Community health and safety involves the protection of the community from natural hazards such as floods, earthquakes, landslides, and fires. It also involves protection from man-made hazards such as air pollution, noise, and hazardous materials. This chapter describes the scope of these hazards, as well as the actions that the City can take to eliminate or reduce the probability of these hazards leading to a major disaster. The overall purpose of this chapter is to--

Action Program [4]:   Designate alternative facilities for post-disaster assistance in the event that primary facilities become unusable.

Action Program [5]:   Correct known structural deficiencies.

Goal 3:   Avoid development in areas prone to natural hazards.

Policy e:   Allow land uses in areas prone to natural hazards only with appropriate mitigation.

Action Program [6]:   Continue to regulate development to assure the adequate mitigation of safety hazards on sites having a history or threat of slope instability, seismic activity (including liquefaction, subsidence, and differential settlement), flooding, or fire.

Most of the flood zone is so designated because of the potential for flooding during a 100-year high tide, rather than from excessive rainfall.

Flood Control and Storm Drainage Improvements
In 1968, the Army Corps of Engineers began to dredge, straighten, and line Corte Madera Creek with concrete. By 1972, the project had been completed from the mouth of the creek to the Town of Ross, a distance of about 3.5 miles (Flood insurance Study). However, Ross opposes placing the creek in a concrete channel, and work on the last phase, through the town, has been stalled. Until the last phase has been completed, Larkspur will not be adequately protected from freshwater flooding. Even then, the improvements will not provide protection from tidal flooding.

The portion of Larkspur Creek (also known as Arroyo Holon) between Monte Vista Avenue and about Meadowood Drive has been diverted to a culvert. A 1989 study also recommended an earthen berm and a concrete floodwall be constructed along a section of the creek near Meadowood Drive.

In 1988, Larkspur undertook a Storm Drain Master Plan Study of the older portion of Larkspur - the area south of Corte Madera Creek and west of Highway 101. Historically, most of Larkspur's flooding has occurred in this area. Also, certain areas adjacent to Corte Madera Creek have experienced significant settlement over the years, exposing them to increased flooding (Storm Drain Study).

Most of the storm drain system in the study area is 30 to 40 years old. The system evolved piecemeal as it was gradually extended upstream to serve new development and as problems occurred. A significant portion of the system has settled and deteriorated, causing maintenance and flooding problems. Also, because much of it is located on private property, the system lacks adequate capacity to convey runoff.

The study recommended a series of improvements (trash racks, catch basins, use of concrete pipe, and annual inspections of open channels on private property) to reduce flooding problems. It also concluded that more pumping stations will be needed to protect against high tides inundating low-lying developed land.

Flooding problems east of Highway 101 and south of Corte Madera Creek have been somewhat alleviated by the installation of a new storm drain system and pump station. However, it is still necessary to sandbag the low-lying area near the creek because of tidewater flooding. Flood walls and a tide gate are needed to provide permanent protection.

Rise in Sea Level
Recent scientific studies indicate that sea level is rising at an accelerated rate. A global climate change caused by the accumulation of "greenhouse" gases in the atmosphere (carbon dioxide, methane, and chlorofluorocarbons) is projected to raise the temperature of the earth's atmosphere, melting some of the earth's glaciers and polar ice caps. If recent historic trends continue, global sea level should increase between four and five inches in the Bay in the next 50 years and could increase approximately one and one-half to five feet by the year 2100.

Although the phenomenon is not fully understood, the rising sea level has implications for Larkspur and other cities along San Francisco Bay. Tidal circulation could change and wave action could increase. Drainage would be impeded, and ground water could be contaminated. The Bay Conservation and Development Commission, which regulates land use along the Bay, recommends that local governments take the predicted rise into consideration in land use planning and development review (Marin General Plan, Environmental Hazards Element, Draft Technical Reposrt #1, Flood Hazards: Existing Conditions and Recent Studies, Feb. 1988; and Amendment 3-88 to "San Francisco Bay Plan," Bay Conservation and Development Commission).

Flood Hazard Goals, Policies, and Programs

Goal 4:   Protect Larkspur from the risk of flood damage.

Policy f:   Seek to have the Corte Madera Creek flood control improvements completed upstream from Larkspur.

Action Program [7]:   Work with the Marin County Flood Control District, the Army Corps of Engineers, and the Town of Ross to develop and implement an improvement plan that protects against flooding.

