Planning decisions can set in motion eventual massive land form impacts The establishment of use designations and zoning densities provides an underlying endorsement for land division and development. The earliest action point for many land alteration decisions occurs at the planning stage, particularly when a large parcel is proposed for subdivision into smaller, supposedly developable lots. Development expectations go hand-in-hand with lot creation. For this reason, subdivision decisions must be made with as complete an understanding of all development-related impacts as possible. The following are checklists of the information that should be used in making subdivision decisions and identification of the minimum information required by the Commission to review a subdivision application.
2.A. Some areas should never be subdivided Some areas or parcels of land have such high environmental value or such high risk of instability that no subdivision of a parcel in one of these areas should ever be considered. For example, it is unlikely that a dune parcel would ever be approved for subdivision. If planners are aware of efforts to subdivide such a parcel, the developer should be told clearly that no subdivision would be recommended for approval, regardless of the study results. This will avoid the expense and time of hiring technical experts and preparing a subdivision plan. In some areas, too, it may be necessary to merge lots or adjust lot lines to provide acceptable building areas.
2.B. Characteristics of a good parcel for subdivision approval The ideal parcel for subdivision would be able to support all elements of the subdivision while completely avoiding all hazardous and sensitive resource areas, and minimizing land form alteration. If there are any resources on the parcel which can be avoided by the proposed subdivision plan, but which could be harmed by some unintentional activities associated with the subdivision, it may be appropriate to require these areas and adequate buffers be placed in permanent open space while approving legal lots for the areas with no siting restraints. If there is any resource which may be impacted by approval of the subdivision and this area cannot be protected fully by open space dedications, deed restrictions, or other acceptable action, it must be assumed that the resource will be damaged or impacted by the eventual subdivision development. Since there is no requirement to approve a subdivision, this possible resource damage is a basis for denial of a subdivision request.
3.A. No subdivision should be approved without complete information on impacts When reviewing a subdivision, the parcel should be treated as a single lot with the minimum allowable development potential until the developer can thoroughly demonstrate that the creation of additional lots will not have an unacceptable adverse environmental impact. This requires that the developer show, in detail, all the impacts which may occur if any further lot division is permitted and all possible steps which can be taken to minimize these impacts. In addition, this requires the developer to delineate all future building areas, such as access roads, driveways and building pads, for all created lots to determine the feasibility of the total development. Table 1 lists the types of studies which should be considered for a parcel, prior to subdivision. The subdivision should not be approved until all aspects of the possible future development have been reviewed and determined to be acceptable. The major land form related concerns associated with subdivision and subsequent development are direct and indirect disturbance of sensitive or environmentally significant habitat areas, modifications to the visual character of the area, direct and indirect transportation demands, changes to or enhancement of natural erosion and sedimentation, and modifications to surface and subsurface drainage. These impacts are discussed at length in Attachment 1: Land Form Alteration Policy Concerns, March 1993, and will not be reiterated further in this report. In addition, other environmental concerns ranging from socioeconomic effects to increases in noise or nighttime glare need to be reviewed prior to any approval of a subdivision request, but since these concerns are tied less clearly to land form alteration, they will not be included in the remaining discussion.
3.B. All resources on the parcel must be thoroughly identified and mapped One reason for concern about the impacts from any type of land form alteration is that the alteration efforts may affect significant resources that occur on the site. To fully determine the impacts that site development may have on the resources, all significant resources must be identified. The Commission is currently preparing guidance on wetlands and non-point source pollution which must be followed for those concerns. All other location-specific concerns -- riparian habitat, rare threatened or endangered plant or animal species, oak habitat, old growth timber, chaparral, native grasslands, streams, wildlife corridors, feeding or nesting areas, public and equestrian trails, archaeological sites, or any other aspect of the site that should be protected -- must be surveyed, located and mapped by qualified botanists, wildlife biologists, ecologists, terrestrial biologists, archaeologists, historians or other appropriate specialists. The subdivision review should identify all the experts hired to inspect the property, their expertise and their findings, with a statement from the property owner or their legal representative that all of the studies or reports for the site have been provided to staff and no report or study was intentionally ignored or suppressed.
