4.6 HYDROLOGY AND FLOODPLAIN - Golden State Web Solutions · 2016-02-12 · and Avoidance, Minimization, and Mitigation Measures 4.6 Hydrology and Floodplain 4.6.2 Affected Environment

Chapter 4 Affected Environment; Environmental Consequences; and Avoidance, Minimization, and Mitigation Measures 4.6 Hydrology and Floodplain

4.6 HYDROLOGY AND FLOODPLAIN This section evaluates potential environmental effects related to hydrology and floodplain issues associated with the Proposed Action. Two technical studies related to these issues were prepared for the Proposed Action, including: (1) an analysis of hydrologic and hydraulic conditions in applicable waterways (Appendix F); and (2) a Drainage Study and Storm Water Management Plan (SWMP; Appendix B). 4.6.1 Regulatory Setting Executive Order 11988 (Floodplain Management) EO 11988 directs all federal agencies to refrain from conducting, supporting, or allowing actions in floodplains unless it is the only practicable alternative. Specific directives identified in EO 11988 to achieve this goal include evaluation of the following considerations: The practicability of alternatives to any longitudinal encroachments.

Risks of the action.

Impacts on natural and beneficial floodplain values, including: (1) water resource values such as flood/erosion control and water quality maintenance; (2) biological resource values including habitat and biodiversity; and (3) social resources such as scientific, educational and recreational opportunities.

Support of incompatible floodplain development.

Measures to minimize floodplain impacts and to preserve/restore any beneficial floodplain values impacted by the project.

Provision of opportunities for early and adequate public review of proposed floodplain encroachments.

The “base floodplain” is defined as the area subject to flooding by the flood or tide having a one percent chance of being exceeded in any given year (i.e., a “100-year” event), while “encroachment” is defined as an action within the limits of the base floodplain. Federal Clean Water Act/National Pollutant Discharge Elimination System The Proposed Action is subject to applicable elements of the CWA, including the NPDES. Specific NPDES requirements include conformance with pertinent hydrology and drainage criteria in the NPDES Phase II Municipal Storm Water Permit, as outlined in Section 4.3, Water Quality. With respect to hydrologic criteria, the Phase II Permit is focused primarily on avoiding or minimizing changes to predevelopment runoff rates and amounts, along with related potential issues such as hydromodification and storm drain system capacity.

San Dieguito River Bridge Replacement, Double Track, and Del Mar Fairgrounds Special Events Platform Environmental Assessment 4.6-1 October 2014


4.6.2 Affected Environment The following analysis incorporates applicable information from the above-referenced technical studies (Appendices B and F), as well as pertinent data from other published and unpublished sources. The study area used for the hydrology and floodplain analysis (study area) includes all areas proposed for grading, excavation construction, and other disturbance, as well as applicable portions of the off-site watershed areas outlined below. Watershed and Drainage Characteristics The study area is within the San Dieguito Hydrologic Unit (HU), 1 of 11 such drainage areas designated in the 1994 (as amended) San Diego RWQCB Basin Plan. The San Dieguito HU is divided into a number of hydrologic areas and subareas based on local drainage characteristics, with the study area encompassing portions of the Rancho Santa Fe Hydrologic Subarea (HSA) of the Solana Beach Hydrologic Area (HA). Figure 4.6-1, Proposed Action Location within Local Hydrologic Designations, shows the Proposed Action site relative to local hydrologic areas. Drainage in the San Dieguito HU is predominantly through the San Dieguito River and associated tributaries, with the river flowing generally west through the study area at the existing/proposed railroad bridge site and discharging into San Dieguito Lagoon. Average annual precipitation in the vicinity of the Proposed Action is approximately 10.4 inches per year, with the majority of this precipitation (over 83 percent) occurring during the period of November through March (weather.com 2013). Much of the Proposed Action alignment and study area encompass existing development, including the railroad corridor, portions of the Del Mar Fairgrounds property, and residential and commercial sites. Undeveloped open space is also present in the study area and vicinity, primarily within the San Dieguito River Valley and Lagoon. Existing drainage facilities within the alignment and surrounding areas include numerous NCTD and municipal (Solana Beach and Del Mar) surface and subsurface storm drain facilities (e.g., pipelines, channels, and related inlet/outlet structures), as well as crossings such as culverts, the railroad bridge over the San Dieguito River and Lagoon, and the Camino Del Mar bridge. Surface drainage within the study area and associated off-site watersheds is variable in direction with local conditions such as topography, with all flows ultimately entering the San Dieguito River and Lagoon and/or the Pacific Ocean. These flows occur through a number of defined drainages (including the San Dieguito River and Stevens Creek near the Del Mar Fairgrounds), as non-point (overland or sheet) flow, and through existing storm drain facilities. The study area is divided into three principal segments in the referenced Drainage Study/SWMP based on existing drainage basins, and includes the excavated trench segment (Segment 1) that extends from the northern project boundary near Lomas Santa Fe Drive to just south of Villa de la Valle, the raised embankment and bridge segment (Segment 2) located between the southern boundary of Segment 1 and the San Dieguito River, and the at-grade segment (Segment 3) that extends between the San Dieguito River and the southern project terminus just south of 17th Street. Summary descriptions of existing drainage conditions, including mapped drainage basins, within these three segments are provided below.


