GEOGRAPHY UNIT 1 Module 3 natural events and hazards. section 5 response to hazards

Module 3: Natural Events and Hazards. Section 5 Response to Hazards

1. Hurricanes
In the Caribbean island countries hurricanes cause more damage and disturb the lives of more people than
any other natural hazard. In Mexico and Central America they are second only to earthquakes. From 1960
to 1989 hurricanes killed 28,000 people, disturbed the lives of 6 million people, and destroyed property
worth US$16 billion in the Greater Caribbean Basin (excluding the United States and U.S. possessions).
Small countries are particularly vulnerable to hurricanes, since they can be affected over their entire area,
and major infrastructure and economic activities may be crippled in a single event.

More significant, however, is the record of reducing this impact. Hurricane intensity has not abated. Thus,
with population density increasing, the number of deaths would be expected to increase over time. In fact,
it has decreased. In 1930 three people were affected by hurricanes for each person killed. By 1989 that
ratio had risen to 100,000 to one. The ratio of dollar value of damage to people killed rose from 5,000 to
20,000,000 in the same period. These reduced death rates are due almost entirely to improved warning
systems and preparation. Some progress has been made toward reducing damage, but that is a more
difficult issue.

A hurricane is defined as a large non-frontal tropical depression or cyclone with wind speeds that exceed
119 km/hr (a tropical storm has wind speeds of 63 to 119 km/hr). The hurricane season of the Greater
Caribbean Basin is June through November, although 84 percent occur in August and September.
Hurricanes cause damage by their high winds, heavy rainfall, and storm surge. Winds up to about 162
km/hr cause moderate damage such as blowing out windows. Above that velocity winds begin to cause
structural damage. Heavy rainfall can cause river flooding, putting at risk all structures and transportation
facilities in valleys, and can also trigger landslides.

Storm surge is a rise in sea level due to on-shore winds and low barometric pressure. Storm surges of 7.5
meters above mean sea level have been recorded, and a surge of over 3 meters is not uncommon for a
large hurricane. Storm surges present the greatest threat to coastal communities. Ninety percent of
hurricane fatalities are due to drowning caused by storm surges. If heavy rain accompanies a storm surge,
and the hurricane landfall occurs at a peak high tide, the consequences can be catastrophic. The excess
water inland creates fluvial flooding, and the simultaneous increase in sea level blocks the seaward flow
of rivers, leaving nowhere for the water to go.

To assess risk as a step in the process of preparing a hurricane hazard mitigation plan, a planner first
determines whether the study area lies within the belt of commonly occurring hurricanes. If it is located in
"Hurricane Alley" (see Figure 9), the planner studies records of past storms and land uses and correlates
them with probable future land use and population changes. Most cities in the West Indies are in low
coastal zones threatened by storm surge, and population movement to these high-risk zones greatly
increases vulnerability. The economic sectors most affected by hurricanes are agriculture and tourism.
Bananas, one of the most important Caribbean crops, are particularly vulnerable. The tourism sector in the
Caribbean is notorious for its apparent disregard of hurricane risk. Not only does a hotel built with
insufficient setback risk damage by wave action and storm surge, the building also interferes with the
normal processes of beach and dune formation and thus reduces the effectiveness of a natural protection
system
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Source: Munchener Ruck. Mapa Mundial de los Riesgos de la Naturaleza. (Munich, Federal Republic of
Germany, MunchenerRuckversicherungs: 1988)

Once the risks are defined and quantified, planners and engineers can design appropriate mitigation
mechanisms. Obviously, these are most cost-effective when implemented as part of the original plan or
construction. Examples of effective mitigation measures include avoiding areas that can be affected by
storm surge or flooding, the application of building standards designed for hurricane-force winds or the
planting of windbreaks to protect crops. Retrofitting buildings to make them more resistant is a more
costly but sometimes viable option, but once a project is built in a flood-prone area it may not be feasible
to move it to safer ground.

In the past three decades the ability to forecast and monitor these storms has increased greatly, which has
had a dramatic effect on saving lives. The time and location of landfall and the resulting damage can be
estimated. The U.S. National Hurricane Center uses this information to issue track prediction and
intensity forecasts every six hours for tropical storms and hurricanes in the Atlantic/Caribbean region.
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The U.S. Oceanographic and Atmospheric Administration (NOAA) has developed the model Sea Lake
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Overland Surge from Hurricanes (SLOSH) to simulate the effects of a hurricane as it approaches land.