3.C. Drainage characteristics of the parcel must be studied A second reason for concern about land form alteration is its potential for impacting drainage and site hydrology. Since any development will cause some change to hydrology, the predevelopment hydrology should be studied. If there are any streams or natural drainages on the site, seasonal and high flow characteristics should be determined. Areas of erosion and sediment deposition should be mapped. If the site has potential landslides, drainage to and near these areas should be carefully delineated.
3.D. Site topography and geologic hazards must be studied thoroughly Any review of a subdivision in any area which may be unstable should start with site topography and a survey of geologic hazards. The survey, identification and mapping of geologic hazards should be prepared by a licensed geologist or geotechnical engineer, and the work should follow, at a minimum, the guidelines of the State Board of Registration or California Division of Mines and Geology. If the survey area is prone to landslides or faults, aerial photographs, satellite imagery or other remote sensing tools should be considered, in addition to ground surveys, borings and other traditional survey techniques. The surrounding area should be reviewed for signs of large-scale instability, even though the final report may only address the subject site. All hazard areas should be fully identified on a topographic map or other appropriate base map and setbacks, if appropriate, should be specified and noted. Since the geology of a difficult site may be subject to different interpretations, a property owner may decide to hire more than one geologist or engineering geologist to inspect a site. If this is done, the property owner should provide the names of all experts who inspected the site, copies of the reports, and a commitment by the property owner or their legal representative that no review or study has been intentionally ignored or suppressed.
3.E. Developable areas should be identified from resource and hazards maps The subdivision plan cannot be started until all areas of possible concern have been identified and mapped. The areas free of hazards or sensitive resources are the only areas that should be considered for development activity. If the parcel contained many acres which the developer intends to maintain in open space, it may be appropriate to study the resources and hazards only in the areas which would be considered for development. If impacts are identified, the study area should be expanded to find alternative areas which might be free of impacts. The subdivision must fit the resources and limitations of the parcel.
4.A. Site Access Must be Provided in Detail Site access is critical for development -- to get construction equipment into the subdivision and to the individual lots, to let property owners get safely to their property, and to let emergency vehicles and fire equipment get to the property if there is a fire or disaster. In flat areas or area with gentle slopes, access can often be fit to the contours of the site. In steep terrain, access if often wedged into hillsides with the roadway straddling engineered fill material and engineered cut slopes. Properly designed cuts and fills require fairly gradual slopes (usually 1.5 or 2 to 1, horizontal to vertical) which means that the width of disturbance for a roadway can be much wider that the actual roadway itself. Grading for access can often be the most significant land form alteration component of site development.
4.B. Access must be reviewed relative to site drainage Once roadways are established, they often serve, intentionally or otherwise, as drainage conveyances for surface flows. Roads can impound small flows, diverse existing flows to other areas and concentrate surface flows to new locations. Drainage modifications can trigger changes in sedimentation, erosion, habitat characteristics and overall site stability. Some examples of options for access across a drainage area are provided in Section VII.
4.C. Only areas with safe access should be considered developable Since access is both a critical component of site development and a potentially critical element in overall site impacts, site access must be considered carefully in the initial efforts to determine buildable areas. While it may be possible to establish foot paths, horse trails or even four-wheel drive roads through a property, it may be very difficult to establish a safe all-weather road, suitable for public traffic, heavy trucks and fire equipment. When the construction of conventional access is found to pose significant environmental problems, alternative access locations and types of access should be examined.
5.A. In most cases, a geotechnical or soils engineer should do early site review Geotechnical site information is most useful when it is available as a tool in determining the overall subdivision layout, rather than used to determine the stability of the areas that have been pre-identified for development. In the book, Forensic Engineering: Environmental Case Histories for Civil Engineers and Geologists, the authors, Shuirman and Slosson, discuss some of the more publicized devastating landslides and floods, and describe many of the human actions which may have lead to these "Act of God" disasters. They found, "fundamental flawed design policies we have seen repeated again and again is the sequence of consultation a land developer often uses when building on hillside property: (a) architect/land planner; (b) civil engineer; and (c) geologist and geotechnical engineer.[5] " If the geologists and geotechnical engineers are hired early, they can work with the architect/planner to identify problem areas in advance of development concepts.
