HomeMy WebLinkAboutFlood Operations Volume 1 of 2_Oct2006 (2)
SUMMERL Y DEVELOPMENT
LAKE ELSINORE BACK BASIN
FLOOD STORAGE, DETENTION AND OPERATION PLAN
(Volume 1 Of 2)
October 2006
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Prepared For:
Laing-CP, Lake Elsinore, LLC
31900 Mission Trail
Suite 225
Lake Elsinore, CA. 92530
prepared by:
Van Dell and Associates, Inc.
17801 Cartwright Road
Irvine, CA 92614
Summerly Development
Lake Elsinore Back Basin
Flood Storage, Detention and Operation Plan
SUMMERL Y DEVELOPMENT
LAKE ELSINORE BACK BASIN
FLOOD STORAGE, DETENTION AND OPERATION PLAN
(Volume 1 Of 2)
October 2006
Prepared For:
Laing-GP, Lake Elsinore, LLC
31900 Mission Trail
Suite 225
Lake Elsinore, CA. 92530
prepared by:
Van Dell and Associates, Inc.
17801 Cartwright Road
Irvine, CA 92614
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TABLE OF CONTENTS
I. INTRODUCTION ...................................................................................................................... 3
A. Background ....................................................................................................................... 3
B. HEC-5 Analysis ................................................................................................................... 3
C. Purpose of Report .............................................................................................................. 4
11. DESCRIPTION OF SITE GRADING PLAN .............................................................................. 4
A. General Mass Grading Requirement ................................................................................... 4
B. Project Elements (Refer to Exhibit 8) .................................................................................. 4
Ill. FLOOD STORAGE, DETENTION AND OPERATIONS PLAN ................................................. 6
A. Management of Off-Site Storm Runoff ................................................................................ 6
B. Management of On-Site Storm Runoff ................................................................................ 8
C. Golf Course Flood Management. ...................................................................................... 1O
D. Pump Station Design, Operation and Maintenance ........................................................... 12
E. Back Basin Dewatering ..................................................................................................... 12
IV. EMERGENCY FLOOD WARNING SYSTEM ......................................................................... 15
A. Need for a Flood Warning System .................................................................................... 15
B. Emergency Warning System Protocol................................................................................ 16
V. IMPACT OF BACK BASIN FLOODING ON PROJECT INFRASTRUCTURE .......................... 16
A. Geotechnical Assessment of Impacts ................................................................................ 16
VI. EXHIBITS ............................................................................................................................... 18
VII. APPENDICES ........................................................................................................................ 19
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I. INTRODUCTION
A. Background
John Laing Homes (JLH) is in the process of developing its Summerly Project, a
planned residential/recreational community located within the Lake Elsinore Back
Basin. The project area, comprising 706-acres, is located between Mission Trail
Road to the east, Lake Elsinore to the west, Diamond Stadium and Malaga Road
to the north and the U. S. Army Corps of Engineers (COE) wetland mitigation
project to the south. The project location is shown on Exhibit A. The project is
subject to the conditions set forth in Amendment 6 to the East Lake Specific Plan
that was approved by the City of Lake Elsinore in 1993. JLH is currently in the
process of mass grading the first phase of the Summerly Project.
As a part of the regulatory permitting required for construction of the Back Basin
Levee, a Lake Management Plan was established that defines the long-term
management of the Lake and Back Basin. The California Department of Fish
and Game issued a Streambed Alteration Agreement in 1989 that established its
jurisdiction to elevation 1265 feet MSL and the COE issued a Section 404 Permit
in 1988, subsequently amended in 1989 and 1995, that established COE
jurisdiction within the Back Basin to elevation 1246 feet MSL. The Section 404
Permit includes Conditions L and M related work proposed within the Back Basin
below elevation 1260 feet MSL. Condition L requires that a HEC-5 analysis be
performed to demonstrate that the100-year flood level in the Back Basin
elevation, established as elevation 1263.3 Feet MSL by the LMP, would not be
exceeded as a result of proposed development. Condition M requires a
demonstration that any proposed development within the Back Basin would not
reduce the LMP established flood storage capacity of 30,750 acre-feet, that the
development would convey floods up to the 100-year frequency, and that the
hydrologic conditions resulting from development would sustain the COE
jurisdictional 356-acre wetlands. Subsequent surveys of the Back Basin have
shown that the existing condition flood storage volume is 30,525 acre-feet. The
COE has agreed that this is the storage volume that must be preserved in the
Back Basin as a part of any proposed development project.
B. HEC-5 Analysis
HEC-5 analyses have been performed for the existing condition (FEMA adopted
model) and for each of the three project phases. The results of theses analyses
show that there is no adverse affect on the 100-year water surface elevation of
1263.3 feet MSL due to the construction of any individual phase, or due to the
final build-out of the proposed development. The analyses also show that that
the Main Lake water surface elevations and the Lake outflows that will occur
between the weir crest elevation of 1262 feet MSL and the peak Lake elevation
of 1263.3 feet MSL have not been adversely affected due to construction of any
individual phase, or due to the final build-out of the proposed development.
The HEC-5 analyses have been reviewed and accepted by the COE. The HEC-
5 report entitled "Lake Elsinore Flood Routing Study, John Laing Homes
Summerly Project, Tract 31920, HEC-5 Analysis to Determine Effect of
Summerly Project on the 100-year Lake elevation", dated August 2004, Revised
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December 2005, as prepared by Van Dell and Associates, Inc., will be made
available upon request.
