TD 603Water Resources

Milind Sohoniwww.cse.iitb.ac.in/∼sohoni/

Lecture 10: Minor Structures for Ground and Surface Water

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Classification by Purpose

We may classify structures/procedures by their primary objective.

Groundwater Recharge: These areimplementations to enhance therecharge of groundwater or toimprove soil moisture. This isusually done either by

(i) reducing the velocity ofwater-flow

(ii) increasing the infiltrationcoefficient

(iii) explicit groundwaterrecharge structure

Examples: Contour bunding,furrowing, well-rechargestructures, percolation tanks.

Reducing Soil Erosion: This isusually to improve agriculture,protect building etc., or to protectdownstream water structures.Examples:

Terracing, contour bunds.

Gabions and gully plugs.

Surface Storage: These intend tostore water on the surface. Someexamples are:

Check-Dams, Weirs

Rainwater harvesting

Bunds, Farm-ponds.

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Contour Trenches

source:FAO

Hill

Contour Lines

L

wd

PitBerm

Saturated region

Parameters L, d ,w depend on theslope, rainfall etc.Working: Pits fill with water andremain so till the end ofmonsoon. This creates a localsaturated layer which helpspercolation. Also used alongsidetree-plantation.

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Hill-sides

Baner, Pune. source:http://stuffido.wordpress.com/2009/07/

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Contour-bundsThis is formed by firming the berms to create obstructions to water flow. It isespecially useful for tree-planting.

source: Ray Weil, Picasa() March 23, 2010 5 / 31

Terracing

This is largely about preventing soil-erosion and utilizing the land for agriculture.It is used when the gradients are small.

source: FAO

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Terracing and gullies

Top view source: FAO

Terracing is an involved anddelicate construction. Specialcare must be taken for the inletand outlet of water.

Here is an example of gullyformation in an agricultural field.Gullies may form in a singlemonsoon in fields with even asmall gradient. These getreinforced and cause substantialloss of water and soil, besidesdamaging the field.

source: FAO

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Furrowing

Soil may get compacted by overgrazing and animal/human use. This reducesinfiltration coefficients substantially. For level lands, furrowing is a usefultechnique for increasing infiltration. In fact, agricultural land is excellent forrecharge.

source: FAO

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Percolation Tanks and Bunds

source:

http://forest.ap.nic.in/Sparks of

Success

APFD-02-05/007-Nallavally.htm

Dug-outs or obtained bybunding an existing flow tocreate a pond.

If bunded, then the design ofthe bund needs some care. Itshould have a spillway, andusually a foundation.

Primary objective is torecharge groundwater byholding it during themonsoons and after it.

Also serves as farm-ponds toprotect kharif crops.

Periodic de-silting importantfor purpose.

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Percolation Tank and Check-Dams

source: http://test1.icrisat.org/

satrends/ jan2006.htm

Note the spillways and pitching.

Most dry up in 3 monthspost-monsoon.

source: http://washim.nic.in/DOC/

Egs files/image007.jpg

A check-dam is designed differently.

The bund is deeper with a clay core.

The above from Washim isprobably a check-dam.

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The principle

During the monsoons,connect the water-table andthe pond.

Increase recharge during themonsoons.

Helps reduce crop stress inlean periods in monsoons.

Post-monsoon, a perchedsaturation region.

Increases soil moisture.

Silts are small size particlesand have low conductivity.Must be removed from tankbottoms to aid percolation.

Evaporation losses about5mm/day.

Evaporation losses about 5mm/day.

For long monsoons, tank mayrecharge several times its ownvolume.

Poor ambient conductivity =⇒ Wetlonger

WTmonsoon

regionsaturated

post−monsoon

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Vanrai Bandhara

Temporary, must be installed after every monsoon.

< 2m in height, and may be used on top of existing bunds.

Installed just after monsoons get over, but stream flows remain.

Mainly to achieve/increment some recharge and some storage.

source: http://washim.nic.in/DOC/ Egs files/image010.jpg

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Gabions

Gabions are loose rock structures toprevent soil erosion.

Located along/across gullies orstream banks.

They trap soil and reduce watervelocities.

They help maintain and controlstream flow.

Typically built using wire-meshes.

A cage is built which encloses rockssuitably arranged.

Manual construction.

