Lecture 7 Urban Flooding

CIV 457 - Lecture 7

Urban Flooding

Kodwo Beedu Keelson Msc Env Eng

MODULE CONTENTS

Types of Urban Flooding

Causes of Urban Flooding

Climate Change and Flooding

Flood Mitigation Measures

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Types of Urban Flooding

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Local Floods

Riverine Floods

Coastal Floods

Flash Floods

Types of Urban Flooding

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Local floods occur in built environments when surface run-off is in excess of local drainage capacity

Local drainage capacity is primarily made up of a localstormwater drainage system composed of stormdrainpipes, curb inlets, manholes, minor channels, roadsideditches and culverts

This system is intended to convey storm flows efficiently tothe communitys primary drainage system, such as themain river channel or the nearest large body of water.

Rubbish and debris tend to clog the bottlenecks of drainagefacilities, thus reducing the drainage capacity and leading toincreased surface runoff and back up effects.

Local Floods

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Local floods are generally confined to rather smallgeographical areas and are normally not of long duration.

However in regions of extended rainy seasons (monsoon

climates), local floods may last for weeks.

Depending on the local hydro-geological situation,

groundwater rising or subsurface flows can be other causes

in the generation of local floods.

Localised flooding occurs many times a year in slum areas

because there are few drains, most of the ground is highly

compacted and pathways between dwellings become

streams after heavy rain.

Local Floods (2)

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River floods occur when the river run-off volume exceedslocal flow capacities.

The river levels rise slowly and the period of rise and fall is

particularly long, lasting a few weeks or even months,

particularly in areas with flat slopes and deltaic areas.

River floods are triggered by heavy rainfall or snow melt in

upstream areas, or tidal influence from the downstream.

Failure or bad operation of drainage or flood control works

upstream can also sometimes lead to riverine flooding.

Urban areas situated on the low-lying areas in the middle or

lower reaches of rivers are particularly exposed to

extensive riverine floods.

Riverine Floods

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High tides and storm surges caused by tropical depressionsand cyclones can cause coastal floods in urban areaslocated at estuaries, tidal flats and low-lying land near thesea in general.

Coastline configurations, offshore water depth and estuary

shape can influence the intensity of coastal floods.

Tidal effects in the estuarine reaches can keep the river

levels high for long periods of time and sustain flooding.

Tsunamis, mainly triggered by powerful offshore

earthquakes, can also cause coastal floods though

infrequently.

Coastal Floods

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Flash floods occur as a result of the rapid accumulation andrelease of runoff waters from upstream mountainous areas,which can be caused by very heavy rainfall, cloud bursts,landslides, the sudden break-up of an ice jam or failure offlood control works.

They are characterized by a sharp rise followed byrelatively rapid recession causing high flow velocities.

Discharges quickly reach a maximum and diminish almostas rapidly

Flash Floods

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Flash floods are particularly common in mountainous areasand desert regions but are a potential threat in any areawhere the terrain is steep, surface runoff rates are high,streams flow in narrow canyons and severe thunderstormsprevail.

They are more destructive than other types of flooding indensely populated areas because of their unpredictablenature and unusually strong currents carrying largeconcentrations of sediment and debris, giving little or notime for communities living in its path to prepare for it andcausing major destruction to infrastructure, humans andwhatever else stands in their way.

Flash Floods (2)

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Causes of Urban Flooding

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Meteorological Factors

Hydrological Factors

Human Factors

Causes of Flooding

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Rainfall

Cyclonic storms

Small-scale storms

Temperature

Snowfall and snowmelt

Meteorological Factors

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Soil moisture level

Groundwater level prior to storm

Natural surface infiltration rate

Presence of impervious cover

Channel cross-sectional shape and roughness

Presence or absence of over bank flow, channel network

Synchronization of run-offs from various parts of watershed

High tide impeding drainage

Hydrological Factors

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Land-use changes (e.g. surface sealing due tourbanization, deforestation) increase run-off and may besedimentation

Occupation of the flood plain obstructing flows

Inefficiency or non-maintenance of infrastructure

Too efficient drainage of upstream areas increases flood

peaks

Climate change affects magnitude and frequency of

precipitations and floods

Urban microclimate may enforce precipitation events

Human Factors

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Climate Change and Flooding

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Climate change is a significant and lasting change in thestatistical distribution of weather.

Climate change is caused by factors that include oceanicprocesses, biotic processes, variations in solar radiationreceived by Earth, plate tectonics and volcanic eruptions,and human-induced alterations of the natural world; theselatter effects are currently causing global warming, and"climate change" is often used to describe human-specificimpacts.

Factors that can shape climate are called climate forcingsor "forcing mechanisms".

What is Climate Change?

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Climate change is making weather less predictable, rainsmore uncertain and heavy storm rainfalls more likely.

Urban areas may help to increase thunderstorm activity

because their built-up surfaces attain higher temperatures

than surrounding areas and create a local air circulation

that produces an urban heat island.

Dust particles caught up in that circulation act as nuclei on

which moisture in clouds condenses, forming rain droplets

that eventually may develop into the large rain drops of a

major thunderstorm.

Sea-level rise increases the risk of coastal floods,

particularly in case of storm surges.

Climate Change and Flooding

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Flood Mitigation Measures

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Flood control measures are classified as either structural ornon-structural.

Structural measures are related to the change of the basinand/or the river, such as dams, dikes, channel conveyanceand basin forestation.

Non-structural measures are based on measures related toflood mitigation, such as: insurance, flood zoning and floodforecasting.

Structural solutions are costly and are feasible only when

flood damage costs would be greater than the development

of structural measures or when they provide intangible

social benefits.

Classification of Flood Control Measures

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Structural measures are classified as either extensive orintensive.

The extensive measures are developed at basin level, sucha reforestation and soil conservation, among others.

These extensive measures can usually be developed only

in small basins because of the costs of the intervention.

The intensive measures are developed in the river, for

example reservoirs, dikes, river section changes and

slopes, and river deviation.

Structural Control Measures

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Intensive Structural Control Measures

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The main non-structural measures usually are flood zoning,flood alert or forecasting, insurance and individualprotections.

Flood zoning is the planning of the soil occupation and

regulation in order to decrease future flood impact, since

part of the population will still be living in risk areas;

Flood forecasting is developed in order to alert the

population and decrease damages by floods;

Insurance is a preventive procedure used to recover the

cost damage of floods.

Flood proofing is an individual measure by a property

owner to protect his/her physical assets.

Non-Structural Flood Control Measures

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