appendix c
APPENDIX C
Mr. Cameron Jeffs September 24, 2013 Page 5
Table 1. Predicted flood elevations at the Church Street Infill site, City of Half Moon Bay
Predicted Flood Elevation (ft, NGVD) 1-percent Dam Failure Combined Flood Flood Event Chance Flood
RAS Model Station
Location
10+80
Western portion of site
48.4
58.6
59.4
13+60
Central portion of site
48.8
58.7
59.5
16+95
Eastern portion of site
49.8
58.8
59.6
Notes: 1-percent chance flood = 3,580 cfs, dam failure flood = 16,000 cfs, combined flood event = 19,580 cfs. See modeling workmap (Figure 2) for cross-section locations.
•
1-percent flood event plus dam failure inundation. As discussed previously, an additional flood surface profile was generated for the highly unlikely coincident occurrence of peak dam failure discharge and 1-percent chance flood event. The peak flow values in this case were simply added together, yielding a peak discharge of 19,580 cfs. Although, this combined discharge is fully 22 percent higher than that for the dam failure scenario alone, the local topographic control of the downstream Highway 1 roadway limits the increase in flood elevations at the site. These are predicted to range from a low of 59.4 feet in the western portions of the site to a high of 59.6 feet at the eastern end. Thus, predicted flood elevations for this unlikely combined scenario are less than 1 foot higher than those for the dam failure scenario alone. Projected finished floor elevations are as high or higher than the predicted flood elevations.
•
Impacts of the Main Street bridge replacement. The HEC-RAS modeling results show conclusively that flood elevations at the project site will almost exclusively be set by conveyance at the Highway 1 crossing, both by the bridge there (for the 1-percent chance event) as well as over the roadway (for dam failure scenarios). This is consistent with the model runs which show that there would be essentially no change in anticipated flood elevations at the project site associated with replacement of the Main Street crossing. A new clear-span structure would have significantly more conveyance capacity than the existing structure, which would tend to better contain extreme flood events within the floodplain. However, with the crossing located over 600 feet upstream from the end of Church Street and the large available floodplain conveyance, there is ample space for flood flow velocities and elevations to adjust downstream of Main Street with the end result that changes at the latter location will not have a significant impact other than reducing the likelihood of flood flow breakouts in the downtown area.
213089 Letter Report Text 092413
Photo 1B. Existing Main Street Bridge from downstream.
Photo 1A (above). Typical channel segment upstream of Highway 1. Note the relatively open bankfull channel and heavily vegetated inner floodplain.
Photo 1C (right). View of Highway 1 bridge from upstream. Note north pier wall and also high conveyance capacity for flood flows on paved recreational trail.
Figure 1.
Photographs of key features along Pilarcitos Creek in the vicinity of the Church Street Infill project, City of Half Moon Bay © 2013 Balance Hydrologics, Inc.
1 Program PeakFq Ver. 5.2 11/01/2007
U. S. GEOLOGICAL SURVEY Annual peak flow frequency analysis following Bulletin 17-B Guidelines
Seq.000.000 Run Date / Time 08/21/2013 18:00
--- PROCESSING OPTIONS --Plot option Basin char output Print option Debug print Input peaks listing Input peaks format
= = = = = =
None None Yes No Long WATSTORE peak file
Input files used: peaks (ascii) - P:\2013\213089 320 CHURCH FLOOD HAZARD MODELING\213089 HYDROLOGY\PILARCITOS PKFQ specifications - PKFQWPSF.TMP Output file(s): main - P:\2013\213089 320 CHURCH FLOOD HAZARD MODELING\213089 HYDROLOGY\PILARCITOS PKFQ 1
Program PeakFq Ver. 5.2 11/01/2007
U. S. GEOLOGICAL SURVEY Annual peak flow frequency analysis following Bulletin 17-B Guidelines
Station - 11162630
I N P U T
PILARCITOS C A HALF MOON BAY CA
D A T A
S U M M A R Y
Number of peaks in record Peaks not used in analysis Systematic peaks in analysis Historic peaks in analysis Years of historic record Generalized skew Standard error Mean Square error Skew option Gage base discharge User supplied high outlier threshold User supplied low outlier criterion Plotting position parameter
********* *********
Seq.001.001 Run Date / Time 08/21/2013 18:00
= = = = = = = = = = = = =
46 0 46 0 0 -0.300 0.550 0.303 WEIGHTED 0.0 --0.00
NOTICE -- Preliminary machine computations. User responsible for assessment and interpretation.
WCF134I-NO SYSTEMATIC PEAKS WERE BELOW GAGE BASE. WCF198I-LOW OUTLIERS BELOW FLOOD BASE WERE DROPPED. 1 WCF163I-NO HIGH OUTLIERS OR HISTORIC PEAKS EXCEEDED HHBASE.
