by

Lim Soon Heng, BE, PE, FSSS, FIMarEST

Founder President, Society of FLOATING SOLUTIONS (Singapore)

Abstract

Amazing as it seems, there is a case for growing rice on floating platforms in the sea. The capital expenditure to develop this is offset by the opportunity to repurpose the land for a number of commercially attractive activities. It would also eliminate crop failures due to droughts and floods and soil degradation due to higher salinity and lower water table.

This paper examines ways crops have been cultivated in the past. Crops are almost exclusively grown on land. Urban farming in recent years has led to the development of structures enabling crops to be produced in multi-tiers over the same plot of ground. Crops have also been grown over water surfaces on floating and semi-floating structures on freshwater bodies. Apart from seaweeds, no vegetables have ever been successfully cultivated in the sea.

While low-salt tolerant rice is being developed, rice has never been grown in the sea. The paper proposes a system where rice is grown on floating platforms with freshwater fed from a freshwater storage facility founded on the seabed.

The key to success for the system is to develop a water storage system fed by the runoffs from the rice fields and supplemented by lakes or rivers on land.

Keywords Rice · Drought · Flood · Salinity · Climate-resilient · Sub-sea water storage

1   Rice is Life

“Rice is the staple food of more than half of the world’s population – more than 3.5 billion people depend on rice for more than 20% of their daily calories. Rice provided 19% of global human per capita energy and 13% of per capita protein in 2009. Asia accounts for 90% of global rice consumption, and total rice demand there continues to rise.”¹ 

“Global rice consumption is projected to increase from 450 million tons in 2011 to about 490 million tons in 2020 and to around 650 million tons by 2050. However, supply projections seem to agree that the global rice supply is unlikely to be able to meet this 44% projected growth in demand. The global rice area harvested increased by 1.38% per year between 1961 and 1977, but since then slowed to just 0.33% per year.”

---

Soon Heng, Lim

Founder President, Society of Floating Solutions (Singapore)

7, Temasek Boulevard

#12-07 Suntec City Tower 1

Singapore 038987

President Floating Solutions LLP

Currently, the world’s rice fields cover a total area about the size of Saudi Arabia (2 million sq. km). With coastal erosion and rising sea levels, the coverage will decrease rather than increase. This is exacerbated by the migration of millions of people from inland to the coast each year resulting in increasing pressure on agricultural land.

Many today still remember Elvis’ hit “Are you lonesome tonight?” back in 1960.  Incredulously, since then the world population has more than doubled from 3 billion to 7.8 billion. It is clear, growing rice the way, it has been done in the past three thousand years is hardly sustainable.

The migration of the global population towards the coast puts huge pressure on land use. It also stresses the biodiversity of flora and fauna on the planet. The next leap is to go beyond where the land ends.

2         Growing Food in Multi-tier Farms

The past few decades see the emergence of the cultivation of crops above ground. In cities from New York to Tokyo, residents are growing not only ornamental plants but vegetables and fruits on rooftops and balconies, more as a pastime than with economic objectives.

Bringing food from traditional farms to the consumer requires long and complex logistics which add costs to the product but not its value. It requires the use of trucks, railways, ships, aircraft, refrigerated containers, and refrigerated warehouses before it reaches the grocery. To reduce the cost of that supply chain the idea of growing food on a commercial scale near the city seems attractive.          

In 1999 Columbia University professor Dr. Dickson Despommier developed the idea of a multi-story building in which layers of crops could be grown on each floor⁵. He still champions vertical farms.            

The idea is good, but it cannot be easily scaled up because there is too much competition for use of urban land. In many cities land is already too expensive for housing, let alone farming.

Vertical farming faces many challenges. Plants need sunlight. Growing them in the vertical direction reduces the amount of sunlight each plant receives. To overcome this artificial light is needed. Alternatively, plants can be grown in troughs that are rotated by a conveyor belt, but this is “dilutes” the share of sunlight for each plant as it is moved into the shaded side in each cycle.

Plants cannot be grown in the usual way with nutrients mixed in the soil, so aquaculture is needed. The system requires nutrient-loaded water to be circulated. Pumps and pipes need to be installed. The building structure to support the plants degrade with time. The entire system requires regular maintenance. Energy is needed to keep it running. The carbon footprint is naturally higher per unit of output.

In most cases, even with the cost of transportation factored into the equation, growing crops on land in remote locations, is still cheaper than growing them in vertical farms in urban areas nearer the consumer.

