OVERHEAD RCC WATER TANK  

SOME OF PROJECTS IMPLEMENTED

02INTAKE WELL

03ANICUT / SMALL DAMS   CONSTRUCTION

04FIRE FIGHTING WATER TANK

05OVERHEAD TANK DESIGN 

06RCC ELEVATED TWIN SERVICE RESERVOIR

07FOUNDATION CONSTRUCTION OF   FACTORY  

08REPAIR AND WATER PROOFING

09ERECTION OF COMMUNICATION TOWER

10WATER FILTER PLANTS

11GENERAL CIVIL CONSTRUCTION WORK

12ELECTRIC GRID AND POWER SUB STATIONS

DESIGN AND CONSTRUCTION INTAKE WELL OR WELL SCREEN

• A reliable and sustainable water supply consistent with your needs and the capability of the aquifer • Good quality water that is free of sediment and contaminants • Increased life expectancy of the well • Reduced operating and maintenance costs • Ease of monitoring well performance. Although you need to hire a drilling contractor to design, drill and construct the well and choose the appropriate materials, it is important for you to know what is going on. You can then work with the drilling contractor to ensure you get the well design you need. 

AS A Drilling Contractor

We can be a drilling contractor with an experience in this area. We complete a survey of existing wells in the existing area before drilling work starts. It Provides Usefull Information

• Typical yields and water quality • Which aquifer to tap into • Trends in well construction methods • Prior drilling success rates. Surveys of existing wells are available for a nominal fee from the Groundwater Information Centre.

Choosing a Well Site

Choice of well site will affect the safety and performance of your well. We can examine various sites.WE take care while we choose a site for intake well design that no contaminants enter the well either through the top or around the outside of the casing. Sewage or other contaminants that may percolate down through the upper layers of the ground surface to the aquifer.

Well Design Considerations

Well design and construction details are determined after a test hole has been completed and the geological zones have been logged. There are many components to well design the driller must take into account. Decisions will be made about: • Well depth • Type of well • Casing material, size and wall thickness • Intake design • Formation seal • Monitoring and preventive maintenance provisions.

Well depthDuring the test hole drilling, the drilling contractor will complete a formation log. Soil and rock samples are taken at various depths and the type of geologic material is recorded. This allows the driller to identify aquifers with the best potential for water supply. Some drillers also run an electric or gamma-ray log in the test hole to further define the geology. This gives them more accurate information about aquifer location.

Generally a well is completed to the bottom of the aquifer. This allows more of the aquifer to be utilized and ensures the highest possible production from the well.

Casing size and typeDecisions about the diameter and type of well casing are made after the driller considers the following:

• Aquifer characteristics • Hydraulic factors that influence well performance • Drilling method • Well depth • Cost (in discussion with the well owner).

The casing must be large enough to house the pump and allow sufficient clearance for installation and efficient operation.

It is recommended that the casing be at least one nominal size larger than the outside diameter of the pump. The more space there is between the pump and the casing, the easier it will be to service and repair the pump in the future.

There are two common materials used for casing: steel and plastic. Steel casing is the strongest but is susceptible to corrosion. Plastic casing is becoming more popular because of its resistance to corrosion. Intake designWater moves from the aquifer into the well through either a manufactured screen or mechanically slotted or perforated casing.

Screens are manufactured with regularly shaped and sized openings. They are engineered to allow the maximum amount of water in with minimal entry of formation sediments. Stainless steel screens are the most widely used because they are strong and relatively able to withstand corrosive water. Screens are manufactured with various slot sizes and shapes to match the characteristics of the aquifer.

Slotted or perforated casing or liner is made by creating openings using a cutting tool or drill. Pre-slotted plastic pipe is also available.

Slot openings and perforations are spaced further apart than screen openings. This reduces the amount of open area to allow water into the well. The openings tend to vary in size and may have rough edges depending on how they were made. This impedes the flow of water into the well and may not hold back the formation sediments.

The drilling contractor examines the cuttings from the borehole and makes a judgement whether to use a screen, or slotted or perforated casing/liner. While a screen is the more expensive alternative, it is necessary if the aquifer is composed of loose material such as fine sand, gravel or soft sandstone. A slotted or perforated casing/liner can be used when the aquifer formation is more consolidated, such as hard sandstone or fractured shale.

After a choice is made between a screen, or slotted or perforated casing/liner other decisions is  made regarding: • Size of slot opening • Total area of screen or perforation that is exposed to the aquifer • Placement of the screen or perforations within the aquifer.

Slot size openings The slot openings must be small enough to permit easy entry of water into the well while keeping out sediment. The slot size chosen will depend on the particle size of the earth materials in the producing aquifer.

As a drilling contractor we select a slot size that allows 60 percent of the aquifer material to pass through during the well development phase of drilling. The remaining 40 percent, comprising the coarsest materials, will form a natural filter pack around the perforations or screen.

Total open area of screen The total area of the slot openings is dependent on the length and diameter of the screen. While the length of the screen is variable, the diameter of the screen is determined by the diameter of the well casing. The yield from a well increases with an increase in screen diameter but not proportionately so. Doubling the screen diameter raises the well capacity only 20 percent.

The amount of open area of the screen or slotted or perforated casing/liner is calculated to ensure the water from the aquifer does not enter the well too quickly. A larger amount of open area allows the water to enter the well at a slower rate, causing a lower drop in pressure in the water as it moves into the well. If the water flows too quickly, there will be problems with incrustation.

Placement in the aquifer The screen or perforations on the casing/liner is placed adjacent to the aquifer. If improperly placed, the well may produce fine sediment which will plug plumbing fixtures and cause excessive wear on the pump. To drill we use geophysical logging equipment to accurately identify the boundaries of the aquifer, the exact placement will be easier.

