HomeMy WebLinkAboutApn I2 - Drainage StudyAPPENDIX I2
Drainage Study
Preliminary Drainage Study
For
Otay Ranch
Village 8 West
Chula Vista Tract No. 09-XX
John A. Hayes, P.E.
R.C.E. # 58003
Expiration Date: June 30, 2014
Prepared For
Otay Land Company, LLC
1903 Wright Place, Suite 220
Carlsbad, CA 92008
(760) 602-3777
Prepared By
Hale Engineering
7910 Convoy Court
San Diego, CA 92111
(858) 715-1420
Date: December 8, 2011
i H.E. Job No. 08020
Executive Summary
The following report was prepared in support of the drainage system shown on
the Tentative Map entitled "Otay Ranch Village 8 West". Within this report are the
necessary hydrologic calculations required for the design of the backbone storm
drain system proposed per the project.
Runoff from the project area drains to one of two receiving waters: Wolf Canyon
or the Otay River.
Discharges to Wolf Canyon are subject to hydromodification criteria. The
calculations within this report are based upon the criteria in the “County of San
Diego Hydrology Manual”, 2003 edition and include basin routing calculations for
the hydromodification basin proposed as part of the backbone storm drain
system. Calculations based upon the criteria in the “County of San Diego Final
Hydromodification Management Plan”, 2009 edition (HMP). The detention basin
design is based upon the more conservative values calculated via the HMP
methodology. Due to the inclusion of the hydromodification basin within the
backbone storm drain system, the 100-year storm event post-construction flows
volumnes and rates discharging into Wolf Canyon are less than those of the
existing condition. Thus, the scour potential is not increased by the project. The
channel composition and integrity have been reviewed by the geotechnical
engineer, Advanced Geotechnical Solutions, and determined to have low erosive
potential1.
Per the Final Hydromodification Plan, the lower Otay River Valley is listed as a
river reach that is exempt from hydromodification analysis. The calculations
within this report are based upon the criteria in the “County of San Diego
Hydrology Manual”, 2003 edition.The post project peak flow is anticipated to
increase over that of the existing condition. However, it is a minor portion of the
total flow within the Otay River at that point. The peak flows with the river and
those from the discharge point do not coincide; the impact of the increased flow
at the discharge point is negligible at peak river flow2. To minimize erosion at the
outlet point, the proposed outlet point to the Otay river shall consist of a USBR
Type IV Energy-Dissipater-Impact Basin at the end of a proposed energy
dissipater with additional erosion control provided by a section of 1 ton rip rap.
The proposed energy dissipater shall be located above the water surface with the
1 ton rip rap outfall below the calculated 100-year flood surface.The scour
potential is not increased by the project. The channel composition and integrity
and outlet structure design have been reviewed by the geotechnical engineer,
Advanced Geotechnical Solutions, and determined to have low erosive
potentials3.
1 See Appendix M for the AGS geotechnical opinion letter 2 See Appendix L for the Hunsaker & Associates Otay River Analysis3See Appendix M for the AGS geotechnical opinion letter
ii H.E. Job No. 08020
Table of Contents
1 Vicinity Map ..................................................................................................... 2
2 Introduction..................................................................................................... 3
3 Hydrologic Method and Criteria ....................................................................... 4
3.1 Rational Method Criteria and Methodology:.............................................. 4
4 Hydrologic Results .......................................................................................... 5
4.1 Pre-Project Condition ................................................................................ 5
4.1.1 Pre-Project Drainage Area A ............................................................... 5
4.1.2 Pre-Project Drainage Area B ............................................................... 5
4.1.3 Pre-Project Drainage Area C .............................................................. 5
4.2 Post-Project Condition .............................................................................. 6
4.2.1 Post-Project Drainage Area 1 ............................................................. 6
4.2.2 Post-Project Drainage Area 2 ............................................................. 6
4.2.3 Post-Project Drainage Area 3 ............................................................. 6
4.2.4 Post-Project Drainage Area 4 ............................................................. 7
4.2.5 Post-Project Drainage Area 5 ............................................................. 7
4.2.6 Hydrologic Data of Pre & Post Project Condition ................................ 8
4.2.7 Drainage Areas 4 & 5 Basin Routing Results .................................... 10
Conclusions ........................................................................................................ 12
