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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)). Floodplain Evaluation Report Page 2 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. Floodplain Evaluation Report Page 3 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. Floodplain Evaluation Report Page 4 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. Floodplain Evaluation Report Page 7 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. Floodplain Evaluation Report Page 8 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. Floodplain Evaluation Report Page 9 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. V. X / \i \ \\\ -^f • ^ n \ \ x.x XN \ \\\\\\\\^ ^•^^\x^^\\ V \\x \ 11/ I 1 i