Sources of Water

The source should be chosen so that the scheme gives the best water at the least cost and with long-term reliability. The main criteria are:

1. Quantity: The yield of the source in the driest season must meet the demand at the end of the design period, preferably with a margin. Springs and streams should be gauged in the dry season.
2. Quality: The water should be wholesome or easily treated. Sources free from pollution risk, such as springs in forested catchments, are preferred. Ground water is usually free from turbidity and bacteria.
3. Elevation and location: A source at a higher level than the town allows a gravity scheme, avoiding pumping costs. The distance from the consumers should be short.
4. Cost: Capital cost (intake, pipe, treatment, pumping) and operation and maintenance cost should be low. Compare alternatives on a life-cycle basis.
5. Reliability and sustainability: The source should not dry up or get polluted in future and should not be threatened by landslides or floods. The catchment should be protected.
6. Ownership and water rights: Legal rights of existing users, irrigation and mills should be respected; no social conflict. Permission is needed under the Water Resources Act.
7. Accessibility and safety: Site should be reachable for construction and maintenance and have good, stable foundation.
8. Environmental effect: Minimum effect on downstream users, ecology and land.
9. Future expansion: The source should permit increase in supply later.

Where several sources exist, the order of preference is usually: protected spring or gravity source, ground water, then surface water that requires treatment and pumping.

A well is a hole or shaft sunk into the ground to reach water-bearing strata. Wells are classified as shallow or deep by the stratum they draw from.

Sketches

  SHALLOW WELL            DEEP (TUBE) WELL
  |  |  ground level      |  | ground level
  |  |                    |  |
  |~~|~ water table       |  | unconfined layer
  |__|                    |==|== impervious layer
  /////  impervious       |  |
                          |__| confined aquifer
                          (strainer at bottom)

Differences

Suitability

• Shallow well water is filtered only by a thin soil layer, so bacteriological pollution from latrines and surface drainage is likely. The quantity is limited and varies with the water table.
• Deep well water is naturally filtered through long distance of soil and protected by the impervious cover. Quantity is large and dependable, though it can be hard or have iron.

Preferred choice

For a public supply scheme a deep well (tube well) is preferred because of its larger and more reliable yield, better quality (less treatment) and long life. Shallow wells serve only individual or very small groups.

An infiltration gallery is a horizontal, porous conduit (tunnel or pipe) laid in a water-bearing stratum, usually along the bank or bed of a river or lake or in a water-bearing sand layer, to collect ground water by seepage. It gives naturally filtered water and is also called a horizontal well.

Construction

• A trench is dug below the water table, parallel to the river, about 3 to 6 m deep.
• Perforated pipes (or porous masonry conduit with open joints) of diameter about 0.3 to 0.9 m are laid with a slight slope (about 1 in 500 to 1 in 1000) toward a collecting (sump) well.
• The pipe is surrounded by graded gravel and coarse sand layers of increasing fineness, to prevent entry of fine particles.
• The trench is back-filled with the excavated soil and sealed with clay at the top, to prevent surface pollution.
• Manholes are provided at intervals for inspection and cleaning.
• Water flows to the sump well from where it is pumped to the treatment or supply system.

