HomeMy WebLinkAboutEIR 91-01; Rancho Santa Fe Rd Bridge Replacement Project; Floodplan Evaluation Report; 1997-03-24FLOODPLAIN EVALUATION REPORT
RANCHO SANTA FE ROAD BRIDGE REPLACEMENT PROJECT
Submitted To:
CITY OF CARLSBAD
2075 Las Palmas Drive
Carlsbad, CA 92009-1576
ATTN: Doug Helming
Prepared By:
DUDEK & ASSOCIATES, INC.
605 Third Street
Encinitas, CA 92024
ATTN: Gail Masutani, P.E.
March 24, 1997
TABLE OF CONTENTS
Page
No.
LIST OF FIGURES i
LIST OF TABLES i
1.0 INTRODUCTION 1
1.1 Background 1
1.2 Purpose of Report 2
2.0 PROPOSED PROJECT 3
2.1 Project Description 3
3.0 RISK ANALYSES OF BRIDGE CONSTRUCTION 5
3.1 Existing Floodplain Conditions 5
3.2 Bridge Construction 6
4.0 FLOODPLAIN EVALUATION REPORT SUMMARY 8
5.0 REFERENCES 10
LIST OF FIGURES
Figure 1 Site Map
Figure 2 Floodplain Boundary
LIST OF TABLES
Table 1 Peak Flows
Table 2 Summary of 25-, 50-, and 100-year Water
Surface Elevations (WSEL) Within San Marcos
Creek at the Existing Rancho Santa Fe Bridge .
1.0 INTRODUCTION
1.1 GENERAL BACKGROUND
Federal Highway Administration Guidelines
Projects receiving federal funding must meet project implementation guidelines stipulated by the
Federal Highway Administration (FHWA). The FHWA is directed, per the May 1977 Executive
Order (EO) 11988, "Floodplain Management", to avoid long-term and short-term adverse
impacts associated with the modification of floodplains and to avoid direct or indirect support
of floodplain development wherever there is a practicable altemative. The FHWA's revised
Federal-Aid Highway Program Manual (FHPM) 6-7-3-2, paragraph 4q, provides the basis of
the guidelines for the compliance of Caltrans projects with EO 11988. The proposed project will
require funding from Caltrans, therefore these guidelines are applicable to the project.
The FHPM 6-7-3-2, paragraph 4q, defines a significant encroachment as "a highway
encroachment and any direct support of likely base floodplain development that would involve
one or more of the following construction or flood-related impacts:
1. A significant potential for interruption or termination of a transportation facility which
is needed for emergency vehicles or provides a community's only evacuation route;
2. A significant risk; or
3. A significant adverse impact on natural and beneficial floodplain values."
The natural and beneficial uses of the floodplain include, but are not limited to: visual (open
space, natural beauty), biological (fish, wildlife, plants), and hydrological uses (i.e., discharge
of storm events).
Federal Emergency Management Agency (FEMA) - Minimum Federal Standards
Local jurisdictions must meet the National Flood Insurance Program (NFIP) guidelines to be
eligible for Federal Disaster relief monies. This project is located within the jurisdictional
boundaries of the City of Carlsbad. The NFIP encourages state and local governments to adopt
sound floodplain management programs. The City of Carlsbad is part of the FEMA program,
therefore FEMA standards are applicable. In addition, Caltrans funding requires FEMA
standards be applicable. Therefore, each FEMA Flood Insurance Study provides 100-year flood
elevations and delineation ofthe 100- and 500-year floodplain boundaries and 100-year floodway
to assist communities in developing floodplain management measures. FEMA floodplain
boundaries provide a national standard without regional discrimination. For floodplain
management purposes, the base flood adopted by FEMA is the 100-year (one percent annual
change) flood. The 500-year flood (0.2 percent annual chance) is used by FEMA to indicate
additional flood risk areas.
Floodplain Evaluation Report Page 1
March 24, 1997 1330-02
Encroachment into a floodplain, such as structures and fill, reduces the flood carrying capacity,
increases flood heights and velocities, and increases flood hazards beyond the encroachment
area. One aspect of floodplain management involves balancing die economic gain against the
resulting increase in flood hazard. For the purposes of the NFIP, a floodway is used as a tool
to assist in balancing the economic gain against the hazards. Therefore, the 100-year floodplain
area is divided into a floodway and a floodway fringe. The floodway is the channel of the river,
plus any adjacent floodplain areas, that must be kept free of encroachments such that the 100-
year flood can be carried without substantial increases in flood heights. The minimum Federal
standard limits such as an increase to 1.0 feet, provided that hazardous velocities are not
produced. The FEMA floodway for the San Marcos Creek is a mimmum standard that can be
adopted directly by local jurisdictions or that can be used as the basis for additional studies.