Appendices
Appendix A Pre-Project Condition Results, Wolf Canyon
Appendix B Pre-Project Condition Results, Otay River
Appendix C Pre-Project Drainage Map
Appendix D Post-Project Condition Results, Wolf Canyon
Appendix E Post-Project Condition Results, Otay River
Appendix F Post-Project Drainage Map
Appendix G Rational Method Analyses Supporting Data
Appendix H Otay Basin, Slope, & Soil Maps
Appendix I Hydromodification Study
Appendix J Post-Project Condition Results, Otay Valley Road
Appendix K Hydrology Routing Results (Hydrology Manual Methodology)
Appendix L Hunsaker Engineering Otay River Analysis
Appendix M Advanced Geotechnical Solutions, INC Geotechnical Opinion
Letters for Wolf Canyon and Otay River
Appendix N Excerpts from supporting Off-Site Hydrology Reports prepared by
others
2 H.E. Job No. 08020
1 Vicinity Map
3 H.E. Job No. 08020
2 Introduction
The proposed project is located within the Otay Watershed, specifically the Otay
Valley Hydrologic Area. Village 8 West is a 320-acre SPA (Special Planning
Area) portion of the larger Otay Valley Parcel of the City of Chula Vista currently
used for light agrarian (dry farming) purposes. An existing reservoir occupies a
central portion of the property. The Otay Valley Parcel is the largest parcel of
Otay Ranch, comprising 9,449 acres bounded by Telegraph Canyon Road on the
north, Heritage Road and the Otay Landfill Site on the west, Brown Field on the
south, and Lower Otay Lake on the east.
The northern portion of the proposed project will ultimately drain to Wolf Canyon
via two of five proposed storm drain systems. The other three storm drain
systems drain the remainder of the project to the Otay River southerly of the
southern boundary of the proposed project.
This report is prepared in support of the drainage system shown on the Tentative
Map entitled "Otay Ranch Village 8 West". Within this report are the necessary
hydrologic calculations required for the design of the backbone storm drain
system proposed per the project. All preliminary calculations and analyses within
this report are carried out in conformance with the criteria in the “County of San
Diego Hydrology Manual”, 2003 edition.
The mitigation of peak flow increases in Wolf Canyon is performed in a separate
hydromodification analysis (see Appendix I) and is based upon the “County of
San Diego Final Hydromodification Management Plan”, 2009 edition (HMP). The
study demonstrates that there are no peak flows which exceed the existing flow
conditions of storms using historic rain gauge data. The storage routing analysis
for project shown in this Hydrology Report is based upon the methodology
outlined in the San Diego Hydrology Manual. Refer to Appendix K for this routing
analysis and Section 4.2.6 for a comparison of the Wolf Canyon post-project
discharge obtained via the two methodologies. Note, the detention basin design
is based upon the more conservative values calculated via the HMP
methodology.
The discharge to the Otay River is downstream of the Otay reservoir. The first
Otay dam failed in an event which degraded the downstream Otay River. Per the
Final Hydromodification Plan, the lower Otay River Valley is listed as a river
reach that is exempt from hydromodification analysis (see section 6, page 6.5,
table 6.1 of the Final HMP; the WQCB approved the Final HMP on July 14,
2010, Resolution No. R9-2010-0066.) Therefore no calculations for
hydromodification or storage routing have been performed for discharge into
Otay River.
4 H.E. Job No. 08020
3 Hydrologic Method and Criteria
The Advanced Engineering Software (AES) Rational Method computer program
based upon the 2002 City of Chula Vista Subdivision Manual is used in the
determination of the 50-year storm event pre- and post-project flow rates.
3.1 Rational Method Criteria and Methodology:
Independent node-link models of each interior drainage basin are created and
linked together at confluence points to create a hydrologic model of the project in
its entirety. The AES program is capable of performing calculations for 15
hydrologic processes. For simplification, these processes are assigned code
numbers that appear in the program results. The code numbers and their
significance are as follows:
Code Subarea Hydrologic Process
1 Confluence analysis at a node
2 Initial subarea analysis
3 Pipe flow travel time (computer-estimated pipe size)
4 Pipe flow travel time (user-specified pipe size)
5 Trapezoidal channel travel time
6 Street flow analysis through a subarea
7 User-specified information at a node
8 Addition of the subarea runoff to mainstream
9 V-Gutter flow through a subarea
10 Copy mainstream data onto a memory bank
11 Confluence a memory bank with the mainstream memory
12 Clear a memory bank
13 Clear the mainstream memory
14 Copy a memory bank onto the mainstream memory
15 Hydrologic data bank storage function
Information obtained from the County of San Diego Hydrology Manual including
land use, soil type, runoff coefficients, and rainfall intensity is used to perform the
hydrologic analysis. Please see Appendices A and C for the pre- and post-project
hydrologic analyses; see Appendices B and D for the respective Drainage Maps.