Sealing The WellSealing the well protects the well’s producing zone from contamination. The diameter of the bore hole is usually slightly larger than the casing being installed. The space between the bore hole and the casing is called the annulus of the well. It must be sealed to prevent any surface contamination from migrating downward and contaminating the water supply. It also prevents any mixing of poor quality aquifers with the producing aquifer of the well (see Figure 3, Annulus Seal).

Well developmentWell development is the process of removing fine sediment and drilling fluid from the area immediately surrounding the perforations. This increases the well’s ability to produce water and maximize production from the aquifer.

Jetting, surging, backwashing and over pumping are methods used to develop a well. Water or air is surged back and forth through the perforations. Any fine materials that are in the formation become dislodged and are pumped or bailed from the well. This procedure is continued until no fine particles remain and the water is clear. Coarser particles are left behind to form a natural filter pack around the screen, slot openings or perforations.

If the aquifer formation does not naturally have any relatively coarse particles to form a filter, it may be necessary to install an artificial filter pack. The pack is placed around the screen or perforations so the well can be developed. For example, this procedure is necessary when the aquifer is composed of fine sand and the individual grains are uniform in size.

It is important to match the grain size of the filter material with the size of the slot openings of the screen to attain maximum yield from the well. Typically the slot size of the screen is selected so that 85 percent of the artificial pack material remains outside of the screen.

Yield testA yield test, often called a pump test, is important because the information gathered during the test assists the drilling contractor to determine the: • Rate at which to pump the well • Depth at which to place the pump. Provincial regulations outline the minimum yield test for all new wells. After drilling and developing a well, the drilling contractor must remove water from the well for at least 2 hours. If a pump is used to remove the water, then water level measurements can be recorded as the water level draws down. After 2 hours, water removal stops and the recovery of the water level is monitored and recorded. Measurements must be taken at specific time intervals for a 2 hour period or until the water level returns to 90 percent of its original level.

Once the yield test is complete, the drilling contractor will decide at what rate the aquifer can be pumped without lowering the water level below the top boundary of the aquifer, the top of the perforations or below the pump intake. The pump that is installed in the well should have a capacity equal to, or less than, the rate at which the well can supply water for an extended period of time without lowering the level below the pump intake. That rate is considered the safe pumping rate for the well.

Disinfecting the well

Provincial regulations require the drilling contractor to disinfect new wells with chlorine. The concentration is calculated on the volume of water that is in the well. The concentration must be at least 200 milligrams of chlorine per litre of water present throughout the water in the well and must be left in the well for at least 12 hours to ensure any bacteria present are destroyed. Chlorination is done after the pumping equipment is installed and before the well is put into production. The yield test provides a benchmark of your well's performance. Repeating this test at a later date can be used to assess any changing conditions of the well and determine when maintenance is required.

WATER INTAKES - SITING AND DESIGN APPROACHES

 

ADNAN M. ALSAFFAR and YIFAN ZHENG

 

Bechtel Corporation 9801 Washingtonian Blvd.

Gaithersburg, MD 20878, , Maryland, USA

Tel: 301-417-3175 , Fax: 301-963-2878 , E-Mail: aalsaffa@bechtel.com

 

 

ABSTRACT

The function of a water supply intake is to extract and deliver water to the users. Therefore the design of water intakes require a series of hydraulic design consideration in order to arrive at a desirable concept that can obtain and deliver the water economically with an acceptably low impact on the environment. Due to variability of site conditions, the environmental hydraulic engineer is faced with several challenges when assessing water supply availability. The major factors that can affect the selection of a concept and design development for a water intake are: water availability, bathymetry, sediment transport, environmental regulations, climatic conditions, constructability, initial and maintenance dredging requirements, and operation and maintenance. The paper examines these factors and discusses their importance in selecting a suitable concept.

To demonstrate the approach in evaluating the various variables, case studies at four sites with differing conditions are presented. The rationale for selecting each concept is presented along with illustrations. Site conditions considered in the cases are; Intake on rivers with high water level fluctuation, intakes on tidal rivers, intakes in mountainous streams and offshore velocity cap intakes.

The paper stresses the importance of appropriate hydraulic design to provide acceptable flow conditions at the pumps.

 

Keywords: Water Intake, Site Hydrologic Conditions, Hydraulic Analysis, Bathymetry, Sediment, Constructability, Environmental Regulation, Tide, River, Offshore

 

INTRODUCTION

The function of a water supply intake is to extract and deliver water to the users. Therefore, the design of water supply intakes requires a series of design considerations in order to arrive at a desirable concept that can obtain and deliver the water economically with an acceptably low impact on the environment. Due to variability of site conditions, the environmental hydraulic engineer is faced with the challenges when assessing water supply availability. The major factors that can affect the selection and design of an intake are site hydrologic conditions, site access, ease of construction, and operation and maintenance. Without a careful and responsible evaluation of various design factors, an intake may be designed and constructed but may not be operable due to lack of adequate water supply or may be adversely impacted due to degraded environment.

This paper examines the major factors that can affect the design and presents examples of design concepts encountered at various sites.

 

DESIGN CONSIDERATIONS

The following factors are considered of primary importance in siting and designing an intake:

 

        Water Availability

        Bathymetry

        Sediment Transport

        Environmental Regulation

        Climatic Conditions

        Constructability

        Initial and Maintenance Dredging

        Operation and Maintenance

 

By far the most important of these factors is availability of water to meet the required demand without creating an environmentally and physically adverse effect on the water body. This is particularly important for fresh water supply. Therefore, detailed hydrologic studies including analysis of historic data must be performed. In areas where no historic data on stream flow are available, rainfall data should be analyzed to determine rainfall frequency. Hydrologic modeling can be used to estimate the runoff.

