Hydrological hazards (Floods)

When does water become a hazard

When water is present in places that is not normal on a usual basis

What creates the hazard of a flood

The need/expectation for an area with certain infrastructure and development to stay dry

what is the most common hazard type

floods

what percent of all hazard events are floods

43%

What percent of all hazard events are storms

28%

What percent of all hazard events are earthquakes

8%

What percent of all hazard events are extreme temperatures (heatwaves or cold)

6%

What percent of all hazard events are landslides

5%

What percent of all hazard events are drought

5%

What percent of all hazard events are fires

4%

What percent of all hazard events are volcanic activity

2%

Floods are characterized by…

High losses (1990-2010 there were 200,000 deaths caused by floods and 3 billion people affected)

What percent of flood related deaths occur in Asia

90%

What percent of flood-related losses occur in Asia

50%

Where have the five most disastrous floods all occurred

China

What is New Zealand’s #1 hazard in terms of frequency and cost

Floods

Why are many New Zealander’s at risk of floods

Most people live close to the coast or on active floodplains

Who is primarily affected by floods

people in lower socio-economic positions and in specific places, minorities

In 2023 extreme weather was responsible for…

3,875.5 million dollars in insured damages (most of the cost was incurred by two events, Auckland anniversary flooding and Cyclone Gabrille)

Variables of floods that influence threat level

Depth of water, velocity of water, duration of flooding, quality of water

Flood damage thresholds based on what

Velocity and Depth (largely about stress on structures)

Threats after flooding

Infrastructure damage, disease, crop damage and failure, sediment transportation, debris, erosion, deposition

Advantages of floods

Flood pulses and habitat maintenance in wetlands, flood sediments and soil fertility on floodplains, important role in agriculture in specific regions (flood-retreat agriculture)

Coping capacity of flooding

a level of flooding can be managed and taken advantage of, flood in excess of this level becomes a hazard (more water than normal, persistent levels of water

What does LDC stand for

Less developed country

What does MDC stand for

More developed country

How do floods impact MDCs and LDCs

Impact them differently, LDCs experience more loss of life, MDCs experience more threat to infrastructure and cost damages

What percent of declared disasters in the US are from river flooding

66%

What percent of the flood risk in the UK is from rivers

33%

What percent of towns in NZ have a river flooding problem

70%

Why is there a difference between river flood risks in the US and NZ compared to the UK when they are all MDCs

Differences in magnitude/frequency distributions, US and NZ have high magnitude rainfalls compared to UK which also has smaller and more passive rivers

What are some flood prone environments

Low-lying floodplains, low-lying coasts and deltas, areas subject to flash floods, areas below unsafe dams, low-lying inland shorelines, alluvial fans

What makes low-lying floodplains flood prone

Low lying land adjacent to rivers (MDC/LDC contrasts — flood protection, social mitigation)

What makes low-lying coasts and deltas flood prone

Combined effects of high tide and high flood flows, deltas often densely populated

What makes areas subject to flash floods so flood prone

Semi arid, unvegetated, high intensity consecutive rainfall

What makes areas below unsafe dams flood prone

overtopping and failure, engineered hazard, construction and maintenance standards

What makes low-lying inland shorelines so flood prone

Oscillations in lake levels storage elements in hydrological system, Wet/dry periods over the years

What makes alluvial fans so flood prone

Special case of flash floods, high stream instability, high stream power

Alluvial fan definition

An alluvial fan is a fan-shaped deposit of sediment that forms where a stream or river exits a narrow canyon and spreads out onto a flatter plain.

Characteristics of coastal flooding

Low barometric pressure during large storms allows sea level to rise, high wave heights + wave energy, winds pile seawater onto the coast,

What influences coastal flooding severity

timing of tides (high/low/spring) and arrival of the flood wave

Storm surge defitition

Abnormal rise in sea level generated by a storm, exceeding the predicted astronomical ride, which can cause severe coastal flooding

What maintains glacier lakes

Dammed by a combination of glacial ice and moraines. Lakes and damn may be ephemeral or ling-lived features

Moraine definition

A mass of rocks and sediment carried down and deposited y a glacier, typically as ridges as its edges or extremity

Characteristics of glacier dams

Can form or change function spontaneously through jamming of ice in outlets or rapid break up of the glacier terminus, Failure of glacier dams can happen without warning and release large volumes of water into downstream catchments and settlements. Limits ability to warn downstream communities

Glacier terminus definition

The lowest end of a glacier where ice meets land, water, or another ice mass, and it dynamically advances or retreats (also called toe or snout)

Characteristics of River floods

Caused by high rainfall (intensity/duration relationships) Are somewhat predictable (seasonal patterns like monsoon seasons), spatial variability is an issue when predicting, Connective (localized high intensity short duration)

Factors that create high rainfall

Frontal lifting, orographic, cyclonic (particularly tropical, hurricanes, typhoons, tropical cyclones)

Frontal lifting definition

Warm less-dense air is forced to rise over a colder air mass along a frontal boundary, leading to cloud formation and often precipitation

Orographic effect

Air is forced to rise over elevated terrain, cooling and condensing to produce clouds and precipitation on the windward side while creating dry conditions on the leeward side

Where is the term hurricane used

North Atlantic and eastern North Pacific regions

Where is the term typhoon used

Northwest Pacific Ocean

Where is the term tropical cyclone used

Indian ocean and South Pacific

Important characteristics of catchments (watersheds)

Catchment relief (steep or low relief), catchment geology and soil (infiltration capacity), prior conditions (soil saturation, slope cohesion, sediment availability, bank capacity)