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Figure 4.6-1SAN DIEGUITO RIVER BRIDGE, DOUBLE TRACK,

AND DEL MAR FAIRGROUNDS SPECIAL EVENTS PLATFORM PROJECT

Proposed Action Locationwithin Local Hydrologic Designations

San Dieguito Hydrologic Unit (905)Solana Beach Hydrologic Area (905.10)Rancho Santa Fe Hydrologic Subarea (905.11)

Source: CalWater 1999

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Segment 1 This segment encompasses portions of four mapped drainage basins (and associated subbasins) within the study area and adjacent off-site properties, including Basins SB-A and SB-B within and east of the study area, and Basins SB-C and SB-D within and west of the study area (Figure 4.6-2, Existing Drainage Basins). Segment 1 includes a topographic high near its midpoint, with flows north of this point in Basins SB-A and SB-D moving north through a series of lined/unlined ditches and catch basins, and flows south of this location in Basins SB-B and SB-C draining south through similar facilities. Flows from off-site developed areas to the east and west enter the “trench” through several pipelines that are part of the City of Solana Beach storm drain system, and discharge into the drainage ditches and catch basins located at the toe (bottom) of the excavated trench slopes (with these flows continuing north or south as described for on-site flows). Flows moving north within Segment 1 enter the City of Solana Beach underground storm drain system at Lomas Santa Fe Drive, and continue west to the Pacific Ocean via a 60-inch diameter pipeline. Flows moving south in Segment 1 discharge into either Stevens Creek (Basin SB-B) east of the existing railroad tracks, or San Dieguito Lagoon (Basin SB-C) west of the tracks, with both outlets located approximately 240 feet south of Via de la Valle (and flow characteristics within Stevens Creek described below). Segment 2 Segment 2 incorporates portions of nine mapped drainage basins (and associated subbasins) within and east of the study area, including Basins DMF-1 through DMF-8, and DMF-13 (refer to Figure 4.6-2). The areas located west of the study area in Segment 2 are within the San Dieguito River and Lagoon. The noted drainage basin areas east of the railroad alignment are located within the Del Mar Fairgrounds, with associated storm water flows entering the alignment via five discharge points that outlet into a portion of Stevens Creek extending north-south between the Fairgrounds and the existing railroad tracks. Stevens Creek extends generally east-west along the northern boundary of the Fairgrounds, before turning south and flowing parallel to (and within) the railroad corridor (next to the raised embankment) for approximately 1,400 feet (with this portion of the creek receiving the described flows from the Fairgrounds). At this point the creek crosses under the railroad alignment via the existing wooden trestle bridge and flows west into the San Dieguito River. Flows in Basin DMF-7 (which includes the main Fairgrounds track area) are regulated via a number of low-flow detention ponds within the track infield, with flows from larger storms pumped into an associated storm drain system that outlets into Stevens Creek as described (with additional discussion of drainage within the Fairgrounds provided in Appendix B). A number of water quality related facilities and activities are also associated with surface drainage in the Fairgrounds, as outlined in Section 4.3. Because the Fairgrounds are owned by the State of California, all associated drainage and water quality facilities are operated and maintained by a designated State agency, the 22nd DAA. Segment 3 This segment encompasses portions of two mapped drainage basins (and associated subbasins) within the study area and adjacent off-site properties, including Basin DM-A within and east of



the study area, and Basin DM-B within and west of the study area (refer to Figure 4.6-2). Basin DM-A includes commercial and residential development in areas east of the railroad corridor, as well as a natural low-lying area known as the Del Mar Wye (Wye). The Wye is a triangular-shaped property that is enclosed by higher elevation features including the railroad corridor to the west, the City of Del Mar Public Works Yard to the north, and commercial development to the south (refer to Figure 4.6-2). As a result, the Wye serves as a natural retention basin for local drainage. Specifically, storm water flows on the east side of the track in Segment 3 move north in an earthen drainage channel and discharge into the Wye near 24th Street. Surface drainage from the adjacent commercial area in Basin DM-A sheet flows into the Wye, while flows from the residential site in Basin DM-A (east of Jimmy Durante Boulevard) enter the City of Del Mar storm drain system and are conveyed generally west and north to the Wye. The Wye has an estimated storage capacity of 512,100 cubic feet (or approximately 11.8 acre-feet1), which exceeds the associated predicted 100-year storm event volume of approximately 12,900 cubic feet (with related overflow from the Wye therefore not anticipated during a single storm event). The noted volume could potentially be exceeded by the combined flows from multiple storm events, however, with the lowest points on the described Wye perimeter occurring along the Public Works Yard boundary. Accordingly, in the event that the Wye storage volume is exceeded as described, overflow would first begin to sheet flow over the existing railway embankment through the houses to the west and toward San Dieguito River. As the floodwaters recede and the flow depths decrease to less than the existing railway berm elevations, the floodwaters would be directed northerly along the railway berm and the Public Works Yard to the San Dieguito River. Basin DM-B includes the west side of the railroad alignment and residential development in areas west of the railroad corridor. Flows within the alignment are conveyed north via an earthen channel, and discharge into the San Dieguito River north of 28th Street. The outlet to the river is controlled by a flap gate during high water events to prevent back flow into the channel. When the gate is closed, standing water in the channel west of the railroad corridor is pumped into the river. Flows from the adjacent residential areas in Basin DM-B move generally east through the City of Del Mar storm drain system, enter the earthen channel within the railroad alignment, and continue north to the river as described for on-site flows. Floodplain The study area and vicinity have been mapped for flood hazards by the Federal Emergency Management Agency (FEMA 2012a and 2012b). The entire study area located north of Via de la Valle is mapped as Zone X, or areas determined to be outside of mapped 500- and 100-year floodplains. Much of the study area and vicinity south of this point are within the mapped 100-year floodplain associated with the San Dieguito River (including areas within the

1 One acre-foot is the amount of water needed to cover an area of one acre to a depth of one foot, and equals approximately 326,000 gallons.