3       Challenges to Ground-based Crops

Many crops such as rice, wheat, potatoes, corn, and soya beans are not adaptable for vertical farming. They are grown in vast quantities and require huge land masses.

The lands that they have been cultivating for the last two hundred years have been severely degraded. Natural nutrients on the topsoil are washed out by floods. Groundwater is depleted by its extraction during times of drought. Coastal and riverine lands are eroded as sea levels rise.  Rising levels lead to higher soil salinity which is detrimental to crops, especially to rice.

In January 2020, the Thai Ministry of Agriculture declared that rice farmers in 22 provinces might not have enough water to grow rice in an area of over 3600 sq. km in size (2.25 million rai.) This is because the extraction of subterranean water to grow rice is prohibited as there is insufficient water for drinking and for industries.

In 2011, 20,000 sq. km of the Thai nation was inundated for six months affecting 13.6 million people and resulting in economic damage worth US$46.5 billion according to the World Bank.

A 41-month drought was recorded in India beginning in 2015.

According to Scientific American referencing Bangladesh, “A three-foot rise in sea level would submerge almost 20 percent of the entire country and displace more than 30 million people—and the actual rise by 2100 could be significantly more.” Severe floods killed at least 61 people, displaced nearly 800,000, and inundated thousands of homes across a third of Bangladesh, after two weeks of heavy monsoon rains in July 2019 in that country.

“Salt stress often causes photosynthesis decrease, plant growth inhibition, biomass loss, and partial sterility, all of which lead to yield reduction.”

Electrical conductivity (EC) of irrigation water is the attribute most often used for monitoring the salinity of irrigation water, by the practicability of its measurement and high correlation with the number of soluble salts, since the EC is the measure of resistance passage of electric current between electrodes in a solution where ionic solutes (cations and anions) are present (Doneen, 1975). Grattan et al. (2002) estimate a yield loss of 50 % with an EC of around 7.4 dS m-1. In some cases, however, the salinity of soil solution from 1.9 dS m-1 is already sufficient to significantly reduce the seedling’s biomass and an EC of 3.4 dS m-1 compromises their survival (Zeng & Shannon, 2000).

Research into the extraction of salt by mechanical and botanical means has not met with measurable success. It is costly and not a satisfactory solution.

3.1 Salt-tolerant rice

“Growing rice in swamps, bogs, and clay-like or salty coastal waters, which comprise about a third of the total arable land in China, has typically been impossible because salt stresses the plants. That makes photosynthesis and respiration a challenge for the stalks, causing them to stop growing and die. An increasing amount of land is expected to face this problem as sea levels rise.

China leads the world in research to genetically modify the DNA of natural rice to produce new strains that are saltwater tolerant. More than 200 strains are experimented on with diluted salt water. The strains can tolerate up to a salt concentration of about “10% of the level in seawater”

To forestall undue exuberance Ren Wang, assistant director general for agriculture at the United Nations’ Food and Agriculture Organization said, “It’s still only maybe 10% the level of salt in sea water,” – “quite far” from any practical application.

China’s legendary agronomist Yuan Longping works tirelessly to hybridize rice since the 1970s. He is the authority on the genetic manipulation of rice cultivars and the creator of the Green Super Rice which has a yield of 17 tons per hectare, twice the national average. He reckons commercial saltwater-tolerant rice is eight to ten years away¹⁰.

4         Hydraulic vs genetic engineering to increase rice yield

In the meanwhile, we need to go forward with a way to grow proven strains of rice in floating rice fields in lakes where there is fresh water and in the sea by devising means of storing fresh water to overcome prolonged droughts and rising sea levels.

This is a mechanical solution, using hydraulic engineering rather than genetic engineering. It is a solution that applies the principles of buoyancy known to Archimedes more than 2000 years ago.

Rice would be a good crop for engineers, particularly naval architects, to put their knowledge and skill to the test, as it is under imminent threats from rising sea levels, coastal erosion, droughts, and soil salinity.  They should team up with agronomists specializing in rice cultivation, pest infestation, harvesting, and storage to address all phases of the rice cycle.

4.1      The Economics of Relocating Rice Fields Away from Land

Is it worth the effort economically? Given that land where rice is grown is cheap, does it make sense? Will landowners obstruct such a move for fear of losing income from farmers? How would it change the life of farmers and their families?

The current wholesale price of rice is about USD 600 per ton and a less-than-optimal yield is about 10 tons per ha per year. Over a ten-year period, ignoring inflation, the value of the output is USD 60,000 per hectare. It would seem that any system to be economically viable needs to address this as the ceiling barrier to entry.