This makes it possible to determine which areas should be abandoned and to plan evacuation routes.
Tailored to specific areas, operational SLOSH is available in the United States and Puerto Rico and is
being developed in the Virgin Islands. It can be expanded to other Caribbean and Central American
countries.

At the national level non-structural mitigation strategies include campaigns to create a public awareness
of warning services and protective measures, since informed citizens are more likely to check the
condition of their roofs and other structures at risk. Good examples of such campaigns can be seen in The
Bahamas, Barbados, and Jamaica. Taxation of investment in high-risk land is a potentially important
strategy that has not been tried widely. Insurance can also be structured to encourage sound land use and
structural mitigation actions. Among the important structural strategies are codes that control the design
and construction of buildings and, in public works, the construction of breakwaters, diversion canals, and
storm surge gates and the planting of tree lines to serve as windbreaks.

All these approaches may be effective in the largest urban settings where communications are good and
institutional arrangements are firmly in place. But national emergency preparedness offices usually do not
have the resources to function effectively in areas of low population density when faced with widespread
catastrophes such as hurricanes. An alternative is to prepare small towns and villages to respond to
emergencies by their own means. The approach followed by the OAS in collaboration with the Pan
Caribbean Disaster Preparedness and Prevention Project (PCDPPP) in several Eastern Caribbean
countries involves training local disaster managers and community leaders of both urban and rural
settlements in organizing disaster risk assessment and mitigation in their communities. A training manual
and accompanying video produced for this purpose are available. The focus is on lifeline networks
(transportation, communications, water, electricity, and sanitation) and critical facilities (health and
education facilities, police and fire stations, community facilities, and emergency shelters). Combining
the disaster preparedness efforts of the PCDPPP program with disaster prevention through the integrated
development planning of the OAS clearly illustrates the interface of disasters and development.

The process of preparing community leaders to cope with hurricanes consists of six steps:

 Preparing an inventory of lifeline networks and critical facilities.
 Learning the operation of these networks and facilities and their potential for disruption by
hurricanes.
 Checking the vulnerability of the lifelines and facilities through field inspection and investigation.
 Establishing an effective working relationship with the agencies and companies that manage the
infrastructure and services of the community.
 Developing an understanding of the total risk to the community.
 Formulating a mitigation strategy.

Communities can use the OAS-PCDPPP training manual and video to train themselves, but often they
find it more effective to have outside help. The best approach is to set up a unit for local training in the
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national government which then travels regularly to each small community first to train the local
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leadership and then to give updates and practice sessions.


2. Geologic hazards
The most damaging geologic hazards are earthquakes, volcanic eruptions, and tsunamis (large sea waves,
erroneously called tidal waves, which are usually caused by earthquakes).

Geologic hazards are characterized by (1) very rapid onset; (2) geographically limited impact (the
phenomena occur in limited and clearly defined zones in Latin America and the Caribbean); (3) lack of
predictability except in the most general sense; and (4) extreme destructiveness (in spite of their relative
rarity, earthquakes in urban areas, pyroclastic flows and mudflows caused by volcanic eruptions, and
flooding due to tsunamis are some of the most damaging and feared natural hazards).

This combination of characteristics makes the non-structural strategy of avoidance the best way to cope
with geologic hazards. As has been emphasized, the avoidance strategy requires information about the
threat of the hazards as early as possible in the development planning process. The information
requirements are very general early in the process, becoming more explicit with each successive stage so
as to provide answers to the following questions in order:

- Does the hazard pose a threat in the study area?

- Is the danger great enough to merit mitigation?

- What kind of mitigation mechanisms are appropriate?

- What are the costs and benefits of a particular mitigation measure, in terms of both economics and
quality of life?

Scientific data to answer the first question exist for the principal geologic hazards in most of Latin
America and the Caribbean, but up to now they have not been readily accessible. One of the services of
the OAS has been to compile this information in a form suitable for use by planners. This section
summarizes that information for earthquakes, volcanic eruptions, and tsunamis.

Earthquakes

Two kinds of information are needed to evaluate earthquake threat: the potential severity of an earthquake
and the likelihood that a damaging earthquake will occur during a specific time frame. When either type
of information is not available, a partial evaluation can be made with the information that does exist.