Policy g:   Work with other cities in the Ross Valley to develop a comprehensive master plan for flood control and management of Corte Madera Creek.

Policy h:   Regulate land uses in flood-prone areas and allow development in those areas only with appropriate mitigation.

A map of possible inundation resulting from a failure of the dam at Phoenix Lake has been prepared by the Marin Municipal Water District (MMWD). (The dam was buttressed on the downstream side in 1970, to survive a "design earthquake." In 1988, the spillway was built and lowered six feet. Any flood damage resulting from flood inundatio would largely be confined to areas northwest of Larkspur-i.e., to the narow Ross Valley down to and including the College of Marin and the College park subdivision (18 homes). From there downstream, the canyon widens into a broad alluvial flood plain where dam flood waterss would rapidly dissipate. Source: Dana Raxon, Marin Municipal Water District, August 1989 and May 14, 1990).The map is available to the public at the City Planning and Public Works departments and the MMWD offices.

Action Program [8]:   Establish standards for minimum grades and minimum finished floor elevations that take into consideration the rising sea level during the expected life of the project.

Policy i:   Continue to upgrade the City's drainage system.

Action Program [9]:   Implement the recommendations of the 1988 Storm Drain Master Plan study.

Action Program [10]:   Construct flood walls and a tide gate in the area east of Highway 101.

Action Program [11]:   Balance required protection measures with the need to protect environmental resources, and do so in such a way as to integrate design improvements with the protection of natural resources.

Action Program [12]:   Require site plans to locate structures outside or above the 100-year flood zone wherever possible.

A large part of the Franciscan Formation underlying Larkspur is Franciscan melange, a mix of rock types embedded in crushed rock materials. Melange has highly erratic slope stability characteristics.

Fingers of alluvial stream deposits (clay, sand, silt, and gravel) extend down the hillsides in swales and creek beds to gently sloping alluvial fans and floodplains. These soils were eroded from the steep slopes and transported by flooding streams. The older area of Larkspur (Downtown and the nearby residential neighborhoods) lies in a wide flat alluvial valley. Occasional erosion-resistant hills or knolls such as Palm Hill and some of its small neighbors protrude above the otherwise flat plain.

The third type of geological material is Bay mud, which lies in a broad band between Magnolia Avenue on the south and Sir Francis Drake Boulevard on the north (excluding Bon Air Hill). The older Bay mud reaches a thickness of 40 feet at the mouth of Corte Madera Creek. Bay mud is an unconsolidated jelly-like material that is both highly compressible, and subject to lateral flow when loads are placed on it (Information on geology was obtained from Geologic Report and Selected Geologic Aspects of Larkspur, James C. Bangert, 1974; Preliminary Geologic Map of Marin (and other) Counties, United States Geological Survey, 1974; and Marin County General plan, Environmental Hazards Element, Draft Technical Report #3, Seismic and Geologic Hazards in Marin County, August 1988).

Seismic Hazards
A seismic hazard is the effect of an earthquake such as surface faulting, ground shaking, ground failure, or tsunami or seiche (tidal waves). All of these must be addressed in the general plan.

Larkspur is not at risk from surface rupture. The San Andreas fault, which is the only active fault in Marin County, lies eight miles to the west of the city. The Hayward fault, also active, lies 13 miles to the east.

Larkspur is at risk from ground shaking - underground vibrations or waves generated by the breaking and snapping of rocks along a fault line during an earthquake. Most damage associated with past - and future - earthquakes is from ground shaking. Ground shaking causes direct damage to buildings, roads, and utilities. The greatest losses solely from ground shaking may occur where tall structures are built on thick, relatively soft, saturated sediments, and the least where they are built on firm bedrock (Marin County Tecgnical Report #3). Ground shaking can also trigger liquefaction, landslides, and tsunamis, indirectly affecting these same facilities.

Larkspur is also at risk from ground failure, in the form of liquefaction, settlement, and landslides. Liquefaction is a process by which water saturated clay-free sands or silts are transformed from a solid to a liquid state. Areas susceptible to liquefaction in Larkspur are those underlain by saturated, loosely compacted granular materials such as old stream beds (alluvium) [Geologic Report].

Settlement is the drop in elevation of a ground surface caused by settling or compacting of the underlying material. The most severe and damaging settlement is most likely to result from liquefaction and landsliding. Settlement may occur without seismic activity, as discussed under Geologic Hazards.