4.1.1   Cottage Industries

That however is an oversimplified benchmark. It does not account for the fact that for every hectare of a rice field on land that is replaced with a hectare of rice field floating in the water, the vacated plot on land can be put to much higher economic use. The land can be used to keep livestock (goats, cattle, ducks, chicken), build factories to produce goods, and keep many employed, even if these factories are only cottage industries producing clay pots or rattan furniture. The farmers would enjoy an additional income in the long months between sowing and harvesting.

4.1.2   Eco-tourism

Other possible revenue-generating ideas include eco-tourism. In Ubud, Bali, 5-star hotels with a panoramic view of rice fields charge rates in excess of USD 1,000 / night. Villagers benefit from finding employment as hotel staff, tourist guides, and selling local handicrafts.

4.1.3   Compatible Crops

The land could also be used to grow other crops that are more climate-resilient and drought-tolerant. A suitable crop to cultivate would be coconut, as it grows naturally on the seafront with very little need for fertilizers or maintenance and copra can be turned into cooking oil. Another is mangrove plants (Rhizophora). This plant grows naturally in swamps, acts as a defense against erosion, produce timber that is useful as piles, and provides habitats for crabs, eels, and other edible creatures.

When rice is grown on floating platforms, the space beneath the platforms can be put to good use for shellfish aquaculture to rear crabs, cockles, mussels, and oysters. See Fig. 2

Current wholesale price of rice is about USD 600 per ton and a less than optimal yield is about 10 tons per ha per year. Over a ten-year period, ignoring inflation, the value of the output is USD 60,000 per hectare. It would seem that any system to be economically viable need to be address this as the ceiling-barrier to entry.

4.1.1   Cottage Industries

That however is an oversimplified benchmark. It does not account for the fact that every hectare of rice field on land that is replaced with a hectare of rice field floating in water, the vacated plot on land can be put to much higher economic use. The land can be used to keep livestock (goats, cattle, ducks, chicken), build factories to produce goods and keep many employed, even if these factories are only cottage industries producing clay pots or rattan furniture. The farmers would enjoy an additional income in the long months between sowing and harvesting.

4.1.2   Eco-tourism

Other possible revenue generating ideas include eco-tourism. In Ubud, Bali, 5-star hotels with panoramic view of rice fields charge rates in excess of USD1,000 / night. Villagers benefit from finding employment as hotel staff, tourist guides and selling local handicrafts.

4.1.3   Compatible Crops

The land could also be used to grow other crops that are more climate resilient and drought tolerant. A suitable crop to cultivate would be coconut, as it grows naturally on the sea front with very little need for fertilizers or maintenance and copra can be turn into cooking oil. Another is mangrove plants (Rhizophora). This plant grows naturally in swamps, acts a defence against erosion, produces timber that is useful as piles and provides habitats for crabs, eels and other edible creatures.

When rice is grown on floating platforms, the space beneath the platforms can be put to good use for shellfish aquaculture to rear crabs, cockles, mussels and oysters. See Fig. 2

Instead of exporting rice landowners, in say Vietnam, may find it more profitable to export energy by undersea cables to Singapore. Australia is already planning to do so. Vietnam would be more competitive because of its cheaper manpower cost and shorter distance to Singapore. Singapore has less diversity in its choice of energy sources than it has in procuring rice.

In summary, if rice is grown in floating farms in lakes or in the sea, the freeing up of the land would bring about a higher level of wealth to the community as a whole.

5         Traditional Floating Farms

Artificial floating farms are not new. Fruits and vegetables have been cultivated successfully and organically for many years in various parts of the world.

The cultivation of crops on floating decaying vegetable matter makes enormous sense. It is less labor intensive, one need not plow the land, the thriving microorganic ecosystem till the soil every hour of the year. The water is always there so there is no need to construct irrigation systems.  Boats move produce from farms to markets without a trace of greenhouse emissions.

Inle Lake in Myanmar is well-known for a community whose economy is built around the production of fruits and vegetables on floating platforms. These platforms are formed by bundling masses of water hyacinths and kept in position with bamboo stakes. There are 3000 hectares of such farms in the 260 sq. km lake. The hyacinths grow in huge profusion rapidly all year round, so the locals are never in short supply of “building material,” or organic fertilizers.  Tomato is the main crop owing to their demand, but other crops are also grown.