OAS work with geologic hazards has been confined largely to pre-event planning and non-structural
mitigation measures. The study for the integrated development of the San Miguel-Putumayo river basin
on the Colombia-Ecuador border, for example, included a comprehensive examination of all natural
hazards that could affect the projects identified. Active fault zones-the locus of potential earthquakes and
unstable ground unsuitable for locating infrastructure-were one of the elements studied.

The principal earthquake hazards are ground shaking, fault rupture, and propensity to liquefaction (see the
section below on landslides). Once it is recognized that an area is prone to earthquakes, it is important to
prepare maps of high-risk areas delineating zones subject to the particular hazards. Some hazard and risk
mapping has been completed in Latin America and Caribbean countries, but in general it is not very
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reliable or useful to engineers, government officials, or planners for site-specific engineering design work.
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Some national and regional projects have begun to incorporate recent scientific and technological


advances into seismic hazard and risk mapping, and are producing work of much higher quality. The
availability of existing information, and more particularly the quality of that information, must be
determined for areas under seismic threat and supplemented as necessary.

The science of earthquake engineering has devised building techniques and materials that resist all but the
strongest earth shaking. Building codes stipulate the application of these structures. Retrofitting may
provide important economic benefits for large buildings and public infrastructure. It is also of great
importance in saving lives of millions who live in non-engineered mud constructions. Basic do-it-yourself
techniques can prolong resistance to shaking of these structures long enough to allow people to escape
before collapse. With regard to fault rupture, the best way to cope with this hazard is to avoid the narrow
zones prone to movement along faults.

Volcanic Eruptions

The principal volcanic hazards are pyroclastic flows, mudflows (or lahars), ash falls, projectiles, and lava
flows. These hazards usually do not constitute a serious problem more than 30 km from the source,
although in exceptional cases, a lahar or an ash fall can cause serious damage as much as 60 km away.
Because some of the most terrible eruptions have come from volcanoes that had been regarded as
dormant, an active volcano is defined as one that has erupted in the past 10,000 years (the Holocene
Epoch of geologic time). The degree of threat is gauged by periodicity, with short-term periodicity
(interval between eruptions of less than 100 years) posing a greater threat than long-term periodicity. The
information given for each volcano includes location, periodicity, date of last eruption, a measure of the
size or "bigness" of the largest historic eruption, and the hazards associated with its eruptions.

If a study area lies within 30 km of a volcano with short-term periodicity, a volcanic hazard zonation map
showing the likelihood of occurrence and severity of each hazard in the vicinity of the volcano should be
prepared as part of the planning process. Few mitigation measures other than avoidance are effective in
resisting volcanic hazards such as lava flows or pyroclastic flows. Steeply sloping roofs help to reduce
damage from heavy ash falls.

Tsunamis

These awesome seismic sea waves are caused by large-scale sudden movement of the sea floor, usually
due to earthquakes. In Latin America they are a significant threat only on the west coast of South
America, where every off-shore earthquake over a magnitude 7.5 is potentially tsunamigenic. While they
occur in the Caribbean, they are so infrequent and cause so little damage, mitigation is difficult to justify
economically. Even where tsunamis pose a significant threat, mitigation is feasible only for urban
concentrations. The construction of sea walls along low-lying stretches of coast, planting tree belts
between the shoreline and built-up areas, and zoning restrictions provide some measure of help, but
effective warning and evacuation systems are the most reliable defense against this intractable hazard.

3. Floods
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Floods are usually described in terms of their statistical frequency. For example, the flat land bordering a
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stream that is inundated at a time of high water is called a "100-year floodplain" if it is subject to a 1


percent probability of being flooded in a given year. Commonly, any risk this great or greater is
considered significant.

Development practices may unwittingly increase the risk of flooding by increasing the amount of water
that must be carried off, or decreasing the area available to absorb it. Drainage and irrigation ditches, as
well as water diversions, can alter the discharge into floodplains and a channel's capacity to carry the
discharge. Deforestation or logging practices will reduce a forest's water absorption capacity, thus
increasing runoff. Large dams will affect the river channel both upstream and downstream: the reservoir
acts as a sediment trap, and the sediment-free stream below the dam scours the channel. Urbanization of a
floodplain or adjacent areas increases runoff because it reduces the amount of surface area available to
absorb rainfall. In short, integrated development planning must examine the potential effect on flooding
of any proposed change and must identify mitigation measures that would avoid or minimize flooding for
inclusion in investment projects.