Locating and selecting the specific type of intake requires adequate knowledge of the bathymetric condition of the river, estuary or sea bottom in the vicinity of the intake. Without this information, no specific intake concept can be selected. Making assumptions could lead to erroneous cost and schedule estimates for the project.

The type of sediment can be either bed load or suspended load in a river, and littoral drift in a coastal environment. The existence of sediment affects the design concept and the suitability of the site for locating an intake.

Other important factors to consider are any water withdrawal limitations as well as the feasibility of dredging and disposal of dredge spoil. In some situations, water may be physically available, however, because of water rights, water required for aquatic habitats or waste assimilation may not be legally available. In addition, dredging and disposal in areas where there are endangered species or contaminated soil, could be harmful to the environment. These factors and others could affect the selection of a desired intake site and may affect the feasibility of a project.

Climatic conditions such as severe winter weather can affect the concept and details of the pump intake structure. A region with below freezing air temperature requires protection for traveling screens and trash racks against the formation of anchor and/or frazil ice. Such protection will affect the design concept and should be considered in the planning phase. For power plant intakes warm water recirculation into the intake is commonly used. However, if the intake is remote from the power plant or if the intake is for water supply, electrical heating elements will be required which will increase the power demand. Alternatively the design could be made to encapsulate the intake and prevent air circulation. However, this concept can not eliminate the need for protection against frazil ice.

Construction, maintenance and access are also important factors to be considered in selecting the intake location. Availability of access road, potential for local and riverine flooding and access to the intake equipment all year round should be considered.

 

DESIGN CONCEPTS OF INTAKE STRUCTURES

 

i.                    GENERAL:

Experience in the design and operation of various water supply intakes indicates that no single design concept is suitable for all locations. Therefore, any intake design must be based on site specific information. This may not be possible at the planning phase of the project due to the absence of specific site data. Therefore, the hydraulic engineer must develop design parameters from the limited data that may be available, and develop programs for the field data collection and analysis for use in detailed design.

Lack of site specific information generally occurs in remote areas of the world where no historic data, studies or maps are available to help in the planning and design. The most practical approach for work under these conditions is to make a site visit and obtain aerial photographs. An important aspect of this effort is the identification of river banks and shoreline conditions and the presence of erosion and deposition. Aerial photos can best be utilized in assessing the presence of shoreline changes and of river meanders.

 

ii. CASE STUDIES:

The following section addresses the approach utilized in selecting types of intakes at various locations and with differing hydrologic conditions.

 

INTAKES ON RIVERS WITH HIGH WATER LEVEL FLUCTUATION:

This type of river can be found in regions where rainfall and runoff occur in a short duration during the year such as the monsoon season. Designing a conventional intake in this type of environment may not be technically or economically feasible. To overcome this condition, an intake structure was designed as a super structure with an access pier connecting the intake to the shoreline as shown on Figure -1. This structure was also used in a lake with large water level variation and can be used in a coastal area where an offshore intake with a buried pipe is not practical.

 

Figure -1 : Hydraulik design of a river wate intake with high water level fluctuations

 

Fish protection is accomplished by installing wedge wire screens with air back wash systems. The design of the intake caisson and the supporting piles for the pier must be based on geologic and geotechnical considerations.

 

INTAKES ON TIDAL RIVER:

Locating an intake in a tidal river requires extensive evaluation of the method of installation and dredging. In a river with a wide tidal flat, dredging and disposal of sediment could be the most controlling factor since it could create disturbance to tidal habitats, increase river turbidity and cause contamination of the river if the soil is contaminated. In certain rivers, the tidal flat can be very wide and dredging can be very costly if not impossible to achieve. This situation was encountered in a tidal river in the United States. The tidal flat extend approximately 500 m from the shore line and the range of the normal tidal water level fluctuation is 1.75 m. Because of the environmental concerns about dredging and disturbance of

aquatic and birds habitats an innovative design technique was required. Extensive visits to the site and meetings with regulators were made. Several alternatives were evaluated and the concept that was finally selected included a pier supported jetty and locating the pump intake at its end. The pump motor, control and the pier deck were set above the 100-year flood level. To protect the pumps from floating debris and to provide mechanism for screening the water, a caisson was installed to house the pumps. Wedge wire screens were used with air back wash. This intake concept is shown on Figure -2.

 

Figure -2 : Hydraulik design of an intakeon a tidal river

 

INTAKES IN MOUNTAINOUS STREAMS:

Intakes on mountainous streams require special designs to exclude or to separate the heavy sediment load that can be carried by the flow which occurs as a flash flood. Sand can form bars during the flood and cause extensive deposits which can block the flow path. During the low flow season these streams carry generally low flow which can affect water availability at the intake. Therefore an intake must be designed to abstract water under all conditions without excessive sediment load. In most cases, particularly when water is pumped, sand exclusion must be made before reaching the pumps.

These requirements were applied in the design of several intakes in Andes Mountain. The approach consisted of estimating the low flow and 100 -year flood flows and water levels. Sediment samples from the river beds were collected and analyzed for gradation. Based on these information and considering space availability at the various sites, some intakes were designed with settling basins, some with sediment exclusion and by-passing. The intake presented in this paper is located in a very narrow river channel and the river has a very low flow during the drought season. Therefore, the design is based on abstracting all the river low flow and by-passing the extra flow with the sediment. The intake

consists of a diversion dam across the stream with inlet grating. Sand is by passed through a sluicing pipe to the stream. The de-sanded water flows over a weir, through a pipe before reaching the pump forebay. This concept is shown on Figure -3.