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SAN DIEGUITO RIVER BRIDGE, DOUBLE TRACK,AND DEL MAR FAIRGROUNDS SPECIAL EVENTS PLATFORM PROJECT

Railroad Right-of-Way!( Existing Discharge Point (DP)

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Source: David Evans Associates 2013

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floodway2), however, including the existing bridge and track (portions of which would be overtopped during a 100-year storm event), as well as much of the proposed bridge and platform site. Figure 4.6-3, Floodplain/Floodway Boundaries, shows the mapped limits of the 100-year floodplain and 100-year floodway within the vicinity of the Proposed Action. Groundwater Portions of the study area and vicinity are located within the mapped extent of the San Dieguito River Groundwater Basin, which includes a surface area of approximately 5.6 square miles along the central and lower portions of the San Dieguito River Valley. The San Dieguito Basin includes shallow alluvial aquifers, as well deeper aquifers associated with a number of underlying sedimentary and metasedimentary/metavolcanic bedrock units (California Department of Water Resources [DWR] 2003). Due to the Proposed Action’s location near the coast, groundwater is anticipated to be at or near mean sea level (MSL) in most or all of the alignment. Groundwater was encountered at depths of approximately 7 to 11 feet below existing grade in all six exploratory borings conducted as part of the Geotechnical Evaluation for the Proposed Action, with these borings located in the vicinity of the proposed rail bridge and platform in the San Dieguito River Valley/Lagoon. From these data, groundwater levels at the proposed bridge abutment sites are expected to be approximately 10 to 20 feet below the grade of the existing rail line, although groundwater levels are also anticipated to vary with tidal and seasonal conditions (Appendix J). 4.6.3 Environmental Consequences Action Alternatives East Side-Loading Alternative Construction Impacts Potential impacts related to hydrology and floodplain during construction of the East Side-Loading Alternative include drainage alteration associated with excavation, grading and construction of proposed facilities; and groundwater extraction/disposal (dewatering), if required, as outlined below. Drainage Alteration. Implementation of this alternative would result in a number of alterations to existing local drainage patterns associated with the construction of proposed facilities, addition or modification of storm drain facilities, and the proposed relocation of Stevens Creek. None of these activities would result in substantial impacts to local drainage patterns or directions, however, with the referenced Drainage Study/SWMP concluding that “The project has been designed to maintain the overall drainage patterns…” (Appendix B). Summary discussions of drainage alteration impacts associated with the East Side-Loading Alternative are

2 The floodway is generally defined as the area within and adjacent to the main channel of a river or stream that is required to convey the base flood flow without cumulatively increasing the water surface elevation more than a designated height. The associated floodplain includes the floodway, as well as the adjacent areas subject to inundation during a flood event, termed the floodplain fringe.



provided below by segment. It should also be noted that, while the described impacts to drainage alteration would occur during construction of the Proposed Action, they would represent long-term (permanent) changes to local drainage patterns and are, therefore, also included below under the discussion of Operational Impacts. Segment 1. The overall drainage patterns and directions in Segment 1 would be retained under this alternative, with several modifications to existing drainage facilities. Specifically, the existing storm drain outlets that convey flows into the alignment from developed areas to the east and west would be reconstructed behind the proposed retaining walls. These facilities would drain into a reconstructed series of lined ditches and catch basins within the expanded trench (similar to the existing facilities), with associated on- and off-site flows continuing north or south similar to existing conditions. Concrete brow ditches would also be constructed along the tops of the trench slopes and retaining walls to control erosion, along with a series of downslope drains to convey flows into the reconstructed ditches and catch basins. Flows within the alignment moving north under this alternative would enter the previously described 60-inch pipeline in Lomas Santa Fe Drive and continue west to the Pacific Ocean as described for existing flows (with no modifications to the City of Solana Beach storm drain system). Flows moving south in the trench would be collected in a new underground storm drain system north of Villa de la Valle, and conveyed to a proposed box culvert extending under the alignment. This culvert is associated with the proposed relocation of Stevens Creek under this alternative, as described below for Segment 2, and would discharge to the San Dieguito River and Lagoon via a new low-flow graded channel west of the alignment (refer to Figure 3-3d). Accordingly, flows moving south in Segment 1 under this alternative would continue to enter the San Dieguito River and Lagoon (albeit at a different location), and associated effects to drainage patterns and directions for all flows in this segment would be minimal. Segment 2. The overall drainage patterns and directions in Segment 2 would be retained under this alternative, although associated activities would include relocating Stevens Creek and modifications to a number existing drainage facilities. Specifically, the relocation of Stevens Creek would entail constructing a 160-foot long, triple box culvert (with three 8- by 10-foot openings designed to accommodate 100-year flow) beneath the alignment, approximately 400 feet south of Via de la Valle (i.e., where the creek currently turns south), and redirecting the creek flow into the culvert at this location. The creek flow would then continue west through the culvert and a new low-flow channel and eventually enter the San Dieguito River and Lagoon as noted above for Segment 1. The existing, approximately 1,400-foot long, segment of Stevens Creek south of the proposed culvert would be abandoned and filled as part of the proposed development under this alternative. The two northernmost existing outlets that currently discharge storm flows from the Del Mar Fairgrounds into the abandoned section of Stevens Creek would be extended under the embankment and flow into the San Dieguito River and Lagoon (either directly or via the proposed low-flow channel). The three southern outlets from the Fairgrounds would flow under the proposed bridge and discharge directly into the San Dieguito River and Lagoon. Two of these outlets would be retained in place, while the third would be relocated approximately 140 feet to the south and would discharge into a proposed graded channel that continues south and west under the proposed platform to the lagoon (Figure 4.6-4, Proposed Drainage Basins). The previously described detention (and related) facilities located within the Fairgrounds would not be affected by implementation of the East


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Railroad Right-of-Way!( Proposed Discharge Point (DP)

