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Y
MAST ER
STORM DRAINAGE REPORT
for
THE LANDINGS P.U.D. '
January, 1979
CORNER CONSUlTINq COMPANY
P Y
MANA(4E MENThNC{INEERINq
VICINITY MAP
SCALE: 1": 600
CORNED CONSU[TINq COMPANY po Ix)x b4b IoRi rdIM. (Acwvk) W,22. pl Pt (303)482-bb50am
MAIW(',EMENI FNGINtF RING, '
January 26, 1979
Reid L. Rosenthal
Osprey Homes
1921 Brookwood
.Fort Collins, CO 80525
Dear Reid:
We are pleased to present this storm drainage and detention
report for the Landings P.U.D., located in Fort Collins.
This report was prepared using City of Fort Collins regulations
for storm runoff. Certain calculations pertaining to report
are included in the appendices.
Please feel free to contact us with any questions or comments.
Very truly yours,
/57� v3..
Bill Blackwell, E.I.T.
Harr �,ACornell, P.E.
President
,
TABLE OF CONTENTS
Page No.
I.
Introduction
1
II.
Methods
2
III.
Basin Description and Land Use
3
:7
..
IV.
Harmony Reservoir
5
E
V.
Hydrologic and Hydraulic Investigation
7
s
VI.
Summary and Recommendations
9.
Appendix A
Appendix B
.b
';J
i::��,:�y.}:`.� . ,. -.<.!�!•:..:.:.i;a;..i1"-;R3tt�j.t%!J:?�':'".ail a�'���aa�i�iDi�.�.�i�i x L.
s 11
I. INTRODUCTION
;{
SCOPE: The purpose of this report is to present the developer
s
or his planner with a set of feasible alternatives to the storm
drainage problem in the Landings/Whalers Cove proposed develop-
ments.
This report will analyze the impact of a major storm on the
proposed development. It also will give guidelines as to allow-
able release rates for the separate phases of.development, and
present several alternative solutions to the handling of storm
runoff in the natural drainage swale which lies south of Harmony
Rservoir and north of Colorado State Highway 68.
LOCATION: The study area is.located in the South One -Half of
Section 36, Township 7 North, Range 69 West. The present land
r:
currently use is agricultural. Land
y being Alarmed for, rezoning
as the South College Properties is located upstream of the
Landings. Upstream property is included in the analysis; however,
`3
no recommendations are included for off -site property.
a
DEVELOPMENT SCHEDULE: The approximate development schedule is
as follows: Construction is to begin on The Boardwalk Commercial
a.
Center in the Spring or Surnm er of.1979. Development of the
t
3-
R
A
A
remainder of Landings properties will proceed during the next
1-2 years.. At this writing the development schedule for the
upstream properties is not known. However, total construction
within five years is a conservative estimate.
II. METHODS
Sa In general the analysis was completed using "A Preliminary
Draft of Storm Drainage Design Specifications for the City of
Fort Collins." The Rational Method was used to determine peak
flows. Although the total area is over 200 acres, sufficient
drainage will be retained in Harmony Reservoir to reduce the
area drained to the natural swale to less than 200 acres.
Coefficients (C values) for use with the Rational Method
were taken from Table 2 of the City specifications and were
applied according to zoning. No adjustments were made in the
coefficients for varying densities between different phases of
the overall zoned properties. For instance, any oroDertV 7nnpd
RLP (Ligh Planned Residential) was represented by a coefficient
of 0.5 regardless of the layout of the phases of development.
Time of concentration (Tc) used was derived from Kirpich's
formula. Kirpich's-time of concentration is -more accurate for
2
I
.. ..u. ..... _ .5 ii�. "1 v:Y...'a��"4i�•Y Y� �r}y�^� gyp �}.� ,
�]1:7��.;:ita �r.La�xN Jya2�T�ti36X�YEiwP,
larger basins than are the overland flow curves given in the
.City specifications. Since some of the subbasins are 50-60
acres in size our staff feels using Kirpich is a better approach.
For documentation on the formula refer to Drainage Manual
published by the U.S. Bureau of Reclamation and "Civil Engineering"
June, 1940, for first publication.
Midway through the analysis a meeting was held with the City
staff responsible for review of drainage plans. The staff requested
additional analysis to include storm runoff from all properties
east of the center line of College Avenue. To perform this anal-
ysis, hydrograph routing was necessary. We developed straight line
hydrographs consistent with the Rational Method peak discharge
and the accumulated runoff method. Included in Appendix B is the
development of this hydrograph.. It does not satisfy the "feel"
of a natural hydrograph, but it allows consistent application of
the accumulated runoff method.