 

Figure -3 : Hydraulik design of a water intake on a mountainour streams

 

OFFSHORE INTAKES:

Offshore intake is a submerged structure for withdrawal of water by gravity from the sea, lakes and in some situations from rivers to a shoreline pump intake. This type of intake consists of a velocity cap connected to an offshore buried pipe that conveys the flow to the on shore pump intake structure. This type of intake is used in regions with shallow water depths, with considerable littoral drift, drift ice and where fish protection is of concerns. The velocity cap creates a horizontal

flow path which was developed through experimental work to preclude fish entrapment( Reference-1). The general velocity of approach is in the range of 0.30 m/s to 0.45 m/s. In thermal power plants it has the advantage of creating a selective withdrawal and therefore minimizes warm water recirculation from the discharge into the intake. An important factor that must be considered in the design is wave induced forces on the structure. Various wave theories must be examined to determine the applicable condition for determining the forces. Buried offshore pipes must be protected against movement by waves and currents action. This protection is in the form of granular backfill covered with riprap. Riprap sizing is based on consideration for the wave induced currents in addition to the ambient current following several analytical procedures such as PIANC (Reference-2) and others.

This concept was used to supply cooling water to a thermal power plant in the Mediterranean Sea, as shown on Figure - 4. Three conditions were encountered, namely : littoral drift, flat bathymetry, and severe wave conditions. From the hydrographic and bathymetric survey it was concluded that a shoreline intake with a dredged channel will not be technically feasible. Waves are very active and range in a height of 2 to 3 m. An offshore intake with buried pipe were designed. The design considered all wave and current loading as well as other applicable loads.

 

Figure -4 : Hydraulik design of a offshore intake

 

HYDRAULIC DESIGN OF PUMP PIT

In all the cases discussed in this presentation as well as any other intake, the

selected design concept must not affect the performance of the pumps. Uniform approach flow conditions, adequate pump submergence, and flow free from surface and sub-surface vortices must all be considered in selecting the pit geometry. The design criteria and dimension of an intake for a given flow can be found in various publications such as IAHR( Reference-3), BHRA (Reference-4) and others.

 

CONCLUSIONS

The above presentation leads to the following conclusions:

 

1. Locating and designing a water supply intake requires careful consideration of hydrologic , environmental, geotechnical and economic factors.

2. Several types of intakes should be considered to meet various site conditions and operational requirements.

3. Long term hydrologic data should be collected and analyzed to arrive at the most suitable and reliable concept.

4. Hydraulic analysis must be performed as an integral part of the intake design to provide flow free from objectionable conditions at the pumps.

 

REFERENCES

1.      Design of Water Intake Structures for Fish Protection, American Society of Civil Engineers, New York 1982.

2.      Guidelines for the Design and Construction of Flexible Revetments Incorporating Geotextiles in Marine Environment, PIANC, Bulletin 7879, 1991.

3.      Swirling Flow Problems at Intakes, Jost Knauss, Coordinating editor, IAHR, 1987.

4.      The Hydraulic Design of Pump Sumps and Intakes, BHRA, July 1977.

Ranney well, also called jack well or radial collector well, is named after its inventor Leo Ranney. He was born in USA in 1884 and graduated in Geology from Northwestern University near Chicago in 1912. He introduced this new-well technology initially for oil wells and later extended it to water wells. He installed the first such well for water production in London in 1934 when the city was facing severe water shortage and then in a number of cities in Spain to France to Germany to Czechoslovakia. He then built the first such well in USA in 1936. Today, these wells are in use all over the world including India, with about 250 located within USA. The individual well yields range from 400 kl to 150 thousand kl per day http://www.ranneymethod.com/index.html

Ranney well produces best results in large alluvial aquifers in and adjacent to rivers or in soft rocks such as cavernous limestone (karst) formations. Construction of a Ranney well involves sinking of a central reinforced concrete caisson by the open-end caisson sinking method until the lower section of the caisson reaches a specified depth in the aquifer. A bottom sealing plug is poured to make the caisson water-tight. One or more lateral perforated pipes (screens) are driven into the aquifer with a hydraulic jack kept inside the well in a lateral or radial pattern at one or more elevations to collect water into the well. The caisson is then extended above known or anticipated flood elevations and completed with pumping equipment, controls and conveyance system. Such a well is called ‘radial collector well’ because the cassion acts as a collector of groundwater from the radial screens; and called ‘jack well’ as a hydraulic jack is used to drive the laterals.

In the method used by Ranney, perforated pipes were simply directly driven into the aquifer. A Swiss engineer modified this method in 1940’s by driving first a solid projection pipe into which a well screen (usually of stainless-steel wire-wound design) was installed and later the projection pipe was withdrawn. In the 1950’s, some German engineers refined the screen installation process further to allow an artificial gravel-pack filter to be placed around the well screen. The number of laterals of a Ranney well can be as high as 20 with an aggregate length of several hundreds of metres. Several refinements were introduced by Prof. J.A. Taraporevala, which include use of push-pull method of driving laterals into the aquifer in an efficient and cost-effective manner and use of automatic flap valves instead of ordinary sluice valves to regulate entry of water into the cassion from the screens. These advances allowed construction of these wells with high yields in a large range of geological settings. The life of a Ranney well is around 50 years. As Ranney well draws water from a very large area of the aquifer at depth, the US EPA considers it to be the most environmentally sound intake system having no direct impact on the aquifer.

