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PREPARED FOR:
PREPARED BY:
KWC Engineers
1880 Compton Avenue, Suite 100
Corona, CA 92881
Tel: (951) 734-2130
www.kwcengineers.com
RIVERSIDE LEGACY IV NICHOLS ROAD,
LLC
1505 Bridgeway, Suite 107
Sausalito, CA 94965
(617) 877-7637
Baker Industrial
City of Lake Elsinore, County of Riverside, California
P R E L I MIN A RY DRAINAGE REPORT
January 2024
Preliminary Drainage Report 2
TABLE OF CONTENTS
Section Name Page Number
LIST OF TABLES .......................................................................................................... 3
LIST OF FIGURES ........................................................................................................ 3
LIST OF APPENDICES ................................................................................................. 4
Section 1 - Introduction ................................................................................................. 5
1.1 Purpose of Study .......................................................................................... 5
1.2 Project Description ........................................................................................ 5
1.3 Floodplain Mapping ...................................................................................... 6
1.4 Design Criteria .............................................................................................. 6
Section 2 – Hydrologic Data and Model Development .................................................. 7
2.1 Existing Condition Model .............................................................................. 7
2.2 Proposed Condition Model ........................................................................... 8
Section 3 – Unit Hydrograph Analysis Existing and Proposed Conditions .................... 9
3.1 Introduction ................................................................................................... 9
3.2 Approach and Methodology .......................................................................... 9
3.3 On-site Unit Hydrograph Results ................................................................ 11
Section 4 – Hydraulic Analysis .................................................................................... 12
4.1 Onsite Drainage Facilities ........................................................................... 12
4.2 Street Capacity Analysis ............................................................................. 12
4.3 Outlet Analysis ............................................................................................ 12
4.4 Detention Analysis ...................................................................................... 12
Preliminary Drainage Report 3
Section 5 – Debris Basin Analysis ............................................................................... 14
5.1 Design Criteria ............................................................................................ 14
Section 6 – Conclusions .............................................................................................. 15
Section 7 – References ............................................................................................... 16
LIST OF TABLES
Table 1. Existing Condition Peak Flow Summary ......................................................... 7
Table 2. Proposed Condition Peak Flow Summary ...................................................... 8
Table 3. Proposed vs Existing Condition Peak Flow Summary .................................... 8
Table 4. Precipitation Depth-Duration-Frequency Summary ......................................... 9
Table 5. Existing Condition Watershed Lag Time Parameters .................................... 10
Table 6. Proposed Condition Watershed Lag Time Parameters ................................. 10
Table 7. Proposed vs Existing Unit Hydrograph Peak Flow Summary ........................ 11
LIST OF FIGURES
Figure 1: Vicinity Map
Figure 2: Existing Condition Hydrology Key Map
Figure 3: Proposed Condition Hydrology Key Map
Figure 4: Existing Unit Hydrograph Key Map
Figure 5: Proposed Unit Hydrograph Key Map
Figure 6: FEMA FIRM Panel
Preliminary Drainage Report 4
LIST OF APPENDICES
Appendix A: Vicinity Map
Appendix B: RCFC Precipitation Data & NRCS Soils Report
Appendix C: Existing Condition Hydrology Rational Method & Key Map
Appendix D: Proposed Condition Hydrology Rational Method & Key Map
Appendix E: FEMA Flood Insurance Rate Map
Appendix F: Existing Condition Unit Hydrograph & Key Map
Appendix G: Proposed Condition Unit Hydrograph & Key Map
Appendix H: Storm Drain Hydraulics WSPG Analysis
Preliminary Drainage Report 5
Section 1
INTRODUCTION
1.1 PURPOSE OF STUDY
The purpose of this study is to hydrologically model the project site’s onsite tributary watersheds
to determine the existing and proposed peak runoffs. The hydrologic analysis was prepared using
the Rational Method as specified in the Riverside County Hydrology Manual. The flows are used
to estimate the size of the proposed drainage facilities that support the proposed project
development.
1.2 PROJECT DESCRIPTION
The Baker Industrial project is comprised of 66.23 acres of developed land along Baker Street in
the City of Lake Elsinore in Riverside County, California, adjacent to Pierce Street. Appendix A
shows a vicinity map of the area illustrating the location of the project.
The Baker Industrial project is generally bounded to the northeast by Baker Street. Bounded on
the northwest by Pierce Street. To the southeast and southwest of the site, the area is bounded by
undeveloped hills.