Porous, does not hold water.

source:

http://lh4.ggpht.com/ KsQX iKm6hw/SMZgiE V4HI/AAAAAAAAACA/2JBdx7LQUO8/01062008(002).jpg

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Across streams: an overflowing gabion

source: http://www.bridgetrust.org/images/Gabion (1).jpg

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Rooftop Rainwater Harvesting

This is especially useful in urbanresidential situations. Aim:

To collect rainwater fordomestic use.

also to use it for recharginggroundwater.

Example from Bangalore.Small (250L?) since (i)frequent use, and (ii)frequent rains.

Systems can be big, but costRs. 3-4 per liter!.

At this price, highlyexpensive for general use (∼100 LPD).

source:

http://bangalore.citizenmatters.in/pics/0001/1645/env-rain-water-harvesting small.jpg

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A novel idea: The TIDE polyhouseDrip Irrigation, greenhouse and rainwater harvesting.

source: http://www.tide-india.org/products/06polyhouse.html

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Urban Rainwater collection

Compulsory in many cities.

Collected water either (i)drains into existing wells, or(ii) into a specially designedpercolation structure.

In Chennai, used primarily topush back saline water.

Percolation well: about 1mwide, 5-10m deep, unlined,and filled with boulders,stones and gravel.

About Rs. 10-20 /sq.ft,percolation tank mostexpensive.

Expect to recharge about80% of the rainfall

source: unknown!

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Traditional: Baulis

Historical examples of urbanresidential rainwater storage:Baulis.

Storage in the basement ofhomes/special buildings.

Intricate network of drainsleading to the basement, andalso tapping groundwater.

Popular all over India, esp.Delhi, Rajasthan and also inthe South.

Example here: Rajon kibauli, Mehrauli. InMaharashtra: Panhalgad.

source:

http://www.flickr.com/photos

/ saad/

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Rajon ki Bauli from Outside

source: http://www.flickr.com/photos / saad/

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Masonry Structures

source: http://www.gomukh.org/images/index 02.jpg

GroundBreakerSilt

Water

Wall Boulder and concrete wall with aconcrete breaker.

Foundation and Key-wall to preventleakage around the wall.

Overflow structure, used as storageand silt trap.

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Dams and Weirs-The Kolhapur Type Bandhara

source: http://www.maharashtra.gov.in/english/ gazetteer/

Nanded/images/kholhapur-dam.jpg

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The principle

source: http://ahmednagar.nic.in/html docs/

images Ralegan.png

Largely stone and masonrystructure within the riverbed.

Gates open during monsoonsand shut just after.

This creates a head which istypically used for agriculture.

The storage is largelyconfined to the river bed. Noland need be acquired.

Used by upstream people!

Usually appear as a sequence

Fairly cheap and useful. Verypopular in India.

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A Small Dam

90

100

100

100

90

100

90

90

bund

maindam

stream stream

spillway

The FSL (full storage level) of thedam is the height at which water isstored, in this case, 100.

The dam and the bund are higher.The bund was needed to achieve anFSL of 100.

The storage is the modified contourat 100.

The spillway is at 100 and cuts intothe old contour at 100. Excesswater overflows from here.

The Key-wall protects the damfrom the spillway.

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Cross-Section

� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �

� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �

� � �� � �� � �� � �� � �� � �� � �� � �� � �� � �� � �� � �� � �� � �

� � �� � �� � �� � �� � �� � �� � �� � �� � �� � �� � �� � �� � �� � �

Core

Drains

Storage Water

Water Table

Iso−heads

Hard RockCOT

Casing

Core : a wall of clay/lowconductivity soil.

COT : Cut-off trench as aninsert into hard-rock.

Drains : To keep the damdry and prevent seepageflows.

Casing : Muram like soil,supports the core.

Note the water-table and theiso-head lines.

Note the rapid drop in thewater table in the core.

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Section

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COT-Section

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Gudwanwadi Dam

85m long, 8m high, earthen.

Storage 2 acres, 20K cu.m.

Cost: 24 lakhs.

Construction time: 6 mo.

Note Spillway, and Key-wall.

Note Pitching (stones) onthe dam walls.

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Bushy Dam-Masonry

source: http://travel.sulekha.com/lovely- lovely-lonavala

maharashtra-travelogue-4508.htm

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Nira-Deoghar-Earthen

source: http://www.abilgroup.com/images/Nira-Deoghar.png

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The Earthen Bund at Khubi

source:

http://thdc.gov.in/Writereaddata/Projects/English/Images/PG 291.jpg

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Discussion

1 How would you go about selecting a particular structure/apparatus for watersupply.

2 One item not covered is: piped water supply schemes.

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