A lecture on “Design of radial well” delivered by Prof. Vinod S. Patel, Consulting Engineer on 15th November 2000 at Vadodara in Gujarat for Indian Water Works Association and Institute of Engineers was published in the Journal of Indian Water Works Association, v. 35, no. 4, October-December 2003. A reprint of the article could be requested from him at vpatel_usa@hotmail.com. Besides giving design details of two Ranney wells constructed in Mahi and Tapi rivers, the article highlights the superiority of Ranney wells over other high-yielding intake structures such as infiltration galleries,

infiltration wells and heavy-duty deep tube wells from the point of view of cost, water quality and life of the intake structures. He also gave design details of two Ranney wells constructed in Mahi and Tapi rivers to give a discharge of around 45,000 cubic metres a day.

Contrary to the functioning of Ranney wells in Gujarat as described above, their functioning in the National Capital Territory (NCT) of Delhi by the Delhi Jal Board (DJB) is not that satisfactory. The contribution of water supply from Ranney wells is just 101 thousand cubic metres a day, accounting for 35% of total groundwater supplied and 2.5% of total surface & ground water supplied. Contrary to the general belief that Ranney well water should be of good chemical and bacteriological quality, some of those in NCT Delhi are of poor quality with high ammonia, iron and faecal pollution http://www.adb.org/Documents/Events/2004/WAP/IND/Delhi/Tripathi_presentation.pdf

Dr D.K. Chadha, the then Chairman of the Central Ground Water Authority, expressed in 1998 that deep tube wells rather than Ranney wells are more cost-effective to develop groundwater in NCT Delhi http://www.expressindia.com/ie/daily/19980718/19951464.html

Our work in Sri Venkateswara University has indicated that sanitary infiltration wells could be best constructed in the most cost-effective manner to discharge very large quantities of good quality groundwater free of turbidity and bacteriological impuries. The design of a conventional infiltration well is such that a sand cushion is kept underneath the well kerb for entry of groundwater into the well during pumping. When there is excessive groundwater pumping, there will be excessive welling of sand into the well leading to its collapse. Contrary to this, we have designed a sanitary infiltration well in such a way that its kerb rests at the junction between sand and the underlying rock. The well walls are embedded with permeable well blocks of high hydraulic conductivity for lateral flow of groundwater into the well. The junction between the well walls and the aquifer is filled with sieved coarse sand shrouding. Detailed geological, geophysical and test boring studies have been carried out to pinpoint the well site at a place where sand of high hydraulic conductivity extends to maximum depth. A picture of one such well designed by us in the ephemeral Swarnamukhi river bed near Tirupati can be viewed athttp://www.indiawaterportal.org/blog/wp-content/uploads/2008/01/infiltration-well.jpg The discharge from this well was found to be 10,000 cubic metres a day. As the cost of this well is at least 20 times less than that of a Ranney well, construction of many such wells could be taken up in preference to a single Ranney well.

Sanitary infiltration wells constructed in the Tungabhadra River near Kurnool in Andhra Pradesh on the other hand gave discharges of more than 40,000 cubic metres a day.

7. MAIN WATER INTAKE STRUCTURES

7.0 Introduction

1. Water intake structures depend on the type of pond you have. You learned earlier that a fish pond can be supplied with water from different sources (see Chapter 1). Several types of pond were defined by their intake structures:

sunken pond: no intake required; barrage pond without diversion canal: no intake required; barrage pond with diversion canal: main water intake combined with a diversion

structure in the diversion canal; diversion pond: main water intake with or without a separate diversion structure

downstream to raise the water level in the stream.

Note: if the water supply is provided from a reservoir, the intake structures are usually part of the system that releases the impounded water into the pond feeder canal. They may consist of:

a siphon placed over the dam; the reservoir bottom valve at the downstream side of the dam; an outlet sluice at the upstream side of the dam such as a monk.

 

      

Selecting the water intake structures

2. The main elements of a water intake are:

a diversion structure, to control the water level in the stream and to ensure it is sufficient to supply the intake but not to flood it (see Sections 7.3 to 7.5);

inlet level (and flow) control in the intake structure itself, to control water supply to the ponds (see Section 7.6). It is usually connected to the water transport structure;

entrance protection , such as coarse bars or piling or a range of screens to protect the intake from debris and scour damage (see Section 7.7).

3. There are many designs for water intake structures, some of which can be quite complex and require specialized design and construction. This manual concentrates on relatively simple designs that you can build by yourself or with the assistance of a good mason.

Main water intakes

4. Main water intakes are used for the overall regulation and diversion of water supplies to a pond or group of ponds. In many cases, they are distinct from water transport structures, which are discussed in Chapter 8, and from smaller pond inlet structures, discussed in Chapter 9, which supply and control water flow into individual ponds.

5. The main purpose of an intake is to ensure a constant water supply that can be adjusted to suit local conditions.

  Section AA

Locating the main water intake along a stream

6. The pond site and its water feeder canal usually determine the location of the main water intake. If the pond is to be built along a stream, it is better to select a site that has:

valley sides that are not too steep; a relatively level, stable and smoothly flowing section of the stream, reasonably

free of debris and moving silt; no excessive forest over and around the feeder canal; a straight stretch of the stream.

Never place a water intake on the inside of the curve

of a stream where silt, sand or gravel can build up

  Choose a place on the outside of the curve where

the water flows faster and where silt, sand or gravel

is less likely to build up

Note: avoid large rivers with a fluctuating water level. Be very careful to make sure the intake is not set above the minimum water level of the river.

7.1 How to define the level of the water intake

1. There are two main types of intake:

an open or free-level intake, in which the water supply levels are uncontrolled, and the intake operates in all water flow conditions. This system is simple and relatively cheap, but usually requires a reliable water supply that does not fluctuate excessively;

 

     a controlled level intake,

in which a diversion structure is set up downstream in the water course for the purpose of maintaining water levels throughout a range of flow conditions. This system is more expensive but more reliable and provides a constant supply.

