Monday , November 20 2017

Terracing




  • Terracing:

    “A terrace is an embankment or ridge of earth constructed across a slope to control runoff and minimize soil erosion”.

    A terrace reduces the length of the hill side slope, there by reducing sheet and rill erosion and prevents formation of gullies.

    Types of Terraces:

    There are two measure types of terrace.

    1. Bench Terrace: which reduce the land slope

    2. Ridge Type Terrace: Which remove or retain water on sloping land.

     

    Depending on the width of the base, ridge type terrace may be classified as:

    1. Narrow Based Terraces

    2. Broad Based Terraces.

     

    Broad based terraces are sometimes referred to as magnum terraces after the inventor, priestly magnum, who introduced B.B. terrace by widening narrow ridge.

    Bench Terrace:

    A bench terrace is shelf like embankment of earth with a level or nearly level top and a step or vertical downhill face constructed along the contour of sloping land.

    Broad Base Terrace:

    A broad base terrace has a ridge 25 to 50cm high and s to 9am wide with gently slopping sides and a dish stopped channel along the upper side constructed to control erosion by diverting runoff at anon –erosive velocity.

    It may be level or have a grade towards one or both ends .based on greed; it is divided or classified as:

    a] Graded Terrace
    b] Level Terrace

    A grads terrace has a constant or variable grade along its lengthened used to convey excess runoff at safe velocity into a vegetated waterway or channel.

    A level terrace follows the contour line, in control to a graded terrace and recommended in areas having permeable soil.

    Bench Terraces:

    Bench terracing is one of the oldest mechanical methods of erosion control having been used for manly centuries in many countries .bench terrace, though not very scientifically designed have been extensively used in India in the mountainous regions of Kerla of H.P and Assam.

    Bench terracing consists of transforming relatively steep land into a series of levee or nearly level strips or steps running across the slope .the strips are separated by almost vertical risers. The risers if sloping may be of earth construction .steep risers are supported by masonry [stones ].bench terracing is adopted only on slopes steeper then 15%[for more then 8%]and where soil condition are favorable .the use of bench traces retards erosion losses and makes cropping operations on these slopes possible and safe.

    Types of bench Terraces:

    There are three types of bench terraces

    A] Based on slope

    B]based on use  /application

    1] Level and table top

    1] Hill type

    2] Sloping inwards

    2] Irrigated type

    3] Sloping outwards

    3] Orchard type

    4] Puertorican or California type

     

    A] Classification based on slope

    1] Table top bench terrace:

    Table top bench terrace are suitable for areas receiving medium rainfall which is evenly distributed and which have highly permeable and deep soils .in paddy fields it may be used for slopes as mild as 1% and used where irrigation facilities are available

    2] Sloping outwards bench terrace:

    In heavy rainfall areas, bench terraces of sloping inwards type are more effective. It prevents inponding of water and useful for crops susceptible to water logging.

    3] Sloping outwards bench terrace:

    Bench terraces sloping outwards are effective only in low rainfall areas whit a permeable soil of medium depth at lower ends graded channels are provided for safe disposal of runoff

    4] Puertorican Type:

    In this type of terrace, the soil is excavated little during every ploughing and gradually developing bench by pushing the soil downhill against a mechanical of vegetative barrier. Mechanical or vegetative barrier is established across the land at suitable interval and the terrace is developed gradually over the years ,by pushing soil downhill and subsequent natural leveling.

    B] Classification based on use:

    Depending upon the purpose for which they are used ,bench terraces are classified as follows:

    1. Hill type Bench Terraces:

    It is used for hilly areas whit a reverse grad towards the hill.

    2. Irrigated Bench Terraces:

    Level benches are adopted under irrigated conditions. The level table top terraces are referred to as irrigated bench Terries.

    3. Orchard Bench Terraces:

    Narrow width terraces [about 1 mm] for individual trees are prepared in this type. These are also referred as intermittent terraces and step terraces. The conversion of land into bench terraces over a period of time is referred as gradual bench terracing.

    Steps in design of bench Terraces:

    For the design of the bench terraces for particular area, the average rainfall, the soil type, soil depth, slope and farming practices of the area should be known. The design consists in determining the  

    1. Type of the bench terrace.

    2. Terrace spacing or the depth of the cut.

    3. terraces width and

    4. Terrace cross section.

     
    Step I Selection of the type of bench terrace depends upon the rainfall and soil, condition.

    Step II Terrace SpacingIt is normally expressed in terms of the vertical interval between two terraces. It depends upon soil, slope, surface condition, grade and agriculture use. The vertical interval is dependant upon the depth of cut. Since the depth of cut and fill are to be balanced, V.I. is equal to double the depth of cut. The depth of cut should not be too high as to exposed bed rock

    Consideration:

    1. Find Out the maximum depth of productive soil,

    2. maximum admissible cutting depth—D/2 or d

    3. Given land slope, [S]

    4. From this, The width of the terraces [W] can be computed for a given slope [S]

     By formula 
                              200d 
                  W = —————-
                                 S
    Depth of Cut:

    Case a:

    When terrace cuts are vertical by similar triangles
                              WS
                D = —————–
                              100
    Case b:

    When batter slope is 1:1
     
           D/2                     S                                         WS
    —————    = —————                  D = —————— = V. I. 
       W/2+D/2               100                                    [100-S]

     
    Case C:

             D/2                       S                                   2 WS
    ——————-    =—————-             D =   —————- = V. I. 
         W/2+D/4                 100                                200-S

     

    Step III: Terrace cross section:

    Design of terrace cross consists of deciding
    1] The battler slope
    2] Dimensions of shoulder bund
    3] Inward slope of then terrace and the dimensions of the drainage channel in case of terrace sloping inward, and
    4] Outward slope in case of terraces sloping outward.

    The height of the embankment [bound] should be increased sufficiently to provide for shrinkage of soils, so that the ultimate slope, as per design can be obtained after compaction.

    Step IV: The cross- section of shoulder bund along outer edge of terrace should also be designed suitably to make the bund stable against slipping and overt to piping.

    Source

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