Water Harvesting Techniques

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Water Harvesting Techniques

The Community Engineer
Administrator
Selection of a particular technique should be after giving due consideration to the social and cultural aspects prevailing in that area since they are very important for the success of the technique implemented. It should also fulfill a number of basic technical criteria.

1. Soil - soils should have mainly properties of soil suitable for cultivation. It should not be salty or sodic. The infiltration rate should be less than the rainfall intensity so that it induces runoff. Avoid soils with sandy texture.

2. Slope - For area with slope greater than 5%, it is not recommended to have water harvesting structures since this will involve uneconomical earth work and shall affect from uneven distribution of runoff.

3. Cost - Will depend on the earth work or stone work involved. This shall be discussed separately in each systems with detailed quantities.
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Negarim micro-catchments

The Community Engineer
Administrator
These are diamond shaped basins surrounded by small earth bunds. It will have an infiltration pit at the lowest corner of each. Runoff collected from the basin is collected in the infiltration pit. This method is suitable for small scale tree planting in area which is moisture deficit. These micro-catchments does soil conservation also apart from water harvesting.

Israel has the most widespread and best developed Negarim catchments.These are suitable for even areas with rainfall as low as 100mm to 150mm per annum.

The shape of each unit is normally square.
The soil should be at least 1.5m deep but 2m is preferred for better root development and storage of water. Negarim is suitable for slopes from flat to 5%.
Size of micro catchments vary from 10m2 to 100m2 depending on the plantation. More than one tree can also be planted in a unit. The area may be decided based on the water requirement of the tree.

Maximum size of the bund (at the pit)


The least height of the bund should be maintained at 25cm. The width of the bund should be 25cm with a 1:1 slope on sides, with grass cover to avoid soil erosion.

The pit size can be decided as in table below.


Design variation of allowing overflow from each unit by constructing V shaped bund is also common. However the storage capacity will be less than closed system. These types can be done for broken terrains.
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Construction - Negarim micro-catchments

The Community Engineer
Administrator
1.
Find the contour line. Use a water level tube to establish the contour line. Even out the contour to get a smooth line. If the topography is very uneven, consider smaller units.

2.
Use a tape to measure the tip of the bunds (1,2,3 etc) on the contour line. see the image below.

The distance between the points (1 and 2) corresponding to the unit size is shown below.


3.
With a string of length equal to the length of unit, intersect the point 11 from points 1 and 2 (3m for a 3m x 3m catchment). Now the length 1-11 and 2-11 are equal to the length of the bund wall. Repeat this until the first row alignments have been marked.

4.
Repeat the step 3 with points 11, 12 etc to make the second row. Continue this to make all the rows.

5.
Stake out and dig the infiltration pits at the lower corner as per the dimension in the above table. Leave a small step towards the back of the pit to plant seedlings.
See the image below.


6.
Clear all the vegetation before building the bund. Use the excavated material from the pit to form the bund. Build the bund in two layers, compacting each layer after wetting it. Use a string to keep the bund height uniform.

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Contour Bund

The Community Engineer
Administrator


Contour bund is a simplified form of micro-catchments. Bund follows close contour and are separated in to micro-catchments through earth ties. These are more economical and suits particularly large scale implementation. It is more economical and efficient and their is suitability for cultivation of fodder or crops along with trees.

Contour bund is suitable for arid to semi arid areas where rainfall is 200mm - 750mm. 2m deep soil is preferred with smooth topography.Generally a spacing of 5m to 10m is adopted between the contours. An infiltration pit is dug between ties and bund wall. Diversion ditches are provided where necessary for protection. Common sizes of micro-catchments are 10m2 to 50m2.

Bund heights vary from 25cm to 40cm depending on the slope. Bund should not be less than 25cm height, and the base width must be at least 75cm. Cross ties should be at least 2m long at a spacing of 2m to 10m. Infiltration pit size is commonly 80cm x 80 cm and 40 cm deep. The excavated soil can be used to build ties.
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Construction - Contour Bund

The Community Engineer
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1.
Determine the contour using water levels. Make a uniform contour line.

2.
Mark the alignment of the bund on the ground. Keep a spacing of 10m to 5m between the bunds depending on slope.

3.
Determine the spacing between the cross ties depending on the area required for trees. Make an infiltration pit in the furrow above the bund. The pit should be at least 30cm away from the tie wall to enable planting seedling. Make ties at least 2m long with 25cm height and 75cm base width.

4.
Build a lateral bund at the end to prevent loss of runoff. Connect the lateral bund to the contour bund.

5.
Make a diversion ditch at the top of the block to divert any additional runoff from outside the block. Make it to 50cm deep with 1m to 1.5 width and a 0.25% slope.
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