20180116 Late Correspondence
City Council (CC)
20180116 Late Correspondence
11/13/2018 9:13:17 AM
11/13/2018 8:25:48 AM
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CITY COUNCIL AGENDA, 2018-01-16 <br />Regular Business, Public Hearing 2 <br />Late correspondence. <br />Comments upon the Draft Feasibility Study Update, Portuguese Bend Landslide complex <br />(December 22, 2017 draft.) <br />The Landflow Subcommittee of the Infrastructure Management Advisory Committee (IMAC) <br />performed a preliminary review of the Feasibility Study. These comments are offered <br />because the City Council's public hearing is occurring before the full IMAC has had an <br />opportunity to evaluate the Draft Feasibility Study. Therefore, the following comments are <br />only those of the subcommittee, not of the full IMAC. <br />There are two types of issues: <br />A. Suggestions for minor revisions of or additions to the Feasibility Study, and <br />B. Suggested issues for public evaluation that are prompted by the Feasibility Study. <br />A. SUGGESTED REVISIONS AND ADDITIONS TO DRAFT FEASIBILITY STUDY: <br />1. Feasibility Study, Figure 14 depicts proposed horizontal drains in plan view. It would be <br />helpful if elevation views of horizontal drain placement were provided, similarly to <br />Geotechnical Figures 3 and 9, Section H-H' and Section 1-1'. The elevation views would <br />show what subsurface water levels the horizontal drains feasibly could and could not <br />passively drain. Elevation views also could show what subsurface water levels would require <br />active water extraction, such as pumped wells. Note that on Geotechnical Figure 3 a "O" <br />(zero) elevation contour occurs along Section 1-1' between Sections G-G' and H-H'. This <br />zero-elevation contour is inland, well north of Palos Verdes Drive South . The zero contour <br />appears to indicate that a portion of the approximate basal rupture surface of the landslide is <br />at the same elevation as the ocean surface. This means that there is no gravity gradient <br />between the zero-elevation-contour area and the ocean. If this is approximately correct, then <br />the horizontal drains cannot collect subsurface water from the zero-elevation basal rupture <br />surface and deliver that water to the ocean. (If the zero contour is not the same elevation as <br />the ocean surface, then a clarification of the zero contour elevation would be helpful, and a <br />conversion of all drawings to the same contour-reference point would be helpful.) <br />2. Could there be added to the draft Study an analysis of the feasibility of successful <br />drainage of surface storm waters to be collected north of Palos Verdes Drive South and <br />delivered past the road bed to the ocean?
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