********* ********* 0.0 52.3 6383.0
1
Program PeakFq Ver. 5.2 11/01/2007
U. S. GEOLOGICAL SURVEY Annual peak flow frequency analysis following Bulletin 17-B Guidelines
1
Seq.001.002 Run Date / Time 08/21/2013 18:00
Station - 11162630
PILARCITOS C A HALF MOON BAY CA
ANNUAL FREQUENCY CURVE PARAMETERS -- LOG-PEARSON TYPE III FLOOD BASE LOGARITHMIC ---------------------- ------------------------------EXCEEDANCE STANDARD DISCHARGE PROBABILITY MEAN DEVIATION SKEW ------------------------------------------------------SYSTEMATIC RECORD 0.0 1.0000 2.7971 0.3942 -0.692 BULL.17B ESTIMATE 52.3 0.9783 2.8108 0.3662 -0.401
ANNUAL FREQUENCY CURVE -- DISCHARGES AT SELECTED EXCEEDANCE PROBABILITIES ANNUAL EXCEEDANCE PROBABILITY
BULL.17B ESTIMATE
0.9950 0.9900 0.9500 0.9000 0.8000 0.6667 0.5000 0.4292 0.2000 0.1000 0.0400 0.0200 0.0100 0.0050 0.0020
--147.8 213.1 324.9 472.1 684.2 792.9 1330.0 1826.0 2504.0 3035.0 3579.0 4135.0 4888.0
'EXPECTED 95-PCT CONFIDENCE LIMITS SYSTEMATIC PROBABILITY' FOR BULL. 17B ESTIMATES RECORD ESTIMATE LOWER UPPER 33.9 48.6 120.5 187.0 305.8 465.4 695.3 811.8 1364.0 1837.0 2428.0 2849.0 3247.0 3624.0 4089.0
--139.9 206.2 319.8 469.2 684.2 794.6 1347.0 1869.0 2604.0 3196.0 3822.0 4481.0 5403.0
--100.9 154.5 249.9 377.0 556.7 646.2 1065.0 1428.0 1899.0 2253.0 2607.0 2962.0 3430.0
--197.5 274.4 404.9 579.8 843.8 985.2 1737.0 2492.0 3593.0 4498.0 5462.0 6478.0 7897.0
1
Program PeakFq Ver. 5.2 11/01/2007
U. S. GEOLOGICAL SURVEY Annual peak flow frequency analysis following Bulletin 17-B Guidelines
Station - 11162630
I N P U T
WATER YEAR
DISCHARGE
1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978
1020.0 1290.0 594.0 1110.0 521.0 48.0 878.0 722.0 530.0 99.0 199.0 580.0
PILARCITOS C A HALF MOON BAY CA
D A T A
CODES
L I S T I N G
WATER YEAR
DISCHARGE
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001
94.0 338.0 1200.0 1010.0 129.0 908.0 1000.0 1970.0 1950.0 729.0 1250.0 194.0
2
Seq.001.003 Run Date / Time 08/21/2013 18:00
CODES
1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989
393.0 997.0 221.0 4750.0 2500.0 345.0 744.0 2240.0 742.0 399.0 730.0
2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012
621.0 824.0 1240.0 492.0 1080.0 191.0 769.0 727.0 571.0 1100.0 630.0
Explanation of peak discharge qualification codes PeakFQ CODE
NWIS CODE
D G X L K H
3 8 3+8 4 6 OR C 7
-
DEFINITION Dam failure, non-recurrent flow anomaly Discharge greater than stated value Both of the above Discharge less than stated value Known effect of regulation or urbanization Historic peak
Minus-flagged discharge -- Not used in computation -8888.0 -- No discharge value given Minus-flagged water year -- Historic peak used in computation
1
Program PeakFq Ver. 5.2 11/01/2007
U. S. GEOLOGICAL SURVEY Annual peak flow frequency analysis following Bulletin 17-B Guidelines
Station - 11162630
PILARCITOS C A HALF MOON BAY CA
EMPIRICAL FREQUENCY CURVES -- WEIBULL PLOTTING POSITIONS WATER YEAR
RANKED DISCHARGE
SYSTEMATIC RECORD
BULL.17B ESTIMATE
1982 1983 1986 1997 1998 1968 2000 2004 1992 1970 2011 2006 1967 1993 1996 1980 1995
4750.0 2500.0 2240.0 1970.0 1950.0 1290.0 1250.0 1240.0 1200.0 1110.0 1100.0 1080.0 1020.0 1010.0 1000.0 997.0 908.0
0.0213 0.0426 0.0638 0.0851 0.1064 0.1277 0.1489 0.1702 0.1915 0.2128 0.2340 0.2553 0.2766 0.2979 0.3191 0.3404 0.3617
0.0213 0.0426 0.0638 0.0851 0.1064 0.1277 0.1489 0.1702 0.1915 0.2128 0.2340 0.2553 0.2766 0.2979 0.3191 0.3404 0.3617
3
Seq.001.004 Run Date / Time 08/21/2013 18:00
1973 2003 2008 1985 1987 1989 1999 2009 1974 2012 2002 1969 1978 2010 1975 1971 2005 1988 1979 1984 1991 1981 1977 2001 2007 1994 1976 1990 1972
878.0 824.0 769.0 744.0 742.0 730.0 729.0 727.0 722.0 630.0 621.0 594.0 580.0 571.0 530.0 521.0 492.0 399.0 393.0 345.0 338.0 221.0 199.0 194.0 191.0 129.0 99.0 94.0 48.0
0.3830 0.4043 0.4255 0.4468 0.4681 0.4894 0.5106 0.5319 0.5532 0.5745 0.5957 0.6170 0.6383 0.6596 0.6809 0.7021 0.7234 0.7447 0.7660 0.7872 0.8085 0.8298 0.8511 0.8723 0.8936 0.9149 0.9362 0.9574 0.9787
0.3830 0.4043 0.4255 0.4468 0.4681 0.4894 0.5106 0.5319 0.5532 0.5745 0.5957 0.6170 0.6383 0.6596 0.6809 0.7021 0.7234 0.7447 0.7660 0.7872 0.8085 0.8298 0.8511 0.8723 0.8936 0.9149 0.9362 0.9574 0.9787
1
End PeakFQ analysis. Stations processed Number of errors Stations skipped Station years
: : : :
1 0 0 46
Data records may have been ignored for the stations listed below. (Card type must be Y, Z, N, H, I, 2, 3, 4, or *.) (2, 4, and * records are ignored.) For the station below, the following records were ignored: FINISHED PROCESSING STATION:
11162630
USGS PILARCITOS C A HALF MOON BAY
For the station below, the following records were ignored: FINISHED PROCESSING STATION:
4
it f 4?f
*4
S. '4-
TV . . 3
to
1«L
> A
-5 T
•"'i .•. • ha
1 - V
>t ». I I ^ C L Contract# jftf l6./
S e c t i o n .
i
'
~
£ Z t t >
[
.