The hydrologic conditions are analyzed using the following guidelines:
Design Storm 2-, 10-, 25-, 50-, and 100-year 6-hour storm
Runoff Coefficients County of San Diego Hydrology Manual
Soil Type Varies
Ranfall Intensity County of San Diego Hydrology Manual
Appendix G contains the excerpts from County of San Diego Manual on which
the analyses are based.
5 H.E. Job No. 08020
4 Hydrologic Results
The AES rational method program results are included in Appendices A, B, D,
and E for the pre- and post-project hydrologic analyses. The corresponding Pre-
and Post-Drainage Maps are included in Appendices C and F respectively.
4.1 Pre-Project Condition
In the existing condition, the site is composed of 3 drainage areas with three
distinct discharge points. Drainage areas A and B drain southwesterly toward
outlet points 130 and 320 respectively along the southern boundary of the
project; drainage area C drains westerly toward outlet point 400 along the
western boundary of the project.
4.1.1 Pre-Project Drainage Area A
Pre-Project Drainage Area A is an 83.9-acre drainage area with a high point of
511.7 and a flow line elevation of 310.0 at outlet Node130. Flow through the
drainage area consists of natural mountain channel flow. The flow path from the
highest point in the basin to the lowest is 4,340 feet. This flow exits the project
limits at the MSCP boundary and continues s’ly along the natural channel to Otay
River.
4.1.2 Pre-Project Drainage Area B
Pre-Project Drainage Area B is a 127.1-acre drainage area with a high point of
514.4 and a flow line elevation of 276.7 at outlet Node 320. This basin receives
offsite flow from Village 7 Vista Verde; quantities are determined in the Rick
Engineering “Drainage Study for McMillin Village 7 Vista Verde” dated November
29, 2004. Offsite flow is modeled in node 300; see Appendix N for supporting
data from the Rick Engineering report. Flow through the drainage area consists
of natural mountain channel flow. The flow path from the highest point in the
basin to the lowest is 4,375 feet. This flow exits the project boundary and
continues s’ly along the natural channel to Otay River.
4.1.3 Pre-Project Drainage Area C
Pre-Project Drainage Area C is a 183.6-acre drainage area composed of 89.6
acres of offsite flow and 94.0 acres of onsite flow; with a high point of 430.0 and
a flow line elevation of 367.1 at outlet Node 410. This basin receives offsite flow
from Otay Ranch Village 7; quantities are determined in the Hunsaker &
Associates “Addendum Rough Grading Hydrology Study for Otay Ranch Village
7” dated May 3, 2006. Offsite flow is modeled in node 400; see Appendix N for
supporting data from the Hunsaker & Associated report. Flow through the
drainage area consists of natural mountain channel flow. The flow path from the
highest point in the basin to the lowest is 3,100 feet. This flow exits the project
boundary and continues sw’ly along the natural channel to Wolf Creek which is
tributary to Otay River.
6 H.E. Job No. 08020
As described in Section 3, design data from the County of San Diego Hydrology
Manual was used with Advanced Engineering Software’s (AES) Rational Method
Hydrology Computer Program Package in the hydrologic analysis of the pre-
project Condition. See appendices for rational method data and drainage map.
See table: Hydrologic Data of Pre & Post Project Condition
Please refer to Appendices A and B for the rational method output for Wolf
Canyon and the Otay River respectively. Refer to Appendix C for the pre-project
drainage map.
4.2 Post-Project Condition
In the post-project condition, the site is comprised of 5 drainage areas. Drainage
areas 1, 2 and 3 drain southerly to outlet point 170; drainage areas 4 and 5 drain
westerly to outlet point 620.
4.2.1 Post-Project Drainage Area 1
Post-Project Drainage Area 1 is a 36.7-acre drainage area; proposed
improvements within this drainage area include mass grading of pads and
portions of Streets 'D', ‘E’, ‘H’, ‘I’, & ‘K’ within Parcel P for future development.
Runoff is transported to the proposed storm drain system via proposed catch
basins and curb inlets located at intervals within the proposed roadways. This
storm drain system ultimately confluences with areas 2 & 3 and conveys s’ly
through storm drain to Outlet Node 170 to the Otay River.