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Figure 4.6-4

Source: David Evans Associates 2013

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Side-Loading Alternative. Runoff from the proposed bridge and platform structures would flow directly into the river and lagoon, via a series of deck drains (with these flows to be treated prior to discharge as described in Section 4.3). Based on the described conditions, flows within Segment 2 would continue to enter the San Dieguito River and Lagoon similar to existing conditions under this alternative (with described modifications to several outlet points), and associated effects to drainage patterns and directions would be minimal. Segment 3. The overall drainage patterns and directions in Segment 3 would be retained under this alternative, with several modifications to existing drainage facilities. Specifically, new concrete ditches (including brow ditches in applicable locations for erosion control), storm drains and subdrains would be constructed within the alignment, and the existing City of Del Mar storm drain system that conveys flows into the alignment from the west would be retained in place. Similarly, the previously described City of Del Mar drainage channel, pump station, and outlet structures (e.g., the flap gate) that convey flows from the west to the San Dieguito River would remain in place. The proposed improvements within the alignment would result in drainage from approximately 1.3 acres within and west of the alignment (i.e., from Basin DM-B) being redirected into the Wye. This additional flow would encompass approximately 7,900 cubic feet for a 100-year storm event, and would not adversely affect the ability of the Wye to accommodate such flows (with the Wye exhibiting an excess capacity of approximately 500,000 cubic feet during a 100-year storm, as described in Section 4.6.1). Conversely, the minor reduction of 100-year storm flow to the San Dieguito River via the City of Del Mar drainage channel, pump station and outlet structures would not result in any adverse drainage impacts. The two existing storm drains that convey runoff into the alignment from residential and commercial properties to the east would be reconnected to the new on-site storm drain system to maintain existing flow patterns (i.e., north to the Wye). In addition, a new low-flow channel would be constructed parallel to the railroad tracks on the east to connect the Wye to the San Dieguito River. The outlet from the Wye to this channel would be at a lower elevation than the previously noted low points along the Del Mar Public Works Yard boundary, thereby reducing the potential for overflow from the Wye (or receding floodwaters from the river) to enter the adjacent Public Works Yard. The proposed widened track berm and the centerline shift to the east would result in a slope that encroaches into the existing Wye retention area, resulting in a 99,400-cubic-foot (19 percent) decrease in available retention storage volume. The remaining retention storage volume area of 412,700 cubic feet in the Wye would continue to be more than required to retain the calculated 100-year runoff volume of 202,710 cubic feet from proposed Basin DM-A. The Wye retention area would continue to perform the same as the existing condition in that a single storm event would not overflow into the Public Works Yard or the San Dieguito River. Based on the described conditions in Segment 3, no substantial impacts to drainage patterns and directions would result from implementation of this alternative. Groundwater Extraction/Disposal. Based on the discussion of local groundwater conditions in Section 4.6.1, shallow groundwater is anticipated to be encountered during implementation of the East Side-Loading Alternative. The presence of shallow groundwater in proposed



development areas may necessitate extraction and disposal (dewatering) operations to facilitate excavation and grading. Potential impacts to local groundwater resources (e.g., through drawdown) from the described dewatering operations would be minor, based on the following considerations: (1) construction dewatering would be relatively small-scale in extent and short-term in duration; and (2) disposal of any extracted groundwater would likely occur locally, and would thus be subject to recharge within the same groundwater basin(s) from which extraction occurred. Construction dewatering, if required, would also be subject to applicable NPDES requirements related to water quality concerns, as described in Section 4.3. Operational Impacts Potential impacts related to hydrology and floodplain during project operation under this alternative include long-term drainage alteration, generation of increased runoff rates and amounts from construction of new impervious surfaces (with corresponding effects related to storm drain capacity, associated flooding and erosion), hydromodification, floodplain hazards (such as floodplain encroachment/modification and related flood hazards), scour, and sea level rise. Drainage Alteration. As described above under Construction Impacts, implementation of the Proposed Action would result in alterations to a number of existing local drainage patterns associated with the construction of proposed facilities, the addition or modification of storm drain facilities, and the proposed relocation of Stevens Creek. While these changes to local drainage patterns would occur as a result of (and during) proposed construction, they would be long-term (permanent) in duration and would thus also be indirectly associated with the operational phase of the project. Based on the less-than-substantial nature of these effects, however (as described under Construction Impacts), as well the fact that no additional drainage alteration would occur during project operation, no substantial operational impacts to drainage alteration would result from implementation of this alternative. Runoff Rates and Amounts. Implementation of this alternative would result in a slight increase of impervious surface area, with a corresponding minor increase in post-development runoff rates and amounts (Appendix B). Specifically, such effects would be limited through proposed design measures to limit new impervious surfaces, including the use of pervious materials for applicable structures (e.g., track bed improvements and access/maintenance roads) and provision of unlined channels where feasible. As a result, new impervious surfaces resulting from the East Side-Loading Alternative would be limited to an area of approximately one acre associated predominantly with the special events platform. The overall associated 100-year storm flow within the alignment would increase by approximately 2.1 cubic feet per second (including local increases and decreases within the described drainage basins), with virtually all of this additional runoff draining directly into the San Dieguito River and Lagoon from the platform (and therefore not adversely affecting storm drain capacity, associated flooding potential, or erosion hazards). The proposed design under this alternative would also encompass energy dissipation structures (e.g., riprap aprons) at applicable outlet locations to regulate runoff rates and provide erosion control (with additional discussion provided in Section 4.3). Hydromodification. In general terms, hydromodification consists of the erosive impacts caused by cumulative changes in the quantity and duration of storm water flows resulting from the



increase in impervious surfaces associated with development. Specifically, an increase in impervious area typically generates related increases in both the rate and amount of storm water runoff compared to pre-development conditions. Pursuant to applicable NPDES requirements, the County of San Diego has adopted a Hydromodification Management Plan (HMP), which requires that all applicable projects either demonstrate that the project is exempt from HMP requirements based on the identified criteria, or provide compliance with the associated mitigation requirements outlined in the HMP. Based on the nature of the proposed design under the East Side-Loading Alternative, the Drainage Study/SWMP concludes that: “The project does not increase the unmitigated peak flow to any outlet… This project is therefore considered exempt from the HMP requirements.” Specifically, as previously noted, the Solana Beach basins connect to an existing City conveyance system that discharges directly to the Pacific Ocean, while the Del Mar Fairgrounds and Del Mar basins discharge directly to the San Dieguito River and Lagoon (Appendix B). Floodplain Hazards. As previously noted, hydrologic, hydraulic, and scour analyses were conducted for the Proposed Action, including the proposed relocation of Stevens Creek, the proposed railroad bridge crossing, and the existing crossing at Camino Del Mar (Appendix F). The referenced analysis also evaluated the effects of potential future sea level rise on bridge hydraulics and scour for the proposed new railroad crossing, with the results of the noted analyses summarized below. Stevens Creek. The hydrologic/hydraulic analysis for the proposed relocation of Stevens Creek was based on design data and flood-related information provided in the San Diego County Hydrology Manual (County of San Diego 2003). As outlined in Chapter 3, Proposed Action Description and Alternatives, Stevens Creek would be relocated under this alternative to accommodate the Proposed Action facilities. Specifically, a portion of the creek channel located approximately 400 feet south of Via de la Valle (i.e., where the creek channel currently turns south) would be filled and realigned, with the modified creek to extend west under the rail alignment through the previously described box culvert and low-flow channel, before ultimately discharging to San Dieguito Lagoon. The hydrologic/hydraulic analysis prepared for the Proposed Action concludes that the proposed realignment of Stevens Creek would not change the flow characteristics for a 5- or 10-year storm event3 (with no associated increase in water surface elevations or lateral extent of the floodplain), and would not increase the potential for upstream flooding during 50- and 100-year storm events. The hydrologic/hydraulic analysis for Stevens Creek also includes a sediment study to evaluate the effect of the proposed creek relocation on sediment loading in the San Dieguito River and the areas west of the alignment (i.e., the relocated creek channel) during a 100-year storm event. This analysis used the FLUVIAL-12 computer model, which considers factors such as sediment supply and removal in the associated channel reaches, flood hydrograph data, channel geometry, and sediment characteristics. The sediment study determined that Stevens Creek does not currently deliver bed sediment to the San Dieguito River (due to its generally flat slope and low

