| 4. Impacts on Geology & Soils. Seismicity, Topographical Impacts |
| 4.1. Areas of Study | 4.2. Precedents and Consequences | 4.3. Mitigations |
Geology and soils are very interconnected with hydrology and water issues - it is suggested that this section be referenced to Section 8: Hydrology and Water Impacts.
Please refer to Appendix A, A-4. Resources Map
| 4.1. Areas of Study |
At this point, no grading plans or soils, geology, or seismic reports are available to help focus on project-specific issues. USGS regional Topology Maps and the topo lines from the preliminary project maps are referenced. Various soils and geological reports from past projects are available. Although not complete, as the projects never progressed past the concept plan stage, they indicate the following areas of study:
a) The lower portion of the East (residential) parcel, near the highway is low lying and a natural water collection point. Vegetation in the area would suggest high ground water and a regular, at least seasonal recharging. Impacts of grading, filling and paving of this area should be examined for possible water accumulation redirection toward lands of the school and the highway berm.
b) The creek that runs along the southern side of the residential parcel has been heavily channeled and diverted over the years, affecting the discharge of water from higher lands, the distribution through the property, and sediment discharge into the ocean. Preliminary site plans would indicate this creek will be covered in grading and pavement, the Eucalyptus and Cypress Trees around it removed, and the water presumably pipe-channeled to the Highway culvert. Sediment load from creeks discharging into the ocean can have an affect on the rate of beach buildup and subsequently the overall erosion rate of the surrounding bluffs. Impacts of any alteration of the soil runoff and sediment load increase to the West Parcel and Ocean should be studied.
c) Consideration should be given to the rapid (1-5 ft a year) erosion rate of the West Parcel when examining the value and use of this parcel in the overall project. This erosion rate needs to be mapped out for the economic life of the houses being proposed to forecast future problems if the erosion rate on the West Parcel and the area immediately to the North of it proceeds at a rate that could cause the inland rerouting of Highway 1.
d) All of the West Parcel, and most of the East Parcel, although zoned RM, have been identified as prime agricultural soils. The southern section of the Hammerhead is considered prime and zoned PAD. Although agricultural preservation policies do not apply within the urban boundary, farming and ranching operations are part of the land's history and consideration should be given to the cumulative impact of the loss of the potential of these soils from the San Mateo Coast.
4.1.1. Agricultural Assessment
(From historian Barbara VanderWerf):1. Prime agricultural soils indicated: soils survey assessment needed.
2. Given the evidence (old newspaper accounts and maps), a good case can be made that the east and west Granada parcels were in dry-land agricultural use (hay, red oats, root crops) as early as 1861. Previously, the parcels were used for cattle grazing.
3. The 1928 aerial of Granada (part of a series of the first aerials made of the MidCoast) shows row crops on the east and west parcels. Local memory indicates the crop was generally irrigated artichokes and Brussels sprouts.
4. The USGS 1946 aerial indicates row crops on both parcels.
5. The USGS 1968 aerial shows El Granada School and what appears to be pasture land on the east parcel. The west parcel appears to be in agricultural use.
6. The east parcel pasture land was until recently a horse boarding operation (photo documentation available). Three or four years ago, the pasture was plowed and seeded to rye grass to enhance the horse feed.
7. The east and west Granada parcels have been in ranch/farm use from the early 1860s until recent times. From the 1790s to the 1860s, these two parcels were grazing lands.
e) The overall site is identified as "Marine Terrace and Younger Alluvial Fan Deposits" on the Geotechnical Hazards Synthesis Map for SMCo *, specifically Beach and Sand Dune Marine Terrace Deposits on the proposed residential site. This area would be subject to "Very strong" groundshaking intensity from an earthquake on the San Andreas, Seal Cove or Hayward faults. The impacts of this level of ground-movement with the "weakly consolidated" sand and gravel deposits, in addition to the obvious ground water saturation on the lower areas, should be carefully analyzed in regard to construction and excavation for building foundations.
* source: Mirada Surf Concept Plan, November 19, 1990 - Section II, Chapters 2 &3
| 4.2. Precedents and Consequences |
| 4.3. Mitigations |