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A fresh view on land reclamations

On March 14, 2019, Marloes van Ginkel defended her dissertation on the design,
construction and operation of land reclamations for freshwater storage and recovery.
She states that use of the newly reclaimed land for efficient underground water
storage and recovery should be a guiding principle when designing new lands in the
sea. Their design from scratch provides opportunities to create a more robust water
system, contributing to the availability and sustainable management of water (SDG 7)
and making cities more sustainable (SDG 11). In this era of climate change,
sustainabIe development goals and unprecedented urbanisation it is time to look
afresh at land reclamations and include sustainable development and climate
resilience in their design.

July 2019

Introduction: urban growth by means of land reclamations

Today, more than half of the world’s population already lives in densely populated megacities
along coasts and these cities are still growing (Neumann et al. 2015; Merkens et al. 2016). Urban
growth is most rapid in the developing world and many municipal governments struggle to deliver
water supply and sanitation services for their residents and for economic activities. Space
limitations lead to an increasing number of seaward expansions of megacities by means of land
reclamations for residential, industrial and recreational development, ports and airports.
Examples are Lagos in Nigeria, Jakarta, Surabaya, Luanda, Singapore, Colombo, Manila and
many more.
The opportunities that the construction and hydraulic filling of and land reclamation offer through
the underground storage and recovery of fresh water for the future residents of these new lands
is not normally included in their design and construction. Dredging engineers focus on meeting
the geotechnical requirements, and aim for sufficient bearing capacity and acceptable risk of
liquefaction against the lowest costs and not for a sustainable water supply. While working on the
drainage design of a land reclamation project near Lagos, Nigeria, I became fascinated by the
opportunities that the construction of new lands for urban expansion could offer to optimize their
subsurface for underground storage and recovery of water to sustain the freshwater demand of
the residents and/or economic activities to be developed on the new land.

Fresh water supply on existing land reclamations

At present, most land reclamations are supplied with fresh water by means of a pipeline from the
mainland. This can be a viable choice, when freshwater on the mainland is abundant. However,
this is hardly ever the case. Many coastal megacities suffer from already high and continuously
growing water shortages. They struggle with soil subsidence caused by exploitation of
groundwater resources and the rivers that pass these cities are often severely polluted and do not
qualify as a viable source of drinking water. Furthermore, even when water resources on the
mainland are sufficient now, water shortages will likely arise in the future because of climate
change, continuing urbanisation and competing demands. Supply from the mainland is, therefore,
often not a sustainable resource. Alternative solutions, like desalination of salt water, are costly,
require high amounts of energy and the disposal of brine and chemical waste resulting from these
technologies cause environmental concerns. Furthermore, although reliable as a technology, their
operation is complex in polluted environments.
When suitable aquifers are available on existing land, storage and recovery of fresh water in a
managed aquifer recharge system is a viable alternative resource of drinking water. The
managed aquifer recharge systems in the coastal dunes of the Netherlands are a good example.
The sandy deposits of hydraulic filled land reclamations could serve a similar purpose an can be
especially efficient if their design is optimised for groundwater recharge and recovery.

Subsurface freshwater storage in saline environment

Since the end of the 19th century, we know how freshwater lenses develop naturally in saline
environments like oceanic islands by the combination of natural density difference between fresh
and salt water and Darcy’s Law (Figure 1). The natural development of such freshwater lenses
that exist beneath islands and in coastal dunes, takes many decades and their continued
existence requires a constant inflow of fresh water. To maintain these freshwater lenses and
increase their capacity for recovery, water can also be artificially infiltrated in saline aquifers by
means of groundwater wells. This technique is e.g. applied by the Dutch drinking water
companies as well as by agri- and horticulturists and is promoted as an alternative supply of
drinking water for coastal communities in for example Bangladesh.

Figure 1: A freshwater lens in an oceanic island under natural conditions.