The project site existing conditions is generally flat with some hills coming onto the site along
the southern boundary. The existing project gross acreage is 66.23 acres. The site’s drainage area
flows from the south and southwest to the north and northeast to the north side of Baker Street.
Two new buildings are proposed, building 1 is 206,982 sf and building 2 is 778,423 sf, landscape
areas, driveways, and parking lots.
The proposed buildings will consist of a warehouse and connected office space with the
necessary improvements to facilitate business. The offsite drainage areas will be captured with a
flow-by basin and a debris basin routed through storm drain before discharging at their historical
locations on the north side of baker street. The onsite drainage areas will be captured by CMP
Detention System and treated by MWS Units.
Baker Street will be improved on the project frontage. The full width drainage of the street will
be captured within the catch basin at the low points of Baker Street. The flows will continue into
MWS units within Baker Street.
Preliminary Drainage Report 6
1.3 FLOODPLAIN MAPPING
The National Flood Insurance Act (1968) established the National Flood Insurance Program,
which is based on the minimal requirements for floodplain management and is designed to
minimize the flood damage within Other Flood Areas. The Federal Emergency Management
Agency (FEMA) is the agency which administrates the National Flood Insurance Program. Other
Flood Areas are defined as areas of 0.2% annual chance flood; areas of 1% annual chance flood
with average depths of less than 1 foot or with drainage areas less than 1 square mile; and areas
protected by levees from 1% annual chance flood. Flood Insurance Rate Maps (FIRMs) were
developed to identify areas of flood hazards within a community.
According to the Flood Insurance Rate Map (FIRM) catalog, there are FIRMs produced by
FEMA for the project site:
MAP Number: 06065C2028G
MAP Revised: August 28, 2008
FEMA FIRM Panel (Figure 4) is attached in Appendix E shows the floodplain limits and
mapped flood zones for the Baker Industrial project area. The project is located within Zone AE,
which is an area within the flood hazard areas subject to inundation by the 1% annual chance
flood.
1.4 DESIGN CRITERIA
The following are design criteria for this project, based on the Riverside County Hydrology
Manual.
Protection Levels
1. The 100-year flood shall be contained 1’ below the building pads.
2. The 10-year flood shall be contained within the top of curbs.
1) Loss rates are to be determined for the 2- and 5-year events using an AMC I condition, while
an AMC II are used for the 10-year event and 100-year event.
Preliminary Drainage Report 7
Section 2
HYDROLOGIC DATA
AND MODEL DEVELOPMENT
2.1 EXISTING CONDITION MODEL
The project site existing condition consists of undeveloped land and characterized by steep
topography, generally decreasing in elevation from the south to the north. Several ravines are
present which will convey natural drainage across the project site washing towards the north side
of Baker Street.
The site is comprised of four (4) major drainage areas and four (4) offsite drainage areas to
describe the existing drainage conditions. Refer to Existing Condition Hydrology Key Map
Figure 2 in Appendix C for locations of the drainage sub-areas and peak flows. Hydrologic
calculations to evaluate surface water runoff associated with the 10-year and 100-year storm
frequency were performed for the on-site drainage areas. The Riverside County Rational Method
Hydrologic calculations (as described in the RCHM) were performed using the CivilDesign
Hydrology / Hydraulics computer program package 2005 by Bonadiman and Associates, Inc.
Precipitation point values for the 10-year and 100-year durations obtained from Riverside
County Flood Control Plate D-4.3, D-4.4, and Plate D-4.6 (see Appendix B).
In order to proceed with analysis of the proposed developed condition, it is necessary to first
establish the pre-developed peak runoff rates. Table 1 summarizes the data and results for the
10-year and 100-year storm event for on-site and off-site flows. All calculations can be found in
Appendix C of the report.
Table 1. Existing Condition Peak Flow Summary
Drainage Area Area (AC) Q100 (CFS)
Q10 (CFS)
Q5 (CFS)
Q2 (CFS)
A 168.50 332.50 197.59 132.10 84.23
B 29.87 77.05 46.47 32.46 21.19
C 24.19 58.01 35.01 24.29 15.83
D 194.30 428.57 258.99 166.04 106.37
2.2 PROPOSED CONDITION MODEL
In the proposed condition, the proposed improvements are to add two large industrial/warehouse
type buildings, parking area with asphalt pavements, concrete slabs, landscape areas, and
improvements to Baker Street. Refer to Appendix B for a preliminary soils report demonstrating
soil type and hydrologic group classification. Two new buildings are proposed, building 1 is
206,982 sf and building 2 is 778,423 sf, landscape areas, driveways, and new drainage systems
will all drain to Baker Street. Street improvements are proposed for Baker Street.
