Water & Wastewater Asia March/April 2024

IN THE FIELD
management approaches in
either open or closed systems.
With the increased frequency of
cloudburst events, above-ground
stormwater detention systems such
as reservoirs are useful, although
often not equipped to handle the
quantity of stormwater — the
systems also compete with human
and development needs.
Meanwhile, below-ground systems,
such as detention tanks, enable the
collection and storage of stormwater
runoff before release at controlled
rates to a drainage system. Although
this helps attenuate peak discharge
rates at the catchment site,
there are challenges — multiple,
large-capacity underground tanks
must be available to handle severe
storms, and this requires significant
construction costs, especially in
cities. Other strategies include
upstream stormwater interception
tunnels and upgrading or widening
drainage pipework. However, these
methods do not fully mitigate the
flooding problem, require a long
time to build, and are operation and
maintenance (O&M)-intensive.
SINGAPORE’S NOVEL APPROACH
In Asia, underground water spaces
for flood defences have already
been utilised in cities such as
Tokyo, Japan, which currently has
the world’s largest underground
floodwater management facility
with an impressive network of
tunnels and water storage tanks
that can withstand 356mm of
rainfall within 48hrs. 2
Singapore — a small, highly
urbanised island country where
one-third of its geography is less
than 5m above sea level — is
exploring a novel, below-ground
solution to mitigate the effects
of climate change. The concept
— an Underground Drainage and
Reservoir System (UDRS) — whose
feasibility is still being assessed,
will feature an integrated system
comprising a Stormwater Tunnel
System (STS) and Underground
Reservoir Cavern (URC) for
stormwater detention storage and
Pumped Storage Hydropower (PSH)
— an energy storage system cycling
water between a surface reservoir
and the URC.
The UDRS feasibility study currently
being conducted by Binnies envisions
that in the event of an extreme
storm event, the STS will receive
stormwater from the surface drainage
network and act as (1) a storage
system to contain the stormwater and
(2) a conveyance system to direct the
stormwater to the URC. In tandem,
the STS and URC will provide storage
for stormwater that the STS may
receive. This, in turn, could potentially
help to offset the capacity of pumps
required when a storm coincides
with high tide. To match the capacity
of a UDRS — which has a deeper
and larger system that is managed
centrally — the volumetric equivalent
of hundreds, or even thousands, of
underground detention tanks would
have to be built island-wide.
The feasibility study is also
exploring a PSH to work alongside
these elements for several
potential benefits: as an alternative
to batteries that are needed to
mitigate intermittent solar power,
as a load shifter which can
generate operational savings, as a
contingency for power disruption,
and as green electricity storage to
reduce carbon emissions.
Although in its early stages of a
feasibility study undertaken by
Binnies and jointly commissioned
by Energy Market Authority (EMA);
and PUB, Singapore’s National
Water Agency; a UDRS could offer
advantages in managing floods from
extreme storms. A larger and deeper
underground storage system, with
a central and efficient operation
and management capability, could
allow for more optimised use of land
and energy storage needs. While
the UDRS could potentially be the
first of its kind concept in the world,
it is likely to be a significant cost
and infrastructure undertaking for
Singapore if deemed feasible for
implementation.
WHAT IS NEXT?
Though an underground system is not
new for flood mitigation in Singapore,
it could potentially pave a new way
forward as rainfall intensifies and
above-ground areas are utilised for
urban development. There may also
be an opportunity to treat water
stored in the UDRS into potable
water, further advancing Singapore’s
sustainability goals.
References:
1 IWA. State of Green white papers for a green
transition: Water for smart liveable cities.
2 The Straits Times. Climate change will test
Tokyo’s world-class flood defences.
Cheng Kin Joe
Director – engineering, Binnies
Water & Wastewater Asia | March-April 2024 33