Treated Water Storage

CHAPTER

Treated Water Storage

18

18.1  Functions of treated water storage Treated water storage became important with the expansion of piped water supply systems in the 19th century. Its subsequent development has been driven by the need to protect water supplies from contamination and deterioration and by increasingly sophisticated operation of supply ­systems. Treated water storage may be provided at the treatment works or further downstream; it can be located at ground level or in elevated tanks. Treated water storage is used to balance relatively consistent or stepped changes in source output with the variable and less predictable demand of consumers. The storage covers diurnal demand variations and may additionally, during peak seasonal demand periods, provide a balance over a week. Treated water storage is also used to maintain supplies during failure of a source works or critical pipeline or to meet fire or other emergency demands. Such storage is located strategically to ensure resilience within the trunk main network and to support local demand centres. The need for treated water storage depends on the facilities supplying water to a distribution zone and on variation in demand in the zone. It is seldom possible or economic for a water treatment works to provide a fluctuating output in step with demand; treatment processes need to be run 24 hours a day with only infrequent, carefully controlled changes of output. For maximum efficiency and to avoid risk of cavitation, pumps should be operated near their design duty point; electricity tariffs may influence pump running times (Section 17.23). It is not usually economic for a long supply pipeline to have a capacity large enough to meet the peak demand of a few hours duration. Introducing water storage to the system can reduce whole life costs and overcome such technical difficulties. The various storage, resilience and siting options should be evaluated technically and in terms of whole life costs: financial, environmental and social (Sections 2.15 and 2.20). The minimum water level feeding distribution by gravity should generally be just high enough to maintain the required minimum pressures in the distribution system at peak flows during the planned life of the scheme. A balance has to be struck between having an elevation just high enough to maintain the required pressures and having some reserve elevation to meet future ­forecast needs at the expense of increasing pumping costs. Twort’s Water Supply Copyright information to come.

671

672 CHAPTER 18  Treated Water Storage

18.2  Storage capacity required Minimum Storage to Even Out Hourly Demand A typical graph of the hourly variation of demand for a UK town of about 75 000 population is shown in Figure 18.1(a). The demand reaches a peak typically between 0700 and 0900 and remains high until after midday. It slackens off in the afternoon but rises to a second peak in the evening when people wash and prepare an evening meal. In summer the evening peak may be higher and more prolonged due to garden watering. The demand pattern is usually different at week-ends and on holidays with peaks occurring later and lasting longer. Daily demand profiles in tropical ­countries or where there are different industrial, work or social regimes should be assessed. Garden watering can form a large part of peak demand and may require special attention. The average demand for the day from Figure 18.1(a) is assumed to represent the input to the ­reservoir; Figure 18.1(b) shows the consequent net outflow starting at 0700 when demand rises above input. A total of 3750 m3 of stored water is used for an average demand of 960 m3/h. This is almost 4 hours’ storage and is representative for towns of this size in temperate climates. The ­number of hours of storage needed for balancing demand through the day is more for smaller systems and less for larger systems, particularly those with significant 24 hour industrial demand. In practice a greater storage volume will be required because: a. daily peak demands and diurnal profile will vary from day to day and with the season; b. a full reservoir cannot be guaranteed before the start of the peak demand period; and c. provision must be made for contingencies (see below) and some storage volume must be retained in the bottom of the reservoir for settling ­sediments. As a guide, for a town with a population of about 75 000, adequate flexibility is provided if the capacity of the reservoir is increased by about 50%, i.e. to about 6 hours’ supply or 25%

Figure 18.1 Typical variation in demand and use of storage for 23 Ml/d supply.

18.2  Storage capacity required 673 of the ­average daily demand. This is not, however, sufficient to safeguard the continuance of the supply against all contingencies and applies to locations where flow from source works is continuous.

Contingency Storage Some contingency storage should be provided to cover intermittent source operation, breakdowns at sources, loss of supplies after major bursts, and consumption for major fires. Pollution incidents may cause a source to be shut down until the hazard has passed (Section 7.1). The storage required for these contingencies depends on the ability to change source output, the availability of alternative sources, the layout of the pipe supply network; and what fire regulations or safety precautions may require. Risk assessments should be carried out for critical sources and pipelines; the likelihood of a failure event, its consequences and any possible mitigation should be determined. The results should be covered in emergency action plans. The loss of water following a major burst should also be considered and how such loss would be regained. A major fire can use 5000–15 000 m3 of water, but it should be possible to increase the output from sources accordingly. Allowance should be made for the event to occur when the reservoir is already low. Shut downs for planned maintenance and due to power supply cuts must be allowed for, particularly for sources such as borehole pumps or booster stations with a single source of electricity supply and no standby generator. Such outages may last a few hours. Pipeline bursts cause supply to stop for rather longer—the following figures are indicative only of the least time that may be needed; if things go wrong, double the time may elapse before supply can be restored. Burst Reported

1/2 hour (say)

Mobilising repair gang and closing valves

2 hours

Repairing pipeline: – up to 600 mm diameter

6–8 hours

– over 600 mm

8–12 hours

Refilling and disinfecting

2–4 hours

Taking into account the need for diurnal, seasonal and strategic contingency storage for routine and emergency conditions, network resilience and source works operational constraints, the overall desirable storage within a system is 18 to 24 hours—subject to risk assessment, levels of service commitments and availability of land. This is a useful target but larger amounts of strategic storage increase the risk of water quality deterioration unless the storage is carefully managed to ensure regular turnover. The location of the storage should be selected so that supplies can be re-routed to areas cut-off by any burst pipe or by maintenance work. It may be necessary to employ a water balance/strategic transfer model, especially where a reservoir forms one component of a complex integrated system. Each water authority should decide on suitable peak hour and peak week demand factors; on seasonal demand variations, for example in holiday areas where visitors can cause local

674 CHAPTER 18  Treated Water Storage demands to increase by as much as 50%; on allowance for high seasonal garden watering; and on policy for use of cheap-rate off-peak electricity supplies for pumping to minimize pumping costs (Section 17.23).

18.3  Ground or elevated storage If storage is required there is usually some flexibility in selection of its location. It may be possible to site it a little way from the ideal point of connection to the system. This could allow it to be located on a hill and thus allow it to maintain the pressure required in the distribution system. However, where there is no convenient high ground, some other solution is necessary. An alternative is to provide elevated storage at a water tower, but local objections may prevent granting of necessary consents. If a water tower is unacceptable in such circumstances, the only alternative is to ­provide ground level storage and boost the water into local supply. Except for ­environmental acceptability, the choice of ground or elevated storage is a matter of ­economics and operational factors. Elevated storage is more expensive to construct and maintain but might allow shorter connection pipelines. There is a practical limit to the size of elevated storage so that, in theory, the choice is between a large ground level service reservoir serving a large area and several elevated water towers each serving smaller areas. The choice is then influenced by the configuration of the distribution system (Section 13.2).

18.4  Statutory consents and requirements Reservoirs and associated structures require the consent of the national and local Planning Authority. Regulations vary from country to country but in the UK reference has to be made, under the Town and Country Planning Act 1990, for consent. This action normally attracts comment from various statutory bodies, local organizations, environmental groups and other stakeholders depending on the sensitivity of the location. Early consultation with these bodies is necessary to avoid delay or refusal of the planning application. Section 5.27 sets out the relevant legislation in UK and the types of body that may need to be consulted. Where the proposed works may affect the apparatus of other statutory undertakers, they must be consulted, and Notices may need to be served (in the UK—under the New Roads and Street Works Act 1991) for any street works. Where public roads are affected, the Highway Authority should be consulted at an early stage. Discharge consent from the Environment Agency will be required for overflow and drain-down discharge into a sewer, soakaway or watercourse. A range of non-statutory consultation may also be required (e.g. The Countryside Agency for landscape, and English Heritage for archaeology). In the UK, if a service reservoir will store more than 25 000 m3 above natural ground level of any part of the land adjoining the reservoir, its design and construction have to be supervised by a Construction Engineer appointed by the water company and who is one of the qualified civil engineers on a panel of engineers appointed under the Reservoirs Act 1975 and associated Regulations of 1985 and 1986. The Construction Engineer is required to issue certificates during construction and after completion of the reservoir, specifying the water level(s) to which the reservoir may be filled. During the life of the reservoir, it has to be inspected at intervals not exceeding 10 years by an Inspecting Engineer appointed also from the panel (Section 5.23).