Landslides, the jarring loose of basically unstable hillside materials, are another type of ground failure. Landslides induced by earthquakes will occur generally in the same marginally stable areas as landslides induced by other forces, such as rainfall.

A tsunami is a large ocean wave generated by an earthquake in or near the ocean. A tsunami would be expected to reach approximately 10 feet in the Bay near Larkspur. The wave run-up would generally be confined to the area east of the railroad crossing on Corte Madera Creek. Tideland areas and filled ground near or below sea level could be inundated. A seiche is an earthquake-generated wave in an enclosed body of water, such as a lake, reservoir, or bay. Similar run-up and inundation would be expected from a seiche (Larkspur Seismic Safety Element, 1973).

divides Larkspur into three seismic hazard categories based on the underlying geology. Areas with the least earthquake stability are composed of artificial fill, Bay mud, and landslide and stream deposits. The thick, loose soils of Bay mud tend to amplify and prolong the shaking. Areas of moderate stability are composed of sandstone, shale, and melange. The most stable areas are underlain by hard sandstone.

Effects of Earthquakes
Earthquakes are measured in terms of magnitude and intensity. The measure of magnitude, the Richter scale, is more commonly recognized. It assigns a number to the calculated energy release of an earthquake which is independent of the earthquake's observed effects. The Modified Mercalli Intensity (MMI) scale assigns a Roman number (I to XII) based on a description of the physical effects of earthquakes. The intensity can vary with the magnitude of the earthquake, the distance from the site to the faults, and with geologic materials.

The intensity of the maximum possible earthquake in Larkspur would vary depending on geologic conditions in each location. Bay muds would experience the greatest intensity of shaking, and corresponding severe damage to nearly all structures (MMI XI). The alluvium areas would experience a lesser intensity of shaking with destruction of most masonry and frame structures (MMI X) [ibid.]. The Franciscan rock areas would experience the least shaking, resulting in general damage to foundations and frame buildings (MMI IX).

The ability of buildings to withstand earthquakes depends on when they were built and their structure type. Buildings built before 1933 did not have to meet building code regulations relating to earthquake resistance. Since then, codes have been updated several times, and newer buildings are increasingly more resistant to damage. Generally, older wood frame structures may perform relatively well, while unreinforced masonry buildings (usually brick, stone, or concrete block with no reinforcement) probably do not meet current seismic safety standards, and may not withstand a major earthquake. In conformance with state legislation passed in 1985, Larkspur has identified 15 unreinforced masonry buildings scattered through the area south of Corte Madera Creek. Although several are located downtown, most downtown buildings, including City Hall, are wood frame. State law requires that cities must adopt a mitigation program for buildings of unreinforced masonry.

A major earthquake (8.3 magnitude on either the San Andreas or Hayward Fault), in addition to damaging buildings, can be expected to topple the Highway 101 freeway overpass at Sir Francis Drake Boulevard and buckle pavement on Highway 101 and Magnolia Avenue through Larkspur. The earthquake could set off landslides along Sir Francis Drake Boulevard leaving Greenbrae separated from the rest of Larkspur for up to 24 hours (Scenario developed under the Earthquake Preparedness Program of the California Division of Mines and Geology). Bon Air Road and Doherty Drive could also collapse from liquefaction and settlement. Utility lines carrying water, gas, and sewage could be ruptured by landslides and sudden settlement. If Fires break out, emergency response teams could encounter serious difficulties in fighting them if water lines are broken and landslides block access roads.

Seismic Hazard Goals, Policies, and Programs

Goal 5:   Reduce risks of personal injury and property damage associated with seismic activity.

Policy j:   Establish acceptable levels of risk and life safety standards, and see that buildings are built to, or brought up to, those standards.

Action Program [13]:   Require that all unreinforced masonry buildings are seismically upgraded to protect against loss of life.

Action Program [14]:   Alert owners of homes built prior to a specified date (related to early codes) that building frames should be bolted to foundations.

Action Program [15]:   Establish a basic seismic safety notification process through resale inspections.

Action Program [16]:   As soon as legally permissible, adopt new versions of the Uniform Building Code which contain updated seismic requirements.

Action Program [17]:    Require geotechnical engineering investigations for (a) buildings proposed to be constructed in "high" seismic hazard areas potentially subject to severe ground shaking and ground failure (Bay mud, stream and landslide deposits) as shown on Figure 7-3, and (b) critical structures or structures made of materials other than wood frame.