First, however, the planning study must establish river flow patterns and propensity to flood. This has
commonly been accomplished by gauging rivers and streams, thus directly measuring flood levels and
recurrence intervals over a period of many years to determine the statistical probability of given flood
events. Without a record of at least twenty years such assessments are difficult, but in many countries
stream-gauging records are insufficient or absent. In this situation hazard assessments based on remote
sensing data, damage reports, and field observations can be used to map flood-prone areas that are likely
to be inundated by a flood of a specified interval.

Remote Sensing Techniques for Floodplain Mapping

Integrated regional development planning studies do not traditionally include original flood hazard
assessments but rather depend on available information. If such information is needed but not available,
an assessment should be undertaken as part of the study. If time and budget constraints preclude a
detailed, large-scale assessment, a floodplain map and a flood hazard assessment can be prepared through
the photo-optical method, using Landsat data and whatever information can be found.

Floods and engineering structures on a floodplain cause changes in the river channel, sedimentation
patterns, and flood boundaries. A flood often leaves its imprint, or "signature," on the surface in the form
of soil moisture anomalies, ponded areas, soil scours, stressed vegetation, and debris lines for days or
even weeks after the flood waters have receded. Because satellite imagery can provide a record of these
changes and imprints, up-to-date imagery can be compared with previously collected data to determine
alterations during specific time periods. Similarly, the inundated area can be compared with a map of the
area under pre-flood conditions.

It should be noted that the delineation of flood-plains through remote-sensing data cannot, by itself, be
directly related to probabilities of recurrence. However, when these data are used in conjunction with
other information such as precipitation records and history of flooding, the delineated floodplain can be
related to an event's likelihood of occurrence. This method can reveal the degree to which an area is flood
prone and yield information useful for a flood hazard assessment.

Dynamic features of flooding that can cause changes, e.g., changes in the channel of the river itself or
floodplain boundaries; can be monitored through repetitive coverage of any area by earth observation
satellites. Further, the spatial distribution of the features that have changed can be readily mapped by
techniques of temporal analysis developed since the launch of Landsat 1 in 1972.4/ Slides of full scenes
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and sub scenes can be projected at any scale. The slides can be projected onto a base map, thematic maps,
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and enlarged satellite single-band prints to define hazard-prone areas for further analysis.


The Landsat Program
The Landsat Program is a series of Earth-observing
satellite missions jointly managed by NASA and the U.S.
Geological Survey. Since 1972, Landsat satellites have
collected information about Earth from space. This
science, known as remote sensing, has matured with the
Landsat Program.
Landsat satellites have taken specialized digital
photographs of Earth’s continents and surrounding
coastal regions for over three decades, enabling people to
study many aspects of our planet and to evaluate the
dynamic changes caused by both natural processes and
human practices.

4.Landslides
The term "landslide" conjures up an image of a great mass of rock and dirt roaring down a mountainside,
uprooting huge trees, burying whole villages, pushing before it a howling wind that flattens every
structure. This is a fair description of an avalanche, one type of the earth mass movements grouped
popularly as landslides, but there are several others. Many are less dramatic but nevertheless cause much
damage.

For the purpose of hazard management, three general types of earth mass movement merit consideration:
(1) slides and avalanches, (2) flows and lateral spreads (liquefaction phenomena), and (3) rockfalls. Slides
and avalanches are very rapid movement of colluvium (A loose deposit of rock debris accumulated
through the action of rainwash or gravity at the base of a gently sloping cliff or slope) material on over-
steepened slopes under conditions of high moisture. They occur commonly too frequently, and each event
can cause moderate to great damage, so that collectively they cause very great damage. Liquefaction
refers to rapid, fluid movement of unconsolidated material on gently sloping to nearly flat land. These
earth movements occur commonly and can cause great to very great damage. Rockfalls are free-falling or
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tumbling rocks from cliffs and steep slopes. Each event may cause limited damage, but because they are
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so frequent, cumulatively they cause great damage and loss of life.


Landslides are often triggered by earthquakes but can also be set off by volcanic eruptions, heavy rains,
groundwater rise, undercutting by streams, and other mechanisms; consequently, they occur more widely
than earthquakes.

The best strategies for mitigating landslide hazards are to avoid construction in hazardous areas and to
avoid land uses that provoke mass movement. To build these strategies into development planning
requires information on the likely occurrence of landslides. Such information should be compiled only for
areas of intensive present or planned land use, since mitigation is not needed in areas of non-intensive use
such as extensive grazing or park land.

