UNDERSTANDING AVERAGE RECURRENCE
INTERVAL (ARI) AND ITS APPLICATION IN
RAINFALL AND FLOOD PREDICTION AND
PROJECT DESIGN IN MALAYSIA
Topic : Rainfall Trend & Flood Detention
Panelist 4 : Ir. Dr. Wong Wai Sam
17 Mar 2022
ARI / RP FOR RAINFALL & FLOOD
•
Hydrologic systems are often impacted by extreme events such as
severe floods and droughts.
•
The more severe the event, the less frequently it occurs as compared
to moderate events.
•
The objective of frequency analysis of hydrologic data is to relate the
magnitude of extreme events to their frequency of occurrence through
the use of probability distributions.
•
Used for engineering designs of dams, bridges, culverts, and flood
control structures.
•
Return Period, T = Average Recurrence Interval (ARI) between
events equaling or exceeding a specified magnitude.
•
The probability of occurrence of an event, P = the inverse of its ARI.
P = 1/T
EXTREME RAINFALL TREND
EXTREME RAINFALL TREND
EXTREME RAINFALL TREND
FREQUENCY ANALYSIS
FREQUENCY ANALYSIS
FREQUENCY ANALYSIS
DISCUSSIONS
•
The annual maximum rainfall depth trend is generally
increasing throughout almost 50 years of records
•
Based on the frequency analysis on the data, using later half of
the dataset give much higher rainfall depth for the same ARI
as compared to the whole dataset
•
The possibilities of this observation are due to climate change
and heat island effect in highly urbanised area like Kuala
Lumpur
•
Hence, is it better to use the more recent dataset and disregard
the older one to produce the IDF in HP1?
AREAL DISTRIBUTION AND DURATION OF
RAINFALL EVENT
• Rainfall is not uniformly distributed over an area and also
vary in duration.
• Normally, rainfall of short duration with high intensity is
localised and covers a small area while long duration with
low intensity is widespread and covers a much larger area.
• The former is convective in nature (can occur everywhere
and more severe during inter-monsoon periods) while the
latter is monsoonal (during monsoon seasons especially
North-West Monsoon)
Localised Short Duration High Intensity Rainfall
(Sep 2019)
JPS Infobanjir Rainfall stations data:

Sentul
: 126 mm (20 ARI)

Jln. Tun Razak (Sg. Bunus)
: 143 mm (52 ARI)

Gombak Barrage
: 188 mm (296 ARI)

Gombak km16
: 117 mm (14 ARI)

Gombak Simpang Tiga
: 209 mm (614 ARI)

JPS Wilayah
: 119 mm (16 ARI)

Kolam Takungan Batu
: 156 mm (90 ARI)

Lembah Keramat
: 120 mm (16 ARI)
Source: Jabatan Pengairan dan Saliran Wilayah Persekutuan
Source: Pusat Ramalan dan Amaran Banjir Negara, JPS Malaysia
Weekly average for Kuala Lumpur:
46.2 mm. Some locations reported
very extreme rainfall >100ARI.
Flooding at multiple location in
KL along Sg Klang and Sg
Bunus. Between 1 and 8 pm
10th Sep 2019 Flash Flood at KL
Flood depth up to 1m
SITE PHOTOS
Bank Muamalat
Masjid Jamek LRT
Masjid Jamek
Sg. Gombak (Near
Sunway Putra Mall)
Sg. Gombak (Near PWTC)
Sg. Klang (Monorail Tun
Sambanthan)
Widespread Long Duration Event (Dec 2021)
Source: MMD Malaysia
Widespread Long Duration Event (Dec 2021)
Source: Jabatan Pengairan dan Saliran Wilayah Persekutuan
Widespread Long Duration Event (Dec 2021)
Source: Jabatan Pengairan dan Saliran Wilayah Persekutuan
Widespread Long Duration Event (Dec 2021)
Source: Jabatan Pengairan dan Saliran Wilayah Persekutuan
Widespread Long Duration Event (Dec 2021)
Source: Jabatan Pengairan dan Saliran Wilayah Persekutuan
Widespread Long Duration Event (Dec 2021)
Source: Jabatan Pengairan dan Saliran Selangor
DISCUSSIONS
•
For localised rainfall event with high intensity, many occasions
there are areas where the ARI are very high, exceeded the
design protection levels that cause flash floods. Hence, the
stormwater drainage design should be resilient enough to
manage it such as using roadways as floodways and temporary
storage, larger OSD and rainfall harvesting storages, better
regulation of flood detention ponds, LID strategy, etc.
•
Long duration widespread rainfall events will result in high
flood flows at the low part of the basin and will also cause
flooding in much larger areas and prolong duration due to the
high volume of water, saturation of flood detention ponds and
pervious areas. To mitigate it will require major works such as
large regional ponds / dams, diversions, enlargement of
conveyance channels, bundings, poldering with pumpings, etc.
TYPICAL FLOOD DETENTION POND
Flood Peak Control from Storage Pond
Downstream Flood Peak Control from Detention Pond
Impact towards Downstream Flood Peak (further downstream) for the
requirement to control the Pre- & Post-Development to be the same
normally cannot be met :
– Flood detention ponds do not reduce the runoff volume
– At downstream stretch, the time of concentration is longer.
Longer and higher volume of runoff hydrographs normally
make the individual detention upstream less effective
– Depends on the basin characteristics and rainfall patterns, with
detention ponds upstream may worsen the peak discharges
downstream if not analysed and designed thoroughly
– Should be designed for full range of flood events (from regular
to extremely rare floods) to ensure their placement and function
does not cause adverse impacts both upstream or downstream
areas.
Effectiveness of Detention Ponds
Effectiveness of Detention Ponds
Effect of Increased Post-Development Runoff Volume
with Detention on a Downstream Hydrograph
Effectiveness of Detention Ponds
Thorough hydrological and
hydraulic analysis are
required to avoid any
adverse impact
Detention Timing Example
DISCUSSIONS
•
No doubt, flood detention and retention systems (ponds, OSD &
rainfall harvesting) have provided substantial benefits in
controlling stormwater runoff quantity and quality;
•
Thorough hydrological and hydraulic analysis are required to avoid
any adverse impact of detention ponds at downstream river
stretches although generally they will improve rather than
aggravate the situations;
•
Should be designed for full range of flood events (from regular to
extremely rare floods) to ensure their placement and function does
not cause adverse impacts both upstream or downstream areas.
•
Regular checks on the ponds, OSD and rainfall harvesting tank
conditions by local authorities and relevant authorities to ensure
the functionality and safety of them
•
Allocate enough budget for the maintenance of the systems
THANK YOU