Policy k:   Seek to preserve existing historic buildings under any new standards that are adopted.

Natural conditions which affect slope stability are steepness of the slope, characteristics of the soil, degree of water saturation, and seismic activity. Human activities that can contribute to landslides include steep cuts in the slope, improper placement of fill on slopes, concentrating surface runoff, and overwatering.

During the winter storms of 1982, landslides (debris avalanches) in Larkspur produced over 10,000 cubic yards of soil, rock, and debris (Hillslope Processes and Urban Planning, Paul J. Seidelman and Jeffrey D. Borum, 1983). Slope movements resulted in several million dollars in damage to homes, roads, and other improvements (see Flooding, page 125). After that, the City undertook a study of the nature, extent, and magnitude of slope stability hazards in a 300-acre area of undeveloped land on the city's southwestern slopes. The results of the study ("Hillslope Processes in Urban Planning") can be generalized to the other hillside areas in the city.

The report found that land surfaces in the study area had been shaped predominantly by erosion processes which could be expected to continue to actively change the landscape. Areas underlain by melange (described earlier) tended to produce more landslides. Physical disturbances of the ground resulting from land use activities have exacerbated slope stability problems in some areas. These activities include siting structures and road improvements on - or in the path of - landslides, and the improper design, use, or installation of retaining wall structures, drainage facilities, and cut slopes.

The report includes detailed maps showing landslide and slope movement locations in the study area.

Because debris avalanches result from a sudden failure of natural or human-modified slopes and travel at high speeds, it is difficult to provide advance warning to those in the path of the flow. However, local residents could be warned when heavy rainfall and total storm precipitation indicate increased landslide potential.

Subsidence and differential settlement have occurred in Larkspur near Corte Madera Creek. Differential (or uneven) settlement may occur in poorly consolidated soils during earthquake shaking or over time. Settlement may be the result of poorly engineered fill, or of building on soils which are too weak to assume the load. In 1974, construction of several condominium developments on Corte Madera Creek had to be halted when cracks up to one inch wide were discovered in outside walls (Geologic Report).

When settlement occurs over a large area it is called subsidence. Subsidence may result in flooding as ground levels are lowered (see section on flooding).

shows four slope stability zones for Larkspur. Zone 1 includes areas of artificial fill over Bay mud or stream deposits. Although generally stable, they are prone to settlement. Zone 2 (flat or gentle slopes in valleys and along ridges) is the most stable. Zone 3 is similar to Zone 2 except that slopes are steeper and they are underlain by rock and slope deposits. Zone 4 (landslides, and quarry walls and highway cuts prone to rock falls) is the least stable. It should be emphasized that the zones are generalized.

In 1982-83, more detailed geologic studies were made of two areas west of Magnolia Avenue. These two areas - called out on Figure 7-4 by solid boundary lines - are evaluated and extensively discussed in "Hillslope Processes and Urban Planning, Larkspur, CA," by Seidelman Associates, Lafayette, California, 1983, available in the City of Larkspur Planning Department.

Geologic Hazard Goals, Policies, and Programs

Goal 6:   Limit the exposure of existing and future structures to risk from landslides, debris flow, and subsidence, and minimize the potential for damage.

Policy l:   Provide property owners with information to assist them in addressing their risk from landslides and debris flows.

Action Program [18]:    Make available to developers (and to owners of residences) in areas of steep slopes with seasonal and intermittent drainages, the City's geotechnical reports on and information regarding the potential hazards from debris slides and flows, and encourage them to seek professional advice from registered engineers or certified engineering geologists on how to lessen potential risks.

Action Program [19]:   Develop a program to protect existing structures located in high risk areas by promoting the use of debris fences (to deflect debris flows around structures) or other mechanisms.

Action Program [20]:   Provide residents with information about the geotechnical hazards of heavy rainfall in areas of landslide potential, and the need to evacuate.

Action Program [21]:   Provide property owners with standard public drainage designs that may be retrofitted to existing homes.

Action Program [22]:   For development in hillside areas, establish, by ordinance, standards for foundations and retaining walls that meet or exceed the current state of the art in structural and civil engineering practice. The standards shall include:

(a)   A retaining wall that provides support for the footings of a structure must have the same life expectancy as that of the supported structure.

(b)   A series of stepped or terraced retaining walls should be designed and approved by a qualified engineer even when the height of the individual walls is less than the standard that requires review.