18.7  Instrumentation 675

18.5  Water quality considerations There is increasing awareness of the influence of service reservoirs on water quality. High storage times can be detrimental to water quality by allowing decay of disinfectant residuals and growth of disinfectant by-products such as trihalomethanes (THMs) (Section 6.25). Of particular concern is the potential for stagnant regions to form. Reservoirs with common inlet and outlet pipework are particularly susceptible to water quality deterioration since demand tends to be met direct from the reservoir inlet; this leads to low turnover of reservoir contents and high stored water age. To maintain good water quality, it is important to ensure that there is good turnover of water throughout the reservoir. This can be achieved by: a. installing baffles to promote plug flow; or b. positioning inlets and outlets to create good mixing and prevent stratification; and c. using the operational regime to generate diurnal fluctuations in water level. Perfect mixing or perfect plug flow in a tank is not achievable and even reasonable mixing or plug flow can be surprisingly difficult to engineer. The tank characteristics required for plug flow are very different to the characteristics required for good mixing. It is therefore best to decide which type of flow the basic tank design is most likely to promote and then modify the design to optimize those flow characteristics, using suitable modelling. This is now best achieved with Computational Fluid Dynamics (CFD) (Chapter 12, Appendix). Useful guidance is given in the AwwaRF report Water Quality Modeling of Distribution System Storage Facilities (Grayman, 2000).

18.6  Sampling and water testing Monitoring of water quality in service reservoirs and water towers is required in UK under the Water Supply (Water Quality) Regulations 2000 but, irrespective of any statutory requirement, routine monitoring of water quality is good practice to ensure that water in a service reservoir has not become polluted. At sites with more than one service reservoir or compartment, each unit should be sampled separately unless they are sufficiently inter-connected that a sample at the combined outlet is representative of both storage units. The sampling arrangements should always ensure that the water sampled is from the body of the reservoir or that leaving the ­reservoir. If there is no suitable location for a sample tap at the reservoir site, then a tap should be provided on the outlet main at the nearest ­possible point to the reservoir. Tanks with combined inlet and outlet pipework should be sampled at different depths. Dip sampling is not recommended and should only be used as an emergency measure. Some water companies have particular sampling requirements and may make it necessary to provide several sample points from each compartment.

18.7  Instrumentation The following instruments and equipment are normally required (see Section 18.12 for terminology). 1. A stilling well, either within or external to the reservoir, in which the water level ­measuring equipment is installed. If the reservoir is divided into two compartments, an externallymounted stilling well with valved connections to each compartment or separate stilling wells

676 CHAPTER 18  Treated Water Storage for each compartment should be provided. The well and valving are normally sited in the valve house. 2. Equipment for displaying the level measurement in either digital or analogue form in the valve house or in a weatherproof and vandal-proof enclosure installed adjacent to the stilling well. 3. Level electrodes or float switches at TWL (high) and BWL (low) for normal on/off pump control and alarm purposes. Additional switches at intermediate levels may be required for some pump control schemes. 4. Level electrodes or float switches at OWL (extra high) and just below BWL (extra low) for alarm purposes and emergency control of pumps feeding to or from the reservoir. 5. A display panel (wall- or floor-mounted as appropriate) for level and alarm indications, power supplies and the like. Provision should be made for interfacing with a telemetry outstation which, if required, may be mounted inside the panel. 6. A telemetry link to the pumping station or control centre to transmit water levels, flow rates through the inlet and outlet pipes, valve positions, power supply failure, telemetry failure and to carry signals for an intruder alarm and for remote control of valves, where appropriate. If required by the reservoir’s Construction Engineer or where foundation or structural movements are thought to be a risk, facilities for monitoring should be provided. These may include physical reference points at selected external locations on the structure and at nearby datum points well clear of the zone of influence of the reservoir. If necessary, strain gauges and inclinometers can be installed.

18.8  Overflow and drain down capacity Inflow controls are normally provided to prevent water level rising above a set normal maximum or top water level (TWL). The control may be via telemetry on source pumps or inlet valves or may use mechanical or hydraulic devices such as float or altitude valves. However, failure of such controls is possible so that some fail safe means of restricting water level has to be provided by means of an overflow. Reservoir roofs are not normally designed for uplift and flow out of vents or access openings is undesirable. Both could lead to structural damage or even reservoir failure. The capacity of the overflow should be not less than the maximum likely inflow. For small reservoirs with small inlet pipework in a simple system it may be sufficient to set design overflow rate at the maximum inflow that the source could produce. However, provision of such overflow capacity and disposal in an urban area may be very onerous for a large reservoir. If the system supplying the reservoir is complex a risk analysis should be carried out with the aim of making reasonable provision for overflow without necessarily covering combined risks of very low probability, particularly where overwhelming of the overflow would not lead to loss of life or other serious consequences. When it is necessary to drain a service reservoir down as much of the contents as possible should be released to supply via the usual outlet arrangements. However, separate facilities should be ­provided to allow the rest of the contents to be drained. These may have to be used for ­emptying the complete contents if contamination prevents use of the water in supply. Drawdown times of 8–12 hours for reservoir compartments under 2000 m3, and between 24 hours and 3 days for larger compartments, may be appropriate.

18.10  Water retaining concrete design 677

18.9  Ventilation Ventilation of reservoir compartments is needed to maintain a fresh supply of air above the water surface, for temperature control of that air and to admit or release air displaced by varying water levels in the compartment. The capacity of the ventilation system should be subject to risk analysis. It should be sufficient for the fastest rate of fall (or rise) of water level that is likely. Drain down must be allowed for but pipe burst may also need to be taken into account to avoid roof collapse. The cross-sectional area of ventilation ducts or openings should be based on a suitable air speed (say 15 m/s). The air vents need to be insect proof and allowance should be made for the reduction in effective air passage area caused by insect screens. In many cases, traditional mushroom-type roof ventilators have been found unsatisfactory in long-term service and to be a potential source of pollution. ‘Vented’ access covers have sometimes been used for small reservoirs but are also vulnerable to pollution. Alternatives include piped systems above the reservoir roof, leading to one or more ventilation chambers or ventilation ducts. At unmanned sites or at very sensitive installations and where serious malicious intrusion is a risk ventilation (and access) facilities should be designed against introduction of chemicals and explosives. The way this is done should be agreed with the reservoir owner but should not be made public (Section 18.13).

18.10  Water retaining concrete design British practice since 1987 has been to follow the procedures set out in BS 8007 for the design of liquid retaining structures. However, by 2010 in UK, when BS 8110 and BS 8007 are due to be withdrawn, design of concrete structures will have to follow BS EN 1992-3 Eurocode 2. As with BS 8007 in relation to BS 8110, BS EN 1992-3 is based on limit state philosophy as used for the design of reinforced concrete structures to BS EN 1992-1-1. BS EN 1992-3 defines four classes of water tightness (0, 1, 2 and 3) for checking cracking serviceability. The first applies where some degree of leakage is acceptable or irrelevant. Tightness Class 1 is the case usually applying to water retaining structures where small leakage leading to damp patches or staining is acceptable. Tightness classes 2 and 3 require appearance not to be impaired by leakage or, in the latter case, there to be no leakage. For both classes 2 and 3 measures are needed to ensure that part of the concrete section remains in compression at all times or that a supplementary barrier such as a liner is applied. For Tightness Class 1 the width of any crack that is expected to pass right through the section is to be limited to: wk1 = 0.875 /(hD /h ) + 0.025 (with limiting values of 0.2 and 0.05 mm), where hD is the hydrostatic pressure and h is the concrete section thickness. Such cracks can be expected to heal in time subject to certain provisos. Where Tightness Class 2 applies (perhaps for exposed surfaces of tank walls) the minimum thickness of the concrete to be always in compression is the lesser of 50 mm and 0.2h. The procedure involves the determination of crack widths and the reinforcement needed. Checks are made for other serviceability limit states as well as ultimate limit states. BS EN 1992-3 limits steel tensile stresses and bar spacing for different design crack widths for sections under axial tension.