The area between the floodway and the 100-year floodplain boundaries is the floodway fringe.
The floodway fringe encompasses the portion of the floodplain that could be completely
obstructed without increasing the water surface elevation of the 100-year flood by more than 1.0
foot at any point within the floodway.
1.2 PURPOSE OF REPORT
This floodplain risk assessment has been prepared to comply with the requirements and
guidelines of the FHWA and Caltrans for the proposed project and to assess the impacts of any
encroachment of the proposed project into the base floodplain. For the purpose of this report,
the base floodplain is defined as the 100-year flood discussed in Section 1.1. An encroachment
is defined as "an action within the limits of the base floodplain." Any construction activity, such
as access roads, buildings, fill slopes, or bank or slope protection, constimtes encroachment.
The regulatory floodway is defined as the floodplain area that is reserved in an open manner by
Federal, State or local requirements, unconfined or unobstructed either horizontally or vertically
to provide for the base discharge of the base flood so that the cumulative increase in water
surface does not exceed the minimum standard established by FEMA (1.0 foot rise in water
surface elevation (WSEL)).
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March 24, 1997 1330-02
2.0 PROPOSED PROJECT
2.1 PROJECT DESCRIPTION
The proposed project realigns and widens approximately 3,500 linear feet of Rancho Santa Fe
Road (S-10) from two lanes to an ultimate 6-lane Prime Arterial Roadway from just south of
Questhaven Road to just north of Melrose Drive in northem San Diego County (see Figures 1
and 2). The proposed widening and realignment project is a part of the City of Carlsbad's
General Plan to upgrade Rancho Santa Fe Road to meet its designation as a Prime Arterial
Roadway. A Prime Arterial Roadway has a 126-foot right-of-way containing six traveled lanes,
a bike lane, an 18-foot raised median, sidewalks, curb and gutter. The new bridge over San
Marcos Creek is planned to accommodate the Prime Arterial Roadway. The bridge replacement
project will involve constmction of a new bridge in a location west of the existing bridge.
Along with the new bridge, the project includes constmction of roadway improvements at the
south and north ends of Rancho Santa Fe Road. The southerly roadway approach for the
proposed bridge(s) will extend approximately 1,800 feet which will include reconstmcting the
Questhaven and Rancho Santa Fe Roads intersection, and reconstmcting approximately 600 to
800 feet of Questhaven Road east of the intersection. If the portion of Rancho Santa Fe Road
located south of this project is delayed, then a detour will be used to join the Rancho Santa Fe
Road Bridge approach with the existing alignment of Rancho Santa Fe Road. In this portion of
the southem Prime Arterial Roadway, the project is to constmct a 32-foot width of pavement
on either side of the median curbs, inner two lanes in each direction plus 8-feet of the third lane,
the median curbs, and the appropriate tum lanes.
The northerly roadway approach for the new bridge will be approximately 1,700 feet long and
includes the reconstmction of the La Costa Meadows Drive/Rancho Santa Fe Road intersection,
and the reconstmction of approximately 500 feet of La Costa Meadows Drive east of the
intersection. The realigned Rancho Santa Fe Road will be constmcted to the full width on the
east side of the median, with sidewalks, curb and gutter and street lights from the bridge to
north of Melrose Drive. The west side of the roadway will be constmcted with 32-feet of
paving adjacent to the median curb. There are two altematives for the connection of Rancho
Santa Fe Road with Melrose Drive:
1. The preferred altemative for the realigned Rancho Santa Fe Road is to match into
existing Melrose Drive by curving to the west at the southerly end of the
proposed Meadowland Subdivision (CT 85-19). This arrangement will necessitate
closing the existing Corintia Street access to Melrose Drive and extending Xana
Way southerly and easterly to meet Melrose Drive. Existing Rancho Santa Fe
Road located north of the study area in San Marcos would be curved at its
southerly terminus to "T" into the new Melrose/Rancho Santa Fe Road alignment.
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March 24, 1997 1330-02
The altemative alignment option is for Rancho Santa Fe Road to match into
existing Rancho Santa Fe Road similar to the existing arrangement. Melrose
Drive would then swing east to intersect with Rancho Santa Fe Road in a location
about 450 feet northerly of the present intersection. Corintia Street would extend
east to intersect with the realigned Melrose Drive.
Currently, Rancho Santa Fe Road is a two-lane conventional highway in northern San Diego
County which connects the City of Encinitas in the south to the City of San Marcos in the north.