A map showing landslide potential is suitable for making recommendations on land use intensity, but the
more explicit information afforded by a landslide zonation map is required for land use management.
Methods of preparing both these types of landslide hazard assessments are discussed briefly below.

The best indicator of landslide potential is the evidence of past landslides. The location, size, and
structure of past landslides can be interpreted from remotely-sensed imagery (aerial photography and
satellite imagery). A map showing the aerial distribution of landslides can be compiled, and zones of
differing landslide potential can be interpreted. Since the map is based simply on frequency of occurrence
and not on causal factors, it has limited predictive power.

Slides and avalanches are associated with steep slopes, certain types and structures of bedrock, and
particular hydrological conditions. Maps of these characteristics can be prepared, and a landslide zonation
map can be compiled by overlaying these causal factors. Much of the required data such as bedrock
geology and topography may already be available. The rest can be compiled-again using remote-sensing
imagery. The geology, slope, and hydrology data can be overlaid to compile a map on which each unit is
a combination of the three natural characteristics. Development activities (e.g., the conversion of forest to
grasslands or crops, which increases soil moisture) can increase susceptibility to landslides, and the map
units of natural characteristics can be adjusted to show the effects of these human activities. Each of the
resulting units can then be characterized as to landslide potential, to provide the basis for preparing a
landslide hazard zoning map.

The same process can be followed in evaluating the potential liquefaction, except that for this type of
mass earth movement the critical factors are the presence of unconsolidated Holocene sediments (sands
and silts less than 10,000 years old) and less than 30-foot depth to the water table.

An example is the landslide hazard assessment prepared by the OAS at the request of Dominica.5/The
study found that the volcanic origin of the country, resulting in steep slopes and unstable bedrock, and the
abundant rainfall together create conditions which readily generate landslides. A full 2 percent of the land
area of the country is disturbed by existing landslides, of which the most abundant type is debris flows.
The landslide analysis team first delineated all past landslides on black-and-white 1:20,000 aerial
photographs and prepared a landslide map at a scale of 1:50,000. Next a map of surface geology was
compiled from existing information and overlaid on the landslide map to determine which bedrock units
were associated with existing landslides. Six of the eight bedrock units were found to be so associated.
Next a map of slope classes was compiled, again from existing information. Four classes were defined
that corresponded to present land uses. Hydrologic factors were examined, but no correlation between
rainfall distribution or vegetation zones with landslides could be established. Finally, the bedrock and
slope units were combined, the composite units were compared with the landslide map, and the
proportion of each bedrock-slope unit subject to landslide disturbance was determined.
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Organization of American States. Landslide Hazard on Dominica, West Indies (Washington, D.C.: OAS,
February 1987).

The landslide hazard map was used to locate areas unsuitable for development. Surprisingly, it also
showed that an active landslide area could dam a tributary of the Trois Pitons River, threatening the lives
of the downstream population. The map of the 290 sq. mi country was compiled in six weeks at a total
cost of US$13,000.

The important message here is that by using modern remote-sensing techniques a landslide hazard zoning
map-which greatly enhances the ability of planners to make intelligent choices about future land use-can
be compiled in one or two months at only the costs of technician time and the acquisition of the imagery.

As wise as the strategy of avoiding hazardous areas may be, it is not always possible to follow it. The
poor commonly establish squatter settlements on the slide-prone steeply sloping areas surrounding many
Latin American urban centers. Landslide mitigation mechanisms tend to be very expensive under these
circumstances. At a minimum, squatters should be helped to avoid settling on previous slides, and care
should be taken to avoid cutting off the toe of a steep slope to increase the area of a settlement. These
areas are most susceptible to sliding in heavy rains, at which time preparatory measures should be taken
to deal with large slides that may occur.

Liquefaction can be prevented by ground stabilization techniques or accommodated through appropriate
engineering design, but both are expensive.

As with all mitigation measures, these proposals hold only within the constraints of cost-benefit analysis.
Avoidance mechanisms will almost invariably yield high cost-benefit ratios. The results for other
mechanisms are not as predictable.