678 CHAPTER 18  Treated Water Storage Under BS 8007 water retaining structural concrete should have at least 325 kg/m3 of cement with a maximum water/cement ratio of 0.55, reduced to 0.50 when pulverized fuel-ash (PFA) or GGBS forms part or all of the cementitious material. It also requires cover to reinforcement to be not less than 40 mm. There are no equivalent requirements in BS EN 1992-3 but the durability requirements of BS EN 206-1 and BS 8500, and any general requirements of BS EN 1992-1-1, should be followed in respect of exposure to water as well as aggressive ground, liquids or environments. Additional protective measures (APMs) may be required for protection of concrete; examples are: (a) application of an external waterproof membrane—usually a bitumastic material on a carrier film, protected either by a fibre board at least 10 mm thick or by concrete blockwork or mass concrete; and (b) increased concrete cover. It should be stressed that good quality control is essential for achieving satisfactory water retaining ­concrete. Wherever service reservoirs are built outside UK any relevant local codes of practice should be followed. It is prudent to take account of local constructional abilities and the quality of materials available and amend factors of safety if necessary. Design practices may need to be modified to take account of local prices so that an economic construction results.

18.11  Welded steel plate design Large steel plate tanks are usually cylindrical with their axes vertical. By 2010 steel tank design in UK must follow BS EN 1993-4-2, Eurocode 3, subject to issue of the UK National Annex. Under this Eurocode steel tanks for water come under Consequence Class 1 for which membrane theory may be used for determining principle stresses and simplified expressions may be used to determine local bending effects. Loadings are to be as defined in BS EN 1990, Eurocode 0. The shell is to be checked for plastic limit, cyclic plasticity, buckling and fatigue. Serviceability checks are to be made for deformations, deflections and vibrations. Minimum carbon steel plate thickness for tank bottoms excluding corrosion allowance is to be 5 mm for butt welded plates and 6 mm for lap welded plates. Reinforcement at openings is calculated by the area replacement method. Otherwise design of tanks for storing water can be to AWWA D100 which, for certain details makes reference to API standard 650 (oil tanks). Both these codes include basic and refined design procedures. The basic procedures use conservative allowable stresses (the same in both codes) and are based on simplified design rules. With these, the steel plate selected is usually the cheapest that satisfies the rules for the intended service although a wide range of steel grades are permitted. The refined design procedures recognize the benefits of higher grade steels, an advantage for higher loaded members such as walls. The steel grade must be weldable and suitable for the stress and temperature ranges expected. Toughness needs to be taken into account for higher strength steels; it reduces with increased thickness but is improved for fine-grained steels and for those with higher manganese content. One difference between the AWWA and API codes is that the latter allows the excess ­thickness at the top of one wall plate to be taken into account in determining the thickness of the plate above it. D100 requires the plate thickness to be based on the stress in the highest loaded extremity. Loads to be allowed for in the tank walls include those from the contained liquid and either wind or seismic effects. Shape factors are given in the standards and provide a convenient means of determining wind pressure. Such lateral loads may induce buckling in tall shell cylinders if the roof provides insufficient bracing. The factor of safety against buckling should be calculated and

18.12  Reservoir Shape and Depth 679 wind girder stiffeners included if necessary (Rajagopalan, 1990). AWWA D100 contains factors for determining increases in plate thickness for different tank heights and seismic accelerations. Sloshing of tank contents may need to be considered separately since it can occur in earthquakes, depending on the tank size and the frequency of seismic oscillations. Distortion may result but failure is rare. When a tall tank is empty wind loads can cause floor lift; anchors should be provided to prevent this. Under AWWA D100 roof loads should include dead weight, snow or other live loads, wind and vacuum or internal pressure commensurate with the capacity of the ventilation system. Buckling safety factors should be determined for spherical or ellipsoidal roofs. Minimum plate thicknesses given in the codes are 6.35 mm (1/4”) for floor and 4.76 mm (3/16”) for the roof. Care should be taken with penetrations of the shell, floor or roof where additional stiffening or reinforcement may be needed. Guidance on these and on other details is given in both AWWA D100 and API 650. Care should also be taken at the joints between wall shell and both roof and floor. Five designs of the latter are covered: 1. Tank founded on 75 mm of oiled sand with the shell located on and bedded in grout on a reinforced concrete ring beam. 2. Tank founded on a reinforced concrete slab covered by 25 mm of oiled sand or 13 mm of joint filler. 3. Tank founded on 150 mm of oiled sand with the shell located inside a reinforced concrete ring beam leaving a gap of at least 19 mm. 4. Tank founded on a platform of graded stone or gravel with side slopes of 1 in 1.5 and surrounded by a level berm at least one metre width. 5. As for Type 4 but with a steel retaining ring extending into the gravel platform. Rigorous corrosion protection is required including: meticulous surface preparation; use of galvanized plates; zinc rich coating after welding; and paint systems such as epoxy resin, glass flake resin and elastomeric coatings. Access has to be provided for the inspection and maintenance of coatings in all areas.

Service Reservoirs 18.12  Reservoir shape and depth The most cost effective shape of a reservoir is circular in plan but the area of land required is greater. Except where a storage facility comprises several storage tanks, service reservoirs are generally built with at least two compartments so that one can be drained for maintenance. Reservoirs which are circular in plan are less suitable for subdivision. Nevertheless circular tanks permit the use of pre-stressed concrete or steel, which may offer cost advantages. For a two-compartment rectangular reservoir the most economic plan shape is usually obtained when its length (measured perpendicular to the division wall) is 1.5 times its breadth. These proportions may need alteration in the light of the shape and slope of the site, the cut and fill balance, pipework configuration for circulation, and any future extension likely or amenity requirements. If significant or abnormal soil settlements are expected, there may be advantages in providing two adjacent structurally independent single-compartment reservoirs instead of one two-compartment reservoir.

680 CHAPTER 18  Treated Water Storage There is an economic depth for any service reservoir of a given storage capacity. The greater the depth the less length of wall and area of roof and floor is needed, though the unit cost of the wall increases with increased water depth. There can, however, be other constraints on the depth such as foundations, the character of the available site or the desirable range of distribution pressures. Depths most usually used for rectangular concrete reservoirs are:

Size (m3)

Depth of water (m)

Up to 3500

2.5–3.5

3500–15 000

3.5–5.0

Over 15 000

5.0–7.0

The following parameters are key to the design: top water level (TWL); usually the level at which the supply into the reservoir is to be shut off; the overflow weir level (OWL); giving a small margin above (a); n the maximum water level (MWL) needed to discharge the maximum possible inflow over the overflow weir; n bottom water level (BWL), being the lowest level to which the water should be allowed to fall for the purposes of supply; n lowest roof soffit level—allowing for roof slope. n n

A freeboard between MWL and the roof soffit is required for ventilation and should not be less than 150 mm above MWL or less than 300 mm above TWL. Settlement of precipitated or suspended solids may occur in reservoir compartments. To prevent turbid water being drawn into supply, BWL should be not less than 150 mm above the highest level of the floor. It may need to be higher, depending on the outlet arrangements (Section 18.23).