 River bank
 ~~~~~~~~~~~  river water level
     \  seepage
 =====\==== ground level
   [gravel+sand layers]
   o o o o o  perforated pipe --> sump well
   -------  impervious bed

Features

• Yield is about 1.5 to 5 litre/second per 10 m length depending on soil.
• Water is clear, cool and low in bacteria, needs little treatment.
• Suitable for rural and small town supplies; capacity may reduce with clogging, so periodic cleaning is required.

Sources of water are of two kinds: surface sources (rivers, streams, lakes, ponds, impounded reservoirs) and ground sources (springs, wells, infiltration galleries). Rain water is a minor source.

Surface sources

Surface water is generally soft and has more turbidity, bacteria and organic matter than ground water, so it requires sedimentation, filtration and disinfection.

Ground sources

• Springs: small to moderate yield, clear and bacteriologically good; the best choice in the hills.
• Wells and tube wells: steady yield, hard water, free from turbidity; may contain iron, manganese or arsenic.
• Infiltration galleries: naturally filtered river-side water.

Choice

Surface water suits large urban supplies where big quantity is required, whereas ground sources suit small towns and rural areas where quality is good and treatment is not easy.

Major sources of water

Surface sources are plentiful but need treatment; ground sources are of better quality but yield is limited.

Factors for selection of a source

1. Quantity: yield in the dry season must satisfy demand at the end of the design period.
2. Quality: wholesome, or easily treatable at low cost.
3. Distance and elevation: short pipeline and a higher level for gravity flow, avoiding pumping.
4. Cost: construction, operation and maintenance.
5. Reliability and protection: perennial, safe from pollution, floods and landslides.
6. Water rights and social acceptance: no conflict with existing users.
7. Environmental effect: minimum effect on downstream flow and ecology.

Convert the monthly flows to volumes ($V = Q\times86400\times\text{days}$) and apply the mass curve (analytical) method. Demand $D = 6202\ \text{ML/month} = 6.202\ \text{Mm}^3$ per month (1 ML = 1000 m$^3$).

Monthly inflow volumes

Total inflow $= 74.42$ Mm$^3$; total demand $= 12\times6.202 = 74.42$ Mm$^3$. The two are equal, so the reservoir must carry the whole yearly regulation.

Cumulative deficit (reservoir full at the start of the deficit period)

The inflow is below the demand from Feb to Jul; the cumulative deficit is largest at the end of Jul.

Answer: Capacity of the impounded reservoir = 20.40 million m$^3$ ($\approx 20{,}405$ ML).

The minimum storage is the maximum cumulative deficit of inflow against the constant draft (analytical mass curve method). Losses are neglected. Draft $D = 4.4$ million m$^3$/month; inflow total $= 55.3$ million m$^3$ and draft total $= 12\times4.4 = 52.8$ million m$^3$, so the yearly yield is sufficient.

The deficit period starts after the reservoir has been filled by the surplus of the wet season (August is the last month with a surplus to carry over; the reservoir is full at the end of Sep).

Inflow is less than the draft from Oct to Mar, so the cumulative deficit increases during this period and then the reservoir is replenished from April.

The largest cumulative deficit is 11.8 million m$^3$.

Answer: Minimum storage capacity = 11.8 million m$^3$.

Analytical (mass curve) method: the storage required equals the maximum cumulative deficit of the yield compared with the constant draft of $4.78$ million m$^3$/month.

Total yield $= 57.41$ million m$^3$ and total draft $= 12\times4.78 = 57.36$ million m$^3$, so the draft is within the yearly yield. The reservoir is assumed full at the start of the dry period (end of Aug), when the last surplus ends.

The largest cumulative deficit occurs at the end of Apr and equals 20.73 million m$^3$.

Answer: Storage capacity of the impounded reservoir = 20.73 million m$^3$.

A reservoir can only store water that the river actually brings in. The solution uses the mass curve (analytical) method, with volumes in million m$^3$ (1 ML = 1000 m$^3$, $V = Q\times86400\times$ days).

Demand $D = 9688\ \text{ML/month} = 9.688\ \text{Mm}^3$/month. The October flow in the given data is doubtful (shown as 5.2 m$^3$/s), and I have used it as given.

Total inflow in the year $= 68.93$ Mm$^3$, while the total demand is $12\times9.688 = 116.26$ Mm$^3$. The yearly inflow is much less than the demand, so the cumulative balance falls continuously and reaches -47.33 Mm$^3$ at the end of the year.

Answer: With the data as printed no impounded reservoir can meet this demand, since the annual inflow (68.93 Mm$^3$) is smaller than the annual demand (116.26 Mm$^3$). The average inflow that can be supplied is 5.744 Mm$^3$/month; if the demand is at or below this value, the same table method gives the storage as the largest cumulative deficit. The demand figure (or the table) appears to be misprinted.

In the hilly region of Nepal a spring (protected by a spring box) is the most preferred source, followed by a perennial stream or small river.

