
AVAILABLE
SIZE.
FOR A NEW SEWER, FLOW WAS ANALYZED BY THE
SUGGESTED SEWER SIZE;
OTHERWISE,
'XISITNG
SIZE WAS USED
----------------------------------------------------
DESIGN FLOW NORMAL
NORAML
7--------------------------
CRITIC CRITIC
FULL FROUDE
,SEWER
OMMENT
ID FLOW Q FULL.Q DEPTH
VLCITY
DEPTH VLCITY VLCITY NO.
NUMBER
----------------------------
CFS CFS FEET
FPS ---
FEET
------------------------------------
FPS
FPS
100.0
7.7 19.2 0.66
10.26
1.06
5.76
4.36 2.55
V-OK
101.0
2.8 9.8 0.55
4.77
0.64
10.60
1.58 1.32
V-OK
'200.0
4.9 19.2 0.52
9.08
0.85
2.71
2.77 2.60
V-OK
201.0
2.0 6.0 0.50
4.40
0.57
9.06
1.63 1.27
V-OK
300.0
2.9 10.3 0.45
7.23
0.69
2.90
2.36 2.21
V-OK
1.2 6.0 0.38
3.82
0.44
7.43
0.98 1.29
V-OK
'301.0
400.0
1.7 10.3 0.34
6.22
0.53
2.44
1.39 2.22
V-OK
401.0
1.7 6.0 0.45
4.21
0.53
3.45
1.39 1.28
V-OK
IOUDE
NUMBER=0 INDICATES THAT A
PRESSURED FLOW OCCURS
----------------------------------------------------------------------
SEWER
SLOPE INVERT ELEVATION
BURIED
DEPTH
COMMENTS
ID
NUMBER
UPSTREAM DNSTREAM UPSTREAM
DNSTREAM
(FT)
(FT)
(FT)
(FT)
------------------------------------------------------------=------
100.00
3.85 54.93
52.00
2.45
5.50
OK
101.00
1.00 55.03
55.02
2.35
2.36
OK
3.85 55.85
54.93
1.53
2.45
OK
'200.00
201.00
1.00 56.20
56.19
1.43
1.44
OK
300.00
2.96 59.29
55.86
2.43
1.77
OK
301.00
1.00 59.39
59.38
2.33
2.34
OK
400.00
2.96 59.95
59.24
1.77
2.48
OK
401.00
1.00 60.05
60.04
1.67
1.68
OK
MEANS BURIED DEPTH IS GREATER
THAN REQUIRED SOIL
COVER OF
1 FEET
nun o..✓ n.�
** SUMMARY OF HYDRAULIC GRADIENT LINE ALONG SEWERS
-------------------------------
SEWER
SEWER
SURCHARGED
ID
-NUMBER
LENGTH
LENGTH
FEET
FEET
100.00
76 00
0.00
101.00
1.00
0.00
'
200.00
201.00
24.00
1.00
5.61
0.00
300.00
116.00
6.15
301.00
1.00
0.00
'
400.00
24.00
0.00
401.00
1.00
0.00
------------------------------------------------
CROWN ELEVATION
WATER ELEVATION
FLOW
UPSTREAM
DNSTREAM
UPSTREAM
DNSTREAM
CONDITION
FEET
------------------------------------------------
FEET
FEET
FEET
56.43
53.50
55.99
54.92
JUMP
56.53
56.52
58.57
55.99
JUMP
57.35
56.43
56.70
55.99
JUMP
57.45
57.44
58.80
56.70
JUMP
60.54
57.11
59.98
56.70
JUMP
60.64
60.63
61.65
59.98
JUMP
61.20
60.49
60.48
59.98
JUMP
61.30
61.29
61.83
60.48
JUMP
�RSS'ED=PRESSURED FLOW; JUMP=POSSIBLE HYDRAULIC JUMP; SUBCR=SUBCRITICAL FLOW
1** SUMMARY OF ENERGY GRADIENT LINE ALONG SEWERS
------------------------------------------------------------------------------
UPST
MANHOLE
SEWER
JUNCTURE
LOSSES
DOWNST
MANHOLE
MANHOLE
ENERGY
FRCTION BEND
BEND
LATERAL
LATERAL
MANHOLE
ENERGY
'SEWER
-------------------------------------------------------------------------------
ID NO ID
NO.
ELEV FT
FT K COEF LOSS FT
K COEF
LOSS FT
ID
FT
100.0
2.00
57.63
2.66 0.00
0.00
0.50
0.05
1.00
54.92
101.0
21.00
58.92
1.00 0.25
0.01
0.25
0.29
2.00
57.63
200.0
3.00
57.98
0.06 0.25
0.03
0.25
0.26
2.00
57.63
201.0
31.00
59.10
1.00 0.25
0.01
0.25
0.11
3.00
57.98
4.00
60.79
2.69 0.25
0.02
0.25
0.10
3.00
57.'98
'300.0
301.0
41.00
61.88
1.00 0.25
0.00
0.25
0.08
4.00
60.79
400.0
5.00
61.08
0.20 0.25
0.01
0.25
0.08
4.00
60.79
'
401.0
51.00
62.11
1.00 0.25
0.01
0.25
0.02
5.00
61.08
BEND LOSS
=BEND K* VHEAD
IN SEWER.
