HomeMy WebLinkAboutHydrologic and Hydraulic Analysis for the Bonita Glen Creek Memorandum
TECHNICAL MEMORANDUM:
Hydrologic & Hydraulic Analysis for
Bonita Glen Creek
Prepared For:
Silvergate Development
Prepared by:
Luis Parra, PhD, CPSWQ, ToR, D.WRE.
R.C.E. 66377
REC Consultants
2442 Second Avenue
San Diego, CA 92101
Telephone: (619) 232-9200
TECHNICAL MEMORANDUM
TO: Silvergate Development
FROM: Luis Parra, PhD, PE, CPSWQ, ToR, D.WRE.
David Edwards, PE.
DATE: January 25, 2018. Revised: June 20, 2018.
RE: Hydrologic and Hydraulic Analysis for Bonita Glen Creek
INTRODUCTION
This memorandum summarizes the approach used to model the hydrologic and hydraulics of the creek
located within the proposed residential use site in the City of Chula Vista. The purpose of this study is
determine the proposed creeks flow capacity using standard 2, 10 and 100-year standard 6-hour design
storm events.
Hydrology calculations were performed using methodology outlined within the 2003 San Diego County
Hydrology Manual with additional input data obtained from the National Oceanic and Atmospheric
Administration (NOAA) and the United States Geological Service (USGS) StreamStats website for rainfall
precipitation and watershed tributary data respectively.
PROJECT SUMMARY
The Bonita Glen project site consists of a proposed residential use site that is bifurcated by an existing
natural stream. In developed conditions, the creek is to remain in a natural state with graded
embankments to the east and west of the delineated existing creek while leaving the creek in its natural
existing condition.
HYDROLOGY ANALYSIS
Per the USGS Streamstats analysis undertaken for the project site (see Appendix 1), the existing creek
has a tributary area of approximately 53 Acres comprising of predominantly developed residential areas.
The Streamstats identified the tributary area as having an impervious percentage of 38.1% and an
overland flow length of approximately 1 mile. Per the County of San Diego Hydrology Manual (SDHM),
as the tributary area is less than 1 square mile, the modified rational method was to be used to
determine the peak flow for the channel.
Runoff Coefficient Determination
Using the following equation from section 3.1.2 of the San Diego County Hydrology Manual:
( ) ( )
Where Cp is the pervious coefficient runoff coefficient for the natural hydrologic soil class, in this case
hydrologic soil class D, Cp = 0.35. The overall weighted runoff coefficient for the tributary area is 0.56.
Bonita Glen Creek
January 25, 2018. Revised: June 20, 2018.
Time of Concentration
Per the USGS Streamstats analysis, the watershed tributary to the creek has an overland flow length of 1
mile with a discharge elevation of approximately 60 ft and an upstream starting elevation of 160 ft. It
was determined that an overland flow time of 25 minutes was representative of the tributary area ,
according to the Kirpich Nomograph, figure 3-4, San Diego County Hydrology Manual (see Appendix 1).
Precipitation
The project site was located on the USGS NOAA 14 Atlas website to determine the rainfall precipitation.
Table 1 below illustrates the precipitation for the 15, 25 and 30 minute intervals. It should be noted that
the 25 minute interval is not provided by NOAA and was interpolated from their data set (see Appendix
1 for additional calculations).
Table 1 – NOAA 14 Precipitations
Design Storm 15 Minute (inches) 25 Minute (inches) 30 Minute (inches)
2-Year 0.240 0.307 0.335
10-Year 0.362 0.462 0.504
100-Year 0.563 0.720 0.786
Given that intensity is a function of precipitation and time, we can determine the following relationship:
( )
( )
The rainfall intensities were then calculated accordingly in Table 2 below.
Table 2 – NOAA 14 Rainfall Intensities
Design Storm 15 Minute (in/hour) 25 Minute (in/hour) 30 Minute (in/hour)
2-Year 0.96 0.74 0.67
10-Year 1.45 1.11 1.01
100-Year 2.25 1.73 1.57
Using the rational method equation:
Where C is the runoff coefficient, I is the rainfall intensity (assuming a 25 minute time of concentration)
and A is the tributary area (about 53 acres), the flows tributary to the creek are provided in Table 3.
Table 3 – Rational Method Peak Flow
Time of Concentration 2-Year 10-Year 100-Year
25 Minutes 21.9 cfs 32.9 cfs 51.3 cfs
Bonita Glen Creek
January 25, 2018. Revised: June 20, 2018.