C. Purpose of Report
The purpose of this report is to describe the flood control features of the
Summerly Project and define how these features will be managed to assure 100-
year flood protection for the project and be in compliance with the LMP.
11. DESCRIPTION OF SITE GRADING PLAN
A. General Mass Grading Requirement
Since the flood storage volume provided by the Back Basin in its existing
condition (30,525 acre-feet) above elevation 1240 feet MSL must be preserved, it
was necessary to excavate material from within the basin and to place the
excavated material for development such that there would be no loss of flood
storage volume. Consultation with the Riverside County Flood Control and
Water Conservation District (District) led to the criteria that for each unit-volume
of fill placed below elevation 1263.3 feet MSL, excavation of 1 1/3 units-volumes
must be obtained below elevation 1263.3 feet MSL from another area of the Back
Basin.
It is planned to construct the project in three phases. It is currently envisioned
that the grading of the individual phases will proceed in sequence with no
significant time gaps provided the regulatory approvals of the subsequent phases
are not delayed. The grading requirement described above has been achieved
for each of the three project phases. The HEC-5 report referenced in Section I.B
above contains the documentation confirming that the grading requirement has
been satisfied. The three phases of the project including the various project
elements are shown on Exhibit B.
B. Project Elements (Refer to Exhibit B)
The project elements to be created through site grading include the following:
1. Residential Development Area
The residential development area is placed on approximately 377 acres
and provides for 1955 units placed with finish floor elevations of 1267 feet
MSL or higher. The development area includes approximately 40 acres
of streets. Storm runoff will be collected in the streets and conveyed by
underground storm drains to the Golf Course and Habitat Buffer areas.
Refer to Exhibit D.
2. Golf Course and San Jacinto River Channel
A public 18-hole regulation golf course will be constructed to the west of
the residential area. The golf course will be excavated approximately 30
feet below existing ground elevations to preserve the required minimum
flood storage volume of 30,525 acre-feet above elevation 1240 feet MSL
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in the Back Basin. The golf course will include two lakes that will store
storm and/or reclaimed water as a source of irrigation water for the golf
course area. The golf course will also include drainage swales to convey
surface runoff and storm drain discharges from the residential areas to
lakes and constructed wetlands. These facilities will provide biological
treatment of storm runoff to assure water quality requirements are met.
The existing San Jacinto River bed will also be lowered and widened as a
part of the golf course excavation. The lowered riverbed will be
revegetated to create an enhanced river corridor.
A berm will be constructed along the north limit of the golf course and
along the westerly boundary for a distance of approximately 900 feet
south of the northwesterly corner of the project. The purpose of this berm
is to divert approximately 22 percent of the weir flows to the westerly
portion of the Back Basin west of the project limits. The berm will protect
the golf course and north park area from the main Lake weir flow and
confine the 78 percent portion of the weir flow to the San Jacinto River
Channel as it enters the project area.
3. Riparian Habitat Buffer and Detention Area
A riparian habitat buffer and detention area will be graded and excavated
along southern boundary of the project. The buffer width will be
approximately 250 feet in width. Storm runoff from the easterly and
southerly portions of the residential development areas will be conveyed
to the buffer area. In addition, the off-site drainage areas directly tributary
to the project area will be conveyed by the in-tract drainage systems to
the buffer area. Refer to Exhibit D.
A detention or ponding area will be excavated at the southwesterly limit of
the project area. Storm runoff that enters the golf course area that is not
captured in the golf course lakes or storm runoff that enters the habitat
buffer area will be discharged into this ponding area after passing through
the golf course/buffer area bio-swales and constructed wetlands. The
golf course and the ponding area will be graded so that up to a 50-year
storm runoff volume from the areas tributary to the ponding area can be
stored prior to discharge to the 356-acre wetlands located immediately to
the south. Refer to Exhibit F. A pumping station will be constructed on a
building pad adjacent to the southerly property line. Refer to Exhibit G
and Exhibit H. This building pad will be graded to approximately elevation
1266.5 feet MSL. The stored water will either be pumped back to the
most southerly golf course lake and/or the 356-acre wetlands.
4. Riparian Lake/River Corridor and Water Supply
An open space corridor approximately 165 feet in width will be graded
along the westerly boundary of the project. The corridor will be
vegetated with wetland, riparian, and upland habitat. This corridor will
convey up to 3 cfs of either Lake water or reclaimed water to sustain the
wetland corridor.
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A water intake structure will be constructed on the main levee that
represents the south bank of the Lake inlet channel. The intake structure
will include an operable sluice gate structure with intake elevation of
1239.0 feet MSL, a back-up control valve and manhole vault, and a
delivery conduit aligned from the sluice gate structure to the north limit of
the riparian corridor. In the event the minimum lake elevation of 1240 feet
MSL cannot be maintained due to lack of make-up water, the alternate
reclaimed water source will be discharged into the corridor. A preliminary
design of the intake structure, manhole vault and the delivery pipeline to
the Riparian Lake/River Corridor is shown on Exhibit I.
The responsibility for the operation and maintenance of the intake
structure and its related elements will rest with Elsinore Valley Municipal
Water District (EVMWD).