.„^..A^.t6j&.*;7
- 0,01
.^.t..
> /
:
. . .
;
77
Struck $
leytL
- 3t*& ..
P.*°l
c
- 7/ 5 ^ •
;
p./
Str4ck 1
5gcfr'oy 6...
_
t
_
Time
•
Section 1
1
..;.-L..i
0
=
t
; o\f
t
0
0
• ~
^
(
\
0>/
s;
9-42-
1
W
i.. ......
.
;.ojj
4
~
.
1 ^o^-c
=
,
"
;
.
__ :
----_
1
d ' 7«r .
SfceUk.4.
...J
.
s
. .r4^&r.2#*>
2-9^
.
!
o.|
-
- S e c t i o n to
Str^tck. 5 l
- y ^
L j ^ j ' f ^ v . - ^.crro
17 :_
:
.
,
S^cjiov^ ( 1 _
;
l i m e
. . j . ...1.
!^rJt..M/t
kok-M..:,
:
.;
.....
.. , J...
4 -
•—1
fd
i ; i
r
-t
r
-
C^/cnlsiio^s
of
/riunc/sfion
To J>e. usee/ iri Pt.tyuncfy'Oto M>i$i "/ft*. io& rkif i cj efirjautt Wj* e f . ret 'h>3. «2fe?*»
'
1
4VZJ1U-ZOH
fT,
i
4-
' T
:1 .
H
I f
:1
r
i-
t
V A | _ L .
1
f— —'r
J
- o
4-
~ D
i-l
m •!-r
'
j:
r r .1
J
I
A.
•
L
.
L
. (.
.. i : 1
Ji-
11
4 • I
f-1
f
TT
ll
1
J
.
Section f
/js?*™*.
D = 2o
Assise . 1) " Z i .
USE-
%
,
T> = 15-' ,
,
.
L=
.
Xirr^jc'
cU ^
_.
Ass^y^e
w
se
£k\J.
Cfoss~S4cfl
0
^
-provH ord^s-_^ cttb ^
5^5
L =
L-
2 6_r
=: /£>
L= 2 f i o
,. 3> —*=» .
. . L=_*£o
J).^'
fro^
_ L= ^ J - S *
L^^fS
d - J _ rr /i^-S"
use
ZjO.
,L,.ro'
E/et/,
C f J
1
^
Cws.s-secflo ^ )
^kv/,
34-1
f
^
. use
L=
T)=.2Jc'.
45S^e
S e c t Q t t . 4 -
*
T :
5
220
1
_b - a s '
_J=}O
d^ ^-- IS
Use
7«r
L
^
3|
ab.^jr
R
_
4: /\ssuuv^e.
d ~ s^s"
. Use,
P-lo
e{/
Cf^fk).
6 f a p O
0
L=4&o
L
,c n : r 0
-- -
l>r]J
'&eM.Jf£_
--4So
^ - jo
„
E/ [
}
' ^ / J T
. 4-Eo
2> -/_r '
Use?
T e r r a s - 5 ^ C ^ ' O H )
- -
...
C^jjU-)"
•
- L~S~6o
L^^tcro
gfes/.
)lS
. .
8
Secivow H
^sfw*^
U S E
/_ ~ | Q
^©-o
D M O
Caress-£*c1\ov.)
L -
l ^ o
£57-V,
B> o
HEC-RAS Plan: Plan 02 River: Pilarcitos Creek Reach: Church Street Reach
River Sta
Profile
Q Total
Min Ch El
W.S. Elev
Crit W.S.
E.G. Elev
E.G. Slope
Vel Chnl
Flow Area
Top Width
(cfs)
(ft)
(ft)
(ft)
(ft)
(ft/ft)
(ft/s)
(sq ft)
(ft)
Froude # Chl
Church Street
2520
PF 1
3580.00
43.40
57.30
57.37
0.000366
2.70
1949.08
219.31
0.13
Church Street
2520
PF 2
16000.00
43.40
69.27
69.40
0.000270
3.53
5844.94
430.50
0.12
Church Street
2520
PF 3
19580.00
43.40
70.18
70.35
0.000324
3.96
6235.79
430.50
0.14
Church Street
2400
PF 1
3580.00
42.70
56.23
51.12
57.17
0.004573
9.34
957.88
149.23
0.45
Church Street
2400
PF 2
16000.00
42.70
68.94
61.85
69.32
0.001309
7.83
4393.69
360.80
0.27
Church Street
2400
PF 3
19580.00
42.70
69.79
64.95
70.25
0.001485
8.53
4700.98
360.80
0.29
Church Street
2360
Church Street
2296
PF 1
3580.00
42.20
54.35
51.87
56.15
0.010709
12.98
726.31
163.43
0.67
Church Street
2296
PF 2
16000.00
42.20
63.00
63.00
64.01
0.004813
12.62
2868.45
285.10
0.49
Church Street
2296
PF 3
19580.00
42.20
63.00
63.00
64.51
0.007209
15.44
2868.45
285.10
0.61
Church Street
2150
PF 1
3580.00
41.60
53.90
54.84
0.004952
9.20
996.58
137.57
0.47
Church Street
2150
PF 2
16000.00
41.60
60.68
62.01
0.006255
13.90
2600.70
292.30
0.56
Church Street
2150
PF 3
19580.00
41.60
61.69
63.08
0.006359
14.51
2894.23
292.30
0.57
Church Street
2060
PF 1
3580.00
41.20
52.96
54.28
0.006827
10.32
706.64
138.88
0.54
Church Street
2060
PF 2
16000.00
41.20
60.59
61.46
0.003715
10.73
2588.54
325.10
0.43
Church Street
2060
PF 3
19580.00
41.20
61.59
62.52
0.003713
11.10
2912.98
325.10
0.44
Church Street
1885
PF 1
3580.00
40.30
51.06
52.78
0.010613
11.70
813.33
199.98
0.65
Church Street
1885
PF 2
16000.00
40.30
59.20
60.56
0.006726
13.89
3030.67
367.14
0.58
Church Street
1885
PF 3
19580.00
40.30
60.02
61.58
0.007527
15.13
3344.96
397.20
0.61
Church Street
1695
PF 1
3580.00
39.90
49.76
50.81
0.008316
10.35
1098.93
201.51
0.58
Church Street
1695
PF 2
16000.00
39.90
58.82
59.42
0.003588
10.51
3840.13
451.95
0.43
Church Street
1695
PF 3
19580.00
39.90
59.63
60.31
0.003949
11.34
4220.84
494.51
0.45
Church Street
1504
PF 1
3580.00
39.20
48.88