(c)   A soils engineer or engineering geologist will be required to provide field supervision of the drilling and concrete pouring operations for pier foundations to insure the exclusion of loose debris from the pier holes, insure adequate pier depth, and confirm soil conditions.

(d)   Foundation plans for hillside structures utilizing pier foundations in soil depths of six feet or greater shall be designed to structural and soils engineering calculations based upon passive pressures and shall demonstrate to the satisfaction of the City that the pier will satisfactorily resist shearing.

Policy m:   Ensure that new development in hillside areas takes place only in areas that are relatively free from the threat of landslide and other forms of ground failure.

Action Program [23]:   Require approval of a use permit for building additions or new development in areas with an average percent of slope equal to or greater than 25 percent.

Action Program [24]:   Employ the following standards for assessing the acceptability of new construction in hillside areas and those adjacent areas with a potential risk from landslides and debris flows.

(a)   Areas subject to recent slope movement or within the paths of debris flows are not suitable for the development of occupied structures. Further disruption of these high risk areas will only be permitted by the City for roads, utilities, and other similar facilities after intensive geologic studies have determined that mitigation measures are practical and their costs warranted.

(b)   Generally, parcels with an average slope of 65 percent or greater (or that portion of a parcel with a slope of this magnitude) that show evidence of having been formed by landslide processes in the past are not suited for the development of occupied structures.

(c)   Sites underlain by deep-seated landslides and landslide debris deposits may only be developed with occupied structures if detailed geotechnical investigations demonstrate that any soils creep and future deep seated slide processes will, in the City's opinion, be satisfactorily mitigated.

(d)   High energy flow paths are not suitable for the development of occupied structures. These flow paths are steep seasonal drainages that have been a path for debris flows in the past.

(e)   The development of new occupied structures within debris flow dissipation areas, which are those areas in the vicinity of the mouths of high energy flow paths, will be permitted only if adequate mitigation measures are provided.

Action Program [25]:   Require site-specific geologic and geotechnical reports for new construction in hillside areas and areas subject to settlement or subsidence.

Action Program [26]:   dopt standards for geologic and geotechnical reports that outline the type and extent of investigation required for various stages of the development process, for various geologic and soils conditions, and for the type of land use and structure proposed.

(a)   Proposed development should include detailed plans for drainage facilities. These plans should incorporate a hydrologic and, where appropriate, a geomorphic evaluation of existing drainage courses and City drainage facilities that will be impacted by the project. The evaluation should demonstrate the adequacy of these systems. After adequacy is demonstrated, the drainage facilities should be connected to City storm drains.

(b)   In hillside areas and at the mouths of seasonal and intermittent streams, a geologic report should be required as a part of the site development review process for all structures proposed for human occupancy and situated where geologic hazards may directly or indirectly influence the design, location, and safety of the structure. A geotechnical report should be required where soil engineering and/or geologic conditions may affect the design, location, and safety of a structure proposed for human habitation.

Many homes located in high risk fire areas were built of combustible building materials or with stilt and pole construction. The latter allows suspended floor areas to trap heat, increasing opportunities to ignite the homes.

Besides the loss of property and natural vegetation from a wildland fire, firefighting efforts could scar the land through bulldozing, road cutting, and use of fire retardant chemicals. Erosion and landslides may occur on the denuded slopes during the rainy season.

Fire Goals, Policies, and Programs

Goal 7:   Protect Larkspur residents and property from fire hazards.

Goal 8:   Minimize the risk of wildland and structural fires, and ensure adequate fire protection.

Policy n:   Provide fast and efficient fire suppression service to Larkspur residents.

Action Program [27]:   Establish performance standards such as desired response times for police, fire, and other public services.

Policy o:   Maintain an aggressive fire prevention program.

Action Program [28]:   Continue to inspect all businesses, public buildings and apartment buildings annually for fire and building code violations.

Action Program [29]:   Continue to require that all vacant lots annually be cleared of excessive vegetation.

Action Program [30]:   Continue to require smoke alarms and Class C or better fire retardant roofs for all new construction.

Policy p:   Establish more stringent fire protection standards for private development in high risk fire hazard areas.

Action Program [31]:   Require that automatic sprinkler systems be installed in new residences in areas with difficult access and/or poor water supply.

Action Program [32]:   Require that brush be cleared for a distance of 30 feet from residences in high fire-hazard areas.

Action Program [33]:   Establish landscaping guidelines that encourage the use of fire-resistant plants in high fire-hazard areas.