Ill. FLOOD STORAGE, DETENTION AND OPERATIONS PLAN
A. Management of Off-Site Storm Runoff
1. Off-Site Hydrology Studies
a. The drainage areas that are directly tributary to the project site
include those watersheds East of Mission Trail and North of
Malaga Road. The 100-year peak discharges and runoff volumes
have been computed using the District's hydrology manual. The
results of the hydrologic studies for these drainage areas are
included in Appendix A, Project Hydrology Report. ·,
b. The drainage areas that are directly tributary to the Back Basin
outside the limits of the project total to approximately 5,500 acres
of watershed area. Refer to Exhibit F. The 100-year peak
discharge and total runoff volume for the entire watershed has
been computed using the District's hydrology manual. The result
of the hydrology study for this large drainage area is included in
Appendix B, Back Basin Tributary Area Hydrology Report.
2. Drainage Areas East of Mission Trail and North of Malaga Road
The 100-year frequency runoff from the drainage areas to the north and
directly east of the project will be collected in a 42-inch diameter storm
drain and conveyed through the development to its discharge point at the
easterly limit of the Riparian Habitat Buffer. At this point the off-site flows,
commingled with a portion of the on-site development runoff, will pass
through the habitat buffer and ultimately discharge into the detention pond
at the southwesterly portion of the project area. Refer to Exhibits C and
D.
3. Drainage Areas Tributary to the Back Basin South and East of the Project
Limits
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Storm runoff from the tributary watershed will be conveyed directly into
the Back Basin by way of existing storm drains, surface channels and
sheet flow. Refer to Exhibit E. After reaching the Back Basin these flows
will initially pass through an open channel beginning near Rome Hill and
located adjacent to the main Lake levee and immediately south of the
356-acre wetlands. Should the 100-year local flood event occur, the
inflow volume will be large enough to inundate the wetlands and a portion
of the Back Basin adjacent to the main Lake Levee west of the project
limits.
In order to protect the project from potential inflow across the south
boundary, and to protect the adjacent properties to the south of the
project from potential scour, a concrete cutoff wall will be constructed
along the southerly property line (north of the exiting unimproved Cereal
Street) at the low point. The low point along the south boundary is
located approximately 900 feet west of the south east corner of the
project. The preliminary design of the cutoff wall erosion control
structure is shown conceptually on Exhibit J.
4. Lake Elsinore Inlet Channel Weir Flows
Based on the HEC-5 analyses, it has been estimated the main Lake will
fill to the crest of the inlet channel weir elevation of 1262 feet MSL when a
33-year frequency storm occurs over the regional watershed. When a
100-year frequency storm occurs over the regional watershed, the main
Lake will rise to elevation 1263.3 feet MSL resulting in a peak weir inflow
to the Back Basin of approximately 7400 cfs. Approximately 22 percent
of this total inflow, or approximately 1600 cfs, is diverted westerly to the
Back Basin areas west of the project. The remaining 78 percent, or
approximately 5800 cfs, is directed into the project area. Based on this
split of the weir flow, and based on the total volume in the project golf
course area being 4247 acre-feet and the volume outside of the project
site being 2087 acre-feet at elevation 1246 feet MSL, computations show
that flood water within the project area will begin to flow to the back basin
area from the southerly boundary low point (elevation 1246 feet MSL) 62
hours after weir flow begins. The back basin and project flood elevations
will begin to equalize at about elevation 1249 feet MSL.
The north project boundary west of the lowered San Jacinto River is
protected by a earth levee approximately 4 feet above natural ground and
protected from erosion with riprap placed on the north face of the levee.
A HEC-RAS hydraulic analysis has been performed to determine the
required height of the north property line levee. The hydraulic analysis
documentation is included in the HEC-5 analysis report referenced in
Section I.B of this report.
The lowered San Jacinto River has been graded to create a 250-foot wide
trapezoidal shaped channel with levees on both sides of varying heights
and a bottom width of 40 feet. The channel invert slope is 2 percent. The
levees have been protected with riprap placed on the inside levee slopes
to prevent erosion during the peak inflow. Once the project area fills to
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elevation 1255 feet MSL, the San Jacinto River channel will be
completely inundated. It is estimated that the time to achieve complete
inundation of the river channel from the beginning of weir flow is 122
hours (18,934 acre-feet) based on the historical 1980 storm scenario.
When submergence of the channel occurs, the risk of channel side slope
erosion is significantly reduced due to tail water energy dissipation.
5. Supplemental Worst Case Flood Analysis
Based on the pumping time studies prepared for the project (see
Paragraph E.2 of this Section), it is possible that the Back Basin could be
flooded for several months in the event of a 100-year or lesser flood on
the regional watershed. This situation suggests the unlikely possibility of
a second flood event while reduced flood storage capacity exists within
the Back Basin. In order to evaluate the risk to the project under such a
circumstance, the 1980 historical rainfall data was processed using the
HEC-5 model assuming a starting water surface elevation of 1262.0 feet
MSL, the crest of the weir structure. It was also assumed that for this
starting WSE, the Temescal Outlet channel is flowing in accordance with
the stage-discharge table included in the HEC-5 model and that the Back
Basin is flooded to elevation 1262 feet MSL. The result of this analysis is
as follows:
Worst Case Peak Main Lake Elevation:
Worst Case Peak Back Basin Elevation:
Freeboard on Development FF Elevation:
1263.27 feet MSL
1263.27 feet MSL
3.73 feet
This result indicates that because of the significant flood storage volume
that exists above the weir crest elevation and because of the significant
Temescal Wash outflow at this high stage, the highly unlikely event of
back-to-back 1980 storms will not reduce the available freeboard on the
minimum finish floor elevation of 1267 feet MSL.