18.13  Covering and protecting reservoirs In temperate climates flat-roofed concrete reservoirs are usually covered over with earth and grass for appearance and temperature insulation. This involves maintenance and grass cutting, but an uncovered reservoir may result in amenity objections. The earth cover to the roof should comprise grassed top soil 150 mm thick, over a fabric filter membrane laid over 100 mm of single size 20 mm round gravel forming a drainage layer. The drainage layer is laid over a waterproof ­membrane (­Section 18.21). A 150 mm thick gravel layer with no topsoil can be used in arid ­countries if there are no amenity objections. This provides thermal insulation. The earth banks against the external reservoir walls must be designed to stable slopes and, for ease of grass cutting, should not be steeper than 1 on 2.5. Topsoil cover to banks should be not less than 150 mm vertical t­ hickness. Special attention has now to be paid to ensuring service reservoirs are secure against vandalism, acts of terrorism and theft. Guidance is given in the UK water industry Code of practice for the security of service reservoirs, 1997 (this is a restricted document, accessible through managers

18.15  Rectangular Jointed Concrete Reservoirs 681 responsible for security in each of the water companies). An impact and vulnerability assessment should be undertaken to determine the level of risk and hence the security measures necessary. Secure perimeter fencing can minimize ordinary vandalism but is not sufficient protection by itself. Access manholes to the reservoir can be screwed and locked down; but, if possible, concealment is better, their location being known. Roof air vents are a problem because they are a potential source of pollution and access (Section 18.9). Valve and instrument houses and their doors should be of strong construction and should have no windows. Sampling points are necessary on both inlet and outlet mains and they too should be protected. All reservoirs should be visited frequently to ensure that none of the protective measures have been tampered with.

18.14  Service reservoir structures The earliest service reservoirs were built in masonry (usually brick) on a concrete or masonry base. Roofs were commonly vaulted and supported on masonry columns. Many such reservoirs are still in use in the UK—some, such as Honor Oak in South London, being very large. Smaller brick reservoirs were often mortar lined to assist water tightness. Masonry is a flexible material that can accommodate movement but the cracking that may result and the gradual erosion of mortar lead to ongoing maintenance problems so that old masonry reservoirs may need to be replaced (in reinforced ­concrete). With the advent of reinforced concrete in the first half of the 20th century, this material became very widely used for service reservoirs. Until about 1980 reservoirs were jointed but after that monolithic construction became common because: 1. Joints require considerable care in construction and frequently are the source of poor concrete and leaks. 2. Monolithic construction usually requires less concrete and reinforcement. 3. There is a better understanding (with modern codes) of the shrinkage and stress cracking of concrete. 4. There is a better understanding of soil-structure interaction and a better ability to model it to establish whether monolithic construction can take the movements. 5. Piling has become cheaper and has enabled monolithic construction on poor ground to be feasible. 6. Site practice can now achieve satisfactory concrete with high wall pours and large distances between construction joints. The materials adopted for reservoirs today depend on their availability and unit cost, local skills, client preferences and on the topography and geology of the site. The materials that should be ­considered are the following (starting with those suitable for large reservoirs and ending with those for small tanks): concrete (reinforced or prestressed), steel, GRP and polypropylene.

18.15  Rectangular jointed concrete reservoirs Jointed concrete reservoirs normally have joints: between lengths of wall; between the top of walls and the roof; between floor panels and their junction with wall bases and columns; and in roofs dependent on their area. The floor and roof are usually parallel so that walls and columns are of constant height.

682 CHAPTER 18  Treated Water Storage Walls are usually reinforced concrete free-standing and cantilevered from a substantial base and stable against sliding and overturning (Figure 18.2) under soil or water loadings. An unreinforced mass concrete wall may be used if reinforcement is locally difficult to obtain for some reason. A sliding joint is normally provided between the top of the wall and the roof to prevent transfer of load due to roof thermal movements. Vertical joints with waterstops and sealing grooves are provided in the wall at spacings of about 12.5 m for contraction joints and not exceeding 30 m for expansion joints. Columns of reinforced concrete are normally arranged on a rectangular (usually square) grid pattern. A column spacing of 5 m results in a flat-slab roof of economic thickness without the need for dropped panels. The side dimension or diameter should be not less than 300 or 350 mm, respectively and not less than one-twentieth of the height from reservoir floor to bottom of column head. The floor is cast as a single or two layer slab in square panels having a side length equal to the ­column spacing. The single layer slab, typically 175 mm thick, is suitable for founding on a firm, non-compressible material. It is laid on a membrane of low frictional resistance; it may be ­unnecessary to provide reinforcement if the subsoil is firm and of uniform bearing capacity. The two layer slab has an upper layer, typically 175 mm thick, over a lower layer, typically 100–125 mm thick. A membrane between the layers permits sliding of the upper layer. This design is suitable for a clay subsoil. Usually only the top layer is reinforced, the reinforcement being discontinuous through the contraction joints. With these types of jointed floors, uplift pressures must be prevented by provision of an underdrainage system which has a free d­ ischarge to a lower level. Joints separating floor slabs should be of the ‘complete contraction’ type, incorporating a joint sealant at the water face (Fig. 18.3). Externally placed waterstops are generally used on the ­underside of the base slab since these allow better compaction of the concrete at the waterstop than with the centrally placed type. In a two-layer floor, the joints in each layer should be staggered to avoid vertical alignment. Where possible, the upper floor slab should not be cast until the reservoir, including

Figure 18.2 Section of jointed reinforced concrete service reservoir with 2-layer floor slab.

18.15  Rectangular Jointed Concrete Reservoirs 683

Figure 18.3 Typical joints for reinforced concrete reservoirs (and other water retaining structures).

684 CHAPTER 18  Treated Water Storage the roof, is substantially complete. This helps to avoid excessive shrinkage, temperature movements and joint damage and fouling before the joint sealant is applied. The roof is a reinforced concrete slab of uniform thickness, minimum 200 mm, and is monolithic with the column heads. This is acceptable because the columns are flexible enough to permit roof expansion/contraction. If expansion joints are needed (depending on exposure and insulation) waterstops must be of the centre bulb type; such waterstops must be provided at any other joints such as construction joints. The roof design must allow for the impact loading of construction plant placing gravel and soil on the roof and for any other live loading that may occur.

18.16  Monolithic concrete reservoirs A monolithic concrete reservoir has reinforced concrete walls, floors, columns and roof, in which there are few (if any) permanent movement joints. In some cases the walls and floor are monolithic but there are sliding joints between roof and top of walls. This type of design has been found to be structurally economical in most situations where the underlying ground (after improvement if necessary) can support the load without risk of appreciable differential settlement. The reservoirs are normally rectangular in plan (Fig. 18.4) but circular and other shapes are feasible. External walls are usually vertical or near vertical on the inner face but battered on the outer face to give the tapered section appropriate to the form of loading (Fig. 18.5). Depending on the height of the walls and the length of the roof slab, monolithic connections with floor and roof slabs can result in lower bending moments and shear forces (especially in the vertical plane) than is the case with jointed structures. The roof is normally constructed in two stages; the second stage at the wall interface being cast after the initial thermal shrinkage has taken place in the roof slab. Within the walls, joints are usually restricted to partial contraction joints (discontinuities in the concrete with 50% of the main horizontal reinforcement passing through) with a sealing groove on each face. The maximum spacing of partial contraction joints should be 7.5 m to avoid unacceptable cracking. For operational reasons, the division wall is usually full-height and can therefore assist in supporting the roof. The columns are arranged on a square grid, the span to external walls being typically reduced to three-quarters of the normal spacing.

Figure 18.4 Plan of monolithic reinforced concrete service reservoir.

18.17  Circular Reinforced Concrete Reservoirs 685

Figure 18.5 Sections of monolithic reinforced concrete service reservoir.

An economical form of floor is a reinforced concrete slab of uniform thickness except at the perimeter, where it should be thickened to cater for moments transferred from the walls or resulting from differential vertical movements as between perimeter and centre (Fig. 18.5). Local ­thickening of the floor below columns should be avoided as it can be awkward and costly to construct; instead additional reinforcement under the columns can be used to increase the shear strength. Local ­thickening is usually required at drainage channels and sumps where these are included. Joints are normally restricted to construction joints. Plate 34(a) shows an internal view of a monolithic RC reservoir nearing completion.