Mixing and density stratification between fresh and saline water can both negatively influence the
recovery efficiency, defined as the ratio between injected and recovered fresh water (e.g., Lowry
and Anderson 2006; Ward et al. 2007; Bakker 2010). This recovery efficiency is controlled by the
physical properties of the aquifer, and well design and operation. For existing aquifers, the
physical properties of potential storage-recovery site is fixed, leaving operational factors and well
design as the only steering parameters to control the recovery efficiency. This is not the case for
land reclamations, for which also the properties of the new aquifer can be part of the design and
construction. This offers opportunities to reduce mixing and density stratification of subsurface
fresh and saline water and optimize the recovery efficiency.
Optimal storage concepts for fresh water in land reclamations
In this research, we identified three geohydrological storage concepts that allow for minimizing
mixing and density stratification and thus maximization of the recovery efficiency. These concepts
require specific geohydrological conditions that can be realized through a specific design and
implementation of the aquifer underlying the land reclamation.

1. Flow barriers
2. Balanced recharge and recovery
3. Horizontal layering

We developed these concepts based on the covering laws of groundwater flow and evaluated
their recovery efficiency with numerical groundwatermodelling and laboratory experiments.

Flow barriers
The first storage concept is to surround a stored volume of fresh water in the subsurface by
vertical flow barriers that partly penetrate the sand fill (Figure 2). Flow barriers lead to significantly
improved recovery efficiencies and a faster growth of the freshwater volume, because they
prevent radial expansion of the stored freshwater volume (Van Ginkel et al. 2016)
This can be achieved by for example sheet pilling or grout injection. The density difference
between the infiltrated fresh water and the local saline water causes the lighter fresh water to float
on top of denser saline groundwater; the mixing zone separates the two fluids. In this concepts,
the fresh water is preferably recovered by horizontal wells in a layer of gravel at the top of the
aquifer. A geotextile between the layer of gravel and the underlying sand prevents fines from
being washed into the gravel layer.

Figure 2: Freshwater storage between flow barriers in a saline aquifer.

Numerical simulations showed that freshwater recovery efficiencies are in the order of 65% in the
first cycle and up to to 90% in following cycles are achievable for the studied configurations.
Groundwater flow rates will be considerably higher along the edges than in the middle due to
contraction of stream lines below the flow barrier. The fluctuations in flow velocity may be
counteracted by spatially adjusting the grain size of the sediment within the storage area.
This will further increase recovery efficiencies. While the required grain-size distribution can
readily be designed with numerical groundwater models, methods to realize this in practise
during hydraulic filling have yet to be developed.

Our groundwater models also showed that larger density differences between the fresh and
saline water and higher conductivity increase the recovery efficiency. In practise this means that
the more saline the surrounding seawater the more efficient the recovery. It was also shown that
the ratios of the distance between the flow barriers and their depth, and between their depth and
the thickness of the aquifer impact the recovery efficiency. Also a gravel layer at the bottom of the
storage zone enhances recovery efficiency.

Balanced recharge and recovery

The second storage concept is by combining freshwater storage with saltwater extraction from
below the freshwater stock (Figure 3). The saltwater extraction counteracts the density-induced
buoyancy of the freshwater volume. This storage concept is especially useful in situations where
a continuous saltwater extraction is maintained for desalination, as is often a primary source of
drinking water on land reclamations as well as resorts along desert coasts. This solution
combines desalination with a storage and recovery system, reduces the total cost of the
freshwater supply and is especially interesting in water scarce areas.

Figure 3: Freshwater storage in combination with saltwater extraction from below the freshwater cone.