B. Management of On-Site Storm Runoff
1. On-Site Hydrology Study
Based on the site development plan, a detailed hydrology study was
prepared for design of the on-site drainage system. This study
incorporates the hydrologic studies prepared for the off-site areas
mentioned in Section 111.A.1 above since the off-site flow will be conveyed
through a portion of the project drainage system. The hydrology study
also provides information needed to perform flood routing studies of the
Golf Course and Buffer areas for pump station sizing studies. The on-site
hydrology study is included in Appendix A, Project Hydrology Report.
2. Description of On-Site Storm Drain System
Since the on-site development area is relatively flat, it was necessary to
configure the local streets with periodic high and low points. Catch basins
were located at the low points to collect and deliver the storm water to
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underground storm drains. The storm drains were laid out to generally
follow street alignments draining to the low-lying Golf Course and Habitat
Buffer areas. Refer to Exhibit D.
3. Storm Drain Hydraulic Analyses
The hydraulic analyses of the on-site storm drain system were performed
for two scenarios. The first scenario assumes that the 100-year design
storm occurs on local watersheds and that the Back Basin is not flooded.
The second scenario assumes that the 100-year design storm occurs on
the local watersheds and that the Back Basin is flooded to the weir crest
elevation of 1262 feet MSL.
a. Local 100-year Flood Event/No Back Basin Inflow
For this scenario, the analysis of the on-site storm drain system
assumes that the storm drain discharge locations are not
submerged, i.e., operate with free discharge. This storm drain
system was sized based on this assumption using the WSPG
modeling software. The supporting documentation for the WSPG
calculations is included in Appendix C.
b. Regional and Local 100-year Flood Events/Back Basin Flooded to
Weir Crest Elevation 1262 feet MSL
For this scenario, the analysis of the on-site storm drain system
assumes that the conduits are in-place as sized for free discharge,
but that the discharge points are operating against a tail water at
elevation 1262 feet MSL. The hydraulic analysis determined the
hydraulic capacity of the system assuming the flood level in the
community cannot exceed levels 2.5 feet below finish floor
elevation. Since the storm drain system was designed for free
discharge, the submerged outlets will reduce the hydraulic
capacity of the system. This situation requires the establishment
of secondary surface overflow routes to deliver the surface flows
to the Golf Course and Habitat Buffer areas.
It should be noted that the assumption of the occurrence of a local
100-year flood event at the time the Back Basin is flooded to
elevation 1262 feet MSL is a very conservative or a very unlikely
occurrence. Since the storm drain outlet control elevations are at
elevation 1240 feet MSL or higher, the tail water levels can be
drawn down from elevation 1262 feet MSL to elevation 1240 feet
MSL in approximately 100 days. (See Paragraph E.2 of this
Section.) The probability of a 100-year event occurring during this
period is .01(100/365) = 0.0027. The probability of this event
following a 100-year event is 0.01(0.0027) = 0.000027 or 1 in
37000.
c. Secondary Surface Overflow Routes
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Secondary surface overflow routes have been identified that will
allow the flows which exceed the hydraulic capacity of the
underground storm drain system to escape to the lower Golf
Course and Habitat Buffer areas. The secondary overflow routes
are shown on Exhibit D. Flows being conveyed by these
secondary routes will be at least 2.5 feet below the finish floor
elevations adjacent to or near the flow routes. Secondary surface
overflow hydraulic calculations are included in Appendix C.
C. Golf Course Flood Management
Management of floodwater within the Golf Course and Habitat Buffer area has
been analyzed for two scenarios. The first scenario assumes that 2-, 5-, 10-, 25-
and 50-year flood events occur over the off-site area directly tributary to the
project limits and also on the on-site and golf course tributary areas. This
scenario does not consider the 100-year flood event occurring on the on-site and
off-site local drainage areas since it has been assumed that the Lake Inlet
Channel weir structure will begin to discharge into the Back Basin for a 33-year
frequency regional flood. Such conditions would begin flooding the entire Back
Basin and would result in a condition that would shut down Golf Course
operations.
The second scenario assumes that a regional 100-year flood occurs resulting in
inundation of the Back Basin with flood levels reaching elevation 1263.3 feet
MSL.
1. Local 2-, 5-, 10-, 25- and 50-year Flood Events/No Back Basin Weir
Inflows
Golf Course inflow volumes were computed for the various frequency
storms based on the hydrologic studies and compared with the available
storage volume available within the Golf Course below the tee boxes,
greens and fairway areas. For this analysis, it was assumed that, other
than the normal storage in the Golf Course lakes and wetlands, the flood
storage areas were at elevation 1217 feet MSL and there was no pumped
outflow from the storage areas. The computed volumes and the peak
ponding elevations within the Golf Course are as tabled below.
Peak Golf Course Area Ponding Levels
For the 2-, 5-, 10-, 25- and 50-year Flood Inflows
No pumped Outflow
Flood Frequency Flood Volume, ac-ft Peak Ponding Elev., ft
2-year 89.3 1222.78
5-year 174.7 1224.82
10-year 245.1 1226.17
25-year 323.4 1227.37
50-year 413.2 1228.56
An analysis of the available storage within the Golf Course shows that all
of the various frequency floods can be stored within the lakes and low
areas such that the tee boxes, greens and fairways are not inundated
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given the initial starting water surface elevation. The 50-year ponding
level has been plotted and is shown on Exhibit F. Since the 50-year flood
level will not impact Golf Course play, immediate pumping of the stored
floodwater, as a flood protection measure, will not be necessary.