The prize for the most amazing use of water bodies for growing food goes to the Aztecs of Mexico. They famously build the chinampas as early as the 14^th century in the lakes in the Valley of Mexico¹⁴. Some of these chinampas were large with footprints measuring 90 m x 10 m. These plots of were constructed in marshlands and the plants are sowed on compost material and silt kept in place by timber stockade. The top of the soil is just high enough to keep the roots from being permanently soaked in the water beneath it.

They were laid out in a rectangular grid surrounded by a network of canals serving as “roadways.”  Boats were punt around to collect the produce during harvest time.

The chinampas still exist today. It seems chinampas ownership is legally recognized and can be passed on to succeeding generations, attesting to the durability of the structure and its engineering excellence.  According to the FAO the “chinampas located in Xochimilco, Tláhuac and Milpa Alta comprise more than two thousand hectares in which about 12 thousand people work, mainly cultivating vegetables and flowers, including 51 domesticated agricultural species and 131 species of ornamental plants.”

6         Floating Farms on the Sea

The floating farms mentioned above are constructed over fresh (i.e. non saline) water.  No one has attempted to construct a floating farm over the sea. This is easily explained by the fact that except for seaweeds, all plants for human consumption require fresh water.

Storing freshwater in the sea was the main challenge. Building a floating reservoir up to recent years would entail constructing a rigid steel or concrete box. That is easily done except that it is not easy to hold the box down when it is empty due to the uplift.

In 2018, the College of Science and Technology, Nihon University’s Dr. Shinji Sato, under the auspices of the Society of Floating Solutions (Singapore) presented a paper in Singapore on an experimental project integrating solar energy, effluent gas, and algae in a floating structure in saline water. Unfortunately, we are unable to obtain any published results.

7         Water Conservation in Paddy Fields and Pest Control

The water balance in the rice field comprises the following processes:

Into the ponded field:

• Rain fall
• Irrigation from river and lake sources
• Ground water by osmosis

Out of the ponded field:

• Evaporation
• Transpiration
• Seepage into the soil
• Run offs overflow the embankments nutrients for crop washed away

The above processes are largely uncontrolled. Heavy rainfall may cause flooding. Irrigation may not always be possible in dry seasons. Ground water table is lowered each year due to human activities and extraction of water for drinking.

In land-based fields, enclosures are not often used (even as protection against birds and insects). Evaporation is therefore uncontrolled and exacerbated by wind. Seepage of water is exacerbated as ground water table recedes.

The above are the dilemma of rice cultivators. Floating rice fields offer opportunities to overcome these challenges. To reduce evaporation and insect and bird attacks plastic sheets are easily spread over bamboo pole supports slotted into sockets on concrete base.  This simple solution reduce the need and cost for pesticides. The devastation by marauding locust can be prevented by covering the rice fields with plastic sheets.  When the plants are flowering, beehives may be placed in the enclosures for pollination.  Ducks¹⁸ may be kept inside enclosure to keep snails and other under control without worry that they may fly away and never return.

The water in the rice pond and is in a close loop and does not get flushed out of the system. This the nutrients either organic or artificial is not wasted when a flash flood occurs.

8         Why Might it be Worthwhile to Explore Growing Rice in the Sea?

As far as rice is concerned the demand will continue to grow but the land available to cultivate it is severely being marginalized.  Vertical farms for rice is not a practical solution so the only option that remains is to use the space in the sea.

There are several challenges with using sea space:

• The forces of nature: wind, waves, and current have to be overcome.
• A solution has to reduce water loss through run offs, and evaporation. 
• Water in excess of immediate requirements has to be stored.
• Storing water on land (to support a floating farm) is difficult as there will be loss through evaporation and seepage and contamination by salt laden molecules of water.  Siltation is another problem: it reduces the capacity of the reservoir over time.
• And any plot of land set aside as a reservoir quite obviously is no longer available to cultivate rice.

8.1      Challenges

Storing freshwater in the sea will overcome the challenges mention above.

The following are the considerations. The storage system has to fulfill the following criteria:

• It has to be protected from damage by passing boats or flotsam or willful acts and terrorism.
• It should not be carried away by wind, waves, or tidal currents.
• As the system is charged with water or water is extracted from it, it should remain substantially at the same elevation relative to the seabed. That is to say it does not float higher as it empties or lower as it is being filled.

A rigid tank is fine when it is full of water. It sinks to the seabed and remains there. However, when water is extracted from it will start to float at some point. It can be held down with cables. However, the tension in the cable will escalate from zero to some value and at some point, it may exceed its capacity or the holding capacity of its anchorage.