3 The listed storm events represent the probability of occurrence, rather than specific recurrence intervals. A 100-year storm, for example, is defined as a storm event with a one percent chance of occurring in any given year (rather than a storm that occurs once every 100 years), while a 50-year storm has a two percent chance of occurring in any given year, and so on.



velocities), but notes that the lower reaches of the creek (near the San Dieguito River) are subject to tidal influence. Accordingly, during the dry season, tidal exchange contributes to a net recharge of sediment in the lower creek channel, with this stored sediment flushed out of the creek during larger storm events. This situation would remain essentially unchanged under the proposed creek relocation, with the hydrologic/hydraulic study concluding that “Stevens Creek has no net sediment contribution to the San Dieguito River under both existing and proposed conditions.” The study also notes that the proposed creek relocation under this alternative would not carry bed sediment to the areas west of the alignment. As a result, the flow through this area, in response to the deficit in sediment supply, would remove small quantities of sediment during a 100-year storm (beginning just downstream of the culvert outlets), with this material then deposited in an existing downstream pond (which outlets on the south end and discharges to the San Dieguito River). From this analysis, no substantial adverse effects related to sediment loading in the San Dieguito River or areas west of the alignment are anticipated from relocating Stevens Creek, based on the following considerations: (1) sediment loading in the San Dieguito River from Stevens Creek would not be affected by the proposed creek relocation (with the creek providing no net sediment contribution to the river under both existing and proposed conditions as previously noted); (2) the amount of erosion and deposition in the areas west of the alignment under the proposed creek relocation would be very minor, due to the generally level topographic conditions and low stream velocities; and (3) the identification of erosion and deposition in the areas west of the alignment under the proposed creek relocation are associated with a 100-year storm event, with essentially no erosion or deposition to occur in the noted areas during smaller storms (Appendix F, Chang 2013). Railroad Bridge Crossing. The evaluation of floodplain-related issues for the proposed railroad bridge involved preparation of a Hydrologic Engineering Center-River Analysis System (HEC-RAS) computer model to determine water surface profiles and velocities in the San Dieguito River channel for existing and proposed conditions. Based on this analysis, the hydrologic/hydraulic analysis provides the following conclusions regarding floodplain issues for implementation of the East Side-Loading Alternative: (1) the proposed bridge would pass the 50-and 100-year flood flows without overtopping, and would provide adequate freeboard (two feet or more) to accommodate the passage of drift (debris) for a 50-year storm event; and (2) the proposed bridge would not raise the 50- or 100-year floodplain water surface elevations above existing floodplain levels or result in any adverse flooding effects to neighboring properties (Appendix F). From the hydrologic/hydraulic analysis, the Drainage Study/SWMP concludes that implementation of the East Side-Loading Alternative would slightly reduce the existing 50- and 100-year water surface elevations, with “…bank elevations…still…exceeded under both conditions with similar areas flooded under these events.” This analysis also notes that:

For the existing condition, floodwaters would recede…toward the main channel to the north and over the railroad embankment to the west. Depth of flow over the railroad embankment would reach a maximum depth of approximately 1 foot…during a 50-year storm event and approximately 3 feet…during a 100-year event. For the proposed condition, the floodwater elevations would be slightly lower and the higher embankment would direct receding floodwaters



toward the main river channel instead of allowing receding floodwaters to flow over the embankment toward the homes to the west.

As a result of the described analyses, no adverse effects related to floodplain water surface elevations or associated flood hazards would result from implementation of the East Side-Loading Alternative. Scour. The evaluation of potential scour hazards in the San Dieguito River from the East Side-Loading Alternative included assessment of general and local scour at the proposed railroad bridge, as well as general scour at the existing Camino Del Mar bridge. Specifically, general scour is associated with an imbalance in sediment transport (e.g., increased sediment removal due to increased stream velocity), while local scour is associated with the placement of obstructions such as piers or abutments. General scour at a bridge crossing results in part from the higher velocity at the bridge contraction, which contributes to greater sediment transport. The analysis of general scour at the noted bridge crossings involved using the FLUVIAL-12 computer model, as previously described. Based on this modeling analysis, the project hydrologic/hydraulic analysis provides the following conclusions, with general scour depths to be reduced at both bridges and no associated adverse effects to result from implementation of the East Side-Loading Alternative: At the proposed railroad bridge, the maximum 100-year storm velocity for the East

Side-Loading Alternative would be the same as for the existing railroad bridge (10 feet per second [fps]), while the minimum elevation reached by scour would be minus 12.6 feet MSL, compared to minus 13.3 feet MSL for the existing bridge (a reduction in scour depth of 0.7 foot).

At the Camino Del Mar bridge, the maximum 100-year storm velocity for this alternative would be 11.8 fps, compared to 11.7 fps for the existing railroad bridge; while the minimum elevation reached by scour would be minus 8.8 feet MSL, compared to minus 9.0 feet MSL for the existing bridge (a reduction in scour depth of 0.2 foot).