LATERAL
LOSS= OUTFLOW
VHEAD-JCT LOSS K*INFLOW
VHEAD
FRICTION
LOSS=O MEANS
IT IS NEGLIGIBLE OR POSSIBLE
ERROR DUE TO JUMP.
FRICTION
LOSS
INCLUDES
SEWER INVERT DROP
AT MANHOLE
NOTICE:
VHEAD
DENOTES
THE VELOCITY HEAD
OF FULL FLOW CONDITION.
A MINIMUM JUCTION LOSS
OF 0.05 FT WOULD
BE INTRODUCED
UNLESS LATERAL
K=O.
FRICTION
LOSS
WAS ESTIMATED BY BACKWATER
CURVE
COMPUTATIONS.
I
--------------------------------------------------- I
INLET HYDARULICS AND SIZING
DEVELOPED BY
DR. JAMES GUO, CIVIL ENG DEPT. U OF COLORADO AT DENVER
SUPPORTED BY METRO DENVER CITIES/COUNTIES AND UD&FCD
-------------------------------------------7----------------------
'SER:Stewart and -Associates -Ft Collins Colorado ..............................
ON DATE 12-30-1994 AT TIME 15:33:46
'** PROJECT TITLE: Shadowbrook
*** GRATE INLET HYDRAULICS AND SIZING:
INLET ID NUMBER: 1
'
INLET HYDRAULICS: IN A SUMP.
GIVEN INLET DESIGN INFORMATION:
'
INLET GRATE WIDTH
(ft)=
1.50
INLET GRATE LENGTH
(ft)=
2.00
INLET GRATE TYPE
=Type 16
Grate Inlet
'
NUMBER OF GRATES
=
1.00
SUMP DEPTH ON GRATE
(ft)=
0.50
GRATE OPENING AREA RATIO
(%) =
0.60
IS THE INLET GRATE NEXT TO A
CURB ?-- NO
'
Note: Sump is the additional depth to
flow
depth.
'
STREET GEOMETRIES:
STREET LONGITUDINAL SLOPE (%) =
1.00
STREET CROSS SLOPE M =
2.00
'
STREET MANNING N =
0.016 .
GUTTER DEPRESSION (inch)=
1.50
GUTTER WIDTH (ft) =
2.00
STREET FLOW HYDRAULICS:
WATER SPREAD ON STREET (ft)
= 14.41
'
GUTTER FLOW DEPTH (ft)
= 0.41
FLOW VELOCITY ON STREET (fps)=
3.28
FLOW CROSS SECTION AREA (sq ft)=
2.20
GRATE CLOGGING FACTOR M
= 20.00
'
CURB OPENNING CLOGGING FACTOR(%)=
10.00
'
INLET INTERCEPTION CAPACITY:
FOR 1 GRATE INLETS:
DESIGN DISCHARGE
IDEAL GRATE INLET CAPACITY
(cfs)=
(cfs)=
7.25
9.25
BY FAA HEC-12 METHOD:
FLOW INTERCEPTED
(cfs)=
7.25
'
CARRY-OVER FLOW
(cfs)=
0.00
BY DENVER UDFCD METHOD:
FLOW INTERCEPTED
(cfs)=
7.25
CARRY-OVER FLOW
(cfs)=
0.00
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site conditions and could be significantly influenced by an increase in vertical
load, such as a placement of a structure (house) directly on a bank or an
increase in pore water pressure due to increased lawn watering or irrigation.
To confirm this qualitative investigation of critical bank height, a Culmann
analysis of threshold bank height was performed. The Culmann equation analyzes
the forces acting on a bank versus the forces resisting bank failure and can be
simply stated as follows:
HC-! 4cxsin Bcos Al -Z
I y 1-cos(B+ 11
where H. is the critical height; c is the shear strength of the soils, � is the
stable bank angle; 8 is the angle of the slope; y is the unit weight of the soil;
and Z is the depth of the surface fractures. Soil properties for this analysis
were derived from the Empire Labs reports (1976, 1977), the Mail Creek bank
material properties (Table 4.1) and field observations. In summary, the Culmann
analysis confirmed that for steep to near vertical banks, a threshold bank height
of 8.9 feet could be derived. For banks approaching 15 feet in height,a stable
bank angle would approach 28 to 30 degrees.
In summary, the theoretical Culmann analysis confirms the Mail Creek bank
stability data observed in the field. Typically banks steeper than 45 degrees
(1:1 H:V) and greater than 10 to 15 feet in height are generally unstable. Where
banks are less than 10 feet in height, oversteepening of the banks by local
hydraulic forces will result in bank failure.
4.3.5 Buffer Limits
The geomorphic analysis evaluated the historical trends of the Mail Creek
system. The period of record included a historical aerial photograph analysis,
which spanned 37 years (1950-1987). The channel profile analysis spanned 10
years of record (1981-1991). Field observations and slope stability analyses
carried the historic data base to 1991. Future trends of the system are
difficult to predict and are dependent on the hydrologic and hydraulic influence
of Mail Creek, future development trends and land use planning.