4.2.2 Post-Project Drainage Area 2
Post-Project Drainage Area 2 is a 42.3-acre drainage area; proposed
improvements within this area include mass grading of pads and portions of
Streets ‘E’, ‘M’, & ‘L’ within Parcels T, U, & V for future development. Runoff is
transported to the proposed storm drain system via proposed curb inlet located at
intervals within the drainage area. This storm drain ultimately confluences with
area 1 and conveys s’ly through storm drain to Outlet Node 170 continuing to the
Otay River.
4.2.3 Post-Project Drainage Area 3
Post-Project Drainage Area 3 is a 100.3-acre drainage area composed of 20.1
acres of offsite flow and 82.7 acres of onsite flow. A south east segment of Otay
Valley Road discharges to the east and is modeled separately in Appendix J.
Proposed improvements within this area include mass grading of pads and
portions of Streets ‘A’, ‘B’, & La Media within Parcels M, O, Q, R, S, & the
reservoir for future development. This basin receives offsite flow from Village 7
Vista Verde; quantities are determined in “Drainage Study for McMillin Village 7
Vista Verde” prepared by Rick Engineering. Offsite flow is modeled in node 300.
Runoff is transported to the proposed storm drain system via proposed curb inlet
located at intervals within the proposed roadways. This storm drain ultimately
confluences with area 1 and conveys s’ly through storm drain to Outlet Node 170
continuing to the Otay River. Velocity for the 3 drainage areas is dissipated with a
USBR type VI impact basin and 20’ rip rap energy dissipater.
7 H.E. Job No. 08020
Otay Valley Road – The section of road east of Street A does not combine with
Drainage areas 1 – 3. This flow is modeled in nodes 700 – 710. See appendix J.
4.2.4 Post-Project Drainage Area 4
Post-Project Drainage Area 4 is a 143.6-acre drainage area composed of 89.6
acres of offsite flow and 54.0 acres of onsite flow; proposed improvements within
this drainage area include mass grading of pads & portions of La Media Road,
Street 'A', and Main Street within parcels B, C, D, E, F, G (portion), H (portion),
and W for future development. This basin receives offsite flow from Otay Ranch
Village 7; quantities are determined in “Addendum Rough Grading Hydrology
Study for Otay Ranch Village 7” prepared by Hunsaker Engineering.Offsite flow
is modeled in node 400. Runoff is transported to the proposed storm drain
system via proposed catch basins and curb inlets located at intervals within the
proposed roadways. This storm drain system confluences with area 5 and
ultimately outlets to an existing natural drainage channel after discharging from a
detention basin designed to manage development hydromodification. This
discharge confluences drainage areas 4 & 5 with Basin P (representing Lot ‘A’,
adjacent land to be developed separately as a park and not included in the
hydromodification design). The runoff from Basin P confluences with the project
runoff after the hydromodification outfall and is modeled at node 620 in the
hydrology study. Discharge outlets through energy dissipaters (impact basin and
rip rap) and flows westerly from Outlet Node 620 to Wolf Canyon.
4.2.5 Post-Project Drainage Area 5
Post-Project Drainage Area 5 is a 59.0-acre drainage area; proposed
improvements within this drainage area include mass grading of pads & portions
of La Media Road, Main Street, and Streets 'A', ‘D’, ‘F’, & ‘G’, and within parcels
G (portion), H (portion), I, J, L, & N for future development. Runoff is transported
to the proposed storm drain system via proposed catch basins and curb inlets
located at intervals within the proposed roadways. This storm drain system
confluences with area 4 and ultimately outlets to an existing natural drainage
channel running westerly from Outlet Node 620 to Wolf Canyon.
Please refer to Appendices D and E for the rational method output for Wolf
Canyon and the Otay River respectively. Refer to Appendix F for the post-project
drainage map.
8 H.E. Job No. 08020
4.2.6 Hydrologic Data of Pre & Post Project Condition
The following table lists the Pre- and Post-Project Hydrology data for the Wolf
Canyon discharge point as determined per the County of San Diego Hydrology
Manual Methodology.