Preliminary Drainage Report 8
The developed condition site consists of four (4) major drainage areas and four (4) offsite
drainage areas. The project site runoff will be picked up by a system of gutters and inlets that
will discharge through a storm drain system that outlets to Baker Street. The offsite flows will
discharge in desilting basins to the south of the property and continue as it does in its current
existing conditions through proposed dual 48” storm drains and dual 5x4’ storm drain box.
Offsite drainage area will not comingle with onsite flows. Refer to Proposed Condition
Hydrology Key Map Figure 3 in Appendix D for locations of the drainage sub-areas and peak
flows. Hydrologic calculations were evaluated for surface water runoff associated with the 10-
year and 100-year storm frequency. The proposed condition watershed boundaries were
delineated using the project’s conceptual grading plan. Hydrologic land cover for the
development is considered commercial.
Table 2 summarizes the proposed condition 10 and 100-year rational method results. Proposed
condition rational method calculations can be found in Appendix D of the report.
Table 2. Proposed Condition Peak Flow Summary
Drainage Area Area (AC)
Q100 (CFS)
Q10 (CFS) Q5 (CFS) Q2 (CFS)
A 150.09 272.50 154.35 120.18 76.87
B 41.42 105.27 63.26 50.84 34.79
C 39.22 97.19 59.73 48.78 34.26
D 175.52 391.60 237.02 151.53 97.18
Table 3 summarizes the comparison between the Existing Condition and Proposed Condition
Hydrology results.
Table 3. Existing vs. Proposed Condition Peak Flow Summary
Existing Condition Proposed Condition Note
Drainage Area
Area
(Acres)
Q100
(CFS)
Q10
(CFS)
Area
(Acres)
Q100
(CFS)
Q10
(CFS)
A 168.50 332.50 197.59 150.09 272.50 154.35 △ Q100 = 60.00
QProposed < QExisting
△ Q10 = 43.24
QProposed < QExisting
B 29.87 77.05 46.47 41.42 105.27 63.26 △ Q100 = 28.22
QProposed > QExisting
△ Q10 = 16.79
QProposed > QExisting
C 24.19 58.01 35.01 39.22 97.19 59.73 △ Q100 = 39.18
QProposed > QExisting
△ Q10 = 19.72
QProposed > QExisting
D 194.30 428.57 258.99 175.52 391.60 237.02 △ Q100 = 36.97
QProposed < QExisting
△ Q10 = 21.97
QProposed < QExisting
Preliminary Drainage Report 9
Section 3
UNIT HYDROGRAPH ANALYSIS EXISTING
AND PROPOSED CONDITIONS
3.1 INTRODUCTION
The purpose of this section is to describe the parameters and modeling methodologies used in the
development of the unit hydrograph calculations. KWC Engineers has performed a synthetic unit
hydrograph analysis for the Baker Industrial project in order to mitigate the expected increase
runoff from the site at drainage areas B and C. The difference between the proposed condition
and the existing condition storm volumes are used to preliminary estimate to meet the increase
runoff criteria.
The CivilDesign Unit Hydrograph Analysis version 7.0 computer program was used to perform
the calculations.
3.2 APPROACH AND METHODOLOGY
Unit hydrograph calculations were performed to determine the 10-year for the 1-hour and 24-
hour duration storms in accordance with the procedures of the Riverside County Flood Control
and Water Conservation District Hydrology Manual (RCHM).
Point precipitations were taken from the Isohyetal maps shown on Plates D-4.1, D-4.4, E-5.5 and
E-5.6 of the RCHM (see Appendix B). The centroid of the watershed was used as the location on
the Isohyetal maps to determine the point precipitation values. Table 4 summarizes the point
precipitations used in this study.
Table 4: Precipitation Depth-Duration-Frequency Summary
Frequency Duration
1-Hour 24-Hour
(in) (in)
2-Year 0.55 1.45
100-Year 2.50 6.00
The CivilDesign software performs an interpolation using the 2-year and 100-year
values to determine the intermediate frequencies (5-year and 10-year values).