Consequently, the minimum pumping capacity has been set at the level
needed to meet the golf course irrigation needs of 1 million gallons per
day or approximately 700 gpm. The maximum pumping capacity has
been set at 2 million gallons per day, or approximately 1500 gallons per
minute (3 cfs +/-). It is recommended that a pump of a capacity of 1500
gpm operating with a total dynamic head of approximately 46 feet be
utilized for routine Golf Course and wetland supply pumping operations.
To provide flood storage for subsequent storm events, the 50 acre-foot
ponding area may be pumped down to provide additional storage volume
as considered necessary based on weather forecasts and existing
watershed saturation conditions. Pump station force mains and related
valving will allow dewatering the storage area by delivering storm water to
either the most southerly Golf Course lake or the 356 acre wetlands. The
pump station force main layout plan is shown on Exhibit G.
2. Regional 100-year Flood Event with Back Basin Weir Inflow
Based on the HEC-5 analysis, weir flow will begin when a flood frequency
of 33-years or greater occurs. Lake inflows that exceed the Lake outlet
capacity will cause overflow of the weir structure and flooding of the Golf
Course will begin. As the Lake rises the weir flow will increase to a peak
discharge of approximately 7400 cfs at elevation 1263.3 feet MSL, the
100-year peak Lake elevation. Since approximately 78 percent of the
weir flow will directly enter the Golf Course area, Golf Course operations
cannot continue since complete inundation of the course will occur for the
larger regional flood events.
For those flood events that have sufficient volume to cause the Back
Basin ponding level to exceed elevation 1246 feet MSL (South boundary
weir crest elevation), flow will occur over the south property boundary
weir and into the south portion of the Back Basin. If the flood event is not
sufficient to cause the ponding level in the Golf Course to rise to elevation
1246 feet MSL, a decision may be made to begin dewatering the Golf
Course area. Under this scenario, water within the Golf Course can be
pumped into the Back Basin since the surrounding ground elevations are
at elevation 1248 feet MSL or above. Since the volume of water in
storage within the Golf Course area to elevation 1246 feet MSL will be
4,247 acre-feet, it would take a 1500 gpm pump approximately 650 days
of pumping around the clock to empty the Golf Course area if ponded to
elevation 1246 feet MSL. Consequently, it is likely that several large
pumps will need to be rented to accomplish the dewatering activity in a
reasonable amount of time. As an example, if four 20 cfs capacity
pumps (36,000 gpm total capacity) are utilized on a 24-hour basis, the
Golf Course could be dewatered in approximately 25 days.
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The decision as to the number and capacity of the pumps to be deployed
under this scenario rests with the Golf Course management in
consultation with EVMWD and the City of Lake Elsinore. For such a
dewatering effort to be successful it will be necessary that the dewatering
of the overall Back Basin be initiated in parallel with the Golf Course
dewatering.
D. Pump Station Design, Operation and Maintenance
1. Pump Station Design
The pump station will be located near the project southerly boundary
approximately as shown on Exhibit B. The minimum pad elevation will be
1,266.5 feet MSL. Two 2,500 gpm pumps and a single 500 gpm pump
will be housed in a concrete block building with a roof design that will
allow the pump to be lifted from the building using a crane for
maintenance and repair. All electrical service, motors and switching gear
will be located at or above the minimum pad elevation. Sound reduction
insulation should be considered as a part of the building design.
Pump impellers will be set at elevation 1217 feet MSL, or as determined
necessary to prevent vortexing, enabling the 50-year storage area to be
dewatered to elevation 1219 feet MSL. This configuration will require a
pump column of approximately 50 feet in length.
Access to the pump station will be from the future Bundy Canyon Road.
A concrete paved ramp will be constructed from the approximate existing
ground elevation of 1247 feet MSL to the building pad elevation of 1266.5
feet MSL. Ramp grades should not exceed 8 percent. The building pad
should be of sufficient size to provide parking for a minimum of two
vehicles with adequate turnaround space.
It should be noted that in the event a Back Basin flood event occurs that
results in ponding to elevation 1246 Feet MSL, it may not be possible to
gain access to the pump station. Consequently, consideration should be
given to establishing remote control operating equipment to activate the
pumps should access to the station not be possible.
2. Pump Station Operation and Maintenance
The pump station will be operated and maintained by EVMWD. EVMWD
will coordinate Golf Course irrigation pumping with the Golf Course
management personnel. Pumping of storm water to the 356-acre
jurisdictional wetlands will be coordinated with the COE and the EVMWD.
E. Back Basin Dewatering
1. Back Basin Pumping Scenarios
In order to expedite the return of the Golf Course to operation and to
minimize flood damage to the tees, greens, fairways and vegetation, it is
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recommended that the pumping of the Back Basin to the main Lake be
initiated when flood levels reach elevation 1246 feet MSL. Beginning
such pumping activities would require that Back Basin weir inflow has
ceased and that the main Lake level has receded below the weir crest
elevation of 1262 feet MSL. If weir flow continues and the Back Basin
flood level is rising above elevation 1246 feet MSL, pumping of the basin
to the main Lake cannot begin until weir flow has ceased. Once pumping
to the main Lake has started, it is feasible to commence pumping of the
Golf Course to the westerly portion of the Back Basin north of the
jurisdictional wetlands.