Wolf Canyon
Pre Project Post Project
Qe
Flow
Into
Detention
Basin
Qp
Flow
Out Of
Detention
Basin
Qp
Lot ‘A’
Qp
Total
Qt
Modeling Node 410 600 810 820
Tributary Basins C 4,5 4,5 Lot ‘A’
Area (acres) 183.6 202.6 202.6 16.3
2-Year
Storm
Q (cfs)155.6 243.8 42.6 18.4 61.0
Tc (min)15.4 13.8
10-Year
Storm
Q (cfs)246.0 380.1 136.4 28.7 165.1
Tc (min)14.9 13.6
25-Year
Storm
Q (cfs)261.2 402.8 157.2 30.5 187.7
Tc (min)14.8 13.5
50-Year
Storm
Q (cfs)322.3 493.8 241.2 37.4 278.6
Tc (min)14.6 13.5
100-Year
Storm
Q (cfs)368.3 559.3 313.4 42.4 355.8
Tc (min)14.5 13.5
The following table lists the Pre- and Post-Project Hydrology data for the Wolf
Canyon discharge point as determined by HSPF in accordance with the criteria
per the County of San Diego Final Hydromodification Management Plan (HMP).
Wolf Canyon
Pre Project
Qe
Post Project
(Out of Detention Basin)
Qp
Tributary Basins C 4,5
Area (acres) 183.6 202.6
2-Year Storm Q (cfs)28.6 17.1
5-Year Storm Q (cfs)39.0 31.8
10-Year Storm Q (cfs)55.6 40.1
25-Year Storm Q (cfs)93.0 63.9
Revised 12/8/2011
9 H.E. Job No. 08020
Note, the detention basin design is based upon the more conservative values
calculated via the HMP methodology.
The following table lists the Pre- and Post-Project Hydrology data for the Otay
River discharge as determined per the County of San Diego Hydrology Manual
Methodology.
Otay River
Pre Project
Qe
Post Project
Qp
Modeling Node 130 330 165
Tributary Basins A B A+B 1-3
Area (acres) 83.9 127.1 211.0 179.3
2-Year
Storm
Q (cfs)47.8 70.8 118.6 155.5
Tc (min)18.1 30.6 22.2
10-Year
Storm
Q (cfs)78.7 112.5 191.2 243.9
Tc (min)16.4 28.6 21.5
25-Year
Storm
Q (cfs)84.0 119.5 203.5 258.7
Tc (min)16.2 28.4 21.4
50-Year
Storm
Q (cfs)105.7 147.7 253.4 317.9
Tc (min)15.5 27.6 21.2
100-Year
Storm
Q (cfs)122.3 169.1 291.4 362.5
Tc (min)15.0 27.1 21.0
The proposed outlet point to the Otay river will consist of a USBR Type IV
Energy-Dissipater-Impact Basin at the end of a proposed energy dissipater with
additional erosion control provided by a section of 1 ton rip rap. The proposed
energy dissipater shall be located above the water surface with the 1 ton rip rap
outfall below the calculated 100-year flood surface. Per the above table, the post
project peak flow is 362.5 cfs with a peak flow time of 21 minutes. Per the
Hunsaker and Associates analysis included in Appendix L, for a 100-year 24-
hour storm event the Otay River has a peak discharge of 20,161 CFS and a time
of peak flow of 21 hours 5 minutes at the Village 8 outlet point. As shown above,
the post project peak flow is anticipated to increase over that of the existing
condition. However, it is a minor portion of the total flow within the Otay River at
that point. The peak flows with the river and those from the discharge point do
not coincide; the impact of the increased flow at the discharge point is negligible
at peak river flow. It is anticipated that the increase in discharge from the Village
8 West outlet point will not increase the erosion potential of the downstream Otay
River Channel.
10 H.E. Job No. 08020
4.2.7 Drainage Areas 4 & 5 Basin Routing Results
Refer to the Hydromodification Study (Appendix I) for storm system
improvements as well as pre-project discharge vs. post-project discharge results.
The Hydromodification Management Plan was developed to manage increases in
runoff discharge rates and durations from all priority projects, where increases
are likely to cause channel erosion increases, sediment pollution, or other
impacts to beneficial use waterbodies. Hydromodification describes change to a
waterbody's physical structure as well as its natural function. These changes can
cause problems such as changes in flow, increased sedimentation, higher water
temperature, lower dissolved oxygen, degradation of aquatic habitat structure,
loss of fish and other aquatic populations, and decreased water quality. There
are 3 categories: (1) channelization and channel modification, (2) dams, and (3)
streambank and shoreline erosion. This project has channelization and channel
modification for flood control and drainage. The timing and delivery of pollutants,
including sediment, and increased flows are managed in design. The basic
element is an engineered detention basin and outflow control device that is
modeled by storage routing processes to achieve the elimination of storm water
flowrate increases in all storm events. The HMP for the County of San Diego
prepared by Brown and Caldwell is the guiding document. Wolf Canyon flows are
mitigated by this process to eliminate increases in erosion.