 

2. In both cases, the important points to consider are:

the levels of the water source (river, stream, etc.) in relation to the water supply structure and the ponds themselves (see Section 1.8);

the depth from which you want to take the water (surface, lower levels or the complete depth of the water supply source).

3. In the case of an open intake system, you must make sure that the level of the water in the supply source is sufficient at all times to allow you to take the water to the depth you need. You must also make sure that there is no risk of flooding the intake. As will be shown later, you can use an intake gate to control the incoming water supply.

4. In the case of a controlled level system, you can define the water level by setting the level of the diversion structure. The following points are very important.

(a) Check the longitudinal and cross-section profiles of the valley upstream of the structure to calculate the size of the flooded area that would be created behind the proposed structure (see Chapter 8, Topography).

(b) Aim to set the diversion structure at the approximately minimum water level required for water flowing in the supply channel.

(c) Make sure that flood water can be removed, either over a weir or through a side channel (see Chapter 11). If the structure is made of soft, easily erodible materials (earth or clay), it is better to use a side channel.

Note: if the control structure has to be set lower to reduce the size of the upstream pool, you may have to widen the supply channel to obtain the required flow (see Section 8.2).

Make sure that flood levels do not overtop the control structure

  If the control structure has to be set lower to

reduce the size of the upstream pool you may have to widen

the supply canal to obtain the required flow

5. The methods needed to determine the relative levels are described in Topography for freshwater fish culture, FAO Training Series. Where possible, make use of local information. Ideally you should set up flow gauges and water-level stations. (See for example, Section 3.6 in Water 4).

7.2 The size of the water intake

1. The wider the water intake area, the less will be the head loss* as the water flows to the ponds. This factor may become important when there is very little head available.

2. In most cases, however, the water intake is about the same width as the supply canal connected to it. The size of the supply canal is chosen according to the flow required (see Section 8.2). If the supply canal is particularly wide, or if you want to increase the head

loss at the water intake (for example, when the external water level is much greater than the level required within the supply canal), the intake can be made narrower than the supply canal. Generally, a narrow intake is easier to control, as the sluice boards or gate controls are easier to move.

3. As an approximate guide, Graph 6 gives typical flow rates through intake structures at different head loss. This head loss should be added to the supply canal head loss (Section 8.2) to define the relative levels of the intake and the ponds.

Example

If 0.20 m is available between the minimum intake water level and the pond supply, a flow of 0.25 m3/s is required. It is calculated that head loss in the supply canal due to its bottom slope (see Section 8.2, paragraph 8) is 0.15 m. Possible head loss through intake is therefore limited to 0.20 - 0.15 m = 0.05 m or 5 cm. To ensure the required flow rate, the intake width would have to be at least 0.40 m or

40 cm (Graph 6).

  GRAPH 6 Water flow through sluices

4. The intake control structures are described later (see Sections 7.6 and 7.7). First, you will learn about the diversion structures that are used for intakes (Table 31).

TABLE 31 Diversion structures to control stream water levels.

7.3 Simple diversion structures

1, Simple diversion structures can be constructed from a range of materials. These materials are suitable for holding back water, but should not be used where water regularly overflows.

Earthen barrage dam

2. You can totally block the channel of a small stream with an earthen dam. Proceed in the following way:

(a) Design the dam to be built as if it were for a barrage pond (see Section 6.1).

(b) Divert the stream around the construction site. It is easiest to do this when the stream flow is low, for example, toward the end of the dry season.

(c) Stake out the dam base, set out the earthwork and build the dam across the stream channel (see Section 6.6).

(d) Construct the intake structure, the water feeder canal and its overflow away from the ponds.

(e) Gradually remove the temporary diversion, letting the stream establish itself in its original channel and fill the feeder canal with water.

Note: if necessary, protect the wet side of the new dam with rocks or stones.

 

Bamboo or wooden pole barrier

3. You can also block the channel of a small stream using a double row of wooden or bamboo poles lashed together with flexible lianas or vines, and packed with clay soil between the poles to prevent water seepage.

4. Remember that:

the double row of poles should be placed side by side and driven vertically into the ground;

the barrier should extend well into each of the stream banks; and the barrier will be stronger if you build it curving against the flow of the stream.

Double row of bamboo   Bamboo or wooden pole barrier

Wooden plank barrier

5. There are other ways you can build a barrier using planks and wooden poles. This kind of barrier can easily be removed in the rainy season when the water level begins to rise in the stream channel.

6. Two kinds of plank barriers are shown here. In the first, the planks are placed at a slight angle and braced by timbers. In the second, the planks are held in place between a light structure of logs and can be removed by lifting out one plank at a time.

(a) The planks should be well driven into the ground next to each other.

(b) The joints between the planks may, if necessary, be filled with heavy clay to make the barrier more impervious.

(c) You can also use medium- to heavy-weight polythene sheeting, overlapping bags, old inner-tubes or tarred felt or sacking to reduce seepage.

(d) The water level in the stream channel can be raised to reach a depth of 0.8 to 1 m.

 

7.4 Submergible diversion structures

1. These structures can be used both for holding back water and for overflows.

Wooden pole barrier 2. The purpose of this type of barrier is only to raise the water level in the stream channel without blocking the water flow completely. Some water can escape through the pervious barrier, while the rest flows over the barrier.

3. The barrier is made of two rows of wooden poles driven vertically into the streambed and closely tied together with ropes or lianas. Gravel or rock can be placed downstream of the barrier base to reduce bottom erosion. The barrier should extend well into both stream banks.