9         Using Bladder Tank Under the Sea

To resolve this, a “bladder tank” (sometimes referred to as a pillow tank) is proposed.  This is a flexible collapsible containment bag shaped like a pillow, when inflated, used by the military for water storage²⁰. It can be folded away for transportation.

When used to store water the stress on the membrane skin is a function of the difference in densities of the fluid inside and outside of the tank. On land water in the tank exerts a considerable pressure on the wall of the tank as the density of water is about 900 times larger than air. The fabric is subject to a high tensile stress.

In the sea the situation is reversed; the denser fluid, seawater, is outside and the lighter fluid water, is outside. The water inside is incompressible.

If the tank is connected by a pipe so that the water inside has a free surface experiencing the same atmospheric pressure as the seawater, the pressure on both sides will equalize as the water rises. (The pressure in a liquid is the head x density.)

In seawater, the fabric of the tank experiences a low stress as the difference in densities of the two fluid is small. The fabric itself may experience an uplift if its density is smaller than seawater. If connected to a pipe to above the waterline the fluid inside will automatically adjust itself so that the pressure at any given elevation is equal inside and outside the tank.

While pillow tanks have been used to transport water over great distances on the sea, as storage facilities for freshwater under the sea it has never been done (at least there is no documentary evidence of this application.) The potential of this innovation has yet to capture the imagination of engineers and agricultural economists.

Flexibladder in South Nowra, Australia, manufactures bladder tanks up to 2000 m3 (larger than an Olympic size swimming pool.) For subsea application, we believe the capacities can be much higher if properly protected from mechanical damage and from forces when empty due to its self-buoyancy. Other manufacturers include several from Europe, the US, China, Japan, and South Korea.

9.1        Material and Fabrication

There is a whole range of polymers that may be considered for the fabrication of these tanks: HDPE, LDPE, LLDPE are some of the common ones. The good news is that the prices of these material is likely to decline.  

 Linear low-density polyethylene (LLDPE) has superior tensile strength, superior impact, frictional, and puncture resistance. It is also UV resilient and recyclable. It has a density of 0.910-0.925 g/cm3. It will therefore float making it simple for recovery if necessary, for maintenance or repair. It has good durability and will easily last ten years. The tensile strength at yield and breaking are respectively 10 – 30 MPa and 25 – 45 MPa, which compares favourably with rubber.

It is probably an appropriate choice for our purpose. A 2000 m3 tank would require approximately 2500 m2 of material. Assuming a wall thickness of 1.2 mm, and a density of 0.92 kg/cm3, it would weigh 2760 kg (0.0012m x2500 m2 x920 kg/m3). The material will cost USD 6900. (According to Alibaba’s website the price is about USD 2.5 per kg.) The labour cost to fabricate the tank depends on the local labour cost. See properties in Table 1.

The methods of joining polymers are well documented and readily available online and in the references mentioned at the end of this paper²¹.  They range from application of heat, induction current, ultrasonic, laser, and microwave.  The skill necessary to perform the bonding of the LLDPE can be easily acquired (unlike welding steel structures.) With the supervision of the manufacturer’s engineer they may be performed by local technicians on site.

9.2      The Undersea Water Storage Facility

The Water Storage Facility is preferably placed directly beneath the floating rice field if there is sufficient depth to accommodate it, or a distance away if necessary if the water is shallow. (A floating bladder tank is also an alternative. Its disadvantage is it occupies space on the surface.)

The bladder tank is kept enclosed in a concrete chamber which is secured to a concrete base plate as shown in Fig. 8. To prevent damage by large marine creatures (crabs, sting rays, sharks etc.) the chamber has hinged gates. As the tank inflates, the displaced seawater leaves the chamber through one gate. As it deflates seawater enters via another gate. The gates to be oriented in such a way as to induce a scouring effect in the chamber to flush out silt. These gates can be remotely locked if necessary.

The chamber is of concrete construction. Little reinforcement is needed as the structure is not subject to any significant bending. The main force it will experience is the uplift due to the lower density of the water in it relative to seawater.

It is important to consider the effect of tidal currents. Sites experiencing strong tidal currents should be avoided.

Studies need be carried out to determine the impact of tidal forces at the selected site, as this will affect the design of the station keeping system. to keep the chamber and tank from floating.

The base plate is sized to provide sufficient mass to ensure the chamber is not lifted when the bladder is full.

