Flood Risk Management with Geotextile and Geotube Technology

Floods and coastal erosion are getting worse in many countries. Global mean sea level has risen by about 3mm a year on average since satellite measurements began in 1993, and the rate has increased in recent years. The rise comes from two sources: seawater expanding as it warms, and hundreds of billions of tonnes of ice melting from glaciers and the Greenland and Antarctic ice sheets every year.

Higher seas push storm surges and waves further inland, and eroding coastlines damage roads, homes and public property. Coastal and river authorities now combine structural and non-structural measures to manage the risk, and geotextile products have a growing share of the structural work.

Moderating flood risk with structural measures

• Dams and flood-storage reservoirs
• Coastal structures: seawalls, revetments, breakwaters and groynes
• Permeable pavement that lets rainwater soak into the ground
• River embankments (levees)
• Flood barriers and walls

Moderating flood risk with non-structural measures

• Flood forecasting and early warning
• Urban drainage planning
• Flood hazard and risk mapping
• Community awareness and evacuation planning
• Policy and building regulations
• Land-use zoning in flood plains

As a non-woven geotextile manufacturer in India, we focus here on the structural side: flood and erosion control with geotextiles and geotubes.

Coastal construction is growing, and coastal structures are now expected to perform well and do less harm to the shoreline environment. At the same time, quarried rock is getting scarcer and more expensive to haul, which raises the cost of building and maintaining traditional rock and concrete defences.

That has pushed engineers towards cheaper alternatives. On many projects, concrete units and rock rubble are being replaced by geotextile tubes filled with locally available sand.

Geotubes

Geotubes (geotextile tubes) are large tubes sewn from high-strength woven geotextile. They are bigger than geobags, which are smaller sand-filled bags placed by hand or crane. Besides coastal and river work, geotubes are widely used to dewater sludge on projects of all sizes, because the method is simple and cheap.

The tubes are filled hydraulically with a pumped slurry of sand and water. The pumping pressure carries the sand along the inside of the tube, the water drains out through the permeable fabric, and the sand settles inside the tube under gravity, leaving a solid, heavy unit.

The Geotube system is used for shoreline protection, land reclamation, building islands and wetlands, construction platforms, revetments, dykes, groynes and offshore structures.

For dewatering, the system needs no belts, gears or other moving machinery, unlike a belt press. Tubes come in a range of sizes to suit the volume and space available, and smaller dewatering units can sit in mobile roll-off containers that are brought to the site as needed.

Geotextile tubes also have environmental advantages over conventional rock construction. A geotextile tube breakwater is filled with sand dredged at or near the site, so it cuts the volume of dredged material that has to be disposed of, the amount of excavation, and the number of truckloads of rock brought in. Less haulage means lower fuel use and CO2 emissions during construction.

Functions of geotextile

A geotextile performs one or more of five functions in a flood or erosion control structure:

1. Filtration

Water flowing from fine soil into coarser material, such as from a river bank into rock armour, carries soil particles with it. A geotextile filter placed between the two lets the water pass but holds the soil in place, so the bank is not washed out from behind the armour. Both woven and non-woven geotextiles are used as filters; the opening size is chosen to suit the soil.

2. Separation

A geotextile keeps two different materials apart, for example soft subgrade and the stone fill above it, so they do not mix and each keeps its own strength and drainage properties.

3. Drainage

Thick non-woven geotextiles carry water within the plane of the fabric, so they can act as a drainage layer that collects water from the soil and carries it away while holding the soil back.

4. Protection

A geotextile spreads concentrated loads, such as falling rock during placement or wave impact, and cushions the material beneath it, protecting the soil or a liner from puncture and local damage.

5. Reinforcement

Placed inside a soil mass, a high-strength geotextile works with the soil as a composite. It carries tension that the soil cannot, so reinforced embankments and slopes are stronger and deform less than unreinforced soil.

Compared with conventional rock works, geotextile tube structures:

• Disturb plants and animals less during repair and maintenance over the life of the structure
• Use less fuel and need less transport of excavated material
• Reduce the impact on habitats around the work site
• Cut CO2 emissions from transport and placement

Geotubes compared with rock structures

Geotextile fabric for tubes is manufactured in India. Because a geotube is filled with sand from the site instead of quarried rock hauled in by truck, its carbon footprint is much smaller than that of an equivalent rock structure.

Where sand is available locally, a geotube breakwater or revetment usually costs less to build than a rock one. The fabric does need protection from UV and vandalism where it is exposed, and the design must account for the tube’s flattened shape once filled.