 

    

Rock barrier

4. This is a very simple submergible structure made by piling rocks across the streambed and forming a small porous barrier. You should build this barrier in layers. For each layer, use relatively large rocks first, and then fill the gaps with smaller rocks. The base width of the barrier depends on its final height, which should not exceed one metre. If you work carefully, you can build side slopes with a 1:1 ratio to save additional work. With this method, a one-metre-high barrier requires a base width of about 2.5 m to give a top width of 0.5 m.

 

Gabion barrier

5. You learned how to construct gabions earlier (see Section 3.7). These baskets can be used very effectively in small streams with a maximum flow of less than 100 l/s to divert part of the water and to act as a spillway when floods occur. They are particularly suitable when gravel is found on the streambed and when the stones can be found locally.

6. Proceed as follows:

(a) When the water flow is minimum, divert the stream around the construction site.

(b) Stake out the base of the barrier you wish to build, for example, a rectangular area 3 m wide across the streambed, at a right angle to the flow direction.

(c) Across this area, prepare a horizontal platform at a depth of about 0.5 m below the streambed level.

(d) Build the foundation of the barrier on the horizontal platform, using one layer of thin gabions (2 m x 1 m x 0.5 m), as shown in Section 3.7.

(e) On top of this foundation build the body of the weir using two layers of thin gabions placed across and on the upstream part of the foundation. Anchor these baskets well into the stream banks and into each other.

(f) If necessary, protect the banks above the second layer with additional lateral layers of thin gabions. Fill in the gaps with compacted clayey soil.

Gabion barrier in a stream   Diagram of a gabion barrier with

additional bank protection

7.5 Adjustable diversion structures

1. Adjustable diversion structures are more expensive and more complicated to build, but they provide an easier and more precise control of the water level in the stream channel. They are permanent structures made of reinforced concrete and removable planks. In the next paragraphs, you will learn about two simple designs for adjustable diversion structures. They can be changed to suit local conditions.

Two-pillar barrage

2. You can build a narrow adjustable barrage 2.5 to 3 m long and 1 to 1.5 m high, using reinforced concrete and strong planks 5 cm thick.

3. For a barrage made of 1 m planks and consisting of two columns 1 m high, you will need the following materials:

concrete for the foundation: 2.8 x 0.8 x 0.25 m = 0.56 m3

concrete for two pillars: 0.36 m3 x 2 = 0.72 m3

reinforcement of pillar: steel bars 6 mm in diameter:

for verticals: (14 x 1. 10 m) x 2 =30.8 m for cross-ties: (4 x 1.90 m) x 2 =15.2 m                          (4 x 1.35 m) x 2 =10.8 m

reinforcement of foundations:

steel bars 8 mm in diameter, 4 x 2.70 m = 10.8 m; and

steel bars 6 mm diameter, 14 x 0.60 m = 8.4 m.

Alternatively, reinforcement mesh, such as 10 cm square, 6 mm thickness can be used.

Plan of two-pillar barrage   Section AA

Building a two-pillar barrage

Placement of steel bar reinforcement for two-pillar barrage

Three-pillar barrage   

4. You can build a wider adjustable diversion structure barrage 4 to 7 m long and 1 to 1.5 m high using two lateral concrete pillars and one or more central ones, connected by two series of strong planks 5 cm thick.

5. For a barrage 1 m high made out of 1-m-long planks and with one central pillar, you will need the following materials:

concrete for the foundation: 4.2 x 0.8 x 0.3 m = 1.01 m3

concrete for three pillars: (0.36 m3 x 2) + (0.3 m3) = 1.02 m3

reinforcement of pillars, steel bars 6 mm diameter:for verticals:(10 x 1.10 m) + 30.8 m = 41.8 m

for cross-ties:

4 x 1. 05 m = 4.2 m 8 x 0. 50 m = 4.0 m 4 x 0.60 m = 2.4 m

plus far end pillars: 15.2 + 10.8 m = 26.0 m

reinforcement of foundations: steel bar 8 mm diameter, 4 x 4.20 m = 16.8 m; and steel bar 6 mm diameter, 21 x 0.60 m = 12.6 m; or use reinforcing mesh as mentioned in previous example.

 

   

Plan of three-pillar barrage   Section AA

6. Bury the foundations of the barrage in the dry streambed, anchoring them as far as possible into solid footing. The top level of the foundation should be about 5 cm below the level of the streambed.

7. Build each pillar into the banks of the stream. If necessary, build lateral wings from stones or concrete. You may use additional planks and fill the space between them with well-compacted clay soil. To avoid erosion, reinforce the stream bank next to each pillar with stones.

8. To make the concrete forms and fix the reinforcement well, you may need the assistance of a good mason.

Note: if you are unsure about the stability of the streambed, it may be safer to join the foundations to form a single foundation spanning the stream. This will require more material but will retain a fixed shape if the bed should erode.

 Note: see how to control water flow and how to ensure good water control in section 7.6.

Note: to make the concrete forms and fix the reinforcement well, you may need the help of a good mason

Placement of steel bar reinforcement for three-pillar barrage*

* see two pillar barrage for plan section of end pillar

7.6 Adjustable main water intake structures

Two major types of structure

1. The previous sections described how to define the level and size of major water intake structures. We now consider the types of structures to be used. There are two basic types:

an underflow intake, in which water flows below the control structure, which is raised or lowered to adjust flow;

more commonly, an overflow intake, in which water flows over the control structure, which can also be raised or lowered.

Controlling water flow

2. You can chiefly control water flow in two ways:

with sluice boards, used both for overflow and underflow intakes; with a penstock, or sliding metal door, controlled by setting with pegs or bolts, or

with an adjustable handle. It is used for underflow intakes; it is usually more expensive than sluice boards but offers more precise control.

