Yuk Feng Huang (PhD, PEng, MIEM)
Univerisiti Tunku Abdul Rahman (UTAR)
Malaysia

Topic:
Flood Mitigation Measures

Natural Disaster• A natural disaster is a natural process or phenomenon that maycause:• loss of life, injury or other health impacts;• property damage;• loss of livelihoods and services;• social and economic disruption;• environmental damage• Various phenomena:• floods, landslides, earthquakes, volcanic eruptions, etc.

Flood Disaster• Flood is overflow of water, which submerges land that is usually dry;
• Floods can happen in a multitude of ways:• when rivers overflow their banks due to excessive rain, or• a ruptured dam upstream, etc.
• typhoon• Floods often cause damage to homes and businesses, if they arelocated within the flood zones or floodplains;• In Malaysia – monsoon flood and flash floods

Monsoon Floods in Peninsular Malaysia

• Monsoon floods take hours or even days to develop, giving residents
ample time to prepare;(24 hours)

River Network & Floodplain in River Basinlessrunoff atupstream

8 hours of travelling time

8 hours of travelling timeRiverFloodplain

8 hours of travelling time
morerunoff atdownstream

https://wolfriver.org/whats-a-watershed• However, flash floods can be extremely dangerous, which are
generated quickly and with little warning

Flood Mitigation• The prevention and mitigation of flooding can be studied on threelevels:• individual properties;• small communities, and• whole towns or cities• Flood mitigation involves the management and control of flood water
movement due to a rainfall event• To mitigate floods, first we must identify the locations of floodplain,
and then design the floodplain (flood frequency)

Floodplain:• A floodplain is an area of land adjacent to a river, which stretchesfrom the river banks to the base of the enclosing valley walls andexperiences flooding during periods of high river flow due to heavyrain

http://www.coolgeography.co.uk/GCSE/AQA/Water%20on%20the%20Land/Meanders/Landforms%20Meanders.htm

(Base Flood)

https://www.peachtreecornersga.gov/government/public-works/stormwater/floodplain-management

Floodplain Design (Flood Frequency):• The 100-year return period of floodplain is the land that is predicted
to flood during a 100-year rainfall, which has 1% chance of occurringin any given year
• A return period, also known as a recurrence interval, is an estimateof the likelihood of a flood event to occur
Return Period (year) = 1 / Annual Exceedance Probability (%)
Annual Exceedance Probability (%) = 1 / Return Period (year)• The 100-year is used since this is the adopted standard design
protection level for flood prevention

Over recent decades, possibly due to global climate change, 100-year floods have beenoccurring worldwide with frightening regularity (i.e. the 40-ft flood stage would be seen
more frequent – instead of once in 100-yr, now could be in 10-yr [increased 10 times])http://www.ci.watertown.wi.us/departments/floodplain_information.php#.W5OD--gzbIU

Floodplain : Before and After Developments (bothare 100-year)
100-Year
Pre Development
Floodplain

100-Year
Post Development
Floodplain

Qpeak

(20% to 30%)(higher)

We don’t want
this !

Flood Volume = Area under the hydrograph

Time-lag(< 10%)

We want this !

tc(smaller)

https://www.slideshare.net/savebuzzardsbay/thinking-about-clean-water-and-developmenthttps://auma.ca/advocacy-services/programs-initiatives/water-management/watershed-management/stormwater

Pre Development

Post Development< 10%

20 - 30%

Flood Mitigation - Structural Measures :• Structural flood mitigation (100-year return period design) is where
physical structures are constructed to reduce the impact of flooding;• Conventional methods: levees, additional channels, etc.
• Green technologies: retention/detention pond, swales, green roofs, rain
gardens, permeable paving, etc.

Levees

https://utazom.com/utazas/del-kinai-nagykorut-jangcehajouttal-0

http://www.waterdamageout.com/water_damag
e_sacramento_next.htm

Levees

https://robinmclachlan.com/2016/10/29/the-levee-dilemma/

Detention & Retention Pond

Urban Stormwater Management Manual for Malaysia

Swales

http://kstonehouse.itag.pw/flat-roof-water-poolingsolution

http://earthrepaircorps.org/2013/10/14/how-tobuild-a-swale/Green Roof, Rain Garden,Permeable Paving

http://www.missouribotanicalgarden.org/sustainability/sustain
ability/sustainable-living/at-home/rainscapingguide/permeable-pavement.aspx

https://lrienergysolutions.com/services/waterefficiency/storm-water-management/YouTube (6 minutes):
What is Floodplains by Design?
https://www.youtube.com/watch?v=-PBT4OEJfGs

YouTube:
What is Floodplains by Design?
https://www.youtube.com/watch?v=-PBT4OEJfGs

Flood Mitigation - Non Structural Measures :Land use planning controls• Strategic land use planning will identify the extent of flood impacted land
to limit the construction of urban and rural residential, commercial andindustrial land.
Early warning systems• Early warning systems are extremely important in flash flooding events toprovide residents with the ability to respond to impending flood waters.
River Basin flood modelling• Maintaining up-to-date flood models of developing river basins (land usechange);• To assist the stakeholders to understand the impact of new developmenton existing residents in the river basins

Flood Risk Map (Non-structural flood
mitigation measure)• The most effective way of reducing the risk to people and property isthrough the production of flood risk maps;• Most countries have produced maps, which show areas prone toflooding based on historical flood and rainfall data;• Again, the 100-year of return period is used since this is the adopted
standard design protection level for flood prevention;• The most sustainable way of reducing risk is to prevent further
development in flood prone areas. It is important for those at-risk
communities to develop a comprehensive Floodplain Management
Plan based on the flood risk maps

https://www.mae.gov.nl.ca/waterres/flooding/frm.html

Flood Risk Map
Dungun River Basin (2017)

Dungun River Basin (2030)

Selangor River Basin (2017)

Selangor River Basin (2030)

YouTube (3 minutes):
Flood Mapping Basics
https://www.youtube.com/watch?reload=9&v=bNasdKVeivk

YouTube:Flood Mapping Basics
https://www.youtube.com/watch?reload=9&v=bNasdKVeivk

Impact of Climate Change• The 100-year flood is no longer the 100-year flood; it may become 10year flood now;
• While the 500-year flood is no longer the 500-year flood; it may
become 100-year flood now;
• Possible? Why?

The Answer : YES(Assumed higher rainfall after climate changed):• Before climate changed, the 100-year flood that was expected to occur
once in 100 years (usually it was due to a very heavy rainfall as it occurred
only once in 100 years), was normal;• After climate changed, it may occur more frequent (become abnormal),let’s say 10 years instead of 100 years. So now, the 100-year (1% ofprobability to occur) flood has become 10-year (10% of probability tooccur) flood;• But do not forget, they have the same level of flood (40-ft). Just different
frequencies (100 years vs 10 years) or probabilities (1% vs 10%);• So what will be the new 100-year flood (after climate changed)? Much
higher? Yes• This is possible because the new 10-year flood is already equal to the old
100-year flood. Thus, the new 100-year flood now could be equivalent tothe old 500-year (0.2% of probability to occur) flood (48-ft)

48-ft40-ft
AFTER climate changed

BEFORE climate changed(10-Year)(100-Year)(10-Year)Thank you