Comparison with Chula Vista Drainage Master Plan Q100.
The 100 year peak flow (51 cfs) is 77% of the value determined by the City of Chula Vista Master Plan
Q100 (66 cfs). The difference can be attributed to the difference in intensity:
For small Tc, NOAA updated intensity values are smaller than the recommended Hydrology Manual of
the San Diego, which are based on the intensity equation. For example, in this project, the 6 hr – 100 yr
rainfall event is about 2.65 inches, which determines an intensity of 2.47 in/hr for a time of
concentration of 25 min. This intensity is 43% higher than NOAA’s (or in other words, NOAA’s intensity
in the location for Tc = 25 min is 70% of the old intensities by the County Manual). Therefore, NOAA
intensity in itself explains the difference in the peak. Notice that the use of County’s intensity in our
model will give us a peak flow even larger than 66 cfs (Q= 0.56·2.47·53 = 73 cfs) which means that the
selection of C in this study is more conservative than in the Master Drainage Study.
The author of this study believes that NOAA’s new intensities are more accurate and precise than maps
prepared in the 90’s by the County as more data and a better statistical analysis was used by NOAA in
their web-site analysis. As a consequence, the peak flow of 51 cfs (and all other peak flows) will be
considered more adequate for the purposes of this analysis.
HYDRAULIC ANALYSIS
In order to assess the conveyance capacity of the creek, a normal depth analysis was to be taken at the
most hydraulically limited section throughout the reach of the project site. It was determined that the
most conservative analysis location was at the southernmost structure adjacent to Bonita Glen Road. A
detail of the section is provided below in Exhibit 1.
Exhibit 1 – Critical Creek Section
Bonita Glen Creek
January 25, 2018. Revised: June 20, 2018.
Per the topographic information available, it was calculated that the creek at this location has a channel
slope of approximately 3.3%. A manning’s roughness coefficient of 0.04 was selected to represent the
natural light grass vegetation plus sandy bottom found within the current creek. The creek has a flow
line invert of 58.4 ft and a top of bank elevation of 59.9 ft providing a maximum channel conveyance
depth of 1.5 ft.
A normal depth analysis for the section was undertaken with AutoDesk AutoCAD Hydraflow, the result
of which is summarized below in Table 4 and is provided in detail in Appendix B of this report.
Table 4 – Summary of Creek Flow Depth
Analysis Location 2-Year 10-Year 100-Year Maximum Flow
Depth Allowable
Southern
Parking Lot 1.14 ft 1.32 ft 1.62 ft 1.50 ft
Per the previous environmental assessment of the creek prepared by Dudek (and also confirmed by Lisa
Honma of the RWQCB in a site visit on June 21, 2017) it was determined that a typical flow width of
about 1.5 ft exits, and an additional buffer of 10 ft was recommended (for a total width of 11.5 ft to be
maintained out of reach from development). Table 5 below illustrates the flow width experienced by the
creek section selected for the design storms analyzed within this study.
Table 5 – Summary of Creek Flow Width
Analysis Location 2-Year 10-Year 100-Year Maximum Flow
Width Allowable
Southern
Parking Lot 8.63 ft 9.81 ft 15.61 ft 11.5 ft
Bonita Glen Creek
January 25, 2018. Revised: June 20, 2018.
SUMMARY
This study has demonstrated that the proposed creek within the Bonita Glen project can safely convey
the 2 and 10-year design peak flow without overtopping or exceeding the allowed width buffer, which
demonstrated that the delineation of the creek main section using biological methods is consistent with
the hydraulic calculations performed here.
However, the 100-year flow is shown to be overtopping the channel section and extend beyond the
buffer area, which is consistent with typical 100-year flood analysis where creeks spill out of their banks.
If a floodplain determination is needed in the future within the creek, additional analysis of the system
using HEC-RAS (or an equivalent 1-D hydraulic model) will be required for design purposes.
KEY ASSUMPTIONS
1. Type D Soils is representative of the existing condition site.
ATTACHMENTS
1. Hydrologic Analysis (USGS Streamstats, NOAA 14 Precipitation)
2. Hydraulic Analysis (Hydraflow Normal Depth)
REFERENCES
[1] – “County of San Diego Hydrology Manual”, June 2003.