The majority of the proposed project will lie within the City of Carlsbad with some of the
project within the City of San Marcos. The proposed project alignment traverses rolling hill
terrain and would generally be to the south and west of the existing Rancho Santa Fe Road
alignment. The proposed project area has traditionally had low volumes of intra-regional and
inter-regional traffic. However, there is anticipated to be future growth in the commercial and
residential development along Rancho Santa Fe Road which will decrease the level of service.
This increase in traffic volume due to regional growth, coupled with projected local and regional
increases in traffic volume associated with further development in this area, necessitate the
widening of Rancho Santa Fe Road.
The existing Rancho Santa Fe Road consists of two paved lanes with an asphalt berm on the
west side and a combination of asphalt berm and concrete curb on the east side. A tmck by-pass
route creates a three-plane paved road with asphalt berm on both sides in uphill areas of the
stretch. San Marcos Creek is crossed by an existing bridge which is located between the
Questhaven Road intersection and the Melrose Drive intersection. The Rancho Santa Fe Road
has two northbound lanes and one southbound lane across the bridge.
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March 24, 1997 1330-02
3.0 RISK ANALYSES OF BRIDGE CONSTRUCTION
3.1 EXISTING FLOODPLAIN CONDITIONS
Rick Engineering Company (REC) performed floodplain calculations for the existing San marcos
Creek Bridge and assuming a new raised bridge. Bot of the proposed alignments for the project
discussed above would have the same bridge crossing at Rancho Santa Fe Road. Hydraulic
Report for San Marcos Creek at Rancho Santa Fe Road was prepared on April 27, 1988 by Rick
Engineering. The report performed the floodplain calculations using the U.S. Army Corp of
Engineers' HEC-2 program model. The HEC-2 backwater analysis program was used to
determine flood water profiles for the existing floodplain, the 1986 FEMA defined floodway,
and for the 100-year flood. A supplemental study entitled Hydraulic Re-Study of San Marcos
Creek at the Existing Rancho Santa Fe Road Bridge was conducted by REC in August 1996.
This study focused on the projected water surface elevations for the 25- and 50-year floods and
the effect on the existing bridge.
REC utilized the U.S. Army Corp of Engineers' report entitled Flood Plain Information on San
Marcos Creek dated April 1971 to determine the 50- and 100-year flood volumes. They
extrapolated that information on log-log plot to determine the 25-year flood volume. These data
are summarized in Table 1. These flows were calculated from a drainage area of approximately
29.4 square miles immediately upstream of the bridge.
TABLE 1
PEAK FLOWS
Flood Recurrence Projected Peak
Interval Flows
25-year 4,500 cfs
50-year 8,000 cfs
100-year 13,000 cfs
Source: REC, 1988, 1996.
Table 2 summarizes the findings of the two REC studies. The existing Rancho Santa Fe Bridge
has a low chord elevation of 335.62 feet msl and top of road elevation of 337.2 feet msl at the
downstream face of the bridge, and a low chord elevation of 336.78 feet msl and a top road
elevation of 338.36 feet msl at the upstream face of the bridge.
Floodplain Evaluation Report
March 24, 1997
Page 5
1330-02
TABLE 2
SUMMARY OF 25-, 50- AND 100-YEAR WATER SURFACE
ELEVATIONS (WSEL) WITHIN SAN MARCOS CREEK AT THE
EXISTING RANCHO SANTA FE ROAD BRIDGE
Cross Section
Number
25-Year WSEL
(feet)
50-Year WSEL
(feet)
100-Year WSEL
(feet)
10 329.3 331.9 334.6
20 330.5 333.1 335.8
30 331.6 334.1 336.9
40 332.6 335.1 337.8
50 333.7 336.3 339.2
60 334.5 337.2 340.3
70 335.3 338.0 341.0
75 335.3 338.0 341.0
80 335.3 338.1 341.2
85 335.3 338.2 341.3
90 335.6 338.7 342.0
100 336.0 339.0 342.3
110 336.5 339.5 342.6
120 337.2 340.0 343.1
Source: REC, 1988, 1996.
3.2 BRIDGE CONSTRUCTION
Based on the existing bridge elevations, the following can be concluded from Table 2:
1. 25-Year Flood Event. The WSEL of the 25-year flood event is below the low
chord of the existing bridge.
2. 50-Year Flood Event. The WSEL of the 50-year flood event at the Rancho Santa
Fe Bridge is approximately one foot over the top of the existing Rancho Santa Fe
Bridge.
3. 100-Year Flood Event. The existing Rancho Santa Fe Bridge is completely
submerged during a 100-year flood event.