Data relating to the permeability of LLDPE is not readily available. It is not likely that there will be significant water loss by osmosis.

There are two pipes, one connects the bladder tank to the rice field and the other to a source of fresh water (which may be a river or a lake) on land. Pipe 1 supplies the rice field with water during dry periods and drains away surplus run offs when it rains. This is a virtually close loop system, both for water and any nutrients dissolved in the water.

Pipe 2 replenishes the bladder tank with water from the land source during a prolonged drought. If the land water source as depleted the crop can still be safe by importing water from elsewhere and pumping it into the rice pond which then may be transferred in the bladder tank.

If water depth permits the chamber may be placed directly beneath the floating field. If not, it may be placed at a distance away. See Figs. 9.

Depending on the weather pattern, the regularity of rain, severity of drought, the storage capacity can be designed accordingly, bearing in mind that waste due to evaporation and transpiration of the plant is reduced with the field being under cover. In a prolonged abnormal drought, it is possible to barge water to replenish the bladder tank if the water resources nearby dries up. Charging the storage capacity is easily done by topping the field with water which is then drained into the bladder tank beneath the sea which is not exposed to evaporation.

10. The floating field 

Just as vegetables grow on roof tops of urban buildings, so can rice on floating docks and be free of droughts, floods, or the salt in the ground and pestilence in the air.  Precious runoffs can be stored, and evaporation can be curtailed.  All varieties and strains of rice, long grains, short grains, medium grains, Jasmine, Basmati, Japonica or Arborio that suit any palate can be grown in the sea all that is needed is a dock to keep seawater and no-saline water separate.

             The water level in the fields may need to be adjusted over the growing cycle from planting seedlings to harvesting. This can easily be achieved with a floating pond. Water is either pumped into or out off the ponded field into the undersea storage tank. No plant nutrient is lost in the process.  

Attempts to develop new salt resistant strains of rice take years for results to show and more years for the taste buds to accept. Even though the new strains may tolerate some salinity in the soil, rising sea levels and coastal erosion are too costly to fight against.

“To address seawater intrusion and salinity ingress, every year villagers create an embankment, but the structure is unable to keep the sea at bay and breaches regularly. “The rate at which the sea is coming close to us makes us believe that soon it will engulf our entire village. Our farming has already gone kaput,” laments K. Kannan, 55-year-old secretary of the local farmers’ association. According to him, villagers have been demanding a permanent cement embankment from the public works department (PWD) to keep the village safe from the sea. But no action has been taken on that front.”

10.1 Construction

The dock for floating field may be of any practical size. 50 m wide and 100 m long (1/2 hectare) would be a handy side from the viewpoint of construction, launching, towing, and mooring at its designated site. The moulded depth has to ensure that its freeboard is large enough to overcome the splash of the sea in a storm.  The section is shown in Fig. 10. (It does not show the full width due to the aspect ratio.) The same design can be replicated and of course there is economy of scale if more are manufactured.

10.1.2 Dock body

Of what material can the dock be fabricated? It all depends on the environment where the dock is to be located. It can be of timber, or HDPE (high density polyethylene) or light weight reinforced concrete, respectively in order of cost, strength, rigidity, and durability.  In the case of concrete, and HDPE void spaces are filled with buoyancy material so that seawater will not fill them when a leak occurs.

The dock may not have the rigidity of a steel dock that is used to lift ships for maintenance but there is no need for such a high standard of structural integrity in the interest of affordability. Under sheltered conditions where waves are less than 1.5 meters and wind speeds less than 20 m/sec, the simple construction will suffice. In the worse case scenario, the dock can be submerged so that it is filled with more water from bladder tank to provide better stability and refloated after the storm has abated.  By submerging, the sides are less exposed to wind, the mass is increased, and the center of gravity is lowered which all converged to keep the dock safe.

The design of the dock should be simple, basic, and intuitive. It need not be subject to the many rules and regulation that governs the design and construction of ocean-going vessels. It is a sustainable structure. The cultivation of rice does not pollute the sea. Under heavy weather no one will be working so no lives will be in danger.

The load on the dock consists of the soil to support the roots, the water, and the rice plant when fully grown and bearing rice. The level of water in the dock is kept constantly at the desired level. When water rises beyond the set level it overflows into a sea chest which is connected by a flexible pipe Pipe1 as illustrated in Fig. 8 in the previous section. When it falls to a set level, the pump automatically cuts in to transfer water from the bladder tank via Pipe 2. The pipes are of inexpensive HDPE material, flexible and corrosion free and easily replaced if damaged. A 150 mm diameter size would be adequate. The length and flexibility allow for some degree of motion without unduly straining the pipe.