III. BASIN DESCRIPTION AND LAND USE
The basin thalweg is generally located between 500 and 1,000
feet north of Harmony Road falling at approximately one (1) per-
cent from west to east. In the eastern one -quarter mile it turns
southeasterly to the intersection of Lemay Avenue and Harmony Road,
- 3 -
A
where a 40-inch by.29-inch elliptical concrete pipe culvert
system exists to carry storm runoff under the intersection.
Prior.to any improvements, the basin extended upstream
approximately three miles and drained an area of one and a half
square miles. However, development has altered the historic
.basin.
College Avenue effectively blocks all upstream flow. The
one 24 inch culvert under College will be unused after completion
of a storm sewer along the west side of the street.
The present upstream limit of the basin under consideration
is the Larimer County Irrigation Canal #2 which lies between
College Avenue and the study area. It is unknown whether developed
storm runoff between the canal and College.Avenue will be allowed
to discharge into the canal or be required to bypass it. The
canal discharges into Mail Creek immediately after leaving the
basin under consideration, and lends itself to use as a storm
drainageway both now and after development.
Harmony Road crosses the natural Swale at the intersection
.with Lemay Avenue. The existing drainage structure (a 45X29 inch
elliptical culvert) has a maximum capacity of 70 cfs before
innundating the intersection. This capacity of 70 cfs should
- 4 -
11
yt•i
�J.1-:iLiJ'1��11'Kiiir :, .L-!n: �..: •: ..:�+:. .!ai ,�' ter. .Y . �...♦ a. `{�:w•in('�N f'. wa,�fK..•�}1����y�y-
isi\v"i'.iJ 1'� � - .. .- .. ... %V':': i1♦ �' 1Wa�Yif+LOAS
equal the release amount of the entire basin to avoid flooding
the intersection or having to install additional expensive
culverts at the intersection.
In planning for development in the area of the natural swale
it should be recognized that storm waters have a natural easement
along this swale. Therefore, it should be used as a local storm
drainageway and as a location of major detention areas. A major
detention area is one that routes offsite water in addition to
detaining its own onsite water.
The proposed collector street, Boardwalk Drive, will also
intersect the valley thalweg. In order to size the culvert under
Boardwalk Drive to carry the two (2) year undeveloped runoff
quantity, a major detention area should be located upstream on
the 10 acre apartment site and the City Park site (basin #7 on
the map). The size of this detention area is greatly affected by
the drainage pattern of upstream lands.
IV. HARMONY RESERVOIR
Harmony Reservoir is a man-made irrigation reservoir owned'
and controlled by The Landings, Ltd. It is located on the northern
edge of the drainage basin at the downstream edge of the proposed
-5-
.T.: �:7 t.'. .. .r ... r... ...... - .. .. ... .[r".•.. ..•,^. .. wiY'r.r iiGi�2f�Y 3'Y
Landings Filing IV. It has a surface area of approximately 14
acres and a control gate outlet along the south side. Since the
lake is less than ten feet deep and holds less than 400 acre-feet
.of water,.state agencies will not be involved in the review of
proposed use or improvements for the reservoir.
Presently only 14 acres of land drain naturally into the
reservoir, and it.has no special spillway to discharge storm
water. After development,.the Landings Filing IV, Park Place, and
;j•
part of the City owned park will flow into the reservoir (basins
1 and 2 on the map). Undetained discharge from this 42 acres
of RLP zoned property during a 100 year, 3 hour storm would cause
the water level in the reservoir to rise eight inches. The only
structural improvement necessary to handle this flow would be a
t
spillway or other control device to ensure that the lake returns
to normal operating level. The design of the improvements should
occur prior to development of Whalers Cove downstream of the
reservoir.
If storm runoff is accepted from a portion of the South
College Properties in the northwesterly portion of the basin
?�! (basins 3 and 4 on the map) a portion of the lake surface will
be increased 20 inches during the design storm described above.
;6 _
u
':1:'�. ;ors:.:.. `� ,.. �',:��'.-�•;i/;._�Y�av :.h:�'Ji:ai�.Ci."w�i
Y Development in The Boardwalk and Landings, Filing IV, will
;y
establish the horizontal and vertical limits of the westerly
bank of the reservoir. Obviously building elevations will be
set above the maximum lake elevation. Final design of the develop-
tw
,•i
;.� ment adjacent to the reservoir and of the release structure will
dictate the operating and maximum lake water surface elevations.
