Erosion Control Using GeosyntheticsGeosynthetics
Prof K. RajagopalDepartment of Civil EngineeringDepartment of Civil Engineering
IIT Madras, Chennai 600 036il lk @iit ie-mail: [email protected]
Introduction• Erosion is the process of detaching
soil particles from ground and so pa c es o g ou d a dtransporting it awayErosion forces wind rain currents• Erosion forces – wind, rain, currents, waves, ice
• Soil on slopes is most subject to erosionerosion
• It takes more than 100 years to form 1 f il (H d 1981)1 cm of soil (Hudson 1981)
Problems due to soil erosionProblems due to soil erosion
• Loss of land – loss of shoreline• Loss of land – loss of shoreline• Instability of slopes• Loss of top soil & fertility of soils• Loss of reservoir capacity due to• Loss of reservoir capacity due to
silting• Rubber and tea plantations suffer
most from loss of top soilp
A TNSCB House lost into sea due to coastal erosion in May 2012
Mouth of estuary closed by sand bar frmation
PULICAT LAKE
BAR MOUTHFORMATION
SHAR INLET
Forces causing Soil ErosiongWind, water and gravity forces are theprime agents of erosion Theseprime agents of erosion. Theseelements combine in a variety of waysto perform the dual role of abrading andsimultaneously transporting both soily p gand rock.
Different Types of Erosion Forces• Aeolian erosion: wind blows away fine
cohesionless soil (sand-blasting).( g)• Rain erosion: rain splash and rain wash attack the
land mass so causing surface erosion.• River erosion: hydraulic lifting, scouring,
cavitation and abrasion. River erosion also triggers mass movement in the form of landslips.
• Marine erosion: wind caused waves and currents d tid l d t iand tides may lead to erosion.
• Glacial erosion: ice sheets and glaciers moving l lalong slopes.
Wind Erosion
Wind speed direction cohesionless soilsWind speed, direction, cohesionless soils, large exposed area, lack of protection lead to wind erosionto wind erosion.
Chepil and Woodruff (1963)
Particle Diameter,mm
Wind Susceptibility
< 0.42 Highly erodible0.42 to 0.84 Difficulty erodible0 84 to 6 4 Usually non erodible0,84 to 6.4 Usually non-erodible> 6.4 Non-erodible
Rain water induced ErosionImpact of rain drops dislodges soil particles and surface runoff carries the loosened soil particles
Rainfall Intensity Droplet Kinetic Energy
down the slope
Form (mm/hr) Diameter (mm)
(J/m2/hr)
Drizzle <1 0.9 2Light 1 1.2 10
Moderate 4 1.6 50Heavy 15 2 1 350Heavy 15 2.1 350
Excessive 40 2.4 1000Cloudburst 100 2.9 3000Cloudburst 100 4.0 4000
Factors influencing Rain Water Erosion
• Erosiveness of rainwater - depends on the di i ti f l h d b denergy dissipation of splash and can be assessed
by duration/intensity of rainfall.• Erodability of soil susceptibility of a given soil is• Erodability of soil - susceptibility of a given soil is
guided by the nature of soil (clay content), particle size distribution and soil condition like saturation,size distribution and soil condition like saturation, density, permeability, plasticity etc.
• Physical features e.g., topography and gradient of y g , p g p y gslope, presence of berms, vegetation growth, etc.
Soil Erodability• low in well-graded, coarse gravels.• high in uniform silts and fine sands• high in uniform silts and fine sands.• decreases with increasing clay and organic
t tcontent.• decreases with lower void ratios and higher
moisture contentsMost Erodible Least Erodible
ML > SM > SC > MH > OL >> CL > CH > GM > SW > GP > GW> GM > SW > GP > GW
COASTAL PHENOMENONCOASTAL PHENOMENON• LITTORAL DRIFT: LONGSHORE OR THE LITTORAL DRIFT IS THE TRANSPORTATION OF
SEDIMENTS (SAND MAINLY BUT MAY HAVE COARSER SEDIMENTS) ALONG A COAST ATAN ANGLE TO THE SHORELINE WHICH IS DEPENDENT ON THE WIND DIRECTION SWASHAND BACKWASH. THIS PROCESS TAKES PLACE IN THE ZONE CALLED LITTORAL ZONE.
• IN THE PROCESS SAND IS AFFECTED BY THE OSCILLATORY FORCE OF THE BREAKINGWAVES MOTION OF THE SEDIMENT AND BED SHEAR FROM LONGSHORE CURRENT.