To assess the potential for local scour at the proposed railroad bridge, the analysis utilized the Federal Highway Administration Hydraulic Engineering Circular No. 18, Evaluating Scour at Bridges, Fifth Edition (Publication No. FHWA-HIF-12-003). This guideline provides a mathematical model to evaluate the scour effects of vortices that form at bridge piers as a result of flow obstruction and acceleration. Estimated local scour depths for varying pier widths (including the six-foot pier width proposed for the new rail bridges) derived from the noted assessment are provided in Table 4.6-1, Estimated Local Scour Depths for Variable Pier Widths at the Proposed Railroad Bridges.



Table 4.6-1 ESTIMATED LOCAL SCOUR DEPTHS FOR VARIABLE

PIER WIDTHS AT THE PROPOSED RAILROAD BRIDGES (Feet)

Pier Width Local Scour Depth

4 5.45 5 6.29 6 7.08 7 7.83 8 8.54 9 9.22 10 9.88

Source: Chang 2014 (Appendix F). The hydrologic/hydraulic analysis prepared for the Proposed Action also identifies the fact that local scour can occur at other structures, including bridge abutments. While the analysis notes that “Under the present state of art, there is no reliable method for computing the depth of abutment scour”, it is also generally recognized that “…the depth of abutment scour is directly related to the ratio of channel width to bridge opening width” (i.e., abutment scour can be minimized if the abutment does not protrude into the river channel). Based on the described assessments of potential general and local scour effects at the existing and proposed bridge structures, a number of recommendations are provided in the hydrologic/hydraulic analysis to address these concerns as outlined below in Section 4.6.4. Sea Level Rise. Based on the location of the proposed railroad bridge near the coast, the potential future rise in sea level was also assessed for related effects to bridge hydraulics and scour. This analysis assumed sea level increases of approximately 16 inches by 2050 and 55 inches by 2100, based on modeling conducted by the California State Coastal Conservancy (Appendix F). It is noted that SANDAG prepared a subsequent study, “San Diego Region Coastal Sea Level Rise Final Report,” dated September 2013, which recommends using an increase of 55 inches as the average upper range for projects with service life estimates beyond 2070; this is consistent with the upper range assessed in Appendix F of this EA. For bridge hydraulics, the hydrologic/hydraulic analysis notes that the computed water surface elevations for the 50- and 100-year floodplains at the river mouth are higher than the predicted sea level in 2100, and therefore concludes that “…the bridge hydraulics based on the HEC-RAS model are not affected by the long-term sea level rise.” With respect to related scour effects, the hydrologic/hydraulic analysis notes that a rise in sea level would generally be expected to lower flow velocities, and therefore would “…reduce the channel bed scour at the bridge crossings.” From the described assessments, no adverse effects to bridge hydraulics or scour would result from project increases in sea level.



West Side-Loading Alternative Construction Impacts Potential impacts related to hydrology and floodplain during construction of the West Side-Loading Alternative include drainage alteration associated with excavation, grading and construction of proposed facilities; and groundwater extraction/disposal (dewatering), if required, as outlined below. Drainage Alteration. Implementation of this alternative would result in a number of alterations to existing local drainage patterns associated with the construction of proposed facilities, and the addition or modification of storm drain facilities. None of these activities would result in substantial impacts to local drainage patterns or directions, however, for similar reasons as noted above for the East Side-Loading Alternative (with the exception of effects related to relocating Stevens Creek, which would not occur under the West Side-Loading Alternative, with no associated impacts resulting). Specifically, existing drainage facilities would be preserved, modified or reconstructed to maintain overall drainage patterns. Accordingly, all associated flows would be collected and conveyed to the Pacific Ocean (portions of Segment 1), the San Dieguito River and Lagoon (all of Segment 2 and portions of Segments 1 and 3), or the Del Mar Wye (portions of Segment 3), similar to existing conditions. It should also be noted that, while the described impacts to drainage alteration would occur during construction of the Proposed Action, they would represent long-term (permanent) changes to local drainage patterns and are, therefore, also included below under the discussion of Operational Impacts. Groundwater Extraction/Disposal. Potential effects related to groundwater extraction and disposal under this alternative are the same as those described above for the East Side-Loading Alternative, based on the similar nature and location of local groundwater aquifers and proposed activities/facilities. Specifically, no associated substantial impacts would result, based on the fact that construction dewatering would be relatively small-scale in extent and short-term in duration, disposal of any extracted groundwater would occur locally (and would thus be subject to recharge within the same groundwater basin from which extraction occurred), and dewatering would be subject to pertinent NPDES requirements if applicable. Operational Impacts Potential impacts related to hydrology and floodplain during operation of the Proposed Action under this alternative include long-term drainage alteration, generation of increased runoff rates and amounts from construction of new impervious surfaces (with corresponding effects related to storm drain capacity, associated flooding and erosion), hydromodification, floodplain hazards (such as floodplain encroachment/modification and related flood hazards), scour, and sea level rise. Drainage Alteration. While operation of this alternative would include long-term changes to local drainage patterns as a result of proposed construction, associated operational effects would be less than substantial for similar reasons as described above for the East Side-Loading Alternative.