5.B. Site stabilization will change existing land forms; site geology must be studied Land alteration impacts from a subdivision cannot be determined without detailed information about all the resources and hazards that exist on the site. In many steep slope areas the most critical aspect of land form alteration and thus potential subdivision impacts comes from efforts to create stable slopes for access roads or building pads. Landslide remediation and slope stabilization are often accomplished by removal and recompaction of the slide material. The engineered slope that will be created from the recompacted material should, at a minimum, be constructed in accordance with local code requirements. In most cases, the resulting slope will not look or function like other land in the surrounding area -- the drainage of the area will be altered; the new slope may be more extensive than the original one, covering adjacent habitat; the slope may be flatter and more uniform than the original. The environmental impacts from slope remediation are well recognized. Some indicators of possible need for slope remediation are the presence of landslides, unstable soils or fault zones in the vicinity, or if any part of the site has slopes steeper than 20%. If there is uncertainty over any of these concerns it is better to expect the worst and require a geologic report, rather than assume the site will be stable and find out otherwise after subdivision of the parcel.
5.C. Plans for a subdivision with slope stabilization If any slope stability efforts would be proposed or required to insure a safe subdivision, the subdivision plans should provide details of the stabilization efforts. The hazard area should be highlighted on a map and the stabilization effort must be outlined, diagrammed or sketched to clearly show all technically acceptable methods for stabilization. The interaction of these corrective efforts and pre-development drainage must be analyzed. Both stabilization efforts and drainage analysis must be prepared and stamped by a licensed geotechnical or civil engineer.
5.D. Planning alternatives for slope stabilization If the extent of visual impacts from these stabilization techniques are of concern, the planner may want to provide acceptable parameters for specific locations. For example, if a hazard area would be visible from a scenic vista and the planner has determined that no vertical disturbance should exceed eight feet in height, the planner should specify this parameter to the engineer; it is solely the engineers responsibility to design the stability efforts which would meet the needs of the site and fit within the planning parameters. There is a clear difference between specifying visual parameters or highlighting habitat areas that cannot be affected by the stability effort and redoing engineering designs or specifying a certain type of stabilization; planners should not attempt to do any aspect of engineering design. Attachment 3 provides a overview of the some of slope stabilization efforts that an engineer may propose. This Attachment was prepared to familiarize planners with some of the options which a licensed engineer may consider, but does not replace professional judgment and training.
5.E. Subdivision plans for areas on or adjacent to active faults If there are any active faults which cross the site, the hazards report should locate the fault zone and establish a safe setback for structures from the fault. At a minimum, the geologist should determine if the parcel is within a special study zone and meet the policies and criteria of the Alquist-Priolo Act. The Division of Mines and Geology has been required to prepare special study zone maps for the San Andreas, Calaveras, Hayward and San Jacinto Faults as well as other faults or segments of faults which the State Geologist deems sufficiently active and well-defined as to constitute a potential hazard to structures form surface faulting or fault creep. Maps of all special study areas can be obtained from the Division of Mines and Geology.
5.F. Intent of the Alquist-Priolo Act The Alquist-Priolo Act of 1972 provides "policies and criteria to assist cities, counties and state agencies in the exercise of their responsibility to prohibit the location of developments and structures for human occupancy across the trace of active faults as defined by this [State Mining and Geology] Board. Single family wood frame dwellings, not exceeding two stories and not part of a development of four or more buildings and single family wood frame structures to be built on parcels of land for which geologic reports have been prepared pursuant to the Subdivision Map Act are exceptions from the Act. Alterations or additions to any structure in a special study zone, where the value does not exceed 50% of the structure are also excluded, except for the requirements of disclosure to potential buyers.
5.G. Subdivision of Eroding Parcels If a subdivision is proposed for a site which is undergoing active erosion or adjacent to the ocean, the lot may be modified significantly by the time it is actually considered for development. The questionable long-term stability of the site should preclude its subdivision. If it is subdivided, the lots should be sized to permit extensive erosion of shoreline retreat before any development occurs. It may be appropriate to set a time limit or site modification limit such that if the erosion reaches a point where the lot cannot be developed with adequate setbacks, the lot must be retired.
