Water Feature Design Manual

Introduction

The purpose of this manual is to offer a standard approach and set of calculations to be used in the design of water features, purely from an engineering standpoint. The engineer’s input to the operation of a water feature may be considered in two parts: maintenance of an acceptable quality of water, and provision of an acceptable head of water to operate water features. Water quality considerations in these applications are firstly the health and safety issues such as avoidance of transmission of disease, secondly aesthetic factors of clarity and absence of odour, and thirdly the corrosive and other qualities that impact on the vessels, pipework and so on used to produce the feature. Provision of an acceptable head of water and prediction of the behavior of water under differing conditions is dealt with by means of hydraulic calculations.

 It is the intention of this manual to offer two levels of calculations:

  1. Quick and simple techniques to allow a proposal to proceed with a reasonable degree of comfort in the integrity of the design
  2. More detailed techniques which allow for more accurate design predictions at contract stage.

It should be noted that hydraulics is never an exact science, and whilst the calculations given are acceptable to most practitioners, if it is desired to offer guaranteed figures, the services of an expert should be employed. It is desirable that persons carrying out tender stage design should be as familiar with the contract stage calculation requirements as possible, to allow an appreciation of the requirements of the engineer carrying out the more detailed calculations.

1.0 Tender Stage Design

1.1 Preliminary Hydraulic Design 

1.1.1 Pipework

Rigorous calculations of straight run head losses for circular pipework running full are usually unnecessary for tender stage designs. 

Nomograms for reckoning headloss per 100 m of pipe are available from many of the manufacturers of plastic pipework. These should be adequate for tender stage calculations for plastic pipework. 

As a general rule, flow velocities for Gravity, Suction, and Delivery lines of 1.0, 1.5, and 3 m.s-1 should not be exceeded at this stage. As a general rule, velocities should tend to be lower than these given for pipework runs over 50 metres, and/or containing many fittings or valves.  

Calculation of fitting headloss (where number and type of fittings is known) should also be carried out. The k value method should be used for tender stage calculations: see Appendix II for details. 

1.1.2 Tankage

The feed to the pump should have a buffering tank of a capacity equivalent to three minutes of the pump’s peak flow. This is to permit the disentrainment of air bubbles, which might otherwise contribute to reducing pump or feature performance. The shape of this tank should be designed to minimise the chances of short-circuiting of incoming flows. 

1.1.3 Channels

Calculation of channel headlosses is a bit trickier than pipework calculations, not least because of variation in depth of flow.  

For tender purposes, it usually suffices to assume depth of flow is set by the depth over the outlet weir, if an outlet weir is present. If there is no outlet weir, calculation of depth of flow is more complicated, and it is suggested that 100 mm of water depth be allowed for tender purposes. 

Setting flow velocity at less than or equal to 1.5 m.s-1  at this depth should result in reasonable losses down the channel. 

Allowing a 50 mm fall from the weir edge, or invert of the channel to the next water surface is usually sufficient to account for head loss. 

1.1.4 Weirs

Calculation of depth of flow over a weir is mostly dependent on the shape of the weir.  

Experiments carried out have yielded the recommendation that 6 l.s-1.m-1be allowed for broad-crested weirs, and 4 l.s-1.m-1 for sharp-crested weirs.  

These flows gave depths of the order of 10 -15 mm over the weir. The recommendations contained in Appendix III are to be followed wherever possible.  

1.1.5 Nozzles

Experiments with PEM and OESA nozzles have shown that variability between literature and actual values and between batches of nozzles can be considerable. 

These manufacturers also change specifications of nozzles without notice, and supply nozzles to differing specifications from those in the catalogue. 

We therefore recommend that at least 25% be added to manufacturer’s recommended head requirements for nozzles at the tender stage. 

1.2 Water Quality 

1.2.1 Chemical Composition

The following substances are undesirable components of water to be used for water features:  

      Water should not have a highly aggressive nature, as measured by Langelier index. (See Appendix I for details) 

It may be assumed that all supplies from potable sources are suitable in these respects. 

Any concerns over the level of any of these contaminants should be expressed at tender stage. 

1.2.2 Clarity

The most important parameter with respect to the visual impact on the feature is the clarity of the water. Unlike industrial applications, however, no performance standards are stated, and the clarity of the water is an aesthetic rather than a scientific measurement. 