Runoff Rates and Amounts. Implementation of this alternative would result in a slight increase of impervious surface area, with a corresponding minor increase in post-development runoff rates and amounts. No associated adverse impacts would result from this alternative, however, for similar reasons as described above for the East Side-Loading Alternative. Specifically, such effects would be limited through proposed design measures to limit new impervious surfaces, including the use of pervious materials for pertinent structures (e.g., track bed improvements and access/maintenance roads) and provision of unlined channels where feasible, as well as the installation of energy dissipation structures at applicable locations. Hydromodification. Implementation of this alternative would be exempt from applicable HMP requirements for similar reasons as noted above for the East Side-Loading Alternative. Specifically, this alternative would not increase the unmitigated peak flow to any outlet. Floodplain Hazards. As previously noted, hydrologic, hydraulic, and scour analyses were conducted for the Proposed Action, including the proposed railroad bridge crossing and the existing crossing at Camino Del Mar for this alternative (Appendix F). The referenced analysis also evaluated the effects of potential future sea level rise on bridge hydraulics and scour for the proposed new railroad crossing, with the results of the noted analyses summarized below. Railroad Bridge Crossing. Potential floodplain-related issues for the proposed railroad bridge under this alternative are essentially the same as those described above for the East Side-Loading Alternative, based on the similar nature and location of the associated bridge structures. Specifically, under this alternative: (1) the proposed bridge would pass the 50- and 100-year flood flows without overtopping, and would provide adequate freeboard (two feet or more) to accommodate the passage of drift (debris) for a 50-year storm event and (2) the proposed bridge would not raise the 50- or 100-year floodplain water surface elevations above existing floodplain levels or result in any adverse flooding effects to neighboring properties. Scour. The evaluation of potential scour hazards in the San Dieguito River from the West Side-Loading Alternative included similar assessments of general and local scour as described above for the East Side-Loading Alternative. While the overall conclusion of no adverse effects noted for the East Side-Loading Alternative would also apply to this alternative, some minor differences in velocities and scour depths were noted for the West Side-Loading Alternative as outlined below. At the proposed railroad bridge, the maximum 100-year storm velocity for the West

Side-Loading Alternative would be the same as for the existing railroad bridge (10 fps), while the minimum elevation reached by scour would be minus 12.7 feet MSL, compared to minus 13.3 feet MSL for the existing bridge (a reduction in scour depth of 0.6 foot).

At the Camino Del Mar bridge, the maximum 100-year storm velocity for this alternative would be 11.8 fps, compared to 11.7 fps for the existing railroad bridge; while the minimum elevation reached by scour would be minus 8.9 feet MSL, compared to minus 9.0 feet MSL for the existing bridge (a reduction in scour depth of 0.1 foot).

The assessment and conclusions related to local scour at the proposed railroad bridge under the West Side-Loading Alternative would be essentially the same as those identified above for the



East Side-Loading Alternative, based on the similar nature and location of the associated bridge structures. Specifically, local scour at bridge abutments can be minimized by avoiding protrusions into the river channel, with associated recommendations outlined below in Section 4.6.4. Based on the described assessments of potential general and local scour effects at the existing and proposed bridge structures, the recommendations provided to address these concerns in Section 4.6.4 for this alternative are the same as those identified for the East Side-Loading Alternative. Sea Level Rise. Potential impacts related to sea level rise under this alternative are essentially the same as those described above for the East Side-Loading Alternative, based on the similar nature and location of the associated bridge structures. Specifically, the computed water surface elevations for the 50- and 100-year floodplains at the river mouth are higher than the predicted sea level in 2100, with the modeled bridge hydraulics therefore not affected by long-term sea level rise. Similarly, a rise in sea level would generally be expected to lower flow velocities, and therefore would be expected to reduce channel bed scour at the bridge crossings. East Center-Loading Alternative Construction Impacts Potential impacts related to hydrology and floodplain during construction of the East Center-Loading Alternative include drainage alteration associated with excavation, grading and construction of proposed facilities; and groundwater extraction/disposal (dewatering), if required, as outlined below. Drainage Alteration. Implementation of this alternative would result in a number of alterations to existing local drainage patterns associated with the construction of proposed facilities, and the addition or modification of storm drain facilities. None of these activities would result in substantial impacts to local drainage patterns or directions, however, for similar reasons as noted above for the East Side-Loading Alternative. Specifically, existing drainage conditions and facilities would be preserved, modified, or reconstructed to maintain overall drainage patterns. Accordingly, all associated flows would be collected and/or conveyed to the Pacific Ocean (portions of Segment 1), the San Dieguito River and Lagoon (all of Segment 2 and portions of Segments 1 and 3), or the Del Mar Wye (portions of Segment 3), similar to existing conditions. Groundwater Extraction/Disposal. Potential effects related to groundwater extraction and disposal under this alternative are the same as those described above for the East Side-Loading Alternative, based on the similar nature and location of local groundwater aquifers and proposed facilities. Specifically, no associated substantial impacts would result, based on the fact that construction dewatering would be relatively small-scale in extent and short-term in duration, disposal of any extracted groundwater would occur locally (and would thus be subject to recharge within the same groundwater basin from which extraction occurred), and dewatering would be subject to pertinent NPDES requirements if applicable.



Operational Impacts Potential impacts related to hydrology and floodplain during operation of the Proposed Action under the East Center-Loading Alternative include long-term drainage alteration, generation of increased runoff rates and amounts from construction of new impervious surfaces (with corresponding effects related to storm drain capacity, associated flooding and erosion), hydromodification, floodplain hazards (such as floodplain encroachment/modification and related flood hazards), scour, and sea level rise. Drainage Alteration. While operation of this alternative would include long-term changes to local drainage patterns as a result of proposed construction, associated operational effects would be less than substantial for similar reasons as described above for the East Side-Loading Alternative. Runoff Rates and Amounts. Implementation of this alternative would result in a slight increase of impervious surface area, with a corresponding minor increase in post-development runoff rates and amounts. No associated substantial impacts would result from this alternative, however, for similar reasons as described above for the East Side-Loading Alternative. Specifically, such effects would be limited through proposed design measures to limit new impervious surfaces, including the use of pervious materials for pertinent structures (e.g., track bed improvements and access/maintenance roads) and provision of unlined channels where feasible, as well as the installation of energy dissipation structures at applicable locations. Hydromodification. Implementation of this alternative would be exempt from applicable HMP requirements for similar reasons as noted above for the East Side-Loading Alternative. Specifically, this alternative would not increase the unmitigated peak flow to any outlet. Floodplain Hazards. As previously noted, hydrologic, hydraulic, and scour analyses were conducted for the Proposed Action, including the proposed relocation of Stevens Creek, the proposed railroad bridge crossing, and the existing crossing at Camino Del Mar (Appendix F). The referenced analysis also evaluated the effects of potential future sea level rise on bridge hydraulics and scour for the proposed new railroad crossing, with the results of the noted analyses summarized below. Stevens Creek. Potential impacts associated with the proposed relocation of Stevens Creek under this alternative are essentially the same as those described above for the East Side-Loading Alternative, based on the similar nature and location of the proposed creek relocation and associated project facilities. Specifically, the proposed realignment of Stevens Creek under this alternative would not change the flow characteristics for a 5- or 10-year storm event (with no associated increase in water surface elevations or lateral extent of the floodplain), and would not increase the potential for upstream flooding during 50- and 100-year storm events. Railroad Bridge Crossing. Potential floodplain-related issues for the proposed railroad bridge under this alternative are essentially the same as those described above for the East Side-Loading Alternative, based on the similar nature and location of the associated bridge structures. Specifically, under this alternative: (1) the proposed bridge would pass the 50- and 100-year