49.41
0.005381
8.07
1236.71
270.70
0.46
Church Street
1504
PF 2
16000.00
39.20
58.74
58.95
0.001195
6.14
5266.32
647.60
0.25
Church Street
1504
PF 3
19580.00
39.20
59.52
59.77
0.001377
6.77
5795.87
697.67
0.27
Church Street
1360
PF 1
3580.00
37.90
48.82
48.91
0.001200
4.21
2425.29
418.94
0.22
Church Street
1360
PF 2
16000.00
37.90
58.69
58.80
0.000420
3.83
7871.50
739.20
0.15
Church Street
1360
PF 3
19580.00
37.90
59.47
59.60
0.000487
4.23
8445.48
739.20
0.16
Church Street
1260
PF 1
3580.00
37.60
48.67
44.65
48.77
0.000981
3.80
2526.92
492.48
0.20
Church Street
1260
PF 2
16000.00
37.60
58.65
48.69
58.74
0.000329
3.40
8894.35
821.13
0.13
Bridge
49.74
HEC-RAS Plan: Plan 02 River: Pilarcitos Creek Reach: Church Street (Continued) Reach
River Sta
Profile
Q Total
Min Ch El
W.S. Elev
Crit W.S.
E.G. Elev
E.G. Slope
Vel Chnl
Flow Area
Top Width
(cfs)
(ft)
(ft)
(ft)
(ft)
(ft/ft)
(ft/s)
(sq ft)
(ft)
19580.00
37.60
59.42
49.30
59.54
0.000393
3.81
9528.13
824.49
Froude # Chl
Church Street
1260
PF 3
0.14
Church Street
1080
PF 1
3580.00
37.10
48.39
44.34
48.55
0.001811
5.26
2385.95
491.37
0.28
Church Street
1080
PF 2
16000.00
37.10
58.61
49.05
58.68
0.000383
3.73
9017.91
808.09
0.14
Church Street
1080
PF 3
19580.00
37.10
59.37
49.75
59.46
0.000473
4.24
9643.98
832.20
0.16
Church Street
925
PF 1
3580.00
36.50
47.88
43.23
48.21
0.002152
5.54
916.61
140.02
0.30
Church Street
925
PF 2
16000.00
36.50
58.31
49.81
58.57
0.000899
5.65
4303.24
709.97
0.22
Church Street
925
PF 3
19580.00
36.50
59.04
51.31
59.33
0.000939
5.91
4836.49
733.70
0.22
Church Street
850
Church Street
805
PF 1
3580.00
36.20
46.02
46.66
0.003963
6.70
843.90
133.84
0.40
Church Street
805
PF 2
16000.00
36.20
50.06
50.06
55.38
0.020917
19.73
1441.37
162.20
0.97
Church Street
805
PF 3
19580.00
36.20
52.01
52.01
57.57
0.018591
20.42
1774.66
179.28
0.93
Church Street
730
PF 1
3580.00
36.00
45.93
46.29
0.003624
6.70
1258.91
175.79
0.38
Church Street
730
PF 2
16000.00
36.00
51.30
52.55
0.008467
13.79
2554.27
316.49
0.63
Church Street
730
PF 3
19580.00
36.00
52.13
53.51
0.008967
14.72
2823.05
330.15
0.65
Church Street
595
PF 1
3580.00
35.00
45.83
45.93
0.001367
4.44
2377.44
343.32
0.24
Church Street
595
PF 2
16000.00
35.00
51.54
51.76
0.001950
7.05
5147.29
508.13
0.31
Church Street
595
PF 3
19580.00
35.00
52.39
52.66
0.002153
7.67
5579.68
509.74
0.33
Church Street
450
PF 1
3580.00
33.80
45.66
45.76
0.000979
4.01
2825.50
370.67
0.21
Church Street
450
PF 2
16000.00
33.80
51.15
51.45
0.002232
7.81
5333.64
475.95
0.33
Church Street
450
PF 3
19580.00
33.80
51.94
52.30
0.002593
8.67
5708.10
477.67
0.36
Church Street
360
PF 1
3580.00
33.40
45.61
45.67
0.000733
3.53
2760.36
388.38
0.18
Church Street
360
PF 2
16000.00
33.40
51.02
51.24
0.001834
7.15
5370.08
537.88
0.30
Church Street
360
PF 3
19580.00
33.40
51.79
52.06
0.002141
7.95
5787.99
553.35
0.33
Church Street
204
PF 1
3580.00
32.20
44.79
39.97
45.39
0.003902
8.21
1602.68
273.16
0.41
Church Street
204
PF 2
16000.00
32.20
50.22
48.39
50.80
0.003908
10.50
4155.68
627.33
0.44
Church Street
204
PF 3
19580.00
32.20
50.98
48.94
51.58
0.003905
10.79
4634.40
635.97
0.44
Bridge
Mr. Cameron Jeffs September 24, 2013 Page 5
Table 1. Predicted flood elevations at the Church Street Infill site, City of Half Moon Bay
Predicted Flood Elevation (ft, NGVD) 1-percent Dam Failure Combined Flood Flood Event Chance Flood
RAS Model Station
Location
10+80
Western portion of site
48.4
58.6
59.4
13+60
Central portion of site
48.8
58.7
59.5
16+95
Eastern portion of site
49.8
58.8
59.6
Notes: 1-percent chance flood = 3,580 cfs, dam failure flood = 16,000 cfs, combined flood event = 19,580 cfs. See modeling workmap (Figure 2) for cross-section locations.