The rainfall-runoff transformation was determined using the s-graph method as outlined in
Section E of the RCHM. The RCHM has four S-Graphs (Plates E-4.1 through E-4.4) titled
Valley, Foothill, Mountain, and Desert, respectively, to represent the specific runoff
characteristics of watersheds. The Valley S-graph was selected as the most appropriate S-graph
Preliminary Drainage Report 10
for this study area due to the mild onsite topography. The CivilDesign Unit Hydrograph
computer program performs the transformation of S-graph and lag time to unit hydrograph
ordinances.
Watershed Lag is defined as the elapsed time in hours from the beginning of unit effective
rainfall to the instant that the summation hydrograph for the concentration point of an area
reaches 50 percent of ultimate discharge. Lag time for the watersheds were calculated using the
equation on Plate E-3 of the RCHM. It is calculated from the physical characteristics of a
watershed area by the empirical formula:
Lag = 24n[(L x Lca )/S(0.5)](0.38)
Where:
Lag = Lag time in hours
n = The visually estimated basin roughness coefficient
L = Length of longest watercourse (miles)
Lca = Length along longest watercourse, measured upstream to a point opposite the
centroid of the area (miles)
S = Overall slope of the longest watercourse between headwater and the collection
point (feet per mile)
Tables 5 and 6 summarizes the watershed lag calculations for the onsite existing and proposed
conditions.
Table 5: Existing Condition Watershed Lag Time Parameters
Drainage Area Area L Lca Elev_Up Elev_Dn S n Lag
(ID) (ac) (ft) (ft) (ft) (ft) (ft/mi) (hr)
B 29.90 2,149 1,357 1532.0 1258.0 673.21 0.045 0.133
C 24.20 1,935 1,199 1532.0 1259.0 744.93 0.055 0.146
Table 6: Proposed Condition Watershed Lag Time Parameters
Drainage Area Area L Lca Elev_Up Elev_Dn S n Lag
(ID) (ac) (ft) (ft) (ft) (ft) (ft/mi) (hr)
B 41.41 3,378 1,788 1532.0 1258.0 428.28 0.029 0.123
C 39.20 2,467 1,557 1532.0 1259.0 584.29 0.039 0.131
Infiltration rates (loss rates) were based on Plate E-6.2 in the RCHM using the runoff index (RI)
determined for each soil and land use condition according to Plate E-6.1. Output from the CivilD
rational results provided a composite RI value which is used as input. Per the recommendations
in the RCHM, Antecedent Soil Moisture Conditions AMC I was used for the 2-year and 5-year
storms, and AMC II was used for the 10-year storm.
Preliminary Drainage Report 11
3.3 ON-SITE UNIT HYDROGRAPH RESULTS
Table 7 presents a comparison between the proposed and existing runoff peak flow rates and
volumes at the project outlet point.
Table 7: Unit Hydrograph Results for Project Outlet Point
Existing Condition Proposed Condition Volume Difference
Drainage
Area
Storm
Frequency
(yr)
Duration
(hrs)
Peak
Flow
(cfs)
Storm
Volume
(ac-ft)
Peak
Flow
(cfs)
Storm
Volume
(ac-ft)
Proposed – Existing
(ac-ft)
B
100 1 77.24 3.3 110.14 4.6 1.3
24 20.55 8.5 29.05 13.5 5.0
10 1 46.46 1.9 67.28 2.7 0.8
24 12.20 4.5 14.02 6.3 1.8
C
100 1 58.25 2.6 100.52 4.4 1.8
24 16.53 6.9 28.16 13.7 6.8
10 1 35.74 1.6 62.70 2.7 1.1
24 9.83 3.6 14.01 6.8 3.2
As shown in Table 7, the 100-year 24-hour storm produces the largest difference (proposed
minus existing) of 6.80 acre-feet which can be used as an approximate value to size the CMPs.
The CMP is proposed to be located on the northern side of the project. The onsite storm drain
system will drain to this location using a single pipe outlet. The detention basin will need to be 6
to 8 feet below the ground surface to allow the pipe to outlet at the existing surface elevation.
The unit hydrograph maps and calculations for existing and proposed conditions are included in
Appendices E and F.