Bonita Glen Creek
January 25, 2018
APPENDIX 1 – RATIONAL METHOD ANALYSIS
1/24/2018 Pr ecipitation Frequency Data Server
https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_printpage.htm l?lat=32.6428&lon=117.0612&data=depth&units=english&series=pds 1/4
NOAA Atl as 14, Volume 6, Version 2
Location name: Chula Vista, California, USA*
Latitude: 32.6428°, Longitude: 117.0612°
Elevation: 170.95 ft**
* so urce : ESRI Maps
** sou rce: USGS
POI NT PRECIPI TATION FREQUENCY E STI MATE S
San ja Perica, Sarah Die tz, Sara h Heim, Lillian Hiner, Ka zun gu Maita ria, Deb ora h Ma rtin , Sa ndra
Pavlo vic, Isha ni Roy, Carl Tryp aluk, Da le Unru h, Fenglin Yan, Mich ael Yekta, Tan Zha o, Geoffrey
Bonn in, Dan iel Brewer, L iCh uan Che n, Tye Parzybo k, Jo hn Yarchoa n
NOAA, Natio nal Weathe r Se rvice, Silver Spring, Marylan d
PF_tabular | PF_graphic al | Maps_&_aerials
PF tabular
PDSbased point precipitation frequency estimates with 90% confidence intervals (in inches)1
Duration Average recurrence interval (years)
1 2 5 10 25 50 100 200 500 1000
5min 0.110
(0.092‑0.133)
0.139
(0.116‑0.168)
0.177
(0.147‑0.214)
0.209
(0.172‑0.255)
0.253
(0.202‑0.320)
0.288
(0.225‑0.373)
0.325
(0.247‑0.431)
0.364
(0.269‑0.497)
0.417
(0.296‑0.595)
0.460
(0.314‑0.680)
10m in 0.158
(0.132‑0.191)
0.199
(0.166‑0.240)
0.253
(0.211‑0.307)
0.299
(0.247‑0.366)
0.363
(0.290‑0.459)
0.413
(0.323‑0.535)
0.466
(0.355‑0.618)
0.521
(0.385‑0.712)
0.598
(0.424‑0.853)
0.659
(0.451‑0.974)
15m in 0.191
(0.160‑0.231)
0.240
(0.201‑0.291)
0.306
(0.255‑0.372)
0.362
(0.299‑0.442)
0.439
(0.350‑0.556)
0.500
(0.390‑0.647)
0.563
(0.429‑0.747)
0.630
(0.466‑0.861)
0.723
(0.512‑1.03)
0.797
(0.545‑1.18)
30m in 0.267
(0.223‑0.322)
0.335
(0.280‑0.405)
0.428
(0.356‑0.518)
0.504
(0.417‑0.617)
0.612
(0.488‑0.775)
0.697
(0.544‑0.902)
0.786
(0.598‑1.04)
0.879
(0.650‑1.20)
1.01
(0.715‑1.44)
1.11
(0.760‑1.64)
60m in 0.372
(0.311‑0.449)
0.467
(0.390‑0.565)
0.596
(0.496‑0.722)
0.703
(0.581‑0.859)
0.853
(0.680‑1.08)
0.971
(0.758‑1.26)
1.10
(0.833‑1.45)
1.23
(0.906‑1.67)
1.41
(0.996‑2.00)
1.55
(1.06‑2.29)
2hr 0.514
(0.430‑0.621)
0.646
(0.540‑0.782)
0.821
(0.684‑0.995)
0.963
(0.796‑1.18)
1.16
(0.924‑1.47)
1.31
(1.02‑1.69)
1.46
(1.11‑1.94)
1.62
(1.20‑2.21)
1.84
(1.30‑2.62)
2.01
(1.37‑2.96)
3hr 0.620
(0.519‑0.749)
0.781
(0.652‑0.944)
0.990
(0.825‑1.20)
1.16
(0.959‑1.42)
1.39
(1.11‑1.76)
1.57
(1.23‑2.03)
1.75
(1.33‑2.32)
1.94
(1.43‑2.65)
2.19
(1.55‑3.12)
2.38
(1.63‑3.52)
6hr 0.809
(0.677‑0.977)
1.02
(0.853‑1.24)
1.30
(1.08‑1.57)
1.52
(1.26‑1.86)
1.82
(1.46‑2.31)
2.06
(1.61‑2.66)
2.29
(1.74‑3.04)
2.53
(1.87‑3.46)
2.86
(2.02‑4.07)
3.11
(2.12‑4.59)
12hr 1.05
(0.880‑1.27)
1.33
