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4. ENVIRONMENTAL ANALYSIS 4.7 GEOLOGY,SOIIS,AND SEISMICITY - surface water with adequate flow velocity could initiate rill erosion,gully erosion and debris flows. Typically,the velocity and volume of surface water flow increases with the length of slope. On a 100-foot high slope terraced at a height of 50-feet, surface water would be intercepted by the terrace and drainage system. Without terracing, surface water flows would — combine and increase velocity without interruption thereby increasing the potential for surface down-cutting and rill erosion. The applicant has not proposed terracing for 100-foot high,2:1 (H:V)final reclamation slopes and as discussed above, this is not consistent with Chapter 9-1415-3(k)(1) (Terracing)of the San Joaquin County Development Title, Ordinance for Quarry Excavations(see Regulatory — Background section). It is unknown at this time whether the applicant could apply to San Joaquin County for a variance to construct the proposed slopes. If the applicant does apply for a variance to the slope ordinance,it would likely be required to demonstrate that the slope would remain — stable to avoid erosion in the proposed configuration. Information reviewed for this EIR indicates that the applicant has based its proposal for non-terraced slopes on the success of 2:1 (H:V)slopes at other RMC Pacific Vernalis Quarry mining sites,including Huck Quarry in San Joaquin County. Because it is uncertain whether San Joaquin County would require a variance to permit the applicant to construct 2:1(H:V)permanent reclamation slopes inconsistent with Chapter 9-1415- 3(k)(1),the Mitigation Measure 4.7-3 would ensure that impacts associated with erosion on 100- foot high,continuous 2:1 (H:V)slopes would remain less-than-significant. Mitigation Measure 4.7.3: Erosion Assessment for Permanent Reclamation Slopes — Prior to project implementation,the applicant shall perform appropriate soil and erosion testing to demonstrate that the combination of clay soils and vegetation, proposed for placement on the — 100-foot high, 2:l(H:V)reclamation slopes,can adequately withstand the erosive energy of stormwater runoff under conditions of a 100-year,24-hour storm event. This assessment shall be performed by a California certified engineering geologist and experienced sedimentologist with — appropriate background in erosion control on engineered earthen slopes. The assessment must present assessment methodologies,provide findings, and recommend erosion control measures to protect the slopes. If the results of the erosion assessment indicate that the tractor-compacted clay — soils would be susceptible to significant erosion(i.e.,erosion capable of causing down-cutting, filling,and gullying) with the proposed slope configuration,erosion control measures,as recommended by the assessment, shall become part of the project specifications and incorporated into the final Reclamation Plan. Recommended erosion controls could include measures such as intermediate lateral drainage with drop inlets,increased compaction and clay content of slope — soils,increased infiltration capacity, or changes in vegetation type to reduce surface flow velocity. The assessment shall be prepared with consultation with the San Joaquin County Office of Community Development and its Review Authority. — RMC Pacific Vemalis Quarry Mining and Reclamation Project 4.7-24 FSA 1203015 Administrative Draft Ent March 2004