HAZARD MITIGATION PLANNING: Tools and Techniques
I. Structural measures:
 Alteration of environment
 Strengthening buildings and facilities
II. Non-structural measures:
 Development management
 Information dissemination
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I.
Structural Measures:
 Alteration of the environment (sediment trapping structures)
i) Groins: groins are wall like structures, built of timber, concrete, metal sheet piling or rock,
placed perpendicular to the beach to capture material drifting along the shoreline. A groin’s
effectiveness in trapping sediment is primarily a factor of the length and spacing of the groin
system. The appropriate length for an effective groin depends on the dominate sediment size
shorter groins for larger grain sizes and longer groin for smaller grain sediment. The spacing
between groins must balance being large enough to not undermine the up drift groin and
being small enough to effectively act as a sediment trap.
ii) Jetties: jetties are wall like structures built perpendicular to the coast to stabilize channels,
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inlets and outlets. While the primary function of jetties is to protect navigation channels,
jetties capture sediments by restricting the movement of materials transported by longshore
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currents. The critical factors for channel stabilization are the width of the channel and the


management of the sediment. Width of the channel must balance being wide enough to
reduce current velocity within the channel but narrow enough to restrict shoaling. Likewise,
consideration of the sediment economy plays a vital role in the long-term viability of the
channel, inlet or outlet.

 Alteration of the environment (sediment-moving structures)
i) Beach nourishment: beach nourishment is the artificial replacement and/or addition of
sediment to beaches. The effectiveness of beach nourishment depends on the type of
imported sand, natural slope of the beach, cross shore currents and the frequency of storms.
Consideration of natural erosional process is vital to the long-term cost-effectiveness of any
beach nourishment programme.
ii) Dredging: dredging involves modification of a channel by extracting sediment. Due to the
need to dispose of extracted sediment and likelihood of future sedimentation, dredging is
usually only undertaken to maintain the navigability of channels and waterways.
 Shoreline protection works:
i) Seawalls: seawalls are vertical walls built on the shoreline that are designed to protect against
direct storm wave attack. Seawalls must be constructed of durable, immovable materials to
withstand the extreme, dynamic power of storm waves. Due to size needed to be effective,
seawalls can also be curve or shaped to dissipate smaller waves and reflect larger storm
waves.
ii) Revetments: revetments are designed to protect the backshore from high tides and surges.
Revetments may be constructed out of a number of materials and configurations, from
boulders placed at the edge of a cliff or along the backshore, to securing loose material in
wire gabion, to pre-cast armour units. Revetments are more successful on lower energy
coasts.
iii) Bulkheads: bulkheads are vertical walls on the shorelines, often constructed of wood or steel
and designed to retain loose fill and sediment behind it. Since the bulkheads are to maintain
the material behind it rather than provide protection from the sea or lake, bulkheads are
usually not good protection from storms or other flooding events.
iv) Breakwaters: unlike seawalls, revetments and bulkheads, breakwaters protect the shoreline by
breaking down incoming waves to diffuse and retract the wave fronts. Breakwaters must be
strong to be effective because they receive the full force of the wave energy. Consideration of
materials is especially important for breakwaters due to the environmental forces acting on
them.
v) Construction and stabilization of sand dunes: construction of new sand dunes requires an
understanding of the biological and physical processes to the coastal zone. Most effective
methods of creating new dunes involve disrupting the airflow to encourage sand deposition,
through the use of fences made of porous materials. It is important that the fences alter the
airflow but do not halt it. Artificial dunes can also be built up by the planting of vegetation. It
is important to note the distinction between vegetation used for dune construction and for
dune stabilization, as they are usually of different species. Stabilization, as opposed to
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construction of dunes is aimed at securing bare sand surfaces against deflation. Stabilization
can be achieved through grading, rapid construction of new dunes through the use of earth-
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moving equipment, surface fixing by addition of chemicals and planting of vegetation,
focusing on grasses, shrubs and trees.