For this concept an analytical Dupuit solution of the groundwater flow was presented for the
steady flow of salt water towards a well with a volume of fresh water floating on top of the cone of
depression. The analytical solution can be used to design and manage the managed aquifer
recharge system. The most relevant parameter for this concept is the required discharge of saline
water that keeps a given volume of fresh water in place. For this numerical simulations of the
operation of this system over a longer time span with fluctuating conditions showed that
freshwater recovery rates of up to 70% in the first cycle increasing to 80% in subsequent ones
are achievable.
Horizontal layering
The third storage concept has to do with the soil structure. Contrary to the most commonly
applied methods for land filling, bottom dumping, rain bowing and hydraulic filling, a build-up of
land reclamations by successive thin layers is sometimes applied on clayey ocean floors for
geotechnical reasons, e.g., in Jakarta Bay. Thin-layer placement methods are more time
consuming and expensive than reclaiming land by the common land filling methods. Yet, in
combination with the development of a managed aquifer recharge and recovery system, land
reclamations constructed as a thousand layers cake could be designed to increase the recovery
efficiency. Especially when a low vertical conductivity, achieved through layering, can be
combined with maintaining a normal or higher horizontal conductivity. Such an anisotropic aquifer
design limits both mixing and buoyancy especially when combined with vertical wells.

In the conceptual geo-hydrological models for managed aquifer storage in land reclamations
presented above, the recovery efficiency can be improved through a specific distribution of
horizontal and vertical hydraulic conductivity within the aquifer. Hydraulic conductivity in lose
sediments largely depends on the grain size and grain size distribution of the sediment. Realizing
a specific distribution of sediments during land filling is in practise non-trivial and has not been
applied. It requires the development of land filling methods that make it possible to control grain
size distributions in the land fill.
Samples of grain size distributions in the land reclamations of Maasvlakte II in the Netherlands,
Palm Jumeirah in Dubai, the port of Hong Kong and the airports of Singapore and Hong Kong
have been constructed by a combination of bottom dumping, rainbowing and hydraulic filling. An
analysis of the soil structure of the five land reclamations shows that all placement methods lead
to some degree of structured heterogeneity (Figure 4). Relating these to the process of land filling
shows that some control on the grain size distribution is possible, which opens up opportunities
for more advances and controlled process of land filling with the aim to realize specific grain size
distributions. Figure 4 shows the minimum and maximum grain size distributions of these land
reclamations. During the construction process, the grains segregate due to a difference is settling
velocity, settling depth, grain-size distribution and angularity resulting from grain type. An analysis
of the soil structure of the five land reclamations shows that all placement methods lead to some
degree of heterogeneity, so that the hydraulic conductivity is not uniform in the aquifer underlying
the new lands constructed.
Figure 4: The minimum and maximum grain-size distribution curves of samples taken from the Maasvlakte II,
Rotterdam, the Netherlands, Palm Jumeirah, Dubai, the United Arab Emirates (Lees et al. 2013), Changi Airport,
Singapore, Malaysia (Chua et al. 2007), Chep Lap Kok, Hong Kong, China (Lee 2001), West Kowloon, Hong Kong,
China (Lee et al. 1999).

An example
Consider a to-be-constructed land reclamation off the coast of a megacity that is constructed of
marine sediment by means of bottom dumping, rainbowing and hydraulic filling. How would we
then design the subsurface for freshwater storage and recovery?
In this specific case, fresh water cannot be supplied from the mainland and desalination seems to
be the only feasible freshwater resource for this land reclamation. Geo-hydrologists and water
engineers, recommend to combine storage concept 1 and 2, which entails applying freshwater
storage between flow barriers and saltwater extraction from below the stored volume to create a
recoverable freshwater volume in this man-made subsurface (Figure 5). This will result in a more
sustainable and less costly solution. Combining concepts 1 and 2 solves the problem of the initial
stages which the freshwater stock has to build up and water supply from the managed aquifer
storage and recovery system is still limited. In addition such a strategy nicely lines up with the
growing water demand as the area is further developing and can be covered by the combination
of desalination and increased capacity of the groundwater storage and recovery system. In
addition the combination of these storage concepts is recommended because common land fill
methods can be applied, and their operation is relatively simple and robust.
Figure 5: Future land reclamation with freshening of the saltwater extraction well and vegetation using fresh
groundwater stored between flow barriers.