The water used is to be free of coloured compounds, whether organic, or inorganic. The suspended solids level is assumed to be the factor determining clarity of the water. 

The filters will remove only gross suspended solids. Any fine or colloidal solids will require additional treatment, as will any of the above undesirable contaminants. 

Sand Filtration equipment to reduce suspended solids content is usually provided to address the need to maintain clarity of feature water. Manufacturers usually specify the type and size of unit to be used, but some rules of thumb may prove useful for giving a rough idea of likely sizes for these filters, in order to allow layout to proceed. 

The volume of water to be treated in an hour may be determined by allowing all of the water in the system to be passed through treatment in a time period of between two and six hours. To determine this flowrate, firstly calculate the volume of all of the water vessels, channels and pipes in the system. If there are a lot of weirs in the system, the head of water over the weirs may be a significant proportion of this volume. If there are a large number of nozzles, water in the air may provide a significant part of the volume. These two items are however usually insignificant contributors to the total volume. This total volume of water (v) is to be treated in x hours, and filter flowrate is therefore v / x. x can be of the order of 2 - 6 hours for tender purposes. 

How quickly we turn the system around is a factor of degree of contamination, amongst other things - outdoor systems receiving leaf debris, and street litter will require quicker turnovers. 

High Rate Sand Filter sizes may be roughly estimated by allowing a flow per unit area of the filter of say 30 m3.m-2.h-1, a figure far in excess of those used for more conventional Rapid Sand Filters, but conservative in these applications. 

Sand filters have to be periodically cleaned by means of back-washing. Filter back-washing is likely to result in wash water flowrates to drain of the order of the feed flow rate at the flow per unit area given. Where air scour is used to supplement the water, similar flowrates (in m3.h-1) are used. Filters are back-washed much less frequently than in municipal water treatment, usually at weekly, rather than daily intervals. The actual washing frequency is however dependent on incoming water quality, flowrates per unit area, and dosing rates.  

Any guarantee figures should be referred to an expert. 

There is a number of other filter types which are used for this application, Rapid Sand Filters and pre-coat filters being the most common alternatives. These have the disadvantage of increased space requirements, and more troublesome operation respectively. Note that (confusingly) Rapid Sand Filters are not as rapid as high-rate filters, and are sometimes known in this industry as standard rate filters.  

1.2.3 Biological Quality

In addition to the chemical and physical considerations outlined above, prevention of the growth of organisms in the water is required. 

Of particular concern in water features is the Legionella organism. This bacterium can cause serious pneumonia type illness in susceptible organisms, can survive in water kept below 60oC, and is transmitted well by any fine dispersion of water, such as those generated by sprays, jets and the like. Following the rules for maintenance and cleaning of features to prevent build up of bacterial films, continuous disinfection of feature water and periodic high level disinfectant dosing are usually sufficient to control this hazard.

Lesser problems of water odour and appearance, as well as staining of water feature surfaces may also result from biological growth. The control measures suggested above would also inhibit the growth of organisms which may have these adverse aesthetic effects.  

Filtration to maintain clarity also removes biological material from the system, and is the major factor in prevention of growth of algae within the feature, other than disinfectant dosing. Specific algaecides may be added to the feature, but they may well come with problems resulting from interaction with disinfectants, and the feature water. It is therefore highly unusual to add such algaecides on a regular basis.  

The most commonly used disinfectant agents are Bromine and Chlorine. They have common advantages as follows:  

Disinfection by means of bromine is favoured over chlorine, mainly because it is simpler to control, by virtue of its wider effective pH spectrum. A side effect of the use of bromine is oxidation of organic contaminants and removal of ammonia from the system. Bromine dosing is usually based on systems dissolving bromine containing solid tablets. Manufacturers will be responsible for sizing the systems, but to allow for adequate feed pump capacity, one manufacturer recommends the provision of a flow to the brominator of 1 l.min-1 per 10,000 litres of feature capacity. Brominator capacities of approximately 1 Kg of tablets per 7.5 m3 of feature capacity are usual, to give reasonable filling intervals.  

Ultraviolet light is also used as a disinfection method. High rates of disinfection are possible with UV, but it leaves no residual disinfectant in the water, unlike chlorine or bromine.  