18.17  Circular reinforced concrete reservoirs The circular reservoir makes the most efficient use of materials since it needs a minimum wall length for a given plan area. Part of the water load on the walls is taken in tension by hoop reinforcement. As the reservoir size increases crack control becomes more difficult but this design has been used extensively for smaller reservoirs, both buried and unburied. However, the curved

686 CHAPTER 18  Treated Water Storage formwork required for the walls and the double curvature formwork usually needed for the roof are expensive and tend to outweigh the savings in concrete materials by adopting the circular shape. Inlet and outlet pipework is usually arranged through the base with access and ventilation through the roof. If a division wall is required it can be in the form of a concentric inner wall with an internal radius about 70% of that of the outer wall. If a diagonal division wall is used thickening is needed at the joints with the circular wall to cope with the horizontal moments; this reduces the simplicity of the design. Circular reinforced concrete reservoirs are of monolithic construction with stiffening beams at the top, and bottom if necessary, of the wall. Roofs may be of the self-supporting thin shell type or flat with columns. A full hemispherical roof imposes no radial load on the wall but this shape is undesirable for aesthetic and planning reasons and is difficult to construct. A satisfactory span to height ratio is about 8:1 but requires thrust to be taken at the top of the wall by a ring beam.

18.18  Pre-stressed concrete reservoirs For larger circular reservoirs hoop tension in the walls needs to be resisted by stressed tendons to eliminate cracks in the concrete. The compression in the concrete is usually arranged to match the tension caused by the maximum internal water load plus a margin to ensure that the concrete is always in compression. Two methods of wall circumferential pre-stressing have been used, both strictly speaking being post-tensioning. With the first, tendons inserted in ducts cast into the concrete are later stressed. The second is where tendons or cables are wrapped around the outside of the wall and later covered with sprayed concrete. A particular case of this involves winding the cables under tension. This allows the cable spacing to be varied up the wall to provide exactly the required load profile. Otherwise, the tendons are arranged in groups and provide a stepped load profile which sets up secondary vertical bending moments in the wall. It should be remembered that, as with any post tensioning, tendons stressed earlier in the process lose tension as other cables are stressed later. This has to be allowed for in the design, along with concrete creep, temperature and moisture content effects, either by providing excess tension at the outset or by returning to apply additional load. Friction is also a serious issue and leads to load at the jack exceeding load in the middle of the tendon by a significant margin. Distortion of the cylindrical shell can arise unless the tendon arrangement and order of stressing are carefully planned. Were it not for the need to ensure a good seal at the base of the wall, a frictionless sliding joint would be ideal since it induces no shear or bending in the wall base. However, a completely frictionless joint cannot be achieved so that some horizontal load transfer takes place, even with a sliding joint. To deal with this, additional pre-stress has to be applied at the base of the wall to compensate and thereby achieve the correct wall profile. A more usual joint is the pinned joint. This may take various forms, some involving ‘pinning’ after cable stressing. Sealing is achieved by use of a waterstop and sealants. Where the joint is monolithic the resulting shears and moments have to be allowed for. For pinned or fixed walls over about eight metres high the walls are usually pre-stressed in the vertical direction. Otherwise vertical reinforcement is sufficient. For large reservoirs, where radial loads on the base slab would otherwise be difficult to resist, the slab may be radially pre-stressed, with the cables anchored through the base of the wall in some cases. Precast panel walls have been used, an example is the 4.5 m high 3880 Ml tank in Truro, Cornwall. This

18.19  Steel Plate Reservoirs 687 used plane concrete ‘staves’ with exterior tensioned cables. Joints were cement-mortar packed and grouted after tensioning. Roof construction may be either self-supporting spherical or ellipsoidal shells or flat slabs supported on columns as described in Section 18.16. Precast concrete profiled planks have been used as a permanent shutter for an in situ concrete roof. This reduces the amount of internal falsework needed. In theory, a shell roof need only be lightly reinforced to carry the load. However, thermal effects (usually not uniform) result in movement and cracking. The working of such cracks and their penetration by foreign matter over time can cause gradual increase in load on the wall ring beam and may result in settlement of the roof profile. Such settlement can further increase the load on the top of the wall and lead to failure. While much used in the second half of the 20th century, pre-stressing of water tanks is now less common. One reason is that the thinner concrete sections and the use of pre-stressing require a high degree of skill and control in construction. Another reason is the ongoing inspection requirement, particularly with the ‘wire wound and gunited’ form of construction, as this is subject to concrete spalling and wire corrosion. A significant factor is the increased attention to health and safety in the built environment. High locked-in stresses in pre-stressed concrete represent a considerable hazard during demolition since the energy released can be damaging and unpredictable. Although published in 1961, Creasy’s book Prestressed concrete cylindrical tanks (Creasy, 1961) remains a very valuable reference to those coming across this form of construction.

18.19  Steel plate reservoirs Circular steel above ground reservoirs have been used for water storage since before World War II. Steel reservoirs with capacity up to 100 000 m3 are now in use in many countries, particularly in North America and the Middle East (Plate 34(b)) where that form of construction is also used for petro-chemical storage. The design is of all welded steel plate (Fig. 18.6) and is very similar

Figure 18.6 Welded steel ground tank.

688 CHAPTER 18  Treated Water Storage to that used for oil storage but greater attention is paid to coatings. The floor of a circular steel reservoir is made up of rectangular plates of thickness sufficient to take any radial tension and provide some contingency for corrosion, where required by design standards. The plates are either butt-welded together or lapped and fillet welded on one side only. Walls are made of butt welded rectangular plates but are thicker. Thickness is matched to circumferential tension according to the position in the wall. The wall base is fillet welded to a ring plate at the edge of the floor. This is thick and wide enough to prevent rotation and lifting of the floor under water load. Roofs are either conical or of low rise spherical shape, sometimes with a tighter curvature at the perimeter (the torispherical shape). Purlins and light truss supports on columns may be used or the roof may be self supporting. The tank base is usually founded on a concrete slab for small tanks or on a bed of sand—oiled sand has been used in arid locations—or fine gravel. Where foundation loadings require it the granular bed should be retained by a reinforced concrete ring beam placed centrally under the wall shell. This prevents local shear failure of the foundation. In common with all ground tanks, foundation settlements should be evaluated. These may be more in the centre than at the perimeter. Differential settlements around the perimeter tend to cause the shell to cant or twist and the walls to get out of vertical. Jacking distorted tanks back into shape has been successful where movement was excessive but is best avoided. Overflow and inlet and outlet pipework is usually arranged to penetrate the wall ‘shell’ via circular nozzles around which the shell plate is reinforced. Washout and drain down pipework usually exit via the ring plate into a sump below the wall. However, thermal movements of the tank perimeter may be large and may need to be accommodated at pipework by special expansion and movement joints. Access is via flanged manholes in the shell. Access to the roof is often provided by external ladders or stairs. In both hot and temperate climates the water temperature is increased by action of the sun on the steel surfaces and can lead to bacteriological issues. The light construction cannot resist uplift forces when the tank is empty; therefore the design is not suitable for sites subject to flooding or high ground water. The tank walls should be left exposed, for inspection of coatings, with a level space 1.2 m wide all round. The finished ground should be battered back from the level strip if the tank base is below original ground level and suitable drainage should be provided. The glass coated steel tank has become more popular in recent years. The plates are pre cut, drilled and then coated with a bonded glass coating. The plates are bolted together on site to a ­pattern with a sealer applied at the contact. The tanks appear to perform well: any leaks at joints are visible and can be resealed. However, the coating is delicate and is difficult to make good if ­damaged.

18.20  Other types of ground level tank Panel or sectional tanks were developed for military purposes to allow mass production, adaptation to many capacities and configurations and for easy transport and assembly. Originally, sections were of pressed steel with flanges drilled for bolting together through a gasket. Panel sizes were then 4 foot square and this size is still available (1.22 m) but 1.00 m square and rectangular panels are now produced. Coatings of steel panels are now usually epoxy or borosilicate glass. GRP panels are increasingly used but their flanges are not as robust as those of steel and need to be treated with care. Other plastics have been tried but are not favoured due to temperature distortion.

18.22  Access to Service Reservoirs 689 The use of internal stiffening braces theoretically allows any capacity to be achieved, albeit limited to a depth of about 6 m for steel (or 4 m for GRP). Sectional ground tanks are raised off the ground on dwarf walls at spacings to suit the panel size. These allow access for assembly and maintenance. Panels can be made with flanged nozzles for pipe entries and instrumentation.