(1.11‑1.61)
1.70
(1.41‑2.06)
2.00
(1.65‑2.44)
2.41
(1.93‑3.05)
2.73
(2.13‑3.54)
3.06
(2.33‑4.06)
3.41
(2.52‑4.65)
3.87
(2.74‑5.52)
4.24
(2.90‑6.27)
24hr 1.30
(1.14‑1.51)
1.65
(1.44‑1.92)
2.12
(1.85‑2.48)
2.51
(2.17‑2.96)
3.06
(2.57‑3.71)
3.49
(2.88‑4.31)
3.93
(3.17‑4.97)
4.40
(3.46‑5.71)
5.06
(3.83‑6.81)
5.58
(4.10‑7.75)
2day 1.60
(1.40‑1.86)
2.06
(1.80‑2.40)
2.67
(2.33‑3.13)
3.18
(2.76‑3.75)
3.88
(3.26‑4.71)
4.43
(3.65‑5.48)
4.99
(4.03‑6.31)
5.58
(4.39‑7.23)
6.38
(4.84‑8.60)
7.02
(5.16‑9.75)
3day 1.79
(1.57‑2.09)
2.33
(2.04‑2.72)
3.05
(2.66‑3.57)
3.64
(3.15‑4.29)
4.44
(3.74‑5.39)
5.07
(4.18‑6.26)
5.70
(4.60‑7.21)
6.36
(5.00‑8.24)
7.26
(5.50‑9.77)
7.96
(5.85‑11.1)
4day 1.94
(1.70‑2.26)
2.54
(2.22‑2.96)
3.33
(2.91‑3.90)
3.98
(3.45‑4.69)
4.86
(4.09‑5.90)
5.54
(4.57‑6.85)
6.24
(5.03‑7.88)
6.95
(5.47‑9.01)
7.92
(6.01‑10.7)
8.68
(6.38‑12.1)
7day 2.25
(1.97‑2.62)
2.95
(2.58‑3.44)
3.88
(3.39‑4.54)
4.64
(4.02‑5.46)
5.67
(4.77‑6.89)
6.47
(5.34‑8.01)
7.29
(5.88‑9.21)
8.13
(6.40‑10.5)
9.27
(7.02‑12.5)
10.2
(7.46‑14.1)
10day 2.47
(2.16‑2.88)
3.25
(2.84‑3.79)
4.27
(3.73‑5.00)
5.11
(4.43‑6.02)
6.25
(5.26‑7.59)
7.13
(5.88‑8.82)
8.02
(6.48‑10.1)
8.94
(7.04‑11.6)
10.2
(7.73‑13.7)
11.2
(8.20‑15.5)
20day 2.99
(2.62‑3.48)
3.96
(3.46‑4.61)
5.22
(4.55‑6.10)
6.24
(5.41‑7.35)
7.62
(6.41‑9.24)
8.67
(7.15‑10.7)
9.73
(7.85‑12.3)
10.8
(8.50‑14.0)
12.2
(9.28‑16.5)
13.4
(9.81‑18.6)
30day 3.55
(3.11‑4.14)
4.71
(4.12‑5.50)
6.22
(5.43‑7.27)
7.42
(6.43‑8.74)
9.03
(7.60‑11.0)
10.2
(8.46‑12.7)
11.5
(9.25‑14.5)
12.7
(9.99‑16.5)
14.3
(10.8‑19.3)
15.5
(11.4‑21.6)
45day 4.17
(3.65‑4.86)
5.53
(4.84‑6.45)
7.27
(6.34‑8.50)
8.65
(7.49‑10.2)
10.5
(8.80‑12.7)
11.8
(9.76‑14.6)
13.2
(10.6‑16.6)
14.5
(11.4‑18.8)
16.3
(12.3‑21.9)
17.6
(12.9‑24.4)
60day 4.84
(4.24‑5.64)
6.40
(5.60‑7.47)
8.37
(7.31‑9.79)
9.92
(8.59‑11.7)
11.9
(10.0‑14.5)
13.4
(11.1‑16.6)
14.9
(12.0‑18.8)
16.3
(12.8‑21.2)
18.2
(13.8‑24.5)
19.6
(14.4‑27.2)
1/24/2018 Pr ecipitation Frequency Data Server
https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_printpage.htm l?lat=32.6428&lon=117.0612&data=depth&units=english&series=pds 2/4
1 Precipitation frequenc y (PF) estimates in this table are based on frequency analysis of partial duration s eries (PDS).
Numbers in parenthesis are PF estimates at lower and upper bounds of the 90% c onfidence interval. The probability that precipitation frequenc y es timates (for a
given duration and average rec urrence interval) will be greater than the upper bound (or less than the lower bound) is 5%. Estimates at upper bounds are not
c hec ked agains t probable maximum precipitation (PMP) es timates and may be higher than c urrently valid PMP v alues.