•
1-percent flood event plus dam failure inundation. As discussed previously, an additional flood surface profile was generated for the highly unlikely coincident occurrence of peak dam failure discharge and 1-percent chance flood event. The peak flow values in this case were simply added together, yielding a peak discharge of 19,580 cfs. Although, this combined discharge is fully 22 percent higher than that for the dam failure scenario alone, the local topographic control of the downstream Highway 1 roadway limits the increase in flood elevations at the site. These are predicted to range from a low of 59.4 feet in the western portions of the site to a high of 59.6 feet at the eastern end. Thus, predicted flood elevations for this unlikely combined scenario are less than 1 foot higher than those for the dam failure scenario alone. Projected finished floor elevations are as high or higher than the predicted flood elevations.
•
Impacts of the Main Street bridge replacement. The HEC-RAS modeling results show conclusively that flood elevations at the project site will almost exclusively be set by conveyance at the Highway 1 crossing, both by the bridge there (for the 1-percent chance event) as well as over the roadway (for dam failure scenarios). This is consistent with the model runs which show that there would be essentially no change in anticipated flood elevations at the project site associated with replacement of the Main Street crossing. A new clear-span structure would have significantly more conveyance capacity than the existing structure, which would tend to better contain extreme flood events within the floodplain. However, with the crossing located over 600 feet upstream from the end of Church Street and the large available floodplain conveyance, there is ample space for flood flow velocities and elevations to adjust downstream of Main Street with the end result that changes at the latter location will not have a significant impact other than reducing the likelihood of flood flow breakouts in the downtown area.
213089 Letter Report Text 092413
Photo 1B. Existing Main Street Bridge from downstream.
Photo 1A (above). Typical channel segment upstream of Highway 1. Note the relatively open bankfull channel and heavily vegetated inner floodplain.
Photo 1C (right). View of Highway 1 bridge from upstream. Note north pier wall and also high conveyance capacity for flood flows on paved recreational trail.
Figure 1.
Photographs of key features along Pilarcitos Creek in the vicinity of the Church Street Infill project, City of Half Moon Bay © 2013 Balance Hydrologics, Inc.
1 Program PeakFq Ver. 5.2 11/01/2007
U. S. GEOLOGICAL SURVEY Annual peak flow frequency analysis following Bulletin 17-B Guidelines
Seq.000.000 Run Date / Time 08/21/2013 18:00
--- PROCESSING OPTIONS --Plot option Basin char output Print option Debug print Input peaks listing Input peaks format
= = = = = =
None None Yes No Long WATSTORE peak file
Input files used: peaks (ascii) - P:\2013\213089 320 CHURCH FLOOD HAZARD MODELING\213089 HYDROLOGY\PILARCITOS PKFQ specifications - PKFQWPSF.TMP Output file(s): main - P:\2013\213089 320 CHURCH FLOOD HAZARD MODELING\213089 HYDROLOGY\PILARCITOS PKFQ 1
Program PeakFq Ver. 5.2 11/01/2007
U. S. GEOLOGICAL SURVEY Annual peak flow frequency analysis following Bulletin 17-B Guidelines
Station - 11162630
I N P U T
PILARCITOS C A HALF MOON BAY CA
D A T A
S U M M A R Y
Number of peaks in record Peaks not used in analysis Systematic peaks in analysis Historic peaks in analysis Years of historic record Generalized skew Standard error Mean Square error Skew option Gage base discharge User supplied high outlier threshold User supplied low outlier criterion Plotting position parameter
********* *********
Seq.001.001 Run Date / Time 08/21/2013 18:00
= = = = = = = = = = = = =
46 0 46 0 0 -0.300 0.550 0.303 WEIGHTED 0.0 --0.00
NOTICE -- Preliminary machine computations. User responsible for assessment and interpretation.
WCF134I-NO SYSTEMATIC PEAKS WERE BELOW GAGE BASE. WCF198I-LOW OUTLIERS BELOW FLOOD BASE WERE DROPPED. 1 WCF163I-NO HIGH OUTLIERS OR HISTORIC PEAKS EXCEEDED HHBASE.
