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HomeMy WebLinkAboutPA2023-0034_20230206_1085-7_Tarp Analysis_August 2022TECHNICAL MEMORANDUM 1940 E Deere Avenue, Suite 250 ● Santa Ana, California 92705 ● 949.837.0404 PROJECT NUMBER: 10850007BLUF TO: Shawna Schaffner FROM: Tony Bomkamp DATE: August 26, 2022 SUBJECT: Potential Effects of Seasonal Tarp Installation of Seasonal Tarp Installation to Protect Bare Slopes Between Park Newport and Back Bay Drive, Newport Beach I have reviewed the project site and associated aerial photographs to determine whether seasonal installation of protective tarps on the slopes below residential areas associated with the Park Newport Community that overlooks Upper Newport Bay. The purpose of the tarps is to protect steep unvegetated slopes from erosion, which in turn protects Back Bay Drive, which is heavily used for recreational purposes by Newport Beach and other County of Orange residents. To determine whether the tarps exhibit the potential for impacts the following factors were considered. • Impact of annual tarping on vegetation, specifically vegetation immediately adjacent to the tarps. • Analysis of “No Tarp” condition – would vegetation be supported in these areas given steepness of the slope, lack of topsoil and highly erosive conditions. • Various measures to minimize impact to adjacent vegetation during seasonal installation of the tarps. • Impact to vegetation if alternative using permanent mesh is implemented. Slope Description Exhibits 1 – 3 depict the slopes showing the slopes with the proposed location of the seasonal tarps and Exhibits 1 – 3 showing the bare slopes with no tarps but with the proposed limits of the tarps. The exhibits show avoidance of impacts to vegetation from the proposed tarps. The areas subject to protection by the tarps are unvegetated, supporting no vegetation or very sparse non-native annual grasses and forbs with Location 2 supporting a few small patches of native alkali heath. The single patch of vegetation in the central portion of Location 1 would be avoided during tarp installation by creating a “cut-out” around the vegetated patch. MEMORANDUM August 26, 2021 Page 2 Tarp Installation Tarp installation would occur prior to the rainy season and consists of layering of the black plastic material that is secured in place by a combination of sandbags and wooden stakes. Following the rainy season, the tarps would be removed, and the slopes would be uncovered and returned to the unprotected condition. The tarps are proposed to be installed prior to the start of the rainy season around October 15th and removed at the end of the rainy season on or about April 15th, weather permitting. Impact of Annual Tarp Installation on Vegetation As depicted on Exhibits 1 – 3, the tarps would be installed in a manner that does not encroach into adjacent native vegetation; thus, installation of the tarps would not impact surrounding or adjacent vegetation and once installed is stationary and does not result in impacts during the rainy season while the tarp provides protection from erosion and landslides. Removal of the tarps following the rainy season would be conducted by accessing the tarps from the tarp surface and would accomplished without disturbance of the surrounding vegetation. Thus, both installation and removal of the tarps and associated sandbags used to anchor the tarps can be conducted with no impacts to the vegetation, including the vegetated patch in the central portion of Location 1. Analysis of “No Tarp” Condition The slope areas receiving seasonal protection from installation of the tarps have exhibited significant sloughing and sliding, which has resulted in closure of Back Bay Drive and significant cleanup costs. The sloughing and sliding have also disrupted recreation and coastal access. Based on the lack of vegetation on adjacent slope areas, it is not expected that that slope areas identified for seasonal protection would support vegetation in the absence of the tarps during the rainy season. Thus, the slope protection does not appear to pose an adverse effect vegetation recruitment on the steep barren slopes. Measures to Minimize Impact to Adjacent Vegetation As noted above, the protective tarps would be installed in a manner that avoids native vegetation alliances as depicted on Exhibits 1 – 3. Access for installation of the protective tarps is gained by using the tarps themselves as the tarps are installed. This method limits the potential for any impacts to the adjacent vegetation alliances and where patches of vegetation occur, the tarps are carefully installed to the edge of the vegetation or allow for “cut-outs” (such a in Location 1) where local patches of vegetation are avoided and protected in place. Similarly, during removal, access is gained by using the tarp surface precluding incursion into adjacent vegetation. Given MEMORANDUM August 26, 2021 Page 3 the current method for installing the protective tarp, no additional measures are necessary to protect the adjacent vegetation. Impact to Vegetation if Alternative Using Permanent Mesh is Implemented An alternative to seasonal installation of the protective tarps is to install metal mesh on the areas requiring protection. This approach is effective for limiting large landslides but is most effective at protecting from rock and boulder falls. While the steep slopes are generally rocky and have lost topsoil, and are generally absent of topsoil, the installation of steel mesh could present opportunity for limited vegetation growth that would include both native and non-native plant species. X:\0363-THE REST\1085-07BLUF\GIS\TarpGIS\1085-7_TarpImpacts_Location2_NoTarp.mxd 0 20 4010 Feet ± PARK NEWPORT ROCKFALL MITIGATION PROJECT Location 2 - Tarp Impacts Exhibit 2 Coordinate System: State Plane 6 NAD 83 Projection: Lambert Conformal Conic Datum: NAD 1983 2011 Map Prepared by: B. Gale, GLA Date Prepared: August 25, 2022 1 inch = 20 feet Key Map Not to Scale P ark N ew port BackBayDrive SanJoaquin H i l l s R o a d Proposed Tarp Boundary Aerial dated April 2021 B ac k B ay D r iv e X:\0363-THE REST\1085-07BLUF\GIS\TarpGIS\1085-7_TarpImpacts_Location3_NoTarp.mxd 0 20 4010 Feet ± PARK NEWPORT ROCKFALL MITIGATION PROJECT Location 3 - Tarp Impacts Exhibit 3 Coordinate System: State Plane 6 NAD 83 Projection: Lambert Conformal Conic Datum: NAD 1983 2011 Map Prepared by: B. Gale, GLA Date Prepared: August 25, 2022 1 inch = 20 feet Key Map Not to Scale P ark N ew port BackBayDrive SanJoaquin H i l l s R o a d Back Bay Drive Proposed Tarp Boundary Aerial dated April 2021