However, the maximum water elevation will never reach the visible
limits of the historic high water level (5010t ft). Therefore,
major dam improvements should not be necessary.
V. HYDROLOGIC AND HYDRAULIC INVESTIGATION
:z
Refer to Appendix A for a map of the drainage subbasins and
a schematic master drainage plan showing probable flow patterns.
Subbasins shown on the map are defined with respect to one or
9i
more of the following: zoning, land ownership, topography, and
a}�
y. proposed improvements.
a5 An example of defining subbasins by property line is the
division between basins 2 and 6. Basin 2, owned by The Landings,
Ltd. is drained to Harmony Reservoir. Basin 6 drains to the
natural Swale.
A
Area 10 is a combination of Whalers Cove and a.church site.
a
?� Both are in the swale and cannot be separated because of the
existing topography.
- 7
,
Storm discharges for this report were calculated using the
Rational Method as explained in the "Methods" section of this
report. Discharges calculated for each subbasin include the 2
year and 100 year frequencies for both the developed .and unde-
veloped conditions. .See Appendix B for a list of release rates.
The 2 year discharges given here are to be used as guidelines
in sizing the release from the individual areas. In addition,
the runoff from a major storm of 60 minute duration was calcu-
lated for.basins 3-10. Hydrograph routing in the drainage swale
was necessary to determine detention volumes. A 60 minute storm
was chosen for this routing as it is usually the critical duration
in the accumulated runoff method. Finally, the discharge for
basin 6 was calculated for its present partially developed con-
dition.
All capacity calculations on culverts were based on the
HEC-5 Manual, "Hydraulic Charts for the Selection of Highway
Culverts." At the point of final .release, head was assumed to
build up to the point of overtopping Lemay Avenue.
To determine required detention volumes the Accumulated
Runoff Method was used. For Harmony Reservoir the release rate is
so extremely small that the line representing inflow and outflow
for the Accumulated Runoff Method were still diverging at. three
- B -
0
hours. Therefore, the entire volume of a 3-hour event was
considered to be retained. Impact on the reservoir is expressed
as the rise of the water surface in inches to store the entire
100 year storm runoff volume. The required size of detention
areas in the swale was determined by routing the controlled
release hydrographs of the upstream basin. Lag time was estimated
as one-half the time of concentration, assuming channelization
in the basin during development.. Detention calculations are
included in Appendix B.
Two (2) open grass -lined channels were designed to carry storm•
water flow. Minimum slope was assumed at one-half percent to
minimi.ze ponding of trickle flow. The channel cross sections were
selected using Manning's Formula applied to grassed channels,
as outlined in Open Channel Hydraulics, Chow, 1959. The size and
location of easements required for the master drainage plan as it
affects this project have been coordinated with the planner.
VI. SUMMARY AND RECOMMENDATIONS
The complexity and number of possible drainage patterns is
such that it is unrealistic to investigate only .one pattern.
Therefore, we chose to present five alternatives, demonstrate their
feasibility, and suggest what we feel is the most economical final
solution.
- 9-
SPI
Three alternatives are based on
the
existing capacity of
the culvert at Harmony and Lemay, and
vary
only in the upstream
drainage pattern. The first assumes
that
all developed storm
runoff uphill of the irrigation canal
be
allowed to flow into
the canal and immediately carried to
Mail
Creek. The second
:y
would consider all runoff by-passing
the
canal with areas 3 and
=ti
4 being directed toward Harmony Reservoir.
The third would have
runoff from all property above the canal
bypass it and flow into
the existing Swale.
.
:J
The last two alternatives assume the culvert at Lemay is
improved to carry the historic 2 year flood or overtopping of
Lemay is alowed to create the historic 2 year discharge out of
't the basin at that point. The alternatives accept the same upstream
i contributions as the second and third schemes described below.
Basically, the alternatives are either land intensive to
K� provide extra space for detention or capital intensive in the
amount of improvements required. These five alternatives are
listed in tabular form with the required improvements in Appendix A.