********* ********* 0.0 52.3 6383.0
1
Program PeakFq Ver. 5.2 11/01/2007
U. S. GEOLOGICAL SURVEY Annual peak flow frequency analysis following Bulletin 17-B Guidelines
1
Seq.001.002 Run Date / Time 08/21/2013 18:00
Station - 11162630
PILARCITOS C A HALF MOON BAY CA
ANNUAL FREQUENCY CURVE PARAMETERS -- LOG-PEARSON TYPE III FLOOD BASE LOGARITHMIC ---------------------- ------------------------------EXCEEDANCE STANDARD DISCHARGE PROBABILITY MEAN DEVIATION SKEW ------------------------------------------------------SYSTEMATIC RECORD 0.0 1.0000 2.7971 0.3942 -0.692 BULL.17B ESTIMATE 52.3 0.9783 2.8108 0.3662 -0.401
ANNUAL FREQUENCY CURVE -- DISCHARGES AT SELECTED EXCEEDANCE PROBABILITIES ANNUAL EXCEEDANCE PROBABILITY
BULL.17B ESTIMATE
0.9950 0.9900 0.9500 0.9000 0.8000 0.6667 0.5000 0.4292 0.2000 0.1000 0.0400 0.0200 0.0100 0.0050 0.0020
--147.8 213.1 324.9 472.1 684.2 792.9 1330.0 1826.0 2504.0 3035.0 3579.0 4135.0 4888.0
'EXPECTED 95-PCT CONFIDENCE LIMITS SYSTEMATIC PROBABILITY' FOR BULL. 17B ESTIMATES RECORD ESTIMATE LOWER UPPER 33.9 48.6 120.5 187.0 305.8 465.4 695.3 811.8 1364.0 1837.0 2428.0 2849.0 3247.0 3624.0 4089.0
--139.9 206.2 319.8 469.2 684.2 794.6 1347.0 1869.0 2604.0 3196.0 3822.0 4481.0 5403.0
--100.9 154.5 249.9 377.0 556.7 646.2 1065.0 1428.0 1899.0 2253.0 2607.0 2962.0 3430.0
--197.5 274.4 404.9 579.8 843.8 985.2 1737.0 2492.0 3593.0 4498.0 5462.0 6478.0 7897.0
1
Program PeakFq Ver. 5.2 11/01/2007
U. S. GEOLOGICAL SURVEY Annual peak flow frequency analysis following Bulletin 17-B Guidelines
Station - 11162630
I N P U T
WATER YEAR
DISCHARGE
1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978
1020.0 1290.0 594.0 1110.0 521.0 48.0 878.0 722.0 530.0 99.0 199.0 580.0
PILARCITOS C A HALF MOON BAY CA
D A T A
CODES
L I S T I N G
WATER YEAR
DISCHARGE
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001
94.0 338.0 1200.0 1010.0 129.0 908.0 1000.0 1970.0 1950.0 729.0 1250.0 194.0
2
Seq.001.003 Run Date / Time 08/21/2013 18:00
CODES
1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989
393.0 997.0 221.0 4750.0 2500.0 345.0 744.0 2240.0 742.0 399.0 730.0
2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012
621.0 824.0 1240.0 492.0 1080.0 191.0 769.0 727.0 571.0 1100.0 630.0
Explanation of peak discharge qualification codes PeakFQ CODE
NWIS CODE
D G X L K H
3 8 3+8 4 6 OR C 7
-
DEFINITION Dam failure, non-recurrent flow anomaly Discharge greater than stated value Both of the above Discharge less than stated value Known effect of regulation or urbanization Historic peak
Minus-flagged discharge -- Not used in computation -8888.0 -- No discharge value given Minus-flagged water year -- Historic peak used in computation
1
Program PeakFq Ver. 5.2 11/01/2007
U. S. GEOLOGICAL SURVEY Annual peak flow frequency analysis following Bulletin 17-B Guidelines
Station - 11162630
PILARCITOS C A HALF MOON BAY CA
EMPIRICAL FREQUENCY CURVES -- WEIBULL PLOTTING POSITIONS WATER YEAR
RANKED DISCHARGE
SYSTEMATIC RECORD
BULL.17B ESTIMATE
1982 1983 1986 1997 1998 1968 2000 2004 1992 1970 2011 2006 1967 1993 1996 1980 1995
4750.0 2500.0 2240.0 1970.0 1950.0 1290.0 1250.0 1240.0 1200.0 1110.0 1100.0 1080.0 1020.0 1010.0 1000.0 997.0 908.0
0.0213 0.0426 0.0638 0.0851 0.1064 0.1277 0.1489 0.1702 0.1915 0.2128 0.2340 0.2553 0.2766 0.2979 0.3191 0.3404 0.3617
0.0213 0.0426 0.0638 0.0851 0.1064 0.1277 0.1489 0.1702 0.1915 0.2128 0.2340 0.2553 0.2766 0.2979 0.3191 0.3404 0.3617
3
Seq.001.004 Run Date / Time 08/21/2013 18:00
1973 2003 2008 1985 1987 1989 1999 2009 1974 2012 2002 1969 1978 2010 1975 1971 2005 1988 1979 1984 1991 1981 1977 2001 2007 1994 1976 1990 1972
878.0 824.0 769.0 744.0 742.0 730.0 729.0 727.0 722.0 630.0 621.0 594.0 580.0 571.0 530.0 521.0 492.0 399.0 393.0 345.0 338.0 221.0 199.0 194.0 191.0 129.0 99.0 94.0 48.0
0.3830 0.4043 0.4255 0.4468 0.4681 0.4894 0.5106 0.5319 0.5532 0.5745 0.5957 0.6170 0.6383 0.6596 0.6809 0.7021 0.7234 0.7447 0.7660 0.7872 0.8085 0.8298 0.8511 0.8723 0.8936 0.9149 0.9362 0.9574 0.9787
0.3830 0.4043 0.4255 0.4468 0.4681 0.4894 0.5106 0.5319 0.5532 0.5745 0.5957 0.6170 0.6383 0.6596 0.6809 0.7021 0.7234 0.7447 0.7660 0.7872 0.8085 0.8298 0.8511 0.8723 0.8936 0.9149 0.9362 0.9574 0.9787
1
End PeakFQ analysis. Stations processed Number of errors Stations skipped Station years
: : : :
1 0 0 46
Data records may have been ignored for the stations listed below. (Card type must be Y, Z, N, H, I, 2, 3, 4, or *.) (2, 4, and * records are ignored.) For the station below, the following records were ignored: FINISHED PROCESSING STATION:
11162630
USGS PILARCITOS C A HALF MOON BAY
For the station below, the following records were ignored: FINISHED PROCESSING STATION:
4
it f 4?f
*4
S. '4-
TV . . 3
to
1«L
> A
-5 T
•"'i .•. • ha
1 - V
>t ». I I ^ C L Contract# jftf l6./
S e c t i o n .
i
'
~
£ Z t t >
[
.