 Flood control works:
i) Dams and reservoirs: a dam is a structure built across a waterway to impound water. Dams,
as well as acting as flood control devices, also serve for maintaining water depths for
navigation, irrigation, water supply, hydropower and others. Dams can serve as effective
flood control measures by retaining water and releasing it at a controlled rate that does not
overwhelm the capacity of the channel beyond. Any dam or reservoir should include a
spillway- a feature of a dam allowing excess water to pass without overtopping the dam.
Usually a spillway functions only in a large flood. Storage capacity of a dam or reservoir
should be a primary consideration in design and construction. In addition, the normal
sediment load on the waterway to be dammed plays an important role in the long-term
viability of the dam. Sedimentation can silt up a reservoir and increase its volume, decreasing
its flood storage capacity.
ii) Dikes and levees: dikes and levees are often used synonymously. Dikes are usually earthen or
rock structure built partially across a river for the purpose of maintaining the depth and
location of a navigation channel. Levees are earthen embankments used to protect low lying
lands from flooding. Levees are built between the floodway and the structures to which they
are intended to protect. The effectiveness of a levee to reduce the threat of flood damage on
structures and low-lying areas depends on the levee being located outside of the floodway
and compensating for the flood storage displaced by it. Locating a levee (or any other
structure) within the floodplain can increase the flood height, increasing flood threat both up
and downstream.
iii) Retaining ponds: retaining ponds or retention ponds are basins designed to catch surface
runoff to prevent its flow directly into a stream or river. Retention ponds are frequently a
relatively inexpensive option, provided that ample undeveloped land is available. Retaining
ponds have the added advantage of not altering the character of the stream. Retaining ponds
can also act as groundwater recharging sites and reduce water pollution through soil filtering.
iv) Flood channels: channelization is a general term for various modifications for the stream
channel that are usually intended to increase the velocity of the water flow, the volume of the
water or both. These modifications in turn, increase the discharge of the stream, and the rate
at which surplus water is carried away. The channel can be widened or deepened, especially
where soil erosion and subsequent deposition in the stream have partially filled in the
channel. Care must be taken; however, that channelization does not alter the stream dynamics
too greatly elsewhere. Flood channels or storm sewers are installed to keep water from
flooding streets during heavy rains, and often, the storm water is channeled straight into a
nearby stream. This can become a problem by increasing the probability of flooding
downstream through greater water volume in streams than would occur naturally. This is
particularly a problem when the flood channel is cement lined; thereby further increasing the
rate at which water enters streams.
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v) Floodwall: a floodwall is a reinforced concrete wall that acts as a barrier against floodwaters.
Floodwalls are usually built in lieu of levees where the space between land and the floodplain
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is limited.


 Storm water management: beyond maintenance and improvement of urban storm
water systems, land treatment measures are effective means of counter-acting the
effects of urbanization (particularly the increase in impervious surfaces) on runoff.
Land treatment measures include maintenance of trees, shrubbery and vegetative
cover, terracing, slope stabilization, grass waterways, contour plowing and strip
farming. The use of perennial vegetation, such as grasses, shrubs and trees provide
cover for the soil, prevent erosion, slow the rate of runoff and increase infiltration
and reduce water pollution. Terracing involves a raised bank of earth having vertical
or sloping sides and a flat top for controlling surface runoff. Strip cropping is the
growing of crops in a systematic arrangement of strips or bands along a contour.
 Drainage system maintenance: maintenance of channels and detention basins is
necessarily an ongoing venture due to blockages caused by overgrowth, debris,
sedimentation and aging of systems. Replacement and/or improvement of culverts,
mains, storm water lines, sewer pipes, backup valves etc. may be part of a general
program of maintenance and improvement to reduce flooding hazards.
 Slope stabilization: a number of potential methods are available to stabilize slopes
from landslides including slope reduction, adding retention structures, fluid removal
and others. Slope reduction involves reducing the slope angle, placing additional
support material at the foot of the slope to prevent a slide or flow at the base of the
slope, and/or reducing the load on the slope by removing some of the materials high
on the slope. Retention structures may include ground cover and retaining walls. The
most successful retaining walls tend to be low, thick walls placed at the toe of a slide.
Fluid removal acts to reduce the role water can play in landslide by covering the
surface with impermeable material and diverting runoff away from the slope, as well
as installing a subsurface drainage system. Other methods include cementing the
slide material, bolting a rock slide, and the driving of vertical piles into the foot of the
slope.
 Brush clearing, controlled burns, fuel breaks: brush clearing, controlled burning
and fuel breaks are all ways of mitigating the threat from wildfires by reducing the
material that can be burned and the area in which it can spread.
 Wetland preservation: wetlands are areas that are normally inundated with water.
Many important ecological communities are found in wetlands, including bottomland
hardwoods, swamps, marshes, bogs, sloughs, potholes, wet meadows, river overflow,
mudflats and natural ponds, and are essential for a number of species of fish and
wildlife. Wetlands act as flood control by storing tremendous amounts of
floodwaters, slowing and reducing downstream flows. Wetlands also play an
important role in coastal productivity and the cycling of river-borne material
(pollutants included) by acting as a biogeochemical filter.