flood flows without overtopping, and would provide adequate freeboard (two feet or more) to accommodate the passage of drift (debris) for a 50-year storm event and (2) the proposed bridge would not raise the 50- or 100-year floodplain water surface elevations above existing floodplain levels or result in any adverse flooding effects to neighboring properties. Scour. The evaluation of potential scour hazards in the San Dieguito River from the East Center-Loading Alternative included similar assessments of general and local scour as described above for the East Side-Loading Alternative. While the overall conclusion of no adverse effects noted for the East Side-Loading Alternative would also apply to this alternative, some minor differences in velocities and scour depths were noted for the East Center-Loading Alternative as outlined below. At the proposed railroad bridge, the maximum 100-year storm velocity for the East

Center-Loading Alternative would be 9.9 fps, compared to 10 fps for the existing railroad bridge, while the minimum elevation reached by scour would be minus 12.8 feet MSL, compared to minus 13.3 feet MSL for the existing bridge (a reduction in scour depth of 0.5 foot).

At the Camino Del Mar bridge, the maximum 100-year storm velocity for this alternative would be 11.8 fps, compared to 11.7 fps for the existing railroad bridge; while the minimum elevation reached by scour would be minus 9.0 feet MSL, the same as for the existing bridge.

The assessment and conclusions related to local scour at the proposed railroad bridge under the East Center-Loading Alternative would be essentially the same as those identified above for the East Side-Loading Alternative, based on the similar nature and location of the associated bridge structures. Specifically, local scour at bridge abutments can be minimized by avoiding protrusions into the river channel, with associated recommendations outlined below. Based on the described assessments of potential general and local scour effects at the existing and proposed bridge structures, the recommendations provided to address these concerns in Section 4.6.4 for this alternative are the same as those identified for the East Side-Loading Alternative. Sea Level Rise. Potential impacts related to sea level rise under this alternative are essentially the same as those described above for the East Side-Loading Alternative, based on the similar nature and location of the associated bridge structures. Specifically, the computed water surface elevations for the 50- and 100-year floodplains at the river mouth are higher than the predicted sea level in 2100, with the modeled bridge hydraulics therefore not affected by long-term sea level rise. Similarly, a rise in sea level would generally be expected to lower flow velocities, and therefore would be expected to reduce channel bed scour at the bridge crossings. No Action Alternative Under the No Action Alternative, the described development actions for the Action Alternatives would not occur. The existing bridge and portions of the existing track would continue to be



overtopped during a 100-year storm event. Regardless, no adverse impacts related to hydrology and floodplains would occur. 4.6.4 Avoidance, Minimization, and Mitigation Measures Action Alternatives As previously noted, avoidance, minimization, and mitigation recommendations related to hydrology and floodplain issues would be generally the same for all three for the Action Alternatives, based on their similar nature and location. An exception to this conclusion involves measures related to the relocation of Stevens Creek, which would not occur under the West Side-Loading Alternative (as reflected in the following measures). Incorporation of the measures and considerations identified below would effectively avoid or address potential impacts related to hydrology and floodplain. Construction Impacts Drainage Alteration All proposed construction, modification, and/or relocation of storm drain facilities will

incorporate applicable recommendations from the Drainage Study/SWMP and the Hydrologic/Hydraulic Technical Studies prepared for the Proposed Action, including the design, location, and dimensions of such facilities.

The location, design, and installation of all new or modified storm drain facilities will be coordinated with engineering (or other appropriate) staff at the 22nd DAA, the City of Solana Beach, and/or the City of Del Mar (as appropriate) to ensure proper function and compatibility with existing storm drain systems.

Groundwater Extraction/Disposal All construction-related dewatering activities (if required) will comply with applicable

regulatory requirements, including the appropriate NPDES Groundwater Permit. Operational Impacts Runoff Rates and Amounts The proposed design will include permeable surfaces wherever feasible, including the use

of pervious materials for track bed improvements and access/maintenance roads, and unlined drainage channels.

The proposed design will incorporate appropriately designed and sized energy dissipation devices (e.g., rip rap aprons) at all pertinent locations, including outlets to unlined channels.



Floodplain Hazards The design, location, and dimensions of the proposed railroad bridge and special events

platform will incorporate applicable recommendations from the Drainage Study/SWMP, the Hydrologic/Hydraulic Technical Studies, and geotechnical evaluations prepared for the Proposed Action. Specifically, this will include: (1) appropriate heights and dimensions for the bridge and platform; (2) appropriate design and dimensions for pier, bank and abutment scour protection; and (3) location of the bridge abutments outside of the river channel.

For the East Side-Loading and East Center-Loading alternatives, the design, location, and dimensions of facilities related to the proposed relocation of Stevens Creek will incorporate applicable recommendations from the Drainage Study/SWMP, the Hydrologic/Hydraulic Technical Studies, and geotechnical evaluations prepared for the Proposed Action. Specifically, this will include: (1) appropriate locations and dimensions of the proposed triple box culvert and associated low-flow channel; and (2) provision of appropriate energy dissipation at the culvert outlet.

No Action Alternative Because no impacts were identified for the No Action Alternative, no associated avoidance, minimization, or mitigation measures are proposed.



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