3. Both of these systems are set in a holding structure, which can be built of wood, bricks or blocks, concrete or steel like the adjustable diversion structures described in Section 7.5. The structures are built with one or more sets of anchoring slots or grooves in each side of the control structure, as illustrated.

Note: an intake can also be made with a swinging arm or flexible stand-pipe (see Section 10.3). This alternative is less common as a main intake, but can be convenient for controlling smaller water flows. Typical capacities of such pipes are given

 

in Table 13 in Section 3.8.     

Sluice boards set in wooden grooves

  Sluice boards set in cast grooves

Adjustable penstock set in metal angles   Sliding metal door set in cast grooves

Ensuring good water control

4. While penstocks are usually designed to seal tightly in a range of conditions, sluice boards are difficult to seal properly, especially for wider gates, where boards are more likely to twist and warp. One useful improvement is to use sealing flaps of heavy polythene sheet or old inner tube. Usually, however, three parallel sets of grooves are used, two for slipping one screen and one series of boards in or out and one for adding a second series of boards when the need arises to stop the water flow completely within the feeder canal.

Fold and nail

sealing material

   

Building a main water intake with sluice boards (dimensions suitable for a medium size pond system)

Building a main water intake  

Placement of steel bars for reinforced concrete

Estimating the flow rate through the intake

5. The flow rate through these structures when open can be estimated using Graph 6. For overflow intakes with boards, the control acts like a small weir (see Section 3.6, Water, 4 ) where water flow depends on the width of the board and the depth of the water flowing over it. Table 32 shows typical values. For underflow intakes such as penstocks, the flow depends on the difference in head from one side of the sluice to the other, and on the size of the opening. Table 33 shows typical values.

Protecting the intake from erosion

6. Care must be taken in all cases to minimize erosion, as the speed of flowing water may substantially increase around the gates. As a general rule, unless special designs are used (consult a hydraulics specialist), you should limit the drop across the intake to 80 cm.

TABLE 32

Water flow over sluice boards (m3/s)

Weir structure width (m) 0.3 0.7 1.0 1.3 1.7 2.0

Depth over weir (cm) 1 0.001 0.001 0.002 0.002 0.003 0.004

 

2 0.002 0.004 0.005 0.007 0.009 0.010

5 0.006 0.014 0.020 0.027 0.035 0.041

10 0.016 0.040 0.057 0.074 0.098 0.115

15 0.029 0.072 0.104 0.136 0.179 0.211

20 0.043 0.109 0.158 0.207 0.273 0.323

     TABLE 33

Water flow through penstock sluice (m3/s)

Sluice opening area (m2) 0.1 0.2 0.3 0.5 1.0 1.5

Head loss across sluice (cm) 1 0.027 0.055 0.082 0.137 0.274 0.412

 

2 0.039 0.078 0.116 0.194 0.388 0.582

5 0.061 0.123 0.184 0.307 0.614 0.921

10 0.087 0.174 0.260 0.434 0.868 1.302

15 0.106 0.213 0.319 0.532 1.063 1.595

20 0.123 0.246 0.368 0.614 1.228 1.841

7.7 Screens and intake protection

1. Where conditions are likely to be turbulent, the sides and the outflow end of the structure may be reinforced using wood, light reinforced concrete, brick or boulders set in cement.

2. Intake structures can be protected from debris such as leaves or branches and from erosion by flowing water in several ways. Screens or guards can be used against debris in most cases, while gabions, wooden or bamboo piling, or rock reinforcement can be used for protection against erosion.

Using screens

3. Screens can be set up in a number of ways, the most common being a simple side screen. They can also be set up horizontally, as inclined screens or even in the base of the supply stream.

4. In many cases, a single screen is used, usually made from steel bars 6 to 8 mm in diameter spaced 20 to 35 mm apart. This screen is sufficient for clearing larger objects. If smaller particles need to be removed, an additional screen of finer bar (e.g. 4 to 6 mm diameter) at closer spacing (5 to 10 mm), or steel mesh, can be used. The additional screen may be set up inside the main screen or may be incorporated into the intake structure itself.

5. For simple structures, the screen has about the same cross-sectional area as the main intake. To improve flow and to ensure the screen will operate even when partly blocked, it is frequently made larger than the intake (e.g. by using inclined "V" screens or horizontal screens - see manual, Management 21, Section 2.9).

6. Remember that if the screen starts to become blocked, it may direct water to diversion canals and so reduce the flow to the pond supply.

7. Screens can be cleaned by lifting the screen from its slots and brushing it, or by raising the hinged portion of a horizontal or inclined screen, or by arranging the screen so the passing water current will keep it clean. Mechanized automatic screens are also available, but these specialized installations are outside the scope of this manual.

8. You can learn more about screens in the next manual Management, 21.

 

 

 

Protecting the intake structures

9. Intake structures can be protected in several ways and the principles of construction are given elsewhere in the manual.

10. A light framework of tied bamboo, woven netting, or posts and boards can be used for wall protection. Make sure the framework is well anchored, and do not let the water work its way behind the structure. If it does, erosion can be rapid, and the structure will weaken and lose its effectiveness.

11. Posts, tied planks or pickets can be embedded into the stream or along the sides. If well placed, they reduce erosion. If placed across an intake area, they can also act as a coarse screen, protecting the area from large and heavy debris.

12. Gabions can be used around the intake and to deflect water, if for example it flows strongly against a stream bank.

 13. If large stones or rocks are available, they can also be used. Generally, the larger the stones, the better protection they provide.