'f
The use of Harmony Reservoir as detention plays a major role
in the drainage system. As discussed, the maximum water elevation
,r
will be set by proposed construction near the lake's edge. The
.,y
10
o'W' .7t�Y,?T.•-�.•*:.G; 3�'} �e�i lti:+(.gallt ."�td7r'i';',zs:'c&V)7Wr+
i water line will be set somewhere between the present water
surface and the historic high water line. With the maximum
high water set, the operating elevation will be set at a lower
elevation equal to at least the rise expected from a major storm.
By setting the maximum high water mark below the historic high
water mark, required improvements to the dam at Harmony Reservoir
are minimal. Since cosmetic improvements will be applied to the
dam face, hydraulic improvements should be done at the same time
by controlling the composition and compaction of imported material
We recommend the following:
1. Larimer County. Irrigation Canal #2, due to its immediate
release into Mail Creek be used as a point of final
discharge for upstream storm waters. Development according
to City criteria will actually lessen the impact on the
canal, since a 100-year frequency storm would only release
an undeveloped 2 year runoff rate into the canal after.
upstream detention is provided.
2. The detention pond in Basin #7 be a phased construction
to be reduced from 7.8 ac-ft. to 1.0 ac-ft. at the time.
of development with.detention in Basin #6.
0
3. Whalers Cove provide 3.5
ac-ft. of
detention in addition
to providing an easement
to route
upstream water.
Should the reviewing parties
reject this
recommendation,
the cost of improvements and land
for this
recommendation could
be used as this project's developers least
expense in setting up
cost allocation.
12
t - ON -
Y}�
,r
' APPENDIX A
r'
n�
•'3
tta
x
Y3
;a
t Yt
ti 'I
in
n
FE
-17 r-7
LON
2, A' 5 a LJ
.-
Ip qp�P Pyppnsn`P� p�InPI nmI ➢,[�fP _ ^-WrPA�a �naimi��PnCno ��CffPO1m -0IadP�°1�Csr0P[PotIrTSCpnoor�
Sp
mi
pn pI � '� apOm n �� nC °n➢ Tim'r yn m J'
r AP�
P i N I P�Rr a{0 r 1 _Cq Ort AC h 0
' Z � � L Dry?� yai �n N� �n�j m[P �[ •+
f i P� na APP OT PPr LrP In p
din,P.X p rQuvrI 0
Pi RN N N N N C
L um ra L R P m 0 0 n
►a
m am m m m C C Pi
a Nm 1 > m rri m
i p �19 m n C O
I
' N
- C
rA ofln c.
a pi
0 R t1 z S
0 pi �l
- d
SCI [ 0
n {
• PO I C
P
r
�c r
p i pf n
C
i L Jp n
00 _ P
{F P o
P
n
I
m
P,
�y
TABLE OF REQUIRED IMPROVEMENTS
Alter-
natives
1
2
3
4
5
Item
Phase I det.
-
Area 7 (AC -FT)
7.8
6.6
7.3
6.3
5.7
Final det. Area
7 (AC -FT)
1 0
3.4
7.3
2.0
4.2
Det. vol.
Whalers Cove
3.4
4.7
3.9
1.0
1.3
(AC -FT) _
Culvert Under
27"
42"x27"
421lx27"
42"x27"
45"x29"
Boardwalk
Circular
Oval
Oval
Oval
Oval
Culvert Improve-
27" RCP
27" RCP
ment at Lemay
None
No
None
Added
Added r
Operating Elevation
of Harmony Resv.
07.0
07.0
07.0
05.8
05.8
r
Alternatives:
1) All water upstream of irrigation canal allowed in irrigation canal- �P
2) Areas 1-4 drain to Harmony Reservior, no.improvements to Lemay culvert
3) Areas 1 and 2 drain to Harmony Reservior, no improvements to Lemay culvert
4) Areas 1-4 drain to Harmony Reservior, Lemay culvert improved to carry 109 cfs
5) Areas 1 and 2 drain to Harmony Reservior, Lemay culvert improved to carry
109 cfs _
n
1E
DEVELOPMENT OF RATIONAL METHOD "HYDROGRAPH"
When the accumulated runoff method is used, the detention volume
for any given storm length is assumed to be total runoff for that storm
subtracted by release rate times storm duration. Given mathematically.
(K) 0100 T - (K) 0 rel T = V det kl�
r
with:
r 0 100 = 100 year runoff
0 rel = Actual release rate
T = Time duration of rainfall
V det = Required detention
K = Constant for unit conversion
' expressed as:
K 0 100 (2T) - K 0 rel (2T) = V det
We see the following straight line Hydrograph would give both the
rational method peak and the accumulated runoff detention volume.
t Q
4 rev
VP�t' Q,oO (LT..) �'i Q vel C;T-p)
TV T _ �z )
vv�9J � Q .ao v .