31
RIO
1
1
1
1
1
On a short term basis one can assume that outside bank erosion and the
channel's propensity to increase its sinuosity and decrease .its radius of meander
curvature will continue. Channel bed adjustments will continue to be restricted
by the presence of both natural and artificial grade controls. Further
development and encroachment on the channel banks would impact the evolution of
the system. Future degradation along Fossil Creek will adversely impact the Mail
Creek system. For that reason "buffer limits" should be established to protect
man-made structures from the instability of the river system and to minimize the
influence of man's activities on the acceleration of the channel response.
The greatest lateral adjustments of Mail Creek occurred in the vicinity of
the "culverts" at Mail Creek Lane and Passway Drive. The most laterally active
channel reach is Reach C where historical channel movement has been estimated to
range from 20 to 25 feet. Channel degradation has also been experienced at
several locations in - Reach C. The construction of the Fairway Estates
Reservoirs, which serve as a local sediment trap for the flows in Mail Creek, had
a significant impact on the stability of the channel reaches below the
reservoirs. The most significant impact occurred in Reach B where degradation
and channel widening (bank failure) were the most evident.
The channel pattern of Mail Creek has undergone significant adjustment from
1969 to the present day. The pre-1970 Mail Creek channel was a relatively
straight incised channel through the study reach (Figure 4.1). Since 1969 the
channel has increased its overall sinuosity and its existing meanders have
decreased in radii of curvature. At one location approximately 250 feet upstream
of the Mail Creek Lane culvert, the radius of meander curvature has decreased
from 30 feet to 15 feet over the 40 year period. A meander cutoff is impending
at this location.
As noted in the previous section a threshold bank height was determined.
The maximum bank heights measured as part of this analysis were 24 feet. This
threshold bank height ranges from 10 to 15 feet, depending on the slope angle.
Utilizing Figure 4.2, stable angles for these bank heights would range from 27
to 32 degrees or approximately 1.5:1 to 2:1 (H:V).
Erosion buffer limits were established based on a combination of these two
j methodologies. Average to maximum channel movement over the period of record was
20 to 25 feet. This value was measured from the outside waters edge of the
channel. Twenty-five (25) feet is recommended as the minimum erosion setback for
32
r
1 all lots and watered lawns. The strip of land within this setback should be left
in native grasses and not be irrigated.
With respect to the placement of permanent structures, the buffer limit
' failure.
analysis assumes a worst case scenario of catastrophic bank
Furthermore, the results of the analysis are based on a data base with a period
' of record of approximately 40 years. Given limited increases in the flow
conditions during the next 40 years, this study assumes that the channel has the
migrate 25 feet; leaving aThiseappro chresultsin
undercut
bank
o
potential to laterally a
' similar height to that which is present today.
minimum setback for all structures that consists of 25 feet for the potential
' lateral movement of the channel plus the horizontal distance associated with the if the
2:1 (H:V) failure plane of the existing channel bank. For example,
' existing bank height is 15 feet, the buffer limit or set back for all structures
should be 55 feet or the sum of the potential lateral movement of the channel (25
feet) plus the reduction in slope associated with the 15-foot near vertical bank
(30 feet). The results of the buffer limit analysis are presented in Plates 4
and 5. It is important to note that the results of the buffer limit analysis
' were compared to the limits of the 100-year floodplain for existing conditions.
Limits of the 100-year floodplain have been superimposed on the buffer limits.
Where the floodplain limits exceed the limits of the erosion buffer, the limits
of the 100-year floodplain became the controlling factor.
4.4 Quantitative Analysis
4.4.1 Routing Reach Delineation
The quantitative sediment transport analysis was conducted on a reach-by-
l"`. reach basis using reaches which individually exhibit similar sediment ent transport
delineated based on: (1)
terial
characteristics. Reaches were
composition, (2) the presence of grade controls, and (3) hydraulic conditions.
In addition, the reach lengths were verified using the concept of sand wave
nd wave can travel
celerity. Sand wave celerity is a measure of the distance a sa
, since
over a period of time for a given set of hydraulic conditions. Hoandemodeling
over the course of a storm event hydraulic conditions vary widely,
time increments are generally not constant for a simulation, the sand wave
33
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WAWA
ALL CONSTRUCTION ACTIVITIES MUST COMPLY WITH THE STATE OF
COLORAOO PERMITTING PROCESS FOR 'STORMWATER DISCHARGES ASSOCIATED
WITH CONSTRUCTION ACTIVITY.' FOR INFORMATION, PLEASE CONTACT
COLORADO DEPARTMENT OF HEALTH, WATER QUALITY CONTROL DIVISION,
WQCD-PE-R2, 4300 CHERRY CREEK DRIVE SOUTH, DENVER, COLORADO
80223-1510 ATTENTION: PERMITS AND ENFORCEMENT SECTION
PHONE: (JOJ) 692-059D
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- NOTES
1. The existing south bank of Spring Creek is not to be distureed.
p, /,' f 2. All structures shall be outside the Spring Creek flood plain.
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