.„^..A^.t6j&.*;7
- 0,01
.^.t..
> /
:
. . .
;
77
Struck $
leytL
- 3t*& ..
P.*°l
c
- 7/ 5 ^ •
;
p./
Str4ck 1
5gcfr'oy 6...
_
t
_
Time
•
Section 1
1
..;.-L..i
0
=
t
; o\f
t
0
0
• ~
^
(
\
0>/
s;
9-42-
1
W
i.. ......
.
;.ojj
4
~
.
1 ^o^-c
=
,
"
;
.
__ :
----_
1
d ' 7«r .
SfceUk.4.
...J
.
s
. .r4^&r.2#*>
2-9^
.
!
o.|
-
- S e c t i o n to
Str^tck. 5 l
- y ^
L j ^ j ' f ^ v . - ^.crro
17 :_
:
.
,
S^cjiov^ ( 1 _
;
l i m e
. . j . ...1.
!^rJt..M/t
kok-M..:,
:
.;
.....
.. , J...
4 -
•—1
fd
i ; i
r
-t
r
-
C^/cnlsiio^s
of
/riunc/sfion
To J>e. usee/ iri Pt.tyuncfy'Oto M>i$i "/ft*. io& rkif i cj efirjautt Wj* e f . ret 'h>3. «2fe?*»
'
1
4VZJ1U-ZOH
fT,
i
4-
' T
:1 .
H
I f
:1
r
i-
t
V A | _ L .
1
f— —'r
J
- o
4-
~ D
i-l
m •!-r
'
j:
r r .1
J
I
A.
•
L
.
L
. (.
.. i : 1
Ji-
11
4 • I
f-1
f
TT
ll
1
J
.
Section f
/js?*™*.
D = 2o
Assise . 1) " Z i .
USE-
%
,
T> = 15-' ,
,
.
L=
.
Xirr^jc'
cU ^
_.
Ass^y^e
w
se
£k\J.
Cfoss~S4cfl
0
^
-provH ord^s-_^ cttb ^
5^5
L =
L-
2 6_r
=: /£>
L= 2 f i o
,. 3> —*=» .
. . L=_*£o
J).^'
fro^
_ L= ^ J - S *
L^^fS
d - J _ rr /i^-S"
use
ZjO.
,L,.ro'
E/et/,
C f J
1
^
Cws.s-secflo ^ )
^kv/,
34-1
f
^
. use
L=
T)=.2Jc'.
45S^e
S e c t Q t t . 4 -
*
T :
5
220
1
_b - a s '
_J=}O
d^ ^-- IS
Use
7«r
L
^
3|
ab.^jr
R
_
4: /\ssuuv^e.
d ~ s^s"
. Use,
P-lo
e{/
Cf^fk).
6 f a p O
0
L=4&o
L
,c n : r 0
-- -
l>r]J
'&eM.Jf£_
--4So
^ - jo
„
E/ [
}
' ^ / J T
. 4-Eo
2> -/_r '
Use?
T e r r a s - 5 ^ C ^ ' O H )
- -
...
C^jjU-)"
•
- L~S~6o
L^^tcro
gfes/.
)lS
. .
8
Secivow H
^sfw*^
U S E
/_ ~ | Q
^©-o
D M O
Caress-£*c1\ov.)
L -
l ^ o
£57-V,
B> o
HEC-RAS Plan: Plan 02 River: Pilarcitos Creek Reach: Church Street Reach
River Sta
Profile
Q Total
Min Ch El
W.S. Elev
Crit W.S.
E.G. Elev
E.G. Slope
Vel Chnl
Flow Area
Top Width
(cfs)
(ft)
(ft)
(ft)
(ft)
(ft/ft)
(ft/s)
(sq ft)
(ft)
Froude # Chl
Church Street
2520
PF 1
3580.00
43.40
57.30
57.37
0.000366
2.70
1949.08
219.31
0.13
Church Street
2520
PF 2
16000.00
43.40
69.27
69.40
0.000270
3.53
5844.94
430.50
0.12
Church Street
2520
PF 3
19580.00
43.40
70.18
70.35
0.000324
3.96
6235.79
430.50
0.14
Church Street
2400
PF 1
3580.00
42.70
56.23
51.12
57.17
0.004573
9.34
957.88
149.23
0.45
Church Street
2400
PF 2
16000.00
42.70
68.94
61.85
69.32
0.001309
7.83
4393.69
360.80
0.27
Church Street
2400
PF 3
19580.00
42.70
69.79
64.95
70.25
0.001485
8.53
4700.98
360.80
0.29
Church Street
2360
Church Street
2296
PF 1
3580.00
42.20
54.35
51.87
56.15
0.010709
12.98
726.31
163.43
0.67
Church Street
2296
PF 2
16000.00
42.20
63.00
63.00
64.01
0.004813
12.62
2868.45
285.10
0.49
Church Street
2296
PF 3
19580.00
42.20
63.00
63.00
64.51
0.007209
15.44
2868.45
285.10
0.61
Church Street
2150
PF 1
3580.00
41.60
53.90
54.84
0.004952
9.20
996.58
137.57
0.47
Church Street
2150
PF 2
16000.00
41.60
60.68
62.01
0.006255
13.90
2600.70
292.30
0.56
Church Street
2150
PF 3
19580.00
41.60
61.69
63.08
0.006359
14.51
2894.23
292.30
0.57
Church Street
2060
PF 1
3580.00
41.20
52.96
54.28
0.006827
10.32
706.64
138.88
0.54
Church Street
2060
PF 2
16000.00
41.20
60.59
61.46
0.003715
10.73
2588.54
325.10
0.43
Church Street
2060
PF 3
19580.00
41.20
61.59
62.52
0.003713
11.10
2912.98
325.10
0.44
Church Street
1885
PF 1
3580.00
40.30
51.06
52.78
0.010613
11.70
813.33
199.98
0.65
Church Street
1885
PF 2
16000.00
40.30
59.20
60.56
0.006726
13.89
3030.67
367.14
0.58
Church Street
1885
PF 3
19580.00
40.30
60.02
61.58
0.007527
15.13
3344.96
397.20
0.61
Church Street
1695