Using flow barriers a more effective and dedicated system can be developed. When flow barriers
are not only applied along the contours of the land reclamation but are also used to create a
number of compartments used for different purposes; e.g. for park irrigation, drinking water,
emergency storage, etcetera, within the land reclamation. Flow barriers might be constructed
during reclamation by smart special placement of finer and coarser grains or constructed
afterwards, e.g. by vertically placed high-density poly-ethylene (HDPE)-foils, sheet piles or clay
walls or grout injection. Biomineralization and podsolization are recent developments that can be
applied to construct flow barriers. These technologies use natural processes for in situ reduction
of permeability. Infiltration may be done by collecting and infiltrating rain water from paved areas
using ponds or gravel beds or by means of infiltration wells. Depending on the climate and the
above-ground functions, other sources of freshwater can be used, like piped water, desalinated
water or treated wastewater. In combination with several subsurface compartments that each
take a specific type of water dedicated systems can be developed that fully accommodate local
hydrological, cultural and socio-economic conditions.

The broader perspective

In moderate or tropical areas, rainwater can be a source of fresh water. This requires the urban
storm-water system to be designed to also recharge the subsurface and not to only discharge
directly into the sea. Currently urban water systems are designed to prevent flooding only.
Capitalizing on the potential of subsurface water storage, however, requires a different
perspective of project developers and regulating government bodies. They need to shift their
focus from storm water drainage, which makes the new land completely dependent on piped or
desalinated water, to more integrated concepts of urban water management. Here sub-surface
water storage and recovery offers ample opportunities. Provisions can be made at surface level
to prevent flooding while maximizing the groundwater recharge. For this purpose, drainage
infiltration and transport (DIT)-systems can be implemented, that not only transport and discharge
storm water, but also retain and infiltrate part of it. Retention and infiltration can also be achieved
by wadi-like facilities in green areas, by infiltration basements or by gravel layers under buildings
and roads. Obviously, pollution of rainwater should be prevented and drainage water from
potentially contaminated surfaces should be treated appropriately, e.g. by passing through a
settlement pond, an oil separator and/or a helophyte filter, prior to infiltration.

The time required to fill the storage volume to its design capacity depends on the available water
resources and the capacity of the infiltration facilities. In tropical and moderate climates with
considerable precipitation, the potential growth of the freshwater volume is likely largest during
the construction phase when the reclamation is still unpaved. Smart inclusion of the development
of the freshwater storage in the planning and design of the land reclamation is therefore required.
The growing capacity of the groundwater storage and recovery system can then be aligned with
the growing water demand of the developing area. Another advantage is that gravel packs around
wells and under infiltration facilities can be far more easily realised during construction works of
the land reclamation. This also applies to the construction of the infiltration facilities, i.e. the
infiltration basements, infiltration ponds, storm water attenuation and infiltration crates and wadis.

Fresh water in the subsurface is vulnerable to above-ground spills and contamination. Therefore,
the ownership of the stored fresh water in the subsurface should be stipulated and regulations for
above-ground use and groundwater protection zones are crucial to prevent contamination.
Pollution of the rainwater runoff can be prevented by regulations for i.e. car-washing and
dog-outlet areas, by the choice of materials used in construction works i.e. stainless steel instead
of weathering-sensitive metals such as zinc and copper. Ownership and regulation can be
simplified when location-specific storage is applied, which can be achieved by
compartmentalization of the subsurface by means of flow barriers. Real-time sensoring can allow
for predictive control of the stored freshwater volumes and yields data necessary to analyse
operational problems when they occur.

Recommendation
To advance the development of the potential of water storage in the aquifers of land
reclamations, a showcase that implements the concepts described in a land reclamation project
can be a great asset. Not only to show the potential but also to research and further develop
technologies and operational and management concepts. A successful implementation of
freshwater storage in the subsurface of land reclamations not only requires linking dredging
engineers and water engineers, but especially raising awareness of its benefits among spatial
developers and their consultants. Only when this happens, subsurface freshwater storage will be
incorporated in the specifications for land reclamations.
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