Pleas e refer to NOAA Atlas 14 doc ument for more information.
Back to Top
PF graphical
cur ve plots
Back to Top
Maps & aerials
S mal l scal e terrain
+
–
3km
2mi
1/24/2018 Pr ecipitation Frequency Data Server
https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_printpage.htm l?lat=32.6428&lon=117.0612&data=depth&units=english&series=pds 3/4
Large scal e terrain
Large scale map
Large scal e aeri al
Back to Top
E rror 500: I nternal Server E rror. P lease t ry anot her location.
+
–
100km
60mi
+
–
100km
60mi
+
–
100km
60mi
NOAA Values Interpretation
2‐Year 10‐Year 100‐Year
15 0.24 0.362 0.563
25 0.307 0.462 0.720
30 0.335 0.504 0.786
y = 0.065217x0.481126
R² = 1.000000
y = 0.099358x0.477434
R² = 1.000000
y = 0.152878x0.481394
R² = 1.000000
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 10203040
Series1
Series2
Series3
Power (Series1)
Power (Series2)
Power (Series3)
Bonita Glen Creek
January 25, 2018
APPENDIX 2 – HYDRAULIC ANALYSIS
Channel Report
Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Thursday, Jan 25 2018
<Name>
User-defined
Invert Elev (ft) = 58.40
Slope (%) = 3.30
N-Value = 0.040
Calculations
Compute by: Known Q
Known Q (cfs) = 21.90
(Sta, El, n)-(Sta, El, n)...
( 0.00, 59.90)-(4.40, 59.90, 0.040)-(9.50, 58.40, 0.040)-(14.50, 59.60, 0.040)-(18.20, 61.00, 0.040)
Highlighted
Depth (ft) = 1.14
Q (cfs) = 21.90
Area (sqft) = 4.92
Velocity (ft/s) = 4.45
Wetted Perim (ft) = 8.93
Crit Depth, Yc (ft) = 1.16
Top Width (ft) = 8.63
EGL (ft) = 1.45
-2 0 2 4 6 8 10 12 14 16 18 20 22
Elev (ft)Depth (ft)Section
57.00 -1.40
58.00 -0.40
59.00 0.60
60.00 1.60
61.00 2.60
62.00 3.60
Sta (ft)
Channel Report
Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Thursday, Jan 25 2018
<Name>
User-defined
Invert Elev (ft) = 58.40
Slope (%) = 3.30
N-Value = 0.040
Calculations
Compute by: Known Q
Known Q (cfs) = 32.90
(Sta, El, n)-(Sta, El, n)...
( 0.00, 59.90)-(4.40, 59.90, 0.040)-(9.50, 58.40, 0.040)-(14.50, 59.60, 0.040)-(18.20, 61.00, 0.040)
Highlighted
Depth (ft) = 1.32
Q (cfs) = 32.90
Area (sqft) = 6.58
Velocity (ft/s) = 5.00
Wetted Perim (ft) = 10.16
Crit Depth, Yc (ft) = 1.36
Top Width (ft) = 9.81
EGL (ft) = 1.71
-2 0 2 4 6 8 10 12 14 16 18 20 22
Elev (ft)Depth (ft)Section
57.00 -1.40
58.00 -0.40
59.00 0.60
60.00 1.60
61.00 2.60
62.00 3.60
Sta (ft)
Channel Report
Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Thursday, Jan 25 2018
<Name>
User-defined
Invert Elev (ft) = 58.40
Slope (%) = 3.30
N-Value = 0.040
Calculations
Compute by: Known Q
Known Q (cfs) = 51.30
(Sta, El, n)-(Sta, El, n)...
( 0.00, 59.90)-(4.40, 59.90, 0.040)-(9.50, 58.40, 0.040)-(14.50, 59.60, 0.040)-(18.20, 61.00, 0.040)
Highlighted
Depth (ft) = 1.62
Q (cfs) = 51.30
Area (sqft) = 10.30
Velocity (ft/s) = 4.98
Wetted Perim (ft) = 16.04
Crit Depth, Yc (ft) = 1.66
Top Width (ft) = 15.61
EGL (ft) = 2.01
-2 0 2 4 6 8 10 12 14 16 18 20 22
Elev (ft)Depth (ft)Section
57.00 -1.40
58.00 -0.40
59.00 0.60
60.00 1.60
61.00 2.60
62.00 3.60
Sta (ft)