If flooding of the Back Basin due to weir flows does not reach elevation
1246 feet MSL, pumping of the Golf Course to the Back Basin can
proceed without pumping of the Back Basin to the main Lake. If such
Golf Course pumping activities cause the water level in the Back Basin to
rise to elevation 1246 feet MSL, and the Golf Course has not been
dewatered to the level that would permit the course to operate, pumping
of the Back Basin to the main Lake should be initiated.
2. Main Lake and Back Basin Drain and Dewatering Times
a.) Gravity Drainage Times
Main Lake and Back Basin Without Project (No Pumping)
Elevation 1263.3 feet MSL to Elevation 1262.0 feet MSL: 33 hrs
Main Lake and Back Basin With Project (No Pumping)
Elevation 1263.3 feet MSL to Elevation 1262.0 feet MSL: 36 hrs
Main Lake (No Pumping)
Elevation 1262.0 feet MSL to Elevation 1261.0 feet MSL: 87 hrs
Elevation 1261.0 feet MSL to Elevation 1255.0 feet MSL: 323 days
The gravity drain time data indicate that pumping of the Back Basin
cannot begin until approximately 33 hours following the peak flood
level is reached in the event of a 100-year flood event. This time
assumes that main Lake inflows have ceased at the time the peak
flood level has been reached. The time for the main Lake to drain to
elevation 1261 feet MSL suggests that, to avoid circulation of pumped
basin outflow, pumping should not begin until approximately 36 hours
following the peak flood stage.
b.) Drain Times with Alternative Pumping Scenarios
The following tables summarize the pumping time to dewater the Back
Basin for the indicated scenarios. Note that it has been assumed that
9000 gpm (20 cfs) capacity pumps are available and can be rented for
the various pumping times.
X:\Projects\884_0210\ENGIOOC\REPIREP01_SUMMERLY Fld Oper Rpt.doc 14
Back Basin Drain Times for Alternate Pump Usage
From Elevation 1262.0 feet MSL to Elevation 1240.0 feet MSL
With and Without the Project In-Place *
Number of
9,000 GPM Pumps
Pumping Time
Without Project In-
Place, days
Pumping Time With
Project In-Place, days
1 712 696
3 238 232
5 143 140
7 102 100
10 71 70
15 48 47
22 33 32
*This alternative provides the pumping times to pump the back basin
down to restore the design storage volume.
Back Basin Drain Times for Alternate Pump Usage
From Elevation 1262.0 feet MSL to Elevation 1246.0 feet MSL
With and Without the Project In-Place**
Number of
9,000 GPM Pumps
Pumping Time
Without Project In-
Place, days
Pumping Time With
Project In-Place,
days
1 617 643
3 206 215
5 124 129
7 88 92
10 62 65
15 41 43
22 28 30
**This alternative provides the pumping time to pump the back basin
down to the project low point at the south boundary.
The above tables indicate that a substantial amount of time will be
required to dewater the Back Basin using a single 9000 gpm pump.
The time is considered unacceptable due to the reduced flood
protection afforded to the Back Basin development as well as the
surrounding development adjacent to the main Lake. Consequently, a
judgment must be made as to the required number of pumps (or the
total pumping capacity) to be utilized in order to restore the flood
storage so that flood protection is not compromised. It is considered
prudent and reasonable, in the opinion of the authors, to utilize a
pumping capacity that will restore the Back Basin flood storage
volume in approximately 90 days. Therefore, a minimum of seven
9000 gpm pumps would be required to achieve this objective. Of
course, the availability of such large capacity pumps will control the
actual number of pumps used. The City and EVMWD must agree
ahead of time as to the number of pumps to be used.
X:\Projects\884_0210\ENG\DOC\REP\REP01_SUMMERLY Fld Oper Rptdoc 15
In addition to the number of pumps needed to pump the Back Basin to
the main Lake, as many as four additional 9000 gpm pumps may
need to be obtained to dewater the Golf Course area. The potential
need for up to eleven high capacity pumps suggests the need for an
investigation to determine a source for the pumps and the lead-time
needed to get the pumps to the area. The number of pumps to be
utilized also suggests that a study should be made to develop the
logistics for delivering electric power/gasoline powered generators to
adequately operate the pumping systems. Such investigations and
studies are beyond the scope of this report.
The above tables also indicate that with the project in-place, the drain
times are increased from those without the project in-place. The use
of seven 9000 gpm pumps, however, increases the pumping time by
only 4 days for both scenarios, or an increase of approximately 4
percent. This situation means that the flow in the Temescal Wash
outlet channel will flow for approximately four days longer with the
project in-place than would be the case for existing Back Basin
conditions. This extended flow condition cannot exceed 140 cfs (the
total capacity of the seven pumps). Since the extended flow is only 2
percent of the peak outflow channel discharge during the 100-year
event (6935 cfs), the risk to downstream areas is considered minimal
or non-existent.
3. Back Basin Dewatering Responsibility
The responsibility for dewatering the Back Basin rests with the EVMWD. The
dewatering activities will require coordination with the City of Lake Elsinore,
which will work with EVMWD to locate the placement of temporary rental
pumps to expedite the dewatering of the Golf Course area within the Back
Basin. It is assumed that the EVMWD would be responsible to operating and
managing the dewatering of the Golf Course as well as the pumping of the
Back Basin floodwater to the main Lake.
