River Bank Erosion
Rivers are dynamic structures. They change their flow patterns continuously and naturally. These changes affect river bank soil stabilization because river bank absorbs the energy. River bank erosion is a part of this changing nature. When banks of a river wear away by watercourse, it is called river bank erosion.
Mostly river banks are made of stratified layers with cohesive and cohesion less materials. Again river banks have three zones- toe zone, bank zone and overbank area. Toe zone is situated between the ordinary water level and low water level which makes it vulnerable to erosion. If non cohesive materials are at the toe zone, vulnerability is increased. But at the time of lowering water level, cohesive banks are most susceptible to erosion because of its low water permeability.
River bank erosion are of two types:
Bank scour: It means removal of bank materials by flowing water and carried sediment.
Mass failure: When sections of the banks slides and topples into the water stream.
Causes of river bank erosion:
Though river bank erosion is a natural process, human activities can increase its rate. Following are the natural causes of river bank erosion:
Flood
Flood is the most crucial reason of river bank erosion. The chances of flood increases in the rainy season. During flood the huge amount of water flow with higher velocity brings enough energy to tear away the top layers of soil or even causes mass failure. The water elevation is raised by centrifugal force resulting highest depth of flow at outside bend and gravitational force pulls down the water. This downward velocity against the bank is the erosive force.
Heavy rainfall
Soil can be eroded by heavy and excessive rainfall. Heavy rainfall often causes strong waves which can loosen and wear away non cohesive bed materials.
Sedimentation
Various disturbances in the nature can cause sedimentation. Silt accumulation at the river beneath reduces water holding capacity of a river as saturation of banks occurs. So the direction of river is changed. Thus meandering rivers are formed eroding river bank.
Strong current of rivers
Strong current generates because of the volume of water flowing or the steep stream gradient. Where the river currents are very much high, river bank erosion can occur. In case of cohesion less bed materials, these strong currents create a cantilever overhang of cohesive materials by wearing away non cohesive materials. So at the toe of the bank shear stress exceeds the critical shear and erosion occurs.
Manmade causes of river bank erosion
Deforestation causes River Bank Erosion
Humans facilitate erosion by cutting down trees at the bank for their own use and removing the natural reinforcement which exists there.
Housing near bank
People in rural areas make their home near the bank which adds compressive force to the soil beyond its capacity and thus cause erosion.
Soil extraction & River Bank Erosion
Humans extract excessive amount of soil from the bank for their uses or they extract huge amount of sands and gravels which help to hold back the soil. By doing this they accelerate river bank erosion.
Impacts of river bank erosion
River bank erosion has many social and economic impacts. It makes people homeless as they lose their property and are forced to migrate to elsewhere. Sometimes it causes death. Humans become poor and valuable land is lost due to erosion. Among the countries of the world, Bangladesh has the highest river bank erosion.
In a data, due to the bank erosion of the Padma river, Bangladesh has lost over 660 square kilometers of land since 1966. In a study of the Brahmaputra river erosion in Mymensingh district shows that the erosion rate in the year of 2006-2010 is 1050 hectares in the left bank and 850 hectares in the right bank. Moreover Assam, a state in India has lost nearly 7.4% of the area of the state due to river bank erosion of Brahmaputra and its tributaries.
River bank erosion control methods
There are several methods to control river bank erosion and protect the bank area.
Soil erosion mats or blankets
Soil erosion mats or blankets are stabilization fabric which reinforce the soil and support the bank until plants are ready to take root. They help to slow down the flow velocity of water along the surface.
The main purpose is to retain the soil and prevent from shifting away. They maintain ideal grow condition as well as protect and cover the soil. Thus they prevent formation of gullies. They can be used on a flat surface or a steep slope area.
It is mainly a grid or lattice or open mesh screen supported with random filament looping patterns. Sometimes backing members are provided below and parallel to the lattice as a base for soil retention. With slope anchoring wire mesh and net backing is fastened with the surface. The anchor provides holding capacity and pull out resistance. The anchor is made of ½” diameter ductile iron or aluminum rod with 30” length.
The net backing is made of polypropylene. Then filaments are layered over the cross bars of the lattice and they are interlocked. Thus it becomes a reinforcing structure. But it provides water drainage thus facilitates establishment of vegetation. It is porous enough to grow plants.
As it is a nonwoven sheet, it is weighed with sand, gravel, slags to firmly hold in the structure and prevent from washing out. It is of uniform thickness and the fibers are uniformly distributed. One example of this type of matting in described in German Patent Specification No. 2,321,362. It is a tangled nonwoven matting of helically crimped synthetic microfibers bonded at the intersections.
There are different types of soil erosion mats. Which mat or blanket is to be used is influenced by soil condition, slope of the bed, flow velocity and the time required to grow vegetation. At last a vegetative cover is placed with the application of seeds and fertilizers. Sometimes we need to prepare the bed slope by excavation or filling. The slope size can be 1.5:1 to 3:1. The mats or blankets are to be placed vertically downslope.
Biodegradable erosion control mats
Natural materials are coir coconut fibers, aspen fibers, jute, wood fibers or straw which biodegrade over time. It can take months to year to break down the fibers in the environment depending on the composition of the matting.
The use of coir fiber is more preferable. Coir fibers are strong natural fibers which can last in water from two to five years. It can act as temporary reinforcement by offering soil stabilization.
Photodegradable erosion control mats
In this mat, the netting is made of plastic like materials containing UV stabilizers. They stop functioning after the shelf life has crossed. Then due to the exposure to sunlight, the netting break down.