Wolf Canyon Scour – The flows discharging into Wolf Canyon match existing
conditions as explained above. The scour potential is not increased by the
project. The channel composition and integrity have been reviewed by the
geotechnical engineer and determined to have low erosive potential. See
Appendix G for the HEC-RAS channel flow analysis. See Appendix M for the
geotechnical opinion letter by Advanced Geotechnical Solutions.
Reservoir Jurisdiction – The project basin is not large enough or high enough to
be categorized as jurisdictional size under the division of Safety of Dams. See
chart in Appendix G.
Otay River Scour - For the Otay River discharge, velocity and flow concentration
are addressed with velocity dissipation design at a discharge point near the
existing flow concentrations within natural channels. The discharge enters Otay
River in a mild sloping channel bottom. These velocities are reduced through the
impact basin and a rip rap energy dissipater mitigates the high erosivity of the
storm culvert flow. See calculations and explanation below.
11 H.E. Job No. 08020
Otay River scour was addressed in a previous study submitted to the city. See
Appendix L, portion of reference report, “TM Drainage Study for Otay Ranch
Village 9” by Hunsaker Engineering, submitted to the City of Chula Vista. This
upstream project used a similar design with energy dissipaters; Type VI impact
dissipator equivalent to RSD D-41 discharging to a rip rap energy dissipator per
RSD D-40. The hydrology of Otay River was modeled in HEC-RAS, resulting in a
100 year 24 hour design storm flowrate and velocity of Q=19,400 cfs and
V=6.4fps. This information is used to evaluate the stability of the energy
dissipater in a flowing channel. This velocity is not fast enough to produce lateral
forces capable of compromising the dissipaters. Further armoring of the
dissipator is possible if the need is determined during final engineering.
Channel Calculator
100 year storm event
Rip rap energy dissipator
Given Input Data:
Shape ........................... Rectangular
Solving for ..................... Depth of Flow
Flowrate ........................ 362.0000 cfs
Slope ........................... 0.0160 ft/ft
Manning's n ..................... 0.0450
Height .......................... 48.0000 in
Bottom width .................... 240.0000 in
Computed Results:
Depth ........................... 31.7743 in
Velocity ........................ 6.8357 fps
Full Flowrate ................... 672.8386 cfs
Flow area ....................... 52.9571 ft2
Flow perimeter .................. 303.5485 in
Hydraulic radius ................ 25.1222 in
Top width ....................... 240.0000 in
Area ............................ 80.0000 ft2
Perimeter ....................... 336.0000 in
Percent full .................... 66.1964 %
Channel Calculator
10 year storm event
Rip rap energy dissipator
Given Input Data:
Shape ........................... Rectangular
Solving for ..................... Depth of Flow
Flowrate ........................ 244.0000 cfs
Slope ........................... 0.0160 ft/ft
Manning's n ..................... 0.0450
Height .......................... 48.0000 in
Bottom width .................... 240.0000 in
Computed Results:
Depth ........................... 24.5962 in
Velocity ........................ 5.9521 fps
Full Flowrate ................... 672.8386 cfs
Flow area ....................... 40.9936 ft2
Flow perimeter .................. 289.1924 in
Hydraulic radius ................ 20.4123 in
Top width ....................... 240.0000 in
Area ............................ 80.0000 ft2
Perimeter ....................... 336.0000 in
Percent full .................... 51.2420 %
RCP flow velocity – Maximum velocity in the Otay outfall pipe will approach 40
fps. High velocity is not a concern with clear water in RCP. There are not regional
experiences with scour being a lifetime issue. The storm drain system does not
anticipate conveying significant volumes of sediment or coarse materials that
would be a scour concern. Thicker pipe and other options to ensure long pipe life
can be explored in final engineering.
12 H.E. Job No. 08020
Conclusions
This Preliminary Drainage Study contains pre- and post-project hydrologic
analyses for the 2-, 10-, 25-, 50-, & 100-year, 6-hour storm events. These
analyses are used in the design and sizing of the backbone storm drain
infrastructure. The design also manages the Wolf Canyon hydromodification due
to channelization with a detention basin. The design manages erosive potential
of high flow velocity at the point of discharge into the Otay River with an
engineered energy dissipater.