Floodplain Evaluation Report
March 24, 1997
Page 6
1330-02
REC also did a hydraulic analyses for a new, raised bridge. The analysis was made with the
location of the proposed bridge to the west of the existing bridge and assuming that "the
proposed bridge piers would have the same spacing and alignment as the existing bridge."
Based on that statement it is inferred that the new bridge was modeled assuming the bridge piers
were also of the same shape and size as the existing piers and that there is no encroachment into
the stream channel with the bridge abutments. REC determined that the lowest chord of the
proposed bridge should be at 342.5 feet msl, which would allow for one foot of freeboard with
the 100-year flood. With this unprovement, REC concluded through the hydraulic analyses that
the water surface elevations upstream of the bridge were lowered slightly (approx. 0.1 feet)
because of no backwater increase by the bridge obstmction.
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March 24, 1997 1330-02
4.0 FLOODPLAIN EVALUATION REPORT SUMMARY
1. Is the proposed action a longitudinal encroachment of the base floodplain?
The proposed project does longitudinally encroach upon the base floodplain as defined by the
FEMA defined regulatory floodway. The encroachment is due to the existence of the bridge
piers.
2. Are the risks associated with the implementation of the proposed action significant?
The risk associated with the proposed project is not significant as defined by FHPM 6-7-3-2,
paragraph 4q(l). There are a number of equivalent detour routes available to emergency
vehicles or as an evacuation route for the area around the proposed Rancho Santa Fe bridge.
South of the Rancho Santa Fe Road bridge, Rancho Santa Fe Road leading into the La
Costa/Encinitas area provides several different routes. North of the Rancho Santa Fe Road
bridge, access to the area is provided by Rancho Santa Fe Road and Melrose Avenue.
3. Will the proposed action support probable incompatible floodplain development?
There are no significant risks anticipated with the proposed project as defined by FHPM 6-7-3-2,
paragraph 4q(2). The WSELs are anticipated to be within the FEMA standard 1.0 foot rise.
The bridge crossing will be designed with the lowest chord at 342.5 feet msl which is
approximately 1 foot above the 100-year WSEL. The existing river channel will not be affected
by the bridge constmction and channel velocities are expected to remain as existing. Further
HEC-2 modeling should be conducted when a bridge design is available, especially if there are
encroachments in to the floodplain due to grading or bridge constmction.
4. Are there any significant impacts on natural and beneficial floodplain values?
The proposed project does not support incompatible floodplain development. Both the City of
San Marcos and the City of Carlsbad have restricted land uses within the floodplain and enforce
these restrictions through policies, general plans, and land use ordinances.
5. Routine construction procedures are required to minimize impacts on the floodplain.
Are there any special mitigation measures necessary to minimize impacts or restore
and preserve natural and beneficial floodplain values? If yes, explain.
Routine constmction procedures are required to minimize impacts to the floodplain. These
measures are included in the Water Quality Report (Dudek, 1997) to minimize water quality
impacts. Potential erosion damage to the roadway fill will require erosion protection methods
incorporated into the design of the proposed project.
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March 24, 1997 1330-02
6. Does the proposed action constitute a significant floodplain encroachment as defined
in FHPM 6-7-3-2, paragraph 4q?
The proposed project does not constitute a significant floodplain encroachment as defined in
FHPM 6-7-3-2, paragraph 4q (1), (2), and (3).
7. Are Location Hydraulic Studies that document the above answers on file? If not
explain.
Location Hydraulic Studies are located at the City of Carlsbad.
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March 24, 1997 1330-02
5.0 REFERENCES
Caltrans, (1995). Environmental Program's Environmental Handbook - Volume 1.
Cotton/Beland/Associates, (1991). Draft EIR for the Rancho Santa Fe Road Realignment and
Mass Grading Project.
Geocon, Inc. (1990). Bridge Foundation Investigation for Rancho Santa Fe Road Realignment,
Rancho Santa Fe Bridge Crossing at San Marcos Creek, Carlsbad, Califomia.
Geocon, Inc. (1990). Supplemental Soil and Geologic Investigation for Rancho Santa Fe Road
Realignment, Carlsbad, Califomia. January.
Rick Engineering Company, (1988). Hydraulic Report for San Marcos Creek at Rancho Santa
Fe Road.
Rick Engineering Company, (1996). Hydraulic Re-Study of San Marcos Creek at Rancho Santa
Fe Road Bridge.
Floodplain Evaluation Report Page 10
March 25, 1997 1330-02
I Basemap: USGS 7.5 Minute Series, Rancho Santa Fe and San IViarcos Quadrangles.
^ Basemap: USGS 7.5 IVlinute Series, Ranclio Santa Fe and San IViarcos Quadrangles. Floodplain per Rick Engineering.
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