IV. EMERGENCY FLOOD WARNING SYSTEM
A. Need for a Flood Warning System
Consultations with the District have suggested that the methodology utilized in the
HEC-5 analysis, such as the application of 78 years of rainfall data and utilizing the
plotting position statistical approach to predicting the 100-year peak flood level, is
subject to question as to the degree of conservatism involved in the analysis. The
supplemental worst case flood analysis discussed in Section 111.A.5 of this report
would indicate that such a warning system might not be needed. In any event,
however, the following is suggested as a possible early warning system protocol. It
is anticipated that the final warning system protocol development, if ultimately
needed, will be the responsibility of the City.
X:\Projects\B84_0210\ENG\DOC\REP\REP01_SUMMERLY Fld Oper Rpt.doc 16
B. Emergency Warning System Protocol
The proposed warning system utilizes a color-coded system. The concept involves
physically posting color-coded warning signs at entrances to the project and at the
vehicle and pedestrian access points to the Golf Course and low-lying park areas.
The color code system is related to the main Lake water surface elevations and the
National Weather Service forecasts and District estimates of estimated storm runoff
to Lake Elsinore. The following table summarizes the possible early warning system
protocol. The color code identifying risk is as follows:
Green
Yellow
Orange
Red
No Risk
Low Risk
Moderate Risk
High Risk
Lake Elsinore Back Basin Summerly Development
Early Flood Warning System Protocol
October 15th through_�Qril 15th
Main Lake
WSE (Feet)
Back Basin
WSE (Feet)
Rain
Forecast
Watershed Flood
Risk Yes No Dry Wet
1240 1240 - X X - Green
1240 1240 X - - X Green
1240 to 1255 1240 - X X - Green
1240 to 1255 1240 X - - X Green
1240 to 1260 1240 X - X - Yellow
1240 to 1260 1240 X - - X OranQe
1255 to 1260 1246 X - X - Yellow
1255 to 1260 1246 X - - X Orange
1255 to 1260 1255 X - - X Red
1255 to 1260 1255 - X - X OranQe
1255 to 1260 1255 - X X - Yellow
1255 to 1260 1260 X - - X Red
1255 to 1260 1260 - X X - Orange
1255 to 1262 1260 X - - X Red
1255 to 1262 1260 - X X - Orange
V. IMPACT OF BACK BASIN FLOODING ON PROJECT INFRASTRUCTURE
A. Residential Area
The design pad grades within the residential development will be established at
elevations above the 100-year flood level of elevation 1,263.3 feet MSL. To
accomplish the design pad grades, approximately 13 feet to 18 feet of compacted fill
will be placed across the residential portion of the development.
X:\Projects\884_0210\ENG\DOCIREPIREP01_SUMMERLY Fld Opel" Rpt.doc 17
The short-term flood water level of elevation 1,262 feet MSL in Back Basin during the
100-year storm event may cause the groundwater levels within the residential area to
rise and approach the floodwater elevation and may result in saturation of foundation
soils.
The foundation criteria and floor slab-on-grade recommendations provided by the
geotechnical engineers consider foundation saturation and incorporate
moisture/vapor barriers beneath slab-on-grade to mitigate moisture/water vapor
migration. Therefore, residential structures constructed at the planned design grades
(approximately elevation 1,265 feet MSL to 1,270 feet MSL) will not be affected due
to saturation of foundation soils.
Fill slopes at 2:1 (horizontal to vertical) are planned along the west side of the
residential development, adjacent to the Back Basin. During a 100-year flood event,
these slopes should remain stable when submerged, and therefore will not impact
the planned residential development.
There is potential that for submergence of infrastructures (i.e. storm drains, sewer)
should groundwater levels rise during a 100-year flood event. However, considering
the planned minimum soil cover over these lines (6 and 8 feet, respectively), the
potential buoyancy forces will not adversely affect these lines.
B. Golf Course Area
During and several months after a 100-year storm event, the golf course portion of
the Summerly cut area will be under water. The flooding and submergence of the
golf course and cut areas will cause saturation and softening of the ground and
greens, which may require maintenance after the flood recedes. In addition, there is
potential for fill slope erosion due to wave action. To promote slope stability, the fill
slopes in this area will be protected by rip-rap or other measures, extending
minimum 2 feet above the flood level (elevation 1,262 feet MSL). The flooding in this
area will also affect the stability of infrastructure and improvements (parking, cart
paths, walkways, water/electrical/sewer lines, and lightly loaded structures).
Therefore, the buoyancy effects due to flooding will be adequately considered in the
design and construction of infrastructure and improvements in the golf course area.
Suitable anchoring for lightly loaded facilities may also be necessary to provide
stability against buoyancy.
X:\Projects\884_0210\ENG\OOC\REP\REP01_SUMMERLY Fld Oper Rpt.doc 18
VI. EXHIBITS
A General Location Map
B Site Grading Plan w/Phasing
C Site Hydrology Map
D Storm Drain Plan and Secondary Surface Overflow Map
E Off-Site Hydrology Map for Back Basin Tributary Areas
F 50-year Regulatory Flooding Limits in Golf Course, Habitat Buffer, San Jacinto
River Channel and Riparian Lake/River Corridor
G Pump Station Preliminary Design Drawings
H Pump Station Force Main Concept Plan
I Riparian Lake/River Corridor Inlet Facilities Preliminary Design Drawings
J South Boundary Cutoff Wall Erosion Control Structure
NO .DATE REVISIONS
DRAFTED BY:
I
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(INFEET) 1 inch ""' 200' rt.
