Park Drive is a neighborhood connector street, per the City of Carlsbad’s General Plan Mobility Element, that runs parallel to the Agua Hedionda Lagoon in Carlsbad, California. Park Drive provides one lane of travel in each direction, parking, bike lanes, and sidewalks within the 60-foot-wide right-of-way (ROW) (Figure 1 – Project Location). Park Drive is the only connector road to Bayshore Drive, which has the only public boat launch for kayaks and other non-motorized vessels on Agua Hedionda Lagoon. Bayshore Drive also provides beach access for fishing and other shoreline recreation. Currently, the hillside along the northeast side of Park Drive between Cove Drive and Bayshore Drive experiences significant erosion and drainage issues that affect the function of the roadway and sidewalk, and the safety of pedestrians, cyclists, and drivers, particularly during and after rain events. Road widening in the late 1980s cut the toe of the slope along the northeast side of the road, after which the slope began to show erosion issues, in turn prompting construction of the retaining wall several years later. Over the years, the deposition of sediment along the sidewalk and roadway has created a safety hazard, restricted public access to local public beach areas, and created a maintenance burden for the City of Carlsbad (City). In addition, the existing retaining wall shows signs of structural failure, likely due to ineffective drainage measures, which has affected its function and longevity (please refer to Figure 2, Existing Site Photos).
The proposed Park Drive Slope and Drainage Improvement Project (Project) is needed to stabilize the slope along the north side of Park Drive, and to mitigate surface (surficial slaking, sloughing, and erosion) and deeper-seated instability. Weakly cemented to friable sandstone is exposed at the face of the slope, which is very prone to surface-water-induced erosion, as evidenced by the deep rills in the face of the slope. This erosion creates a large volume of sediment loss. Slope failure and sediment loss from the hillside repeatedly occurs during rain events, requiring the City to close the sidewalk and roadway on a regular basis to clean up large volumes of debris.
Dating back to 2004, the City has recorded work orders and service requests every year to clean up debris within the Project site. Figure 2 depicts sediment deposition along Park Drive after a storm event in December 2018. The closure of the sidewalk, bicycle lane, and roadway caused by the sediment deposition impacts the public’s beach access and presents continuous safety hazards. These closures occur multiple times a year after even small rain events. This erosion creates a large volume of sediment loss that was estimated to be an average of 35 cubic yards per year. In addition, slope stability analyses completed for the Alternatives Analysis indicated that portions of the slope do not meet generally accepted minimum standards. The analyses indicated that deep-seated stability of the slope should meet generally accepted minimum standards. In addition to the slope stability considerations, the existing retaining wall requires replacement because structural failure increases each year. Figure 2 depicts degradation of the existing retaining wall. Portions of the block wall are degrading to the point where the steel rebar is exposed, and portions of the retaining wall are beginning to lean toward the sidewalk. The failure of the retaining wall could result in additional slope instability issues and would be further exacerbated by storm events, and would require a long-term closure of the sidewalk, bicycle lane, and portions of the roadway until an emergency repair project could be constructed.
To address public safety and coastal access issues associated with the failing hillside, the City proposes to remove the existing wall, install a new retaining wall, and repair portions of the failing slope. All wall improvements are contained within the City’s ROW and the easement areas, including the open space easement, and no permanent impacts to the adjacent private property would occur. Construction impacts would occur within the private property to replace the slope to the existing condition due to the wall replacement.
The proposed wall design includes several types of retaining walls based on the varying site conditions, easements, and access. For this analysis, the Project site has been sectioned into three improvement zones: Zone A, B, and C, which are shown on Figure 3, Proposed Improvement Zones, and described below.
Zone A is located within the Park Drive ROW (Assessor’s Parcel Number 20-101-01) and private property (Assessor’s Parcel Number 207-100-57) within the open space easement and 25-foot-wide storm drain easement (Doc. No. 85-207258). Zone A is located at the southeast end of the Project site and currently consists of about 1.5 to 1 (horizontal to vertical [1.5:1]) slope with a low retaining wall, approximately 2 to 5 feet high (please refer to Figure 4, Improvement Zone A). In most portions of this zone, the soil has eroded down to parent material, with little to no topsoil present. The overtopping of the sediment-laden flows, as well as the runoff from the hillside, is straining the existing wall in this area.
Proposed Zone A improvements would include an approximately 330-linear-foot stepped planter block wall with geogrid reinforcement. The geogrid wall in Zone A would replace the existing wall at a maximum height of 12 feet. The geogrid-reinforced segmental retaining wall would consist of masonry facing blocks with proprietary means of vegetating the face, supported by a geogrid-reinforced soil mass behind the wall to create a gravity retaining wall. The slope behind the wall would be contour graded to a 2:1 slope (horizontal to vertical inclination). The slope height within the Project area of Zone A is approximately 30 feet. The wall would be constructed in incremental lifts consisting of stacked masonry facing blocks connected to geogrid reinforcing layers that are embedded in structural backfill behind the wall facing. The geogrid wall would be planted with native and drought-tolerant species and would be maintained by the City. A brow ditch would be located at the top of the 2:1 slope and at the top of the wall to safely convey stormwater runoff to the existing storm drain system. Although erosion is expected to decrease substantially, the potential for erosion would not be eliminated entirely by removing the stormwater runoff from the face of the slope using the brow ditch at the top of the slope. Revegetation of coastal sage scrub along the hillside would commence once construction is complete. Mitigation efforts through revegetation would result in no-net-loss to coastal sage scrub.
The majority of Zone B is within private property, which limits options where the permanent impacts are contained within the existing ROW and easements (please refer to Figure 5, Improvement Zone B). Understanding that reduction of the current public access is unfavorable, options for this zone to replace the existing retaining wall in kind at its existing height are limited.
Proposed improvements for Zone B include installation of an approximately 10-foot-tall solider pile and lagging type retaining wall to replace approximately 180 linear feet of the existing retaining wall in this area. The soldier piles would be drilled behind the existing wall along the majority of the alignment, and sections of the existing wall would be demolished from the top down to allow the lagging to be installed while maintaining the stability of the slope behind the wall. Once the lagging is installed, a finish would be installed on the face of the wall to mimic the existing block wall’s look and color. The proposed retaining wall in Zone B would minimize the temporary construction impacts on the existing slope.?At either end of the solider pile and lagging retaining wall in Zone B, the wall would tie into the proposed wall in Zone A to the southeast, and transition into the proposed wall in Zone C to the north.
Zone C is located at the north end of the Project site (please refer to Figure 6, Improvement Zone C). Proposed improvements for Zone C would include installation of a 2-foot-tall block wall approximately 300 feet in length. Additionally, a drainage ditch and sediment trapping best management practice (BMP) is proposed to reduce the potential deposition of sediment from the slope on the sidewalk after storm events and to reduce the sediment that reaches the storm drain system and ultimately discharges into the Agua Hedionda Lagoon. The drainage ditch and screen wall would collect the eroded slope material until City crews are able to remove it. The proposed wall in Zone C would tie into the existing wall located at the north end of the Project site and is intended to be of similar color and type.?The proposed 2-foot-tall wall in Zone C would transition into the proposed solider pile wall in Zone B. Similar to Zone B, the majority of Zone C is within private property, and the options are limited to improvements that can be made within the ROW.