Characteristics of catchment conditions for Cyclone Gabrielle

Wet summer + cyclone hale a few weeks prior, narrow short catchments, soft erosion prone bedrock, high intensity long duration rainfall, maturation and clearance of pine plantations in the years prior (large volumes of sediment and slash sitting on hillslopes

Role of catchment conditions in the Texas hill country floods

Steep drainage basin, thin rocky soil with limited infiltration capacity, prolonged drought conditions had hardened soils, reducing infiltration capacity further, remnants of a tropical storm stalled out over Texas, >300mm of rain in a few hours, Guadalupe River rose 8 m in 45 minutes

How does land use change influence river flooding

Pasture conversion & deforestation decreased soil stability and can compact soil making it less absorbent, Reforestation can reduce flood damage and risk

How can urbanization influence river flooding

drains can channel flood waters and reduce flood damage or can back up causing flooding, impervious surfaces create more runoff heightening flood severity

How does a forested or natural catchment influence storm runoff

More absorbent with natural vegetation, slows overland flow, gradual delivery of water to channel, lower flood peak, long descending climb

How does an urbanized catchment influence storm runoff

slick and impervious surfaces, network of storm piped can channel runoff but release it to channel more rapidly, “flash” floods

Why are certain groups more vulnerable to flooding

Is a product of historical planning practices and boarder socio-cultural conditions, these groups often have less resources to cope with and recover from these events

Explain the levee effect

Newly built flood defenses increase demand for building on low-lying areas, thought to be fully protected for future floods, land values rise, more structures are built, more assets at risk, claims for higher levels of protection, flood losses continue to rise

What is the most widely used approach in the management of any hazard (even earthquakes)

River engineering

What is the most favored form of flood control

Flood embankments

What do flood embankments do

Increase channel capacity

How are flood embankments constructed

Earth cored most common type, concrete walls, height is based on a specific type of flood (often 100 yr recurrence interval)

Other flood control measures

Flood control dams, flood abatement schemes,

What is a flood abatement scheme

Integrated management of soil, vegetation, and drainage

When was the resource management act established

1991

Functions of regional councils under the RMA

The avoidance or mitigation of natural hazards

Methods of flood proofing

Structures designed to survive flooding, raising, hardening, set back

What requirements do houses have in hurricane prone areas

Often have building codes that mandate minimum home elevations and/or flood spaces to be incorporated into home design

Characteristics hazard insurance

Often not compulsory, individualizes risk, does not encourage preparedness, incentive schemes (premiums reduced for measures taken), actuarial issues (statistics), repeated payouts (higher premiums), loss or lapse in coverage (disaster induced poverty)

Where do international sources of post-disaster aid come from

World bank, NGO’s like red cross, red cresent, save the children (important for LDCs)

Where do local sources of post-disaster aid come from

Central government, regional government, mayoral relief funds

How can we adjust to the hazard in order to minimize the risk

Preparation and planning (education, engineered structures etc.), forecasting (predicting events), land use planning (avoiding hazardous areas or only having low-risk ventures in these areas)

What are some institutions that aid in preparation, planning, and action during disasters

FEMA (Federal Management Authority in the US), Homeland security, Civil Defense (NZ), Armed forces

What are the roles of Civil Defense in New Zealand

Aid in education, declare states of emergency, are regionalized

What are the 4 Rs

Reduction (of risk), readiness, response, and recovery

When was the Civil Defence Emergency Management Act established

2002

Characteristics of the Civil Defence Emergency Management Act

Improve and promote the sustainable management of hazards in a way that contributes to the social, economic, cultural, and environmental well-being and safety of the public.

Enables communities to achieve acceptable levels of risk by identifying risks and managing them

Provide planning and preparation, as well as response and recovery in event of disaster

Requires local authorities to coordinate Civil Defence Emergency Management (CDEM) through regional groups across the 4 Rs

Integrate local and national CDEM planning and activity through the alignment of local planning with a national plan and strategy

Definition if Reduction from the 4Rs

Identifying and analysing long-term risks to human life and property from hazards; taking steps to eliminate these risks if practicable, and, if not, reducing the magnitude of their impact and the likelihood of their occurring

Definition of readiness from the 4Rs

Developing operational systems and capabilities before a civil defence emergency happens; including self-help and response programmes for the general public, and specific programmes for emergency services, lifeline utilities, and other agencies

Definition of response from the 4Rs

Action taken immediately before, during, or directly after a civil defence emergency to save lives to protect property, and to help communities recover

Definition of recovery from the 4Rs

The coordinated efforts and processes to bring about the immediate medium-term and long-term holistic regeneration of a community following an event

Monitoring and forecasting are…

Linked

Important aspects of land use planning

Balancing social and economic facilities, having accurate maps of flooding to know what will be impacted, ensuring only designated activities take place in hazardous areas, implement building codes to reduce damage to structures in vulnerable areas, in areas that are too hazardous prohibit activities and have entities purchase land so it cannot be developed

In a mixed land use approach what should be placed in areas with a very high flood risk

Agriculture, recreation, open space, some commercial

In a mixed land use approach what should be placed in areas with a high flood risk

Flood resistant two-story buildings, high rise residential buildings, industrial, commercial

In a mixed land use approach what should be placed in areas with a moderate flood risk

Flood resistant single story houses

In a mixed land use approach what should be placed in areas with a low flood risk

Standard single story houses

In a mixed land use approach what should be placed in areas with no flood risk

Critical utilities (hospitals, evacuation centres, emergency response centres)