Permanent erosion control blankets or mats
Permanent erosion control mats are made of synthetic fibers which does not break down though the matrix can be decomposed. There are two types of permanent erosion control blankets:
Turf reinforcement mats (TRMs)
TRMs are non-degradable three-dimensional matrices. These mats are used in the place of strong erosive forces where flow velocity can be of 15fps and shear stress can be of 8lb/feet2. They consist of UV and chemical resistant synthetic fibers, wire mesh and nets to form a long-term durable blanket.
To provide a sustainable environment there are supplementary biodegradable materials like straw, coir fibers or wood fibers. After vegetation occurs, stems and roots inter-twined with the mesh create a biotechnical composite. TRMs have been successfully applied to 1:1 steep slope areas. They are available in rolls for easy installation.
High performance turf reinforcement mats (HPTRMs):
These mats have improved reinforcement ensuring higher tensile strengths and superior interlock with both soil and root system. So they can deal with higher water flow velocity. These are pyramid like structures for additional tensile strength. Its ultimate tensile strength is 4000x3000lb/ft and shear stress is 15lb/ft2. It can protect the bank up to 25ft/sec flow velocity.
These mats are made of polypropylene monofilament yarns. As HPTRMs are woven structure, they does not need netting. These are used where strong wave impact exists and no vegetation can occur naturally. They can function under extended UV exposure. A deeper stabilization technique is soil nailing.
PYRAMAT® is a HPTRM available in green or tan. Its features are:
- Width: 8.5 feet
- Length: 90 feet
- Thickness: 0.4 inch
- Roll weight: 86 lb
- Longevity: Permanent (up to 50 years)
Use of Geotextile
Geotextiles are permeable fabrics made of polypropylene, polyethylene, polyamide and polyester. They can be woven or nonwoven. Woven geotextiles are made by inter placement of wrap and weft yarns at right angles. Nonwoven geotextiles are made of mechanical interlocking process or heat treatment or needle punching method.
Woven geotextiles are stronger to protect river bank from erosion. The functions of geotextiles are reinforcement to the weak soil, drainage, separation and filtration. They filter water from one side of the sheet to another without significant loss of soil particles. The physical properties of geotextiles like density, thickness, stiffness, specific gravity mainly depend on temperature and humidity.
The pore size distribution of the geotextiles is small enough to retain erodible soil and permeable enough to permit drainage through the eroded soil. Again the geotextiles have to ensure anti-clogging meaning soil particles does not clog the geotextile pore and reduce permeability. They stabilize the soil by creating a filter layer. They also acts like reinforcement.
Geotextiles are available in roll which can be spread over the slope bed and layered with brick chips. Above them CC blocks are placed to keep them in position. We can also dump geotextile bags filled with sand in the river bed and layer it in the bank slope. The end should be secured at least 2 feet past the toe of the bank. Specification of a 315lbs tensile strength woven stabilization geotextile fabric from GEI works:
- Grab tensile: 315 lbs
- Grab elongation: 15%
- Mullen Burst: 600 psi
- UV resistance: 70%
There are criteria for selecting the right type of geotextile depending on the flow condition whether it is uniform or dynamic.
A potential alternative of traditional geotextile is jute geotextile (JTG). Jute geotextiles are treated with bitumen or adhesive for better tensile strength. It can be as high as 2425 lb/ft. Following is the design of typical river bank protection system by JGT done by Prof. Mohammad Shariful Islam, Prof. Abdul Jabbar Khan, Prof. Abu Siddique, Roman Kabir and Shamima Nasrin from BUET on a jute geo textile project.
Coir logs for River Bank Erosion control
Coir logs are natural product made with coconut fibers to stabilize soil and support along river banks. They offer a high level of reinforcement and tensile strength. Coir logs are mainly densely packed coir fibers inside a tubular coir netting.
The diameter and length of the coir logs depend on the flow velocity and slope ratio. The diameters vary from 12 to 20 inch and length can be as long as 20 feet. Multiple coir logs can be joined together using twine. Coir logs help to restore marshes and provide a durable barrier.
They last from two to five years before biodegrading. They are strong enough to withstand adverse weather conditions. There is no chemical seepage from coir logs and these logs naturally break down providing nutrients to the soil.
Specification of a 12 inch coir logs from GEI WORKS:
- Length: 10 feet
- Unit weight: 5 lbs/ft
- Density: 7 lbs/ft3
- Outer net: Bristle Coir Twine Net 2 in. x 2 in. Openings
- Inner core: Coir mattress fiber
Riprap for River Bank Protection
Ripraps are strategically stacked different types of rocks or modular concrete blocks to protect vulnerable areas like river banks from erosion. It is a permanent solution where vegetation cannot occur. It can endure great amount of impact force. There are two types of ripraps – concrete riprap and stone protection riprap. The stones consist of broken limestone to concrete rubble.
Stone riprap is more flexible as it adapts to the natural distortion of soil. In case of concrete ripraps there is a chance of cracking or erosion at the perimeters. Riprap is a costly method in terms of labor and transportation. It is not recommended to use riprap with a slope greater than 2:1 as stones can fall causing erosion and danger. After installation riprap requires regular maintenance. So riprap is not a desired process where green technologies are available for river bank erosion control.
Concluding thoughts on River Bank Erosion & Control
Apart from these methods, there are various methods like willow spilling or using gabions or soil nailing. The main purpose of all of these methods is to stabilize the soil against the erosive force. Depending on the soil type, materials availability, weather condition, hydraulic impact and cost any of the methods can be chosen for river bank erosion control. It will save many lives and properties from being damaged. Again when choosing any method, we have to evaluate the impact of it on the environment and act accordingly.