Q RQl
2 TP
}
RATIONAL METHOD DISCHARGES
�1
Area
Height
Length
Tc
Acreage
(ft)
(ft)
(min)
1
18
1400
11
41
2
10
900
10
21
3
12
800
10
15
4
18,
1100
10
20
w
j
5
22
2000
15
43
I
.6
20
2500
20
70
1
7
8
700
10
13
8
10
600
10
8
9
4
200
10
4
10
22
200`�
10
45
-
Total Basin
72
6060
34
269
�f
('
Present Basin
40
4800
34
191
i
Tiem of concentration
Formula. Tc = 0.0078
.O.77--0.385
r
%y
STORM DURATION = TC
100
Year
2
Year
Area
Developed
Undeveloped
Developed
Undeveloped
I
' CCf -
0
CCf
Q
I
C
Q
C
0
1
6.9
0.63
176;
0.38
106.
2.4
.50
49,'
.30
30
2
7.2
0.63
108
0.38
65
2.5
.50
30
.30
1$
3.
7.2
0.93
100
0.38
40
2.5
.74
28
.30
11
4
7.2
0.88
126
0.38
54
2.5
.70
35
.30
15
5
6.0
1.00
258
0.38
97
2.1
.95
86
.30
27
6
5.2
1.00
364
0.38
137
1.8
.81
102
.30
38
7
7.2 .,
0.63
59
0.38
35
2.5
.50
16
.30
10
„8
ta
7.2
0.63
36
0.38
22
2.5
.50
10
.30
6
9
7.2
0.63`
18
0.38
11
2.5
.50
5
.30
3
10
7.2
.0.63
203
0.38
122
2.5
.50
57
.30
34
Total
Basin
3.9
0.38
393
1.35
..30
` 109
Present
Basin
3.9
0.38
279
1.35
30
77,
Historic
Historic
6
5.2
0.63
228
.1.8
.50
63
STORM
DURATION
= 60 min
100
Year Developed
2.Year
Developed
E;. Area
I
CCf
0
I
C
0
3
2.6
.93_.
35
1.46
.74
16
. 4
2.6
.63
33
1.46
.50
15
5
2.6
1.00
112
1.46
.95
60
6
2.6
1.00
182
1.46
.81
83
7
2.6
.63
21.
1.46
.50
9
8
2.6
.63
13
1.46
.50
6
9
2.6.
.63
7
1.46
.50
3
10
2.6
.63
73
1.46
.50
33
CULVERT FLOWS
RELEASE FROM AREA #7
FLOW UNDER BOARDWALK DRIVE
�Soo�.Z
roo� s
GROOVE END PROTECTING
Ke = 0.2
ti99s
HEAD =. 103.2 - 98.2 =
4.0'
i
OUTLET SUBMERGED
'
USE OUTLET CONTROL
goo ` --•-i
ALTERNATIVE
DISCHARGE
CULVERT SELECTED
(CFS)
RE: HEC - 5
1
40
27
2
60
421lx27" w/ Orifice
3
70
42"x27"
4
65
42"x27" w/Oriface
5
80
45„x29 to
FINAL RELEASE POINT
CULVERT UNDER LEMAY & HARMONY
EXISTING 45"x29" OVAL
37
ALL MEASUREMENTS SHOWN
_
ARE FIELD SHOT
=
INLET CONTROL"
OUTLET CONTROL
13W/D = 1.82 Ke = 0.2
H
= 87.9 - 82.1 + 1.37 = 4.4'
ems.
L=240'
0 = 70 CFS
0 = 72 CFS
IMPROVEVE NT
0 = 39 CFS If = 4.4
L = 240
D = 27" ADDED OR REPLACE
W/ 53" x 34" OVAL
1
,
OPEN CHANNEL FLOW
SOUTH INLET TO
HARMONY RESERVOIR
CM ANN iS t_ �NtOCt1
ASSUMPTIONS:
Tov cr( —y
3:1 Side Slopes
"V" Channel
` Design 0 = 108 (,FS
Slope = 0:6% y
CHANNEL DIMENSIONS: oE.p.h
18' Top width Depth = 3.0
Freeboard = 2' Total width = 22'
A 27.0 SO FT
R P 18.9 1.43
V = 1.49 (R2/3)(-Sz) 1.49 (1.432/3 ) (.006;�) 4.07
n .036
VR = 5.82 n = 0.036
0 = 4.07 x 27.0 = 110 cfs
CHANNEL THROUGH
WHALERS COVE
ASSUMPTIONS:
f� Size for alternative with largest flow 0 design = 80
Slope = 0.4%
Use section described above
V = 1.49 (1.43 2/3) (0.0041) 3.32
0.036
- 0 = 3.32 z 27.0 = 90 cfs
REF: Chow,Open Channel Hydraulics
PG 179-185.