B. Strengthening buildings and facilities
13

i) Strengthening Buildings
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 Flood proofing: flood proofing can be done in two ways, dry and wet. Dry flood proofing
involves the sealing of a building against floodwaters by making all areas below the flood
protection level watertight. This can be done by coating walls with waterproofing compounds or
plastic sheeting and protecting building openings with removable shields or sandbags. Dry
floodproofing is limited to 2 or 3 feet above the foundation of the building due to the pressure
exerted by deeper water on the walls and floors. With wet floodproofing, floodwaters are
intentionally allowed to enter a building to reduce the pressure exerted by deep water. Wet
floodproofing at minimum involves the removal of some valuable items and extends to the
rebuilding of floodable areas. Wet floodproofing can dramatically reduce damage costs by simply
removing furniture and electrical appliances out of the flood prone area.
 Elevating: elevating a building is the raising of that building above flood level. This is the one of
the best techniques for protecting buildings that are, or for some reason must be located in areas
prone to flooding. Elevation is cheaper that relocation and is less disruptive to the neighbourhood.
Effective elevation should take in consideration the need to wet floodproof everything still
located in the flood prone area, such as basements or garages.
 Windproofing: Windproofing focuses on design and construction of a building to withstand
wind damage. This involves the aerodynamics of a structure, materials used, and addition of
features such as storm shutters.
 Basement protection: basement protection may involve floodproofing of the structure, both wet
and dry, as well as building a barrier around the opening to the basement to protect it from
floodwaters.
 Seismic retrofitting: seismic retrofitting involves adding braces, removing overhangs and
providing flexible utility corrections and tie downs to reduce damage.

ii). Strengthening Facilities

 Flood proofing: flood proofing can be done in two ways, dry and wet. Dry flood proofing
involves the sealing of a building against floodwaters by making all areas below the flood
protection level watertight. This can be done by coating walls with waterproofing compounds or
plastic sheeting and protecting building openings with removable shields or sandbags. Dry
floodproofing is limited to 2 or 3 feet above the foundation of the building due to the pressure
exerted by deeper water on the walls and floors. With wet floodproofing, floodwaters are
intentionally allowed to enter a building to reduce the pressure exerted by deep water. Wet
floodproofing at minimum involves the removal of some valuable items and extends to the
rebuilding of floodable areas. Wet floodproofing can dramatically reduce damage costs by simply
removing furniture and electrical appliances out of the flood prone area.
 Burial: burial can play an important role in protecting necessarily utility connections, particularly
during high winds and ice storms.
 Elevating: elevation of facilities is the raising of the facility above the flood level. Of particular
importance for facilities is the elevation of electrical and mechanical equipment. It may not be
14

possible to effectively raise many facilities, but by elevating electrical and mechanical equipment,
the facility should be able to recover quicker after a disaster.
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 Seismic retrofitting: seismic retrofitting involves adding braces, removing overhangs and
providing flexible utility corrections and tie downs to reduce damage.
 Improvements to storm water/wastewater/water treatment facilities, pump stations:
improvements to these should be undertaken to minimize threat from flooding and other disasters.
Capacity of these systems should be evaluated and, if necessary, increased to meet realistic
demands.
 Upgrading piers/wharves: wharves and piers should be upgraded and retrofitted to match the
storm forces they are exposed to.
 Repair/reconstruction of fuel storage tanks: fuel storage tanks need to be inspected and if
necessary, repaired or reconstructed in the event of flooding or earthquake.
 Storm shutters: storm shutters are an important defense against high winds. Storm shutters
protect a facility by preventing winds from entering a building and possibly damaging it.

Iii) Building codes:

Building codes are laws, ordinances or governmental regulations setting forth standards and
requirements for the construction, maintenance, operation, occupancy, use or appearance of buildings,
premises and dwelling units. Building codes should be designed to ensure that development is built to
withstand natural hazards. Regulatory standards should be created for the following:

 Freeboard
 Foundation design
 Wind standards
 Cumulative substantial improvement
 Lower substantial improvement
 Critical facilities
 Enclosure limits
 Electrical and mechanical equipment

15
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GEOGRAPHY UNIT 1 Module 3 natural events and hazards. section 5 response to hazards

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GEOGRAPHY UNIT 1 Module 3 natural events and hazards. section 5 response to hazards