18.21  Drainage and waterproofing concrete service ­reservoirs Where the reservoir external wall is designed as a free-standing cantilever or is of mass concrete, the backfill against the wall should comprise a vertical drainage layer of gravel about 300 mm, extending the full height of the wall and continuing down over the wall heel to link with a drainage system. Where the wall is monolithic with the reservoir floor it is still advantageous to retain the vertical gravel layer to control the ground water level and also to transmit water draining from the roof (Fig. 18.5). The wall and roof drainage systems must be kept separate from any underfloor drainage system. Wall drainage pipework should discharge into an observation chamber to help locate any leaks. Ingress of water and pollution through the roof must be prevented by a positive waterproof membrane. For new reservoirs an adhesive membrane such as Bituthene DW is recommended, protected by heavy duty polyethylene sheet under the gravel drainage layer. The roof gradient should be no flatter than 1:250 for drainage; the floor slope should be made parallel to it so as to maintain constant wall and column heights. The simplest way to achieve this is to provide the slope in one direction only. Where there is no vertical wall drainage layer and where support from the embankment is essential for wall stability, a low peripheral kerb provided along the lowest edge of the roof can act as a collector for piping the water away. Inside the reservoir, the floor should have a shallow collecting channel leading to a drainage sump to aid cleaning of the floor of the reservoir. The underfloor drainage system is usually laid to a rectangular pattern, normally comprising porous pipes surrounded in gravel in a trench below the floor. The layout should make it possible to observe drainage or leakage flow from separate areas of the floor. The porous pipes from each area are continued in ductile iron piping laid in concrete below the wall and the embankment and discharge individually to collector manholes, from which there is a free outfall pipe to some lower point. With jointed reservoir construction there must be no possibility of the drain outfall being submerged. If, on the other hand, conditions are such that uplift below the floor is unavoidable, then monolithic construction must be adopted, with the unit area weight of the floor, columns and roof being made greater than the design uplift pressure.

18.22  Access to service reservoirs Access to each reservoir compartment is needed for personnel, plant and materials. Access openings are usually sized to allow entry by a person wearing breathing apparatus. Access openings for plant and materials should be larger. Upstands should be provided around each opening to prevent surface water entering the reservoir. Covers to all openings must be robust but they do not normally need to be designed to support heavy loadings. They must be secure to prevent unauthorized access and must not allow rainwater to enter the reservoir. Lift-off covers risk introduction of mud and debris

690 CHAPTER 18  Treated Water Storage into the reservoir; therefore hinged covers are preferred but they must have an effective system for holding them in the open position when the access is in use. For personnel entry into the reservoir the preferred arrangement is an inclined ladder leading to a platform about 2.5 m below the roof and a stairway leading from the platform to the floor. Where a stairway height exceeds 3 m, an intermediate landing is required. Reinforced concrete construction is recommended for platforms and stairways as this needs less long-term maintenance. The platforms can either be supported on columns or, in some cases, cantilevered from the walls. Alternatively the platforms and stairways can be fabricated in galvanized steel or anodized aluminium alloy. The same material should be used for the ladder. Typically two separate human accesses should be provided into each compartment, near opposite corners to assist ventilation of the compartment when work is in progress and to provide an escape route in an emergency. Access for plant and materials has to be unobstructed to allow items to be lowered vertically to the compartment floor. The clear opening needed for small plant and materials for normal maintenance should be not less than 1.5 m × 1.0 m to allow a wheelbarrow to be lowered. ­Consideration should be given to the provision of removable handrailing around such openings, or of sockets into which it could be fitted. For reservoir compartments exceeding about 10 000 m3 a second and larger access for plant and materials should be considered if larger mechanical equipment might be needed for cleaning or major repairs. It is important to ensure that ­unauthorized vehicles cannot reach the roof or be used outside any specially strengthened areas of the roof.

18.23  Service reservoir pipework Reservoir pipework normally comprises: inlet(s), outlet(s), overflow, drawdown, reservoir bypass and drainage pipes. The outlet may comprise a suction main to a site pumping station. An 80 mm diameter valved pipe through the division wall of a two-compartment reservoir should be provided so that water is available for hosing down a compartment when taken out of service for cleaning. Unless separate connecting pipes are used for flow in each direction, the control valve on the connection must be operable from outside the reservoir. Flexible joints should be incorporated between embedded or rigid pipes and external pipelines to accommodate differential settlement (Section 16.25). Inlet and outlet pipes should bifurcate to serve each reservoir compartment equally. The inlet pipe can discharge at top water level (TWL) or near bottom water level (BWL). One of the disadvantages of the latter is that, in the event of a burst on the incoming main, the reservoir contents will be lost unless a suitable non-return valve is provided. On the other hand, if the incoming supply is pumped, a high-level entry will forfeit the energy savings potentially available when the reservoir is operating below TWL.

Inlet Pipework The inlet piping arrangement needs to either achieve complete mixing of the inflow with the stored water or produce plug flow and thus avoid build-up of stagnant water areas. This involves suitable siting of the inlet and outlet pipes and, if necessary, the use of baffle walls. For the design of large reservoirs, and where there are water quality problems, it is becoming more common

18.23  Service Reservoir Pipework 691 to use 3-D (CFD) modelling techniques (Chapter 12—Appendix) to optimize inlet and outlet arrangements and baffle wall (if any) placement. Options for encouraging circulation comprise: placing the inlet and outlet at opposite ends of the compartment (Fig. 18.4); distributing the incoming flow as evenly as possible along an end wall by the use of a long inlet weir; using a tapered diffuser pipe with several openings, or delivering to a semi-circular terminal box with ­slotted outlets.

Outlet Pipework The most common (and simplest) outlet system uses only one draw-off point per compartment, but this is likely to leave some potentially stagnant areas in one (or both) corners at the outlet end of the compartment. To avoid this, the outlet may draw water from a number of points along an end wall if flow distribution is used as the sole means of avoiding stagnation. If the inlet and outlet pipelines are to terminate at opposite sides of the reservoir compartment, it may be appropriate to have separate inlet and outlet valve houses. However, for economy and ease of operation, a single valve house containing controls for both inlet and outlet is usually preferable. With this arrangement, one pipeline (usually the inlet) is normally laid within the reservoir compartment to feed water to the far end of it. This pipeline should be placed alongside the wall and encased in concrete to avoid ‘dead’ spaces and to inhibit external corrosion of the pipes. If the reservoir is of the jointed design, the internal pipework (and its surround) must have flexible joints corresponding with the joints in the structure. The outlet pipe can be laid horizontally, either through the reservoir compartment wall, or under the floor with a 90° vertical bend. It is usual to provide an entry bellmouth, to reduce hydraulic losses. The outlet bellmouth must be sufficiently submerged at BWL to prevent the entrainment of air into the flow, particularly where the flow will be pumped. For a bellmouth in the horizontal plane (i.e. vertical axis), a safe rule for minimum submergence of the bellmouth lip is:

S D = 1.0 + 2.3F



(18.1)

where S = submergence below BWL; D = bellmouth diameter at lip; and F is the Froude number VD / ( gD)0.5, where VD = the average flow velocity through the bellmouth opening. With a bellmouth in the vertical plane (horizontal axis) the same equation may be used, but S is measured from the bellmouth axis. For a gravity supply outlet, the submergence requirement can be somewhat relaxed depending on the acceptability of air entry into the pipeline, but should not be less than D. The required submergence may create an uneconomic depth of ‘dead’ water unless the reservoir outlet is lowered by means of a sump in the floor. The sump should be generously sized to avoid undesirable hydraulic turbulence. The bottom of the sump collects floor deposits and should be not less than 300 mm below the bellmouth lip. Safety features for maintenance personnel need to be considered in the detailed design of a sump. The outlet sump can also serve as the drain sump. Where a service reservoir has a common inlet/outlet main, circulation inside the reservoir can be achieved by dividing the common main into inlet and outlet pipes before these pipes bifurcate to each compartment. If a low-level entry design is used, both inlet and outlet must be fitted with nonreturn valves. With the high-level type entry, a non-return valve is required on the outlet pipes only. Common inlet/outlet arrangements are not preferred due to the risk to water quality from stratification and stagnation (Section 18.5).

