**NATURAL HAZARDS: Tahoe Basin**

Prepared by Victor G. Mossotti

**Statement of Problem**

The potential impacts of natural hazards in the Tahoe basin include a
wide range of adverse effects on life and property, businesses, and the
natural environment. The most apparent potential hazards include:
earthquakes, landslides, rockfalls, tsunamis, flooding (within basin,
downstream from dam), and catastrophic wildfire. Many of these potential
impacts are coupled to each other and have cross-cutting relationships
with other science theme areas. For example, vegetative cover, which is
closely linked to topology and local climate, is essential to the
natural ecology of a watershed; vegetative cover is also a critical
control on wildfire. Wildfire, in turn, drastically alters vegetative
cover and soil structure, both of which control the hydraulic response
of the landscape. Water retention and penetration control soil erosion
rates and affect pore pressure at potential geologic landslide surfaces.
These variables combine to exert control on the seismic response of the
landscape for rockfalls, landslides, and sediment releases, all of which
influence the health of downhill wetland ecosystems. Potential impacts
on the downhill natural environment include: damage to forests, damage
to soils, increased stream runoff, nutrient loading and contamination of
the Lake Tahoe and of lateral lakes and wetlands, alteration of
lake-bottom sediments and chemistry, and modification of lake dynamics
(such as circulation and upwelling). Possible adverse effects on life
and property include: loss of lives, loss of property including damage
to homes, schools, businesses, utilities (e.g., electrical, natural gas
lines, water supplies, sewage lines), transportation facilities,
transportation routes, and the Lake Tahoe dam. Several of these effects
could in turn trigger additional wildfires or floods, and could also
result in short-term isolation of the basin or parts of the basin. All
of these events would have negative impacts on businesses and tourism.

**Objectives**

The goal of the proposed research is two-fold. The first is to fill the
knowledge gaps associated with natural hazards identified with the
following subthemes: 1) Earthquakes and related effects; 2) Landslides
and related effects; 3) Tsunamis and related effects; 4) Wildfires and
related effects; 5) Effects of hazards on vulnerable facilities and
populations; and 6) Integration of scientific findings to inform land
planners of systemic dynamics of Tahoe basin. The second goal is to
develop transferable knowledge such that the Tahoe basin work will serve
as a model study for similar basins around the planet.

**Work Strategy**

*Immediate/short-term activities*

\*Acquisition of high-resolution LIDAR data set for all land area within
the entire Tahoe Basin; \*Database compilation: Compilation of existing
scientific literature on natural hazards at Tahoe; Compile in a GIS
existing data appropriate to subthemes 1- 6 on water wells, historic
earthquakes and landslides, seismic reflection profiles and core
analyses (on land, in lake), and physical properties for major surficial
and bedrock units; \*Geologic maps: Revise, update, and harmonize
geologic mapping in a format designed to serve subthemes and research
questions; \*Forest health maps: Create or obtain GIS layers for
vegetation, forest health, rainfall, wildfire, debris flows in
watersheds, and hydrologic impacts of burned areas; \*Critical
facilities maps: Create GIS layers for infrastructure units that could
trigger wildfire if damaged by hazards covered in subthemes 1- 3
(Earthquakes, Landslides, Tsunamis), and/or that would be adversely
affected by wildfire; \*Hazard Maps/Management response: Create
preliminary GIS hazard maps and derivative emergency response maps that
would serve the goals of Subtheme 5 of identifying vulnerable facilities
and populations. Such maps would integrate the layers of hazard
information in the GIS databases compiled from existing data as outlined
above. In order to compute potential for ground shaking, slope
stability, liquefaction, tsunami, flooding, and forest fire hazards, we
would also require models for the integration of the GIS layers.

*Long-term research activities*

These activities are designed to fill knowledge gaps associated with
subthemes 1-6.

[Earthquakes]{.underline} \*GPS networks to measure tectonic and local
strain; \*Terrestrial LIDAR study sites to determine local movements;
\*Surveying fault offsets of Quaternary deposits (tripod-mounted LIDAR,
conventional surveys, shallow and high-resolution seismic reflection
surveys); \*Trenching of selected faults; \*Coring and dating of
sediments displaced by active faults, on land and beneath lake;
\*Detailed characterization of surficial deposits in selected areas near
faults; \*Monitoring of microseismicity within the Lake Tahoe basin
(subthemes 1 & 2); \*Determine ground-response characteristics of
Quaternary deposits in selected areas (e.g., shaking, liquefaction),
using seismic surveys and refraction microtremor surveys;
\*Probabilistic seismic hazard analysis; \*Chronology of surficial
deposits to calibrate frequency of events (earthquakes, landslides) and
fault slip rates; (Tephrochronology of lacustrine and surficial
deposits, OSL (optically stimulated luminescence) dating of lacustrine
deposits; 14C dating of charcoal fossils in lacustrine and fluvial
deposits; Diatom micropaleontology; Cosmogenic-exposure (10Be, 26Al,
3He) bedrock & boulder surface dating).

[Landslides]{.underline} \*Accurate delineation of landslides and
rockfalls, using submersible ROV mapping of fractures and landslides
beneath lake and surface mapping with LIDAR data and other imagery;
\*Modelling of slope stability; \*Testing to determine strength
properties of materials; \*Detailed surveys of landslide-prone areas;
\*Delineation of fractures and other weak zones; \*Detailed
characterization of surficial deposits in selected areas near
landslides; \*Delineate areas of active soil creep and tilting using
novel methods.

[Tsunamis]{.underline} \*Develop records of past tsunamis from detailed
studies of shoreline areas and shallow submerged areas using ROV
observations; \*Sediment coring in marshes near shoreline to detect and
date tsunami deposits; \*Modelling of earthquake scenarios along
selected faults to predict tsunami effects; \*Modelling of selected
landslide and collapse events to predict tsunami effects.

##### [Wildfires]{.underline} \*Determine the most vulnerable forest and/or watershed areas; \*Refine maps of most vulnerable facilities, such as gas lines, electrical lines or substations, and fuel storage facilities; \*Determine the most vulnerable locations for triggering of wildfires;\*Determine the probability of triggering mechanisms such as natural agents (lightning strikes), human agents (home fires, industrial fires, construction fires, motorists, campers, smokers); \*Intersection of natural hazards with human infrastructure (gas lines, electrical lines or substations, fuel storage facilities); \*Identify mitigation strategies currently in use, whether they are effective, and whether they may be improved (cf. subtheme 6); \*Work with other theme areas, such as Soil Conservation and Water Quality, to evaluate effects of wildfires on soils and vegetation, runoff, infiltration, and nutrient-loading of Lake Tahoe. 

##### [Vulnerable facilities and populations]{.underline} \*Periodically update hazards and vulnerability maps as data from subthemes 1‑ 4 become available;

##### [Integration of scientific findings and management response]{.underline} \*Use modeling tools from complex systems science to develop a framework for understanding the dynamics of the Tahoe basin and for evaluating the consequences of proposed planning scenarios; \*Develop planning scenarios for emergency response to various types of damaging events and their possible coupled effects. 

**Partnerships**

The scope of work identified in this proposal is sufficiently vast so as
to require partnering with academia, governments at all scales, and
stakeholders in the Tahoe basin for advancing this work.