Ozone is a highly reactive form of Oxygen that is gaining in popularity for swimming pool water treatment. Like UV, it leaves no residual disinfectant in the water, and therefore the possibility of growth of organisms within the body of the feature is a concern. This is often overcome in swimming pools by being used in tandem with Chlorine dosing. The advantages of Ozone in the swimming pool application do no apply to water feature use. Ozone is very toxic to humans, stringent and costly provision for avoidance of Ozone poisoning must therefore be incorporated within the design.  

There are a couple of other minor techniques for disinfection that have come from developments to answer the requirements of the US and USSR space programmes for water recycling. The US programme devised a technique where disinfectant metal ions are introduced into the water by electrolysis of the water using precious metal electrodes. Use of this technique is practically limited to small domestic pools, as the effective agent is precious metal ions. The Russians apparently developed electrolysis of salt through a semi-permeable membrane. This system is sold as “Enigma” in the UK. The mode of action of this system is production of hypochlorous acid. There may be trace levels of other oxidised compounds, but these are insignificant with respect to the disinfecting effect of the system. There is no proof whatever of any effects over and above that explainable by the action of hypochlorous acid (the effective agent in standard chlorine disinfection). The system is more expensive than all other chlorine dosing systems. 

2.0 Contract Stage Design 

2.1 Detailed Hydraulic Design  

2.1.1 Pipework

Calculations of head losses for pipework are recommended to be carried out at contract stage. These calculations should be based on the Darcy Equation, using the Colebrook - White Approximation for determination of l. This is the technique used in the Standard Pipework Headloss Calculation Spreadsheet (available from Expertise Limited). Please note that pipework that is not circular and running full will not conform to these standard calculations. An equivalent diameter method may be used for such pipework, in which the cross sectional area for flow is calculated, and converted to an equivalent circular cross section. 

Fittings headloss shall also be calculated. The method used shall be the “k” value method as detailed at Appendix II. This technique forms the basis of the calculation used in the Standard Pipework Headloss Calculation Spreadsheet. 

2.1.2 Channels

Calculation of channel head losses will be required at contract stage. 

The Standard Channel Headloss Calculation Spreadsheet (available from Expertise Limited) may be used for these calculations. In addition to the straight run head losses calculated by the spreadsheet, shock losses from bends, contractions and expansions may be calculated thus:  

Entry B1 > B2             Approximate headloss (m) = 0.015 (V12 - V22)

Entry B1 < B2             Approximate headloss (m) = 0.026 (V12 - V22)

Bend                           Approximate headloss (m) = 0.015 V2  

Where B = Channel Width (m); B1 is upstream of B2; V1 is velocity at B1 (m.s-1) 

2.1.3 Weirs

If it is desired to estimate depth of flow over a weir more accurately, a number of equations are available to determine depth of flow for a number of different weir cross-sections. Note that these standard calculations may not apply to weirs over 10 m long. Advice should be sought from an expert if it is desired to utilise weirs of over 10 m in length. Weirs are generally divided into two classes, being “thin” / “knife edge” and “broad”. This distinction may actually be defined by means of whether the nappe is free or adheres as it leaves the weir edge. This in turn is a function of the relationship between the head of water passing over the weir, and the breadth of the weir. We shall here define broad weirs as being those where weir breadth is greater than three times the head upstream of the weir. 

2.1.3.1 Thin Weirs

A knife-edge or thin weir will have a flow rate at a given depth over the weir according to the following formula, if approach velocity is less than 0.6 m.s-1: 

Q = 1.84 x (L - 0.2 H) x H1.5  

Where L = Weir Length (m); H = Head upstream of the weir (m); Q = Flowrate (m3.s-1) 

For deeper channels, and higher approach velocities, Bazin’s formula (see Appendix V) may be used. 

2.1.3.2 Broad Weirs

Determination of flow over broad crested weirs is more difficult than for knife-edge weirs, and there are a number of complicating factors, such as the degree of detachment of the nappe. A clinging nappe gives a higher discharge for a given head than a free nappe. 

The flow at a given weir breadth and head may be calculated from: 

Q = C x b x H1.5

Where b = weir breadth (m)

C varies from 1.4 to 2.1 according to weir shape and discharge condition. A value of 1.6 may be taken for estimation purposes. 