692 CHAPTER 18  Treated Water Storage

Overflow and Draindown Arrangements Adequate overflow arrangements must be provided in case of an inflow control malfunction. Each compartment should be provided with an overflow capacity equal to the maximum likely inflow possible into that compartment with the other in or out of service (Section 18.8). The simple provision of a vertical pipe with bellmouth attached as an overflow has limited capacity and a horizontal weir is usually required. A convenient arrangement in a two-compartment reservoir is a weir box in the central division wall with weir entries from each compartment. The weir box often discharges to a pipe laid through the valve house, which can also receive the washout pipework connections. The combined overflow/washout system should preferably discharge into an open watercourse or, failing that, to a sewer; both of which must be of adequate capacity. It may be necessary to ­consult with the land drainage authority or sewerage agency concerned for any permissions needed. A break pit must be provided before final discharge to allow levels to be monitored and dechlorination to be carried out if necessary. Drainage pipes should be connected to a drainage sump in each compartment and sized to allow emptying in an acceptable time (Section 18.8). A reservoir bypass (between inlet and outlet pipes) is necessary in the case of a single-compartment reservoir, or where the whole reservoir may need to be taken out of service. It is normally possible to accommodate the valve on this within the valve house.

Valves Stop valves (gate or butterfly) must be provided on inlets, outlets, scour pipes and the reservoir bypass but must not be provided on the overflow or on any wall or underfloor drainage systems. Gate valves become impracticable for normal reservoir use above about 600 mm diameter, when resilient-seated butterfly valves should be provided (Section 16.8). The valve size can be less than that of the pipeline, though the saving in cost of the valve is at least partly offset by the need for tapers and the increased space occupied by the pipework in a valve house. If a smaller size of valve is selected, a check should be made that the maximum velocity through the valve does not exceed that recommended by the valve manufacturer. Autonomous over-velocity valves, designed to close automatically when the water velocity in the pipeline exceeds a predetermined rate, have fallen out of general favour because of their high cost and infrequent use. They may still be appropriate in special circumstances, for example where a large reservoir provides the major supply to a distribution area, or where the loss of water from a failed outlet main would be severe because of high head. The possible need for such valves should therefore be reviewed in reservoir planning and electrically operated butterfly valves should be considered as an alternative. Wherever they are located, all butterfly valves and special control valves should be installed in chambers or houses so that they are accessible for maintenance. Important gate valves (such as the isolating valves on any pipes connecting into the reservoir) should also be placed in chambers but others can be buried. Isolating valves on pipes leading into or out of the reservoir should be bolted to flanged pipes cast into the reservoir wall. Otherwise any differential movement between reservoir and valve could cause a joint to fail and release of the entire reservoir contents. The same principles apply to outlet or drain pipework built into the reservoir floor.

18.25  Baffles in Service Reservoirs 693

18.24  Valve houses for service reservoirs It is often convenient for all reservoir control valves to be concentrated in one chamber or valve house. For security of supply, the valve house should be as close as possible to the reservoir and is usually part of the reservoir structure. Access to the valve house may be by top entry through the roof or side entry through a wall. Top entry may result in the whole of the interior of the house being classified as a ‘confined space’, with consequent safety constraints on entry. These could give rise to unacceptable delays in gaining access if, for example, it becomes necessary to isolate the reservoir in an emergency. Side-entry valve houses are therefore preferred but may give rise to unacceptable visual impact in environmentally sensitive areas. The pipework within the valve house must be arranged so that it is possible to install, maintain or remove any valve without great difficulty. If a valve is too heavy to be manhandled, it is important that there is clearance for a straight vertical lift out of the building (if top entry is provided) or to a position where it can be transferred to a trolley or road vehicle (if side entry is provided). Fixed monorail hoists, lifting eyes, davits or portable hoists may need to be provided. In addition to pipework, valve houses are often used to accommodate sampling pumps and pipework, level recording and indicating equipment, telemetry and site monitoring equipment for flowmeters on inlet and outlet pipework, ventilation and dewatering equipment. Provision must also be made for dealing with any water resulting from spillages during maintenance work or leakage from pipework components. As a minimum, this should comprise a sump into which the suction hose of a portable pump can be inserted.

18.25  Baffles in service reservoirs Baffle walls or curtains aim to achieve plug flow through the reservoir by directing flow from inlet to outlet by a circuitous route. The optimum arrangement of baffles depends on the shape of the compartment and the most convenient positions for the inlet and outlet. For construction convenience, baffles are normally installed between the columns supporting the reservoir roof. Solid baffle walls are normally made of reinforced concrete, brickwork or blockwork. Lightweight, hollow blockwork should be avoided because the free chlorine normally present in a reservoir air-space has been known to cause deterioration of some blocks of this type. Plastic or rubber curtains are sometimes used. Where appropriate, openings must be provided along the bottom of the baffle walls to allow all areas of the reservoir floor to drain and to facilitate cleaning and maintenance. Openings should also be provided at the top of the walls to assist with ventilation. Joints in solid baffle walls must be provided at all points where they bridge movement joints in the floor or roof. Additional joints may be needed to allow for thermal movements or for the flexing of the reservoir walls as water levels rise and fall. Baffle curtains may be less robust but easier to install. The material should be fibre reinforced to reduce elasticity and should be resistant to chlorinated water and approved for use with potable water. The edges of each curtain are formed by a seam in which holes are cut out at intervals for attachment points and through which a stainless steel rod is inserted. The recommended method for supporting the top edge of the curtain is to build a dove-tail galvanized or stainless steel channel into the roof soffit for the subsequent attachment of hangers although, for columns, lashing or strapping may be acceptable. The bottom edge must also be firmly anchored, preferably by fixing it to steel or

694 CHAPTER 18  Treated Water Storage GRP angles bolted to the floor. The use of pre-cast concrete blocks which are not fixed to the floor is not recommended. Curtain alignment should be chosen so that they are not subject to high velocity or turbulent flow such as may occur near an inlet. At the inlet a dwarf concrete wall, bonded to the floor, should be provided below the curtain to protect it when the reservoir is filling. This wall should be about 500 mm high and the bottom of the curtain should be anchored to the top of the wall.

18.26  Testing service reservoirs Service reservoirs should be tested for water tightness before being put into service. The test should be carried out before placing any backfill or banks against the outside walls unless the wall design relies on the embankment to resist hydraulic forces. The roof should be complete including any second-stage concrete. Each reservoir compartment should be tested separately, with the other compartment empty. The compartment should be filled with treated water, to a test level about 75 mm below the overflow sill, at a uniform rate not exceeding 2 m vertical rise in water level per 24 hours. It should then be left to stand for at least 7 days to allow for absorption into the concrete. Longer periods (up to 21 days) may be required by some specifications. The water level should then be measured and recorded using a hook gauge with vernier control, or by other approved means of no less accuracy, and the water allowed to stand under test for 7 days. At least once each day during this period, the water level should be measured and recorded. During the 7-day test period, the effects of evaporation from the water surface can be reduced by closing all air vents and access openings (except for one vent left open for pressure balance). Any flows in the underdrain and wall drain systems should be measured and recorded throughout the test, from a time at least 24 hours before beginning to fill, until 24 hours after emptying or on completion of a final water level measurement. Taking such measurements in chambers on the drain systems normally requires safety precautions appropriate to confined spaces. The outfalls of all pipes connected to the reservoir should be inspected during the test to ensure that all isolating valves are shut tight. Any significant leakage through them should be measured. In some circumstances it may also be necessary to keep records of evaporation losses from the water surface. The test may be deemed successful if the drop of water level over the 7-day test period does not exceed the lesser of 1/500 × average water depth or 10 mm, after deducting any measured leakage through valves and making allowance for any evaporation or condensation. If the test fails, any increase in underdrain or wall drain flow during the test period should be investigated to identify, if possible, the part of the reservoir that leaked. The test compartment should then be emptied and closely inspected for faults likely to cause the leakage. Investigating reservoir leakage can be troublesome and time consuming. The interior of the reservoir, especially any joints, should be closely inspected before filling with water.