6.A. All technical reports must be clear and precise Many technical studies are written by experts and are addressed to a professional audience. These reports must remain useful to the experts who will use the technical information to make design recommendations, site restoration or possible mitigation plans and must be presented in the technical vernacular that is expected from the profession. Nevertheless, technical experts must also recognize that their reports will be used by planners to identify locations or resources on the parcel which must be avoided, protected, or treated in some special manner. Land use planners are not required to be experts in any of the technical areas which they review to make informed land use decisions, and it would be impossible for any one person to achieve broad expertise in all the technical areas which may have some bearing on land use decisions. Technical reports must provide overviews or summaries that any reasonable intelligent lay person can understand. If the reports contain information on areas where sensitive or important resources have been located or where some geologic hazard may exist, the reports must state these conclusions plainly, show clearly where the locations are and indicate, if possible, what activities can and cannot be undertaken on or proximity to these resources or hazards. For example, it is not enough to provide a map of surface geology and assume that a planner can and will find all the indicators of landslides of surface instability; these must be highlighted in some manner so that the average person can understand that there are, or are not, stability problems on the parcel.
6.B. A subdivision plan, complete with all relevant requirements, must be provided All elements of the subdivision must be provided and mapped in a fashion that shows how they will or will not affect the identified resources and hazard areas. The subdivision elements would not just be lot lines, but access roads that will meet fire access requirements, lot areas that meet minimum zoning requirements, driveways and building areas that meet minimum lot line set back requirements, the areas that would have to be cleared of vegetation for fire safety, utility corridors, septic areas, well sites, and open space areas. Whenever there is a law, regulation, code or standard which dictates any element of this subdivision plan, the supporting report must provide a complete reference for this code, current for the time the subdivision is being considered. If the planner does not have copies of these codes, standards, requirements, etc., the developer can be asked to provide a copy with the application. If the design is affected by undocumented professional "standards of practice", the applicant should provide the names or at least three professionals working in the area who can and will substantiate the use of these "standards of practice" for the parcel which is being reviewed.
6.C. Subdivision plans should specify appropriate foundation styles for building pads Once building areas are established, the style of building foundation can affect how the individual lot development may alter the existing land forms. In attempting to fit the development to the general characteristics of the site, every portion of the development must be adapted to the site. For a building this starts with the foundation. In a flood prone area, elevation of the building may be necessary, either through elevating the site or siting the building on pilings. A flat slab foundation may reduce the visual profile of the building, but may require extensive cut and fill if the building area is steep. Different foundation designs will affect the property differently and for many areas it is appropriate to consider various foundation designs, rather than fix the site to accommodate a chosen foundation design.
7.A. TYPES OF INFORMATION FOR SUBDIVISION PLANNING DECISIONS
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TYPE OF INFORMATION |
USES FOR INFORMATION |
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Aerial Photographs |
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Topographic Maps, with contour intervals suitable for site layout. If grading or drainage plans are prepared, some topo maps should be provided at a scale similar to these plans. |
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Geologic Studies Soils Surveys |
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Hydrologic studies |
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Hazards Assessment (for high hazard areas) |
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Vegetation and Wildlife Surveys |
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Cultural Features, Circulation Patterns |
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Utilities and Public Services |
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Visual surveys |
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Regulatory Setting |
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7.B. SUMMARY OF SUBDIVISION AND SITE PLANNING GOALS
Determine if parcel is suitable for subdivision or should remain a single lot.
Examine land for appropriate use consistent with state and local planning and zoning policies
Ensure that every aspect of the subdivision maintains the character of the site through consideration and integration of topographic and natural features into the overall design and layout.
DO avoid areas which could require extensive grading to insure stability.
DON'T rely on engineering solutions to correct poor planning decisions.
DO identify and avoid or setback from all sensitive resources and prominent land features.
DON'T rely on relocation efforts or restoration to replace resources impacted by poor planning decisions.
DO study the parcel thoroughly for all possible resource and hazard concerns.
DON'T assume that omissions in planning can be corrected during site development
DO consider combining subdivision elements, such as access, utilities and drainage.
DON'T overlook the effects to surface hydrology from every modification to the parcel.
DO use natural features to plan all aspects of the subdivision.
DON'T plan development for ridge lines, on dunes, in or adjacent to streams, or on top of prominent features.
DO require the all the technical experts that should have input on the resources and concerns about the parcel have been involved in the subdivision plan; one site development expert cannot cover all the critical technical aspects.
DON'T approve any request for a subdivision unless all potential impacts from possible land form alteration have been resolved in a manner that is guaranteed to succeed whenever site development occurs.
ENDNOTES
5. Shuirman, Gerald and James E. Slosson (1992) Forensic Engineering: Environmental Case Histories for Civil Engineers and Geologists, Academic Press, San Diego.
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