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HomeMy WebLinkAboutChipping_Froom Ranch DEIRSunday,December 22,2019 Attn:Shawna Scott:Senior Planner,Community Development Department City of San Luis Obispo 919 Palm Street,San Luis Obispo,CA 93401-3219 RE:Comments on Froom Ranch SpeciIic Plan Draft EIR From:David Chipping:Los Osos,CA 93402 Prof.Emeritus,Geology:CalPoly,SLO) The following comments are narrowly focussed regarding hydrology and drainage related issues.I will,however,strongly recommend that any approved plan does not violate existing General Plan standards,most importantly the upholding of the 150 ft.contour building limit for all parts of the proposed project. The hydrologic analysis largely rests within Appendix H,the analyses derived therein being presented in the main body of the document.I will present each numbered issue as a bulleted,numbered and underlined point for the purposes of clarity,with any questions and requests for response in regard to that issue in bold font at the end of my discussion on that issue. Hydrology Issues 1:Misstatement concerning original position of Froom Creek. On page 13 I-5)Appendix H states Development of the Speci0ic Plan area will enhance and restore the historic Froom Creek corridor alignment and allow it to traverse future development areas.”It is true that the original alignment joined the Laguna Lake close to the edge of what is now LOVR topographic map from 1900).It then Ilowed through the currently lowest part of property and joined San Luis Creek.In no way did it turn southward and follow the current drainage along the eastern property line.Thus the creek realignment cannot be cited as a signiIicant historic restoration.This is correctly noted in Section 1.4. Topo base map from 1900.The creek crossing of the main highway and railroad is essentially at the current freeway overpass.LOVR and Madonna Rd.are present,but LOVR dead ends short of the current overpass Hydrology Issues 2:Natural slopes along the intended creek realignment’s southern sections tilt against the intended gradient of the creek,thus increasing probability of increased Ilow depth,decreased Ilow velocity,and increased Ilood depths along the Calle Joaquin wetland and developments. The following Iigure shows that the lowest point of the proposed new channel has to be low enough to receive Ilow from the LOVR ditch system,which also carries Home Depot- derived water.and is also designated as Home Depot Water Quality Treatment Area”This same Iigure has detailed topographic contouring showing the creek crossing a contour in the uphill direction. Thus while the original channel would have delivered Froom Creek Ilow directly to San Luis Creek at the overpass area,and the creek alignment from the 1940s still maintained a down-channel gradient of about 1 ft.in 100 ft based on Google Earth-derived elevations), the proposed alignment appears to result in almost no down-channel slope.It appears that the channel will have to be considerably deepened downstream of the Low Point’shown in the above Iigure. The following photograph shows the alluvial fan that is derived from the three small drainages at the southern end of the property,with the white arrows showing the slope direction of the fan surface.See also the contours on the picture above.The picture is taken at the point where the proposed channel with turn to the northwest,with green wetland vegetation in the foreground. Question:How will channel excavation affect subsurface water conditions in the jurisdictional wetlands adjacent to Calle Joaquin.Reference is given to pages.228 and 230 of Appendix H,creek sections 2031 and 1757,which are shown cutting into the edge of the wetland by as much as 4 feet. Question:Please clarify the expected Mlood discharge expected to a)overspill into the Calle Joaquin wetlands during 2-100 year Mlooding,and b)the amount that would be retained in the channel to Mlow to the box culvert and proposed storage basin at the southern end of the project. Hydrology Issues 3:The project intends to remove existing retention basins that currently capture discharge from the existing development to the west.These basins are to be replaced by a new retention basin adjacent to,and south of,the current Froom Creek box culvert. On page 156 of Appendix H,there is the following table: This table appears to show that overbank Ilow rates will almost double for the 2 year Ilood, which appears to conIlict with apparent design standards that seem to contain all peak Ilows up to the 2-year Ilood.The same table shows that the 100 year overbank Ilow rate increases from 980 to 1,240 cfs under the proposed realignment.The hydrology report does not appear to account for the effects of increased Ilow rates on the wetland and Calle Joaquin corridor. Question:The Minal EIR should show analysis of the effects of a)removing the existing upstream retention basins,and b)the effects of the proposed development’s contribution,on total Mlow spilling as portions of the overbank Mlow rate in the table shown above. Question:The Minal EIR should how the proposed retention basin will relieve overbank Mlow rates at the existing wetland area,which is far upstream and upslope of the new basin. Question:Levees in the existing wetland are designed to retain the 2-year storm,and therefore it would appear that larger storms would spill out of the channel at that point.If water is spilling out of channel upstream of the proposed retention basin, why is the channel between the spill point and the basin designed to retain the 100 year Mlood? Hydrology Issues 4(a):Proposed longitudinal gradients along the new channel suggest that sedimentation will accumulate in the area of the current wetland The new creek alignment will result in a steepening of the bed slope at the point where the new channel starts relative to the existing channel.In order to accept water moving southeastward along the edge of LOVR,the channel will Ilatten in the area of the existing wetland to a slope considerably lower than the existing channel.The drop in Ilow velocity will lower steam competence and capacity regarding sediment load. Question:Can it be shown that sedimentation will not build up on the channel Mloor adjacent to the existing wetland,and that there will be no resultant increase in Mlooding or possible stream migration? Hydrology Issues 4(b):A diagram of proposed longitudinal gradients along the new channel on page 215 of the PDF Iiles of Appendix H is confusing The above diagram appears to show an upstream slope for the bed of the stream lower line)and a another proIile which is higher than the wetland and drainage along LOVR. Appendix H does not seen to explain what this all means,and how the proIiles reIlect existing conditions and engineered changes. Question:The FEIR should provide explanation for this Migure,and show the places and elevations where LOVR and Home Depot drainage enter the engineered channel, and where storms greater that the 2-year storm are expected to have planned overbank Mlow. Hydrology Issues 5:There is no analysis of any cross-Calle Joaquin Ilows on Ilooding potential in that area Analysis of photos taken during the 1973 Ilood show that water stands at an elevation at 106 feet,as elevations of the high water mark can be matched with Google Earth elevations. The existing land surface of the proposed drainage capture basin adjacent to the Froom Creek crossing of Calle Joaquin is 102-14 feet,and appears to be underwater in the 1973 Ilood. The above photo clearly shows water Ilowing onto the southbound 101 from the west. Since 1973 there has been signiIicant alteration of the grades along the southbound entry ramp,added impermeable surface around Calle Joaquin,but no signiIicant change in drain capacity connecting the area to San Luis Creek.Raised water levels at the current culvert would also affect the sewer lift station,which will be expanded to greater capacity to meet Calle Joaquin and Froom Ranch demand. Question:Are there public safety and property damage impacts that will result in the Froom Ranch development’s added Mlood discharges across Calle Joaquin?