2.1.4 Nozzles

If at all possible, testing of nozzles to determine their head requirements at the desired appearance is recommended. It is worthy of note that for simple clear stream and aerated jet effects, plain pipe can give effects equal to or better than the far more expensive nozzles.  

2.2 Water Quality 

2.2.1 Chemical Composition

It is assumed that adequate information has been made available by construction phase on compliance of water with the requirements laid out in the previous section on composition.

Plant should have been included to remedy any of the undesirable features of any proposed supply water.

Coagulation and Filtration can remedy many of the Organic and Inorganic Colour problems. This may however involve construction of what is in effect a mini water treatment works, with metered dosing and controlled mixing of coagulants and pH correction chemicals, followed by additional filtration capacity. 

It may be possible to remove odorous compounds, and oxygen demand in the water by means of oxidant chemicals, including some of the disinfectant chemicals. 

Langelier indices in the aggressive range may be corrected by means of passive absorption of hardness from contactors containing magnesium and or calcium carbonates. It may be necessary to correct pH downstream of these contactors.  

2.2.2 Clarity

Since the client requirements for water quality are aesthetic rather than scientific, we recommend that filtration equipment should be purchased from manufacturers with a guarantee of 95% removal of particles > 5 um. While this will not guarantee overall water clarity, it is a common specification for filters producing drinking water. 

Given the preceding specification for filters, the clarity of the feature water is mostly dependent on the turnover, assuming all turbidity is caused by solid particles  > 5um. 

The turbidity of the water is the percentage of light that is absorbed as it passes through a sample of the water in a standard cell. This is therefore a measurement that corresponds inversely to the clarity of the water within the feature. If we assume that a similar amount of turbidity is added each day, estimates are available on the percentage of this added turbidity that remains when the filters have done their work. Table 1 overleaf shows this relationship.  

Table 1: Effect of Turnover Rate on Turbidity of Pool Water

 

Turnovers per day

1

2

3

4

5

6

7

8

9

% Turbidity remaining at equilibrium

58

16

5

2

1

0.5

0.08

0.02

0.01

 It is suggested that for most applications, a turnover rate of 4 will suffice, removing 98% of turbidity at equilibrium. Higher rates would only be required when there is an unusually high load of incoming turbidity.

 In the event that the turbidity is not filterable, advice should be sought from a water treatment expert on how to render the turbid components removable.

 2.2.3 Biological Quality

The importance of the maintenance of water quality has already been emphasised. Common practice has been to leave this area to the manufacturers of chemical dosing systems. It may be however that on some bigger systems, concerns over areas such as pH stability have resulted in the use of pH dosing plant in addition to the disinfectant dosing kit. 

The interaction between alkalinity, hardness and pH is complex, and the interaction between bromine / chlorine and these factors adds more complexity. There is in addition to this a demand for disinfectant chemicals from oxidisable substances, especially ammonia, which makes it difficult to predict with accuracy the exact demand for dosing chemicals. 

It is however necessary to be able to make a rational decision as to whether to install pH dosing equipment in addition to simple disinfectant dosing plant. It would be best to start from the analysis of water to be used within the feature in making this decision. The issue of size of feature is normally considered the major indicator as larger features have a greater chance of harbouring stagnant areas where disinfectant chemicals may be destroyed by sunlight, or chemically used up, resulting in an area susceptible to biological growth. It may actually be best to address this feature by means of ensuring no such dead areas occur. 

The pH and Alkalinity of the water to be used both affect the need to have pH correction equipment. Both chlorine and bromine disinfectants give best results at pH 7.2 to 7.8. Water that enters the system at a pH far from this range may be unsuitable for disinfection without pH correction. In addition to the loss of desirable disinfectant effect, there are a number of undesirable chemical reactions that are promoted by pH values far from optimal values. Tablet brominators of the type commonly used for water features use chemicals that have very little effect on pH. Use of gaseous chlorine, and sodium hypochlorite move pH into acid and alkaline ranges respectively. If these chemicals are used, the advice of an expert should be sought in order to determine the pH that results from their addition. If it is desired to correct pH, the alkalinity of the water must be taken into account in deciding which chemicals to use. Water with less than 50 ppm of Alkalinity will have a very variable, and hard to control pH value. 

The remainder of the document and standard calculation spreadsheets are available on request.