Reasons

1. Gravity flow: Springs occur at high elevation, so water reaches the village by gravity. No pumping or electricity is required, which cuts operating cost.
2. Good quality: Spring water is filtered by soil and rock, clear and almost free from bacteria. Little or no treatment is needed, so only chlorination may be required.
3. Low cost: The intake is simple (spring box), and the scheme is cheap to build and easy to run and maintain by the community.
4. Reliability: Many springs are perennial. After checking dry-season discharge, they give a dependable supply.
5. Local availability: Springs are abundant on hill slopes; rivers lie deep in valleys far below the settlements, making lifting expensive.
6. Simple technology: Locally available materials and skilled labour can be used.

Where springs are insufficient, a stream with a sedimentation tank is used. Wells and rivers are less suitable in hills because of depth and pumping needs.

Possible sources

In the hilly rural areas the sources are springs, streams (khola), rivers, ponds and dug wells, and rain water in some cases. Springs and streams are the main sources of gravity schemes.

Quality of water

• Spring: clear, low turbidity, cool, free of pathogens when protected. Slight hardness, sometimes iron. Needs only disinfection.
• Stream/river: turbid in monsoon and exposed to pollution from upstream settlements, animals, agriculture and landslides. Needs sedimentation, sometimes filtration and chlorination.
• Pond/dug well: polluted by surface drainage and animals; used only when nothing else exists.

River source

• Quantity: large, but varies greatly, with minimum flow in the dry season (March to May), and floods in monsoon. Must be gauged in the lean period.
• Quality: turbid in the rainy season, low in dissolved minerals, bacterially polluted. Treatment is necessary.

Spring source

• Quantity: small, often 0.1 to 2 litre/s, depends on rainfall and catchment; must be measured in the driest period (the yield falls in spring).
• Quality: good, constant, safe if the spring box is protected and the catchment is free of latrines and grazing.

Why ground water is chosen

• It is naturally filtered during passage through the soil, so it is clear and generally free from pathogens.
• Quality and temperature stay almost constant through the year.
• Yield is dependable even in the dry season, and wells can be built near the consumers, saving transmission cost.
• Little or no treatment is needed (often only chlorination), which makes it economical.
• It is less exposed to surface pollution and is easy to develop in phases.

Common ground water quality parameters

Regular testing, especially for arsenic in Terai tube wells, is necessary before use.

Possible actions for sustainable ground water use in the Kathmandu valley:

1. Control and regulate extraction: License all deep tube wells, meter abstraction, set limits and tariff, and stop illegal boring.
2. Artificial recharge: Build recharge pits, ponds and recharge wells; use roof-top rain water harvesting in every building.
3. Protect recharge zones: Keep the northern valley rim and open land (Gokarna, Sundarijal, Chapagaun areas) free from construction and paving.
4. Surface water supply: Bring more surface water by the Melamchi Water Supply Project and other schemes to reduce dependence on wells.
5. Reduce losses: Repair leakage in the distribution system (non-revenue water over 35 percent in Kathmandu) and prevent wastage.
6. Demand management: Water metering, tiered tariff, water-saving fixtures, awareness campaigns, and reuse of waste water for gardening and toilets.
7. Conjunctive use: Use surface and ground water together, extracting more ground water in monsoon and less in the dry season.
8. Monitoring and law: Observation wells to monitor water levels, and strong enforcement of the Groundwater Resources Act and by-laws.
9. Restore traditional sources: Revive stone spouts, ponds and wells that recharge the aquifer.

Formation of springs

A spring is a natural outflow of ground water at the ground surface. It forms when the water table meets the ground surface, or when ground water is forced out by an impervious layer. The main circumstances are:

1. Depression spring: the ground surface dips below the water table in a valley or hollow.
2. Contact (gravity) spring: an impervious layer below a pervious layer on a hill slope blocks the downward movement and water emerges at the contact.
3. Fault spring: a fault or fracture brings a confined aquifer in contact with the surface.
4. Artesian spring: confined water under pressure rises through a crack in the overlying impervious layer.
5. Fissure or solution spring: water comes out through joints and cavities in rock such as limestone.

 Contact spring           Artesian spring
  ___________              ground
 / pervious  \___        ____  ____
/____________    \__ o  ====impervious====
 impervious  -----> spring   confined aquifer

Infiltration gallery