To keep the dock in position against tidal currents, waves and wind, some form of mooring system is needed. The system should allow the dock to rise and fall with the tide. This can be achieved with tubular steel pipes, but it would require a piling machine for its installation. We recommend for simplicity the anchor and cable catenary system. The anchor can be replaced by a large boulder if it is difficult to procure. Instead of steel cables, nylon marine mooring ropes may be just as effective. Ropes should be class approved.  The mooring rope runs beneath the dock to the opposite side at the forward and after end of the dock. To prevent longitudinal drift, the rope should be inclined at about30 degrees to the longitudinal center line of the dock.  The upper end of the rope is lashed to a bollard or bitt with the necessary precaution to avoid damaging the rope at sharp edges. 60 mm dia. (50 ton) marine mooring ropes would suffice. These ropes float in water and may require sinkers to configure the catenary geometry.

The inside surface of the dock (bottom and sides) are lined with geotextile sheets to keep out the seawater. These sheets may be glued easily on site in hours with a small crew of men within hours, easily with spray-on glue such as 3M HoldFast 70.  The bonding strength of the glue is as good as the strength of the textile. The liner further enhances the strength of the bonding between the modules (timber or HDPE) used to form the geometry of the dock.  

No painting is necessary on the exterior of the dock. Rubber tires or timber may be used as fenders to protect the side against barges that may come alongside. Some void spaces may be used as stores for tools and ploughing machines, shelters for farmers in bad weather, as well as stores for seeds and fertilizers.

10.1.3 Hydrostatics of the rice floating dock

Hydrostatically, the floating rice field is similar to a floating swimming pool but with one main difference.  The swimming pool is filled with water to a level that is close to the level of the water outside the pool. It means that each of the four walls of the pool is experiencing similar (though not necessarily identical) pressure distribution inside and outside. This is not the case with the floating rice field.

Another floating structure that has somewhat similar shape is the floating dock commonly used to lift ships out of water for maintenance. It consists of two “wing walls” erected on a flat top pontoon. By ballasting its tanks, the dock is submerged so that a ship can be maneuvered into it between the walls. By subsequently deballasting the same tanks the dock is floated lifting the ship in it above water. In both cases (afloat or submerged) the wing walls experience the same pressure distribution. Because of this identical water pressure distribution on each side, each wall experiences zero tipping moment.

The situation in the floating rice field is rather different. The water pressure distribution acting on each wall is unsymmetrical as shown in the diagram C in Fig. 11. The wall needs a reaction, R to keep it in equilibrium. That reaction is provided by the pontoon.

A good location for developing a cluster of floating fields would be in a sheltered bay.  Breakwaters if necessary, can be constructed with concrete tetrapods. However, it is worthwhile considering floating breakwaters with pillow tanks tethered by marine ropes to rocks. These floating breakwaters double up as additional water reservoirs.

All the hydrostatic forces for the floating field are moderate as the structure and it load are light.  The water in the pond may vary unlikely to overstress the structure. This of course can be confirmed by FEM (finite element method.)

11. Financing

Finding the necessary capital is of course crucial for small time farmers. However, if he owns the land on which he grows his rice he can pledge that as a collateral for a loan and if that is not sufficient to cover the cost of the floating rice field the field itself may also be collateralized until the loan is redeemed. Even if the land is under threat of sinking under rising sea levels, it can be economically productive for instance as fish or seafood farms or as mangrove forest. A floating structure for planting rice can be designed to last for several decades and can be easily repossessed, relocated, repurposed, and sold so it is a low risk collateral.

12. Concluding remarks

There are two unrelated but fundamental reasons why floating rice fields may lead to the betterment of marginalized farmers. The first is an existential one for many farmers as well as consumers of rice. Vast areas of coastal land are so degraded by ground water extraction, increased levels of salinity and drought that its viability for rice cultivation is undermined.  Most rice fields are not short of water but short of reservoirs to harvest and store water for use during the dry seasons. That problem has never been adequately addressed but can be now.

             The second reason is that relocating rice farms to the sea frees up land that can be used to produce goods and services and provide employment for thousands more people benefiting the local economy.  Growing mangrove swamps for example protects land from erosion and the trunk (bakau wood) is useful as piles in low rise buildings and monsoon drains. It is also a natural habitat for a variety of shell fishes including crabs. Another crop worth considering is coconut, the mother of all plants. It grows well in coastal areas, requires little maintenance. Every part of it has a value in one way or another.