• IT PLAYS AN IMPORTANT ROLE IN THE EVOLUTION OF A SHORELINE.THESE ARE DUETO MANY FACTORS LIKE HYDRODYNAMIC FORCES,SEDIMENT BUDGET AND HUMANINTERVENTION.
COASTAL PHENOMENON• GROYNE: THEY ARE SHORE PROTECTION STRUCTURES PLACED AT EQUAL
INTERVALS ALONG THE COASTLINE IN ORDER TO STOP COASTAL EROSION
COASTAL PHENOMENON
INTERVALS ALONG THE COASTLINE IN ORDER TO STOP COASTAL EROSION.• GROYNE STRUCTURES ARE USUALLY USED ON SHORES WITH HIGH ANNUAL
LITTORAL DRIFT.• THE THREE TYPES OF GROYNES USED ARE:
ZIG ZAG GROYNES WHICH DISSIPATE THE DESTRUCTIVE FLOWS IN WAVE– ZIG ZAG GROYNES WHICH DISSIPATE THE DESTRUCTIVE FLOWS IN WAVEINDUCED CURRENT OR IN BREAKING WAVES.
– T HEAD GROYNES WHICH REDUCE WAVE HEIGHT THROUGH WAVEDIFFRACTION.
– Y HEAD GROYNE SYSTEMY HEAD GROYNE SYSTEM.
Deep gulleys formed on highway slopes by rain water p g y g y p yinduced erosion – notice the fine silty soil which is highly susceptible to erosion
Different protection systems• Rip rap
Concrete lining• Concrete lining• Guniting• Gabion mattresses• Geotextile coveringGeotextile covering• Cellular mattresses & nets
Bitumen & asphalt spray• Bitumen & asphalt spray• Vegetation
Components of the Grass Plant (Hall 1992)
Protective Role of VegetationINTERCEPTION b b i f ll d t ilINTERCEPTION: absorb rainfall energy and prevent soil
detachment by raindrop splash.RESTRAINT: root system bind soil particles & filters theRESTRAINT: root system bind soil particles & filters the
sediments above the ground levelRETARDATION: surface roughness increased and slow
d ff l idown runoff velocityINFILTRATION: higher porosity and permeability thus
delaying onset of runoffdelaying onset of runoff.Deep roots prevent shallow mass movements by:
- Mechanical reinforcement - Soil water depletion through transportation and interception, and
B tt i d il hi ti f b dd d- Buttressing and soil arching action from embedded stems.
Armour Layer provided by VegetationArmour Layer provided by Vegetation(Hall, 1992)
Use of coir mat to promote vegetation growth to prevent surface erosion
Courtesy Garware Wall Ropes, Pune
Irrigation canal lining by using grout filled geocells
Innovative use of geogrids for shore protection at Navi Mumbai
Swaminarayan Temple TithalSwaminarayan Temple, Tithal
• Temple is located on the Arabian Sea GujaratTemple is located on the Arabian Sea, Gujarat.• Severe erosion at the site due to wave and
current actionscurrent actions.• Seabed erosion posed a threat to the structure
i lin long run.
Use of stone filled rope net gabions for shoreline stabilization at Swami Narain Temple, Tithal, Gujarat
Courtesy Garware Wall Ropes, Pune
Gabion wall at Swami Narayan Temple, Gujarat
Courtesy Garware Wall Ropes, Pune
Courtesy Garware Wall Ropes, Pune
Courtesy Garware Wall Ropes, Pune
Courtesy Garware Wall Ropes, Pune
Courtesy Garware Wall Ropes, Pune
Courtesy Garware Wall Ropes, Pune
Deep cutting in Konkan Railway lineCourtesy Garware Wall Ropes, Pune
Boulder Net installed on the slope to preventBoulder Net installed on the slope to prevent boulders from falling into railway track
Courtesy Garware Wall Ropes, Pune
Net fixed in the anchor trench at the top of the slope
Courtesy Garware Wall Ropes, Pune
Vegetation growth within two seasonsCourtesy Garware Wall Ropes, Pune
Basu et al. (2011)
G d t ti th ithi th thGood vegetation growth within three monthsBasu et al. (2011)
River bank after heavy rainfall and floods Basu et al. (2011)
Condition of river bank after floodsBasu et al. (2011)
Basu et al. (2011)
Shore protection using stone filled metallic gabions
Stone filled PP rope net gabions for shore protection
Shore protection work using geotextile bags
Rope net gabions filled with sand bags
Flexible Break water unit made of geosynthetics – beach sand filled bags in rope net gabionsg p g
EFFECTS OF Cyclone THANE
Coast line at INS Adyar, Chennai before cyclone
Coastline at INS Adyar, Chennai after cycloneChennai before cyclone Chennai after cyclone