18.27  Searching for leaks It is not easy to make a large reservoir fully watertight and the following notes may help track down the point of leakage. 1. Flows from underdrains should be examined. If they serve known areas of floor and are not joined to one common outlet point they may help narrow the search.

18.28  Use of Water Towers 695 2. The inlet and outlet valves must be observed to ensure that they are not passing water into or out of the reservoir. The only secure way of knowing this is by withdrawing flange adaptors from the valve or by removing or temporarily leaving out a section of pipe next to the valve. Any outflow should be measured. 3. The rate of leakage at full depth, half depth, and with about 0.6 m of water in the reservoir should be measured. It is not likely that any revealing mathematical relationship between rate of leakage and depth of water in the reservoir will be established but leakage is usually less when the depth of water is less. No leakage at all below a certain level points to a leak at higher sections of wall. 4. After attempts to narrow the search for leaks the reservoir must be emptied and subjected to the most careful internal inspection with good lights, adequate ladders, plenty of time and a consistent pattern of examination. It is very easy to miss a faint crack in the wall or floor. Walls (particularly the joints next to the corners) should receive special attention for it is here that there is most likelihood of movement having occurred. After emptying, reservoir walls should be kept under observation when drying off since temporarily visible damp patches may be evidence of water held in underlying cracks or poor concrete. If first inspection does not reveal the causes of the leaks, it is worth repeating the inspection with even more care. 5. The floor joints should be inspected. Jointing material should be examined to see if it has sunk, has holes in it, or has come away from or failed to bond to the concrete of the sealing grooves. The majority of leakages arise from defects of this kind. Wall joints should similarly be examined. 6. If failure still results, about 0.6 m of water should be put into the reservoir and be left to stand until the water is quite still. Then crystals of potassium permanganate may be dropped into the reservoir, widely spaced, and left for a considerable time. Observed from a pre-arranged walkway (so as not to disturb the water) with a good light, streaks of colour may be noticed from the permanganate crystals showing some definite flow towards a point of leakage. 7. As an alternative to method (6), about 150 mm of water can be put into the reservoir, a hole or several holes bored through the floor, and compressed air can be introduced under the floor. In certain conditions of floor foundation air bubbling upwards through the water may indicate where faulty floor joints occur. 8. If, despite all these attempts, the cause of leakage is still unaccounted for then more drastic measures may have to be undertaken, such as digging pits in the bank to inspect the rear of the wall joints, placing further sealing strips (such as glass fibre embedded in bitumen) over joints, or even rendering wall face areas. Sealing of leakage through a reservoir floor has been achieved by gravity grouting. About 450 mm of thin grout mix is put into the reservoir and the cement is kept in suspension by continually sweeping the floor and disturbing the water with squeegees for two successive days. Thus grout passes into the unknown paths of leakage and the cement sets. It should not be necessary, however, to adopt these measures unless poor construction has taken place.

Water Towers 18.28  Use of water towers Water towers are used as a local source of water at times of peak demand where it would not be economical to increase the size of the supply pipeline and add a booster pump installation. In undulating terrain ground level storage could provide the pressure needed but in areas of flat topography

696 CHAPTER 18  Treated Water Storage the storage must be elevated. Many shapes and design features are possible but the designer should aim to produce a structure that meets the requirements of the water supply and planning authorities, bearing in mind that it will become a landmark in the community which it serves. Ancillary equipment including pipework, valves, ladders, instrumentation and booster pumps, if required, can all be hidden in the cylindrical shaft. The optimum depth/diameter ratios should be determined taking into account the most efficient shape and the needs of the distribution system. It is usually advisable to avoid large pressure fluctuations in distribution that may be caused by drawdown or filling in excessively deep tanks.

18.29  Concrete water towers Concrete water towers are built with capacities up to about 5000 m3. They are usually circular in plan although rectangular concrete towers have been built (Plate 34(c)). The diameter of circular water towers is not usually sufficient to warrant the use of pre-stressing since cracks can be controlled by applying normal water retaining concrete criteria. Concrete water towers allow some scope for architectural statement so that the result can be regarded as a visual asset. Typical dimensions adopted for the reinforced concrete design shown in Figure 18.7 are:

Size (m3)

Depth of water (m)

Internal diameter (m)

1200

7.5

17.0

2000

9.1

19.4

3000

10.2

22.6

Figure 18.7 Reinforced concrete water tower.

18.30  Welded Steel Water Towers 697

Figure 18.8 Reinforced concrete water tower (Intze type).

Rectangular water towers are designed as small monolithic service reservoirs with the floor slab supported on some form of open column and beam framework or a hollow vertical shaft, itself founded on a base slab, piled if necessary. Wind and seismic loads should be taken into account in the design of tank, supports and piles. Circular concrete water towers allow more scope for different styling from a simple cylinder with a flat base to a sophisticated form such as the hyperbolic-paraboloid of the 39 m high Sillogue tower near Dublin airport built in 2006. In this case the vase shape resembles an inverted version of the nearby control tower. The Intze type water tower (Rajagopalan, 1990) is designed so that bending moments are as near zero as possible at all sections (Fig. 18.8). The radial thrusts from the outer conical section of base on the supporting ring balance those from the spherical centre section. Roofs may be flat for small tanks, conical or, for larger tanks, spherical as described in Section 18.17.

18.30  Welded steel water towers Relatively small welded tanks have been used for over 100 years for industry and rail transport. These were usually small radius cylinders supported on a framework of steel columns with braces or ties. Welded steel water towers of capacities up to 15 000 m3 are now available and have been widely used all over the world—particularly in North America, the Middle East and the Far East. These are now constructed of butt welded steel plate in several configurations: spheroids or ellipsoids on tubular columns belled out at the base; cylinder or spherical shapes with conical bases and supported on wide steel columns which help resist seismic loads and provide space for plant rooms or offices or on a reinforced ­concrete frame (Plate 34(d)).

n n

Whilst the forms available for welded steel water towers do not offer much scope for architectural treatment, the coatings provide an opportunity for decoration and can be attractive.

698 CHAPTER 18  Treated Water Storage

18.31  Segmental plate tanks The type of steel or GRP panel construction described in Section 18.20 can also be used for elevated storage. However, it is unlikely that segmental plate tanks would be used for anything other than industrial or emergency water storage since their poor visual appearance is exaggerated by height. Where they are used, the bases are placed on a series of beams which are supported on a framework of braced columns.

18.32  Pipework and access for water towers Pipework and access facilities below water towers are usually concealed within or obscured by the tank supports. A dry access shaft in the centre of the tank allows access to equipment above water level such as water level instruments and inlet float valves and may permit access out onto the roof. Old designs used to provide facilities for external access including circular walkways and revolving ladders. However, such facilities themselves need maintenance and cannot easily meet modern safety standards. This means that external coatings for steel tanks must be of the highest quality to minimize time to first maintenance.

Reference Standards AWWA D100 Steel tanks for water storage. AWWA. ANSI/API STD650 Welded steel tanks for oil storage. ANSI. BS 8007 Code of Practice for Design of Structures for Retaining Aqueous Liquids. BSI. BS 8110-1 Structural Use of Concrete. Code of Practice for Design and Construction. BSI. BS 8500-1 Concrete. Method of specifying and guidance for the specifier. BSI. BS EN 206-1 Concrete. Specification, performance, production and conformity. BSI. BS EN 1990 Eurocode 0. Basis of structural design. BSI. BS EN 1992-1-1 Eurocode 2. Design of concrete structures. General rules and rules for buildings. BSI. BS EN 1992-3 Eurocode 2. Design of concrete structures. Liquid retaining and containing structures. BSI. BS EN 1993-4-2 Eurocode 3. Design of steel structures. Tanks. BSI.

References Creasy, L. R. (1961). Prestressed concrete cylindrical tanks. John Wiley & Sons. Grayman, W. M. et al (2000). Water Quality Modeling of Distribution System Storage Facilities. Denver, Colo. ­AwwaRF and AWWA. Rajagopalan, Dr. K. (1990). Storage structures. A. A. Balkema.