PF 1
3580.00
39.90
49.76
50.81
0.008316
10.35
1098.93
201.51
0.58
Church Street
1695
PF 2
16000.00
39.90
58.82
59.42
0.003588
10.51
3840.13
451.95
0.43
Church Street
1695
PF 3
19580.00
39.90
59.63
60.31
0.003949
11.34
4220.84
494.51
0.45
Church Street
1504
PF 1
3580.00
39.20
48.88
49.41
0.005381
8.07
1236.71
270.70
0.46
Church Street
1504
PF 2
16000.00
39.20
58.74
58.95
0.001195
6.14
5266.32
647.60
0.25
Church Street
1504
PF 3
19580.00
39.20
59.52
59.77
0.001377
6.77
5795.87
697.67
0.27
Church Street
1360
PF 1
3580.00
37.90
48.82
48.91
0.001200
4.21
2425.29
418.94
0.22
Church Street
1360
PF 2
16000.00
37.90
58.69
58.80
0.000420
3.83
7871.50
739.20
0.15
Church Street
1360
PF 3
19580.00
37.90
59.47
59.60
0.000487
4.23
8445.48
739.20
0.16
Church Street
1260
PF 1
3580.00
37.60
48.67
44.65
48.77
0.000981
3.80
2526.92
492.48
0.20
Church Street
1260
PF 2
16000.00
37.60
58.65
48.69
58.74
0.000329
3.40
8894.35
821.13
0.13
Bridge
49.74
HEC-RAS Plan: Plan 02 River: Pilarcitos Creek Reach: Church Street (Continued) Reach
River Sta
Profile
Q Total
Min Ch El
W.S. Elev
Crit W.S.
E.G. Elev
E.G. Slope
Vel Chnl
Flow Area
Top Width
(cfs)
(ft)
(ft)
(ft)
(ft)
(ft/ft)
(ft/s)
(sq ft)
(ft)
19580.00
37.60
59.42
49.30
59.54
0.000393
3.81
9528.13
824.49
Froude # Chl
Church Street
1260
PF 3
0.14
Church Street
1080
PF 1
3580.00
37.10
48.39
44.34
48.55
0.001811
5.26
2385.95
491.37
0.28
Church Street
1080
PF 2
16000.00
37.10
58.61
49.05
58.68
0.000383
3.73
9017.91
808.09
0.14
Church Street
1080
PF 3
19580.00
37.10
59.37
49.75
59.46
0.000473
4.24
9643.98
832.20
0.16
Church Street
925
PF 1
3580.00
36.50
47.88
43.23
48.21
0.002152
5.54
916.61
140.02
0.30
Church Street
925
PF 2
16000.00
36.50
58.31
49.81
58.57
0.000899
5.65
4303.24
709.97
0.22
Church Street
925
PF 3
19580.00
36.50
59.04
51.31
59.33
0.000939
5.91
4836.49
733.70
0.22
Church Street
850
Church Street
805
PF 1
3580.00
36.20
46.02
46.66
0.003963
6.70
843.90
133.84
0.40
Church Street
805
PF 2
16000.00
36.20
50.06
50.06
55.38
0.020917
19.73
1441.37
162.20
0.97
Church Street
805
PF 3
19580.00
36.20
52.01
52.01
57.57
0.018591
20.42
1774.66
179.28
0.93
Church Street
730
PF 1
3580.00
36.00
45.93
46.29
0.003624
6.70
1258.91
175.79
0.38
Church Street
730
PF 2
16000.00
36.00
51.30
52.55
0.008467
13.79
2554.27
316.49
0.63
Church Street
730
PF 3
19580.00
36.00
52.13
53.51
0.008967
14.72
2823.05
330.15
0.65
Church Street
595
PF 1
3580.00
35.00
45.83
45.93
0.001367
4.44
2377.44
343.32
0.24
Church Street
595
PF 2
16000.00
35.00
51.54
51.76
0.001950
7.05
5147.29
508.13
0.31
Church Street
595
PF 3
19580.00
35.00
52.39
52.66
0.002153
7.67
5579.68
509.74
0.33
Church Street
450
PF 1
3580.00
33.80
45.66
45.76
0.000979
4.01
2825.50
370.67
0.21
Church Street
450
PF 2
16000.00
33.80
51.15
51.45
0.002232
7.81
5333.64
475.95
0.33
Church Street
450
PF 3
19580.00
33.80
51.94
52.30
0.002593
8.67
5708.10
477.67
0.36
Church Street
360
PF 1
3580.00
33.40
45.61
45.67
0.000733
3.53
2760.36
388.38
0.18
Church Street
360
PF 2
16000.00
33.40
51.02
51.24
0.001834
7.15
5370.08
537.88
0.30
Church Street
360
PF 3
19580.00
33.40
51.79
52.06
0.002141
7.95
5787.99
553.35
0.33
Church Street
204
PF 1
3580.00
32.20
44.79
39.97
45.39
0.003902
8.21
1602.68
273.16
0.41
Church Street
204
PF 2
16000.00
32.20
50.22
48.39
50.80
0.003908
10.50
4155.68
627.33
0.44
Church Street
204
PF 3
19580.00
32.20
50.98
48.94
51.58
0.003905
10.79
4634.40
635.97
0.44
Bridge