Preliminary Drainage Report 12
Section 4
HYDRAULIC ANALYSIS
4.1 ONSITE DRAINAGE FACILITIES
Preliminary onsite drainage facilities for the project were calculated utilizing the rational method
hydrology program and are presented on the Proposed Condition Hydrology Key Maps. The
approximate locations of drainage facilities are intended for conceptual purposes only and will
be refined in the design review and final engineering process. Pipe sizes are based on the design
criteria presented in Section 1.4. Pipes were designed as reinforced concrete pipe, with a
roughness coefficient of 0.013. The proposed onsite storm drain system calculations will be
performed in the Final Engineering phase.
4.2 STREET CAPACITY ANALYSIS
Preliminary onsite street capacities for the proposed project were calculated by the rational
method software. By reviewing street capacity analysis within the rational method, it’s
concluded that the street sections are sufficient to provide the level of protection required by
Riverside County as previously discussed. Since, the storm drain mainline pipes were sized
for the 100-year storm event, the street will only contain local flows until catch basins
intercept the 100-year flow.
4.3 OUTLET ANALYSIS
The proposed development will outlet at four (4) locations: Drainage Area A will outlet into
a proposed two 48-inch CMP, Drainage Area B will outlet into a proposed 48-inch CMP,
Area C will outlet into a proposed 36-inch CMP, and Area D will outlet into a proposed two
5x4’ box. The outlet velocity shall be at or below the existing condition or to a non-erosive
velocity.
4.4 DETENTION ANALYSIS
The developed tributary areas at the most downstream point for each stream within the proposed
project were analyzed for both the existing and proposed conditions. The 100-year and 10-year
storms, for the 1-hour and 24-hour events were analyzed using the Riverside County Unit
Hydrograph Method. The difference in peak flow and volume were compared to determine if and
how much mitigation would be necessary. Refer to the tables on the following pages for the
results of the Detention Analysis. Unit Refer to Appendix F for the Unit Hydrographs for the
Existing Condition and Appendix G for the Unit Hydrographs for the Proposed Condition. The
same overall tributary drainage boundary for each stream was used in the Rational Method and
Unit Hydrograph Method. Refer to the Unit Hydrograph Method Maps for the area delineation,
centroids, length and difference in elevation calculation.
Preliminary Drainage Report 13
The project proposes a detention basin for drainage areas B and C . A combination of WQMP
and detention basin is proposed. Refer to the Hydrology Maps for locations and details of these
basins.
Preliminary Drainage Report 14
Section 5
DEBRIS BASIN ANALYSIS
5.1 DESIGN CRITERIA
This section determines the debris yield for the undeveloped tributary watershed areas upstream
of the development. The corresponding calculated debris yield is then used to determine the size
of the debris basin required upstream of the drainage watersheds. Sediment yield must be
accounted for when designing drainage facilities to convey runoff from undeveloped (natural)
areas. Soil erosion caused by runoff in undeveloped areas increases the amount of sediment yield
or load in storm water discharge. In order to accommodate for the increase in discharge volume a
debris factor must be accounted for when considering design flows. Basin sizing and calculations
will be provided in the final hydrostudy report.
Preliminary Drainage Report 15
Section 6
CONCLUSIONS
This preliminary drainage study has evaluated the potential effects of runoff on the proposed
project. In addition, the report has addressed the methodology used to analyze the existing and
proposed conditions, which was based on the Riverside County Hydrology Manual. This section
provides a summary discussion that evaluates the potential effects of the proposed project.
The project is 66.23 acres of developed industrial land.
Offsite tributary areas will be captured and moved into their original flow location along
the north side of Baker Street.
Baker Street full width improvements will be captured, treated, and discharged to the
north side of Baker Street.
Onsite tributary areas will be captured, treated, and discharged to the north side of Baker
Street.
Preliminary alignment and pipe sizes of storm drain lines were presented.
All storm water runoff will be carried via gutters, catch basins, or onsite storm drain system that
will outlet into the north side of Baker Street. The computed 10-year storm event is contained
below the top of curb and the computed 100-year storm event is contained within the street right-
of-way.
Preliminary Drainage Report 16
Section 7
REFERENCES
Riverside County Flood Control and Water Conservation District. Hydrology Manual. April
1978
Riverside County Flood Control and Water Conservation District. Increased Runoff
Mitigation Workshop. August 24, 1995
State of California – Department of Transportation, Interstate 91 Drainage Plans, December 1,
2008
Appendix A
VICINITY MAP
Appendix
B
RCFC PRECIPITATION DATA &
NRCS SOILS REPORT
SITE