HYDROGRAP11 ROUTING THROUGH FOND IN AREA #7
ASSUMPTIONS
i
- 1)
60 MIN STORM ASSUMED
-
60 11IN STORM
USUALLY GIVES THE
CONTROL-
LING DETENTION
VOLUME
IN THE ACCUMULATED
RUNOFF METHOD
2)
2 YEAR HISTORIC
RELEASE RATE USED
FROM UPSTREAM DEVELOPED
3)
RUNOFF FROM EACH
BASIN
DELAYED
Tc IN PASSING THROUGH
DOWN
STREAM BASINS
TABLE OF
CONTROLLED
RELEASES
AFFECTING AREA #7
AREA
- DEVELOPED
Orel
Orel
Tp* Tend*
Tlag
a 100
0100
3
36
11
0.306
102 392
22
4
33
15
0.455
93 264.
17
5
112
27
0.241
106 298
10 '
6
182
38
0.209
107 574
0
7
21
21
1.00
60 120
0
* See
"Development
of Rational
Method
Hydrograph"
POND IN AREA #7 RECEIVING HISTORIC FLOW FROM AREA #6 AND DEVELOPED RUNOFF
FROM AREA #7.
12.5
10.0
E—•
U
7.5
2
n
2.5
30
60
90 120
TIME (MIN)
100 YEAR STORE^. DATA
T
I
A
CCf
0
V
20
5.20
63
.625
-270
7.43
30
4.18
83
.625
219
8.96
60 .
2.60.
83
.625
135
11.15
90
1.87
83
.625
97
12.03
120
1.43
83
.625
74
12.26
DISCHARGE/VOLUME
RELATIONSHIPS
0
DET VOL
60
6.6
65
6.3
70
6.1
oD
5.7
40
7.0
_I
COMPOSITE HYDROGRAPH
AREA 7 MAXIMUM
UPSTREAM DEVELOPMENT
NO WATER ACCEPTED BY
HARMONY.
120
�'— Volume detained = 4.2 AC -FT
N
80
Volume detained._ 7.3 AC -FT
W
x
40
0
_
-
Inflow
0
0
120 240 360
480 600
TIME (MIN)
COMPOSITE HYDROGRAPH
AREA 7 MAXIMUM
UPSTREAM DEVELOPMENT
AREAS 3-4 ACCEPTED BY
HARMONY
120
^
80
Volume detained = 2.0 AC -FT
r --Volume detained = 3.4 AC4T
l�
x
40
o
Inflow
f;
0
120 240 360
430 600
�J
TIME .(M'IN)
r
120
80
N
t.. H
Q 40
Va
120
80
c�
v
a
x
U
(
H-4
0
40
COMPOSITE. HYDROGRAPH AREA #10
AREA 3 & 4 ACCEPTED TO HARMONY•
RELEASE 104
120 240 360 480
TIME (MIN)
COMPOSITE HYDROGRAPH AREA #10
AREA 3 & 4.ACCEPTED Tn HARMnmy
:TIME (MIN)
1
• S
4C
0
120
-
80
G.
U
G]
U
'
fL
4
40
H-
120
120
COMPOSITE HYDROGRAPH AREA 010
NO WATER ACCEPTED TO HARMONY.
RELEASE 104 cfs.
240
TIME (MIN)
COMPOSITE HYDROGRAPH AREA #10
240
TIME (.MIN)
360 480
360
WE
AREA #7 DEVELOPED BASIN #6
ONLY UPSTREAM CONTRIBUTION '
With flat top hydrograph
a wide range of detention
/ volumes are possible.
N / Assign volume = 1 AC -FT
� 40 • /f
w< :
U
OC
. x
U
N
N
A
20
60 120 180 240
TIME (MIN)
AREA #10 NO RUNOFF ABOVE
120
G.
80
w
a
d
S
U
N
O
40
60 120 180 240
THE (MIN)