An infiltration gallery is a horizontal perforated pipe or porous conduit laid below the water table, usually along a river bank or in a water-bearing sand layer, which collects water by seepage and delivers it to a sump well.

  river ~~~~~~~~~~~~~~~~
  ground ===========\===
       gravel+sand   \
   o  o  o  o  o  o  -> [Sump well]
   perforated pipe, 1 in 500 slope
   ------- impervious layer -------

• Trench 3 to 6 m deep, pipe diameter 0.3 to 0.9 m laid on a slight slope to the sump.
• Pipe surrounded by graded gravel and sand; the top is sealed with clay to stop surface pollution.
• Manholes are provided for cleaning.
• Water is naturally filtered, so it needs little treatment.

Infiltration wells are shallow, open, circular wells dug in the bed or bank of a river (or near a lake) in permeable sand and gravel, to collect water that has been filtered through the bank material. They are sunk in a group and joined to a common sump well by horizontal pipes or by a gallery, and from the sump the water is pumped.

Construction

• Wells of 3 to 4.5 m diameter are lined with brick or concrete rings with perforations (weep holes) in the lower part.
• The bottom is plugged with a layer of concrete, and the wells are sunk some metres below the water level.
• The top is covered above the highest flood level, with a manhole for access.
• Several wells are located in line, 20 to 30 m apart, and connected to a collecting well by siphon pipes.

 river ~~~~~~~~~~~~~
 =========================  bank
  [W1]   [W2]   [W3]
    \     |     /   siphon/pipe
     \    |    /
       [Sump well] ---> pump

Features

• Water is cleaner and clearer than the river water because of filtration in the bank sand.
• The yield is moderate and depends on the permeability of soil and on the river level.
• The supply is cheap to build; clogging reduces the yield with time, so periodic cleaning or development is needed.
• Used for small town and rural supplies, mostly in Terai and along rivers.

The storage of an impounded reservoir must supply the demand during the period when inflow is less than the demand. It is found by the analytical (mass curve) method: take demand $D = 4\times10^6\ \mathrm{m^3}$/month and find the largest cumulative deficit. All volumes are in million m$^3$.

Check: total inflow $= 55.2$ and total demand $= 12\times4 = 48$, so the river can meet the demand over the year.

The reservoir is assumed full at the end of September, when the deficit period starts.

The inflow is below the demand from October to March, and the cumulative deficit reaches its maximum at the end of March.

Answer: Storage capacity of the impounded reservoir = 9.5 million m$^3$ ($= 9.5\times10^6\ \mathrm{m^3}$).

Convert the river flow (lps) to monthly volumes using a 30-day month and compare with the demand. Demand $= 400\ \mathrm{m^3/day}\times30 = 12{,}000\ \mathrm{m^3}$ per month. Volume $= Q(\text{l/s})\times10^{-3}\times86400\times30$ m$^3$. The September flow (1.12 lps) is read as printed.

Total inflow $= 143{,}908$ m$^3$ and total demand $= 144{,}000$ m$^3$, so the river just meets the yearly demand.

The river flow is less than the demand from Sep to Feb. The reservoir is full at the beginning of this period, and the largest cumulative deficit is the capacity needed.

Answer: Capacity of the impounded reservoir $\approx 21{,}404\ \mathrm{m^3}$ (about 21,400 m$^3$).

Disputes on ownership and priority of use are common in Nepal, because many villages, irrigation systems and water mills depend on the same spring or stream. As a team member I would resolve them through an open, participatory and legal process.

1. Identify all stakeholders: Meet all users (existing drinking water users, farmers, mill owners, upstream and downstream villages, women, Dalit and marginalised groups, local government, and ward committees).
2. Measure the source jointly: Do the dry-season discharge measurement with the users, so that everyone sees how much water exists and how much can be taken without harming others.
3. Document the present uses: List traditional rights and uses (irrigation turns, mills, ritual use) and record them in an agreement.
4. Explain the national priority: Under the Water Resources Act 2049 and the Water Resources Regulation, drinking water and domestic use have the first priority, followed by irrigation, agriculture, hydropower and other uses.
5. Seek win-win options: Leave an agreed share for downstream users (for example the spring overflow goes to irrigation), build storage tank to use night flow, look for another source, or reduce loss.
6. Community meetings and mediation: Hold meetings, involve the Water Users' Committee, ward chair, local leaders and, if needed, the District Water Resources Committee for mediation.
7. Written agreement: Sign a written agreement with consent of the users; register the Water Users' Association and the water right with the local government.
8. Social awareness: Show the health and time-saving benefits, assure equal tap distribution, and make users part of the operation and maintenance.
9. Include the matter in the feasibility report with a source-sharing plan and monitoring arrangement.