The ways to repurpose the land are only limited by the imagination. From rearing animals for meat to producing bamboo furniture the land can provide more employment for villagers to supplement income from rice. 

References

1. Ricepedia http://ricepedia.org/rice-as-food/the-global-staple-rice-consumers
2. Rice Farming: Complete Guide from Seeds to Harvest https://www.youtube.com/watch?v=J_mMS3EkHok
3. This is how rice grows in Japan https://www.youtube.com/watch?v=joLWlEd2bPM
4. Rice farmers move towards sustainability, UN Environment Program https://www.unenvironment.org/news-and-stories/story/rice-farmers-move-towards-sustainability
5. The Vertical Farm: Feeding the World in the 21st Century, Dr. Dickson Despommier
6. Off-season rice crop in 22 provinces threatened by drought, The Nation Thailand 13 January 2020
7. India’s Longest Drought: 41-month-long Dry Spell From 2015-18 Was Longest in 150 years,
   https://www.news18.com/news/india/indias-longest-drought-41-month-long-dry-spell-from-2015-18-was-longest-in-150-years-finds-study-2358355.html
8. Rising sea forces villagers to abandon rice farming, by Nidhi Jamwal, https://indiaclimatedialogue.net/2019/01/02/coastal-villagers-abandon-farming-as-sea-level-rises/
9. The Unfolding Tragedy of Climate Change in Bangladesh, Scientific American, Robert Glennon on April 21, 2017
10. An 87-year-old scientist may have just unlocked the secret to growing rice in saltwater, https://www.businessinsider.com/yuan-longping-chinese-rice-scientist-growing-saltwater-rice-2017-11
11. Agrisea Ocean Farms Will Grow Rice In Saltwater This Year, Andrea D. Steffen, https://www.intelligentliving.co/agrisea-ocean-farms-grow-rice-saltwater/
12. Inle Lake and the floating gardens – it’s all about tomatoes, https://www.youtube.com/watch?v=f8p0sx4xRd4
13. Chinampa, https://en.wikipedia.org/wiki/Chinampa
14. Chinampas of Mexico City were recognized as an Agricultural Heritage System of Global Importance, http://www.fao.org/americas/noticias/ver/en/c/1118851/
15. Chinampas, The Floating Gardens of Mexico
    https://www.ancient-origins.net/ancient-places-americas/chinampas-floating-gardens-mexico-001537
16. Water Balance of Flooded Rice in the Tropics by Siva Sivapalan,
    https://www.intechopen.com/books/irrigation-and-drainage-sustainable-strategies-and-systems/water-balance-of-flooded-rice-in-the-tropics
17. Improvement of Salt Tolerance Using Wild Rice Genes, Ruidan Quan et el, https://loop.frontiersin.org/people/463320/overview
18. The Use of Ducks in Rice Fields in the Control of Weed and Pest
    https://www.permaculturenews.org/2016/09/15/use-ducks-rice-fields-control-weed-pest/
19. Filling the World’s Rice Bowl – with Less Water, https://www.voanews.com/east-asia-pacific/filling-worlds-rice-bowl-less-water
20. Smarter Liquid Storage for Industry, https://www.labaronne-citaf.com/fields-of-use/
21. Global Plastic, https://www.globalplasticsheeting.com/hdpe-vs-lldpe-vs-ldpe
22. Geotextile Seaming Solutions using 3M™ HoldFast 70 Cylinder Spray Adhesive https://www.youtube.com/watch?v=d-J0GRSG2ns
23. Coconuts, Plantation International. https://www.plantationsinternational.com/coconuts/

Lim Soon Heng

Disclaimer:

The views, information, or opinions expressed in this article are solely those of the author and do not necessarily represent those of TheNavalArch Pte Ltd and its employees

Lim Soon Heng

President, Society of Floating Solutions Singapore

Lim is a Professional Engineer, a Fellow of IMarEST. He has a long career in the marine industry much of it with Keppel Offshore and Marine. He now runs a consultancy company specialising in the planning and start-up of new shipyards and the revamping old ones.  He is the Founder, President of the Society of Floating Solutions (Singapore). He can be reached at lsh@sixtrees.com.sg. More information on Society of Floating Solutions Singapore (SFSS) can be obtained from http://floatingsolutions.org/

Please register or login to read the full article