1----+-----<--------------� ------------------- 10ESIGNED BY:
1----t---;--, -------------------------------------------------------------- lcHfCKED BY:
1----1-=�:=1--------�==-= ----------------------- toATE:
'-�
LAKE B..SINORE MANAGED LAKE LEIB.. EL 1240-1249
(MAXIMUM LEIB..1272.0)
100-YEAR ..1263.3
::::::7/-- 7
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MAIN , LEVIEfTOP EL 1265
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'?•.N. DE•LL AND ASSOCIATES, INC.
• ¥1 CIVIL ENGINEERS I.AND PLANNERS SURIIEVORS
; :17801 CARTWRIGHT ROAD SITE VICINITY MAP
EXHIBIT
.IRVINE, CA 92614 19491 474-1400 1ENTATIVE TRACT 31920 A
/
EXHIBITC
See Appendix A
Hydrology Study for the Southerly Project Tentative Tract No. 31920
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EXHIBITD
GRAPHIC SCALE
( IN FEET )
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SUMMERLY - JOHN LAING HOMES
STORM DRAIN
PRO.ECT NO.
10001.13
0 '" r---+---+ ------------------------------------------------------------ tCHfCKED BY:
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3 PEIERS CANYON sum: 110 JRI/INE, CA 92606
T: 949-679-0090
F: 949-679-0091 AND SECONDARY SURFACE OVERFLOW EXHIBIT
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OFF-SITE HYDROLOGY MAP FOR
BACK BASIN TRIBUTARY AREAS
LEGEND
"'�.,_,,
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- MAJOR AREA DRAINAGE BOUNDARY
X CENTROID
02,(roo 100-YEAR 24-HOUR PEAK DISCHARGE (cfs)
A WATERSHED AREA
11.25-r --- AREA (SO.Ml.)
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PROJEC
884-0
EXHI E DATE: 02/22/06
VAN DELL AND ASSOCIATES, INC, CML tNruNEERS SUR\t£YORS LANO PLANNERS
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DESIGNED BY:
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949-679-0090
SUMMERLY - JOHN LAING HOMES 50 YEAR REGULATORY FLOODING LIMIT
PROJECT NO.
1000113
SHEET 1
NO. I
DATE
REVISIONS
CHECKED BY:
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SUITE 110 IRVINE, CA 92606 F, 949-679-0091
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VAULT PLAN
DATE
2/06
FIGURE
1-2
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SLUICE GATE OPERATION
VAULT SECTION A-A'
SCALE 1" = 2'-0"
VAN DELL :��!OCIATES, INC. SLUICE GATE OPERATION SURVEYORS LAND PLANNERS
DATE
2/06
17801 CARTWRIGHT ROAD � IRVINE, CA 92614 (949) 4:14-MOO x
VAULT SECTION A-A' FIGURE
1-3
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STRUCTURE PLAN
SCALE: 1" = 3'-0"
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w1, 1 V/\N DELL AND ASSOCIATES, INC.
CML ENGINEERS
� SURVE'fORS LANO PLANNERS
17801 CARTWRIGHT ltOAD
IRVINE, CA 92614 (949) "74-MOO
SLUICE GATE OPERATOR
STRUCTURE PLAN'
OATE
2/06
FIGURE
1-4
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HAND WHEEL
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STEM GUIDE
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s-¢-DRAINTO
11'-0" D/S SLOPE
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STRUCTURE SECTION A-A' �
Cl) SCALE: 1" = 3'-0"
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avN, CA t2'14 &11 47+-1400
SLUICE GATE OPERATOR
STRUCTURE SECTION A-A'
DAlE
2/08
FIGURE 1-5
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STRUCTURE SECTION A-A'
SCALE 1" = 2'-0"
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VAN DELL =��CIATES, INC.
SURYnORS LAND PLANNERS
17801 CARTWRICiHT �
IRVINE, CA 92614 (949) 474-1400
SLUICE GATE INLET
STRUCTURE SECTION A-A'
DAlE
2/06
FIGURE
1-6
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EARTH BERM LIMITS OF
H=3'(VARIES) CONCRETE SLAB S/E PROP. CORNER
900' ± TO APPROX. LOW POINT ON ft
HEAD CUT CONTROL WEIR
SCALE: 1''=30'
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,-
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VAN DELL AND ASSOCIATES, INC.
CIVIL !ENGINEERS
SURVEYORS LAND P1.ANN1ERS
17801 CARTWRIGKT ROAD
IRVINE, CA 92614 1949) '47+-MOO
HEAD CUT CONTROL WEIR PLAN
DATE
2/06
FIGURE
J-1
[I]
ON-SITE
WEIR
END WALL
ct_ WEIR
16' 4' I 4' 16'
8'
TYP
It
OFF===SITE
EXIST.
EL=1246.0 I 3' GROUND
LOOSE
6' ROCK (TYP)
6" CONCRETE SLAB
#4 BAR @ 12"C-C
CENTER OF SLAB
HEAD CUT CONTROL WEIR
SECTION A.-A
SCALE: 1"=10'
X: \PROJECTS\884_0210\ENG\MISC\FLOOD OPERI.TICJN PLAN\J-2...HEADCUTVtEIRSECTION.DWG
VAN DELL AND ASSOCIATES, INC.
CMt. ENCilMEEltS
SURVnORS LANO PLANNERS
'
17801 CAltTWltlGHT ROAD
lltVN, CA 92614 194!11 '474-1400
HE.AD CUT CONTROL WER
SECTION A-A
DATE
2/06
FIGURE
J-2