Department of water affairs and forestry technical guidelines for the development of water and sanitation infrastructure

5.3Water availability

A common cause of scheme failure is the overestimation of the availability of water. Care must be taken that the underlying assumptions of water source availability are proven, especially in the case of ground water and where river abstraction is not controlled by significant upstream storage.

5.4Power availability and alternative pump drives

It is also important that the availability of a power supply for the project is properly investigated. Installation of long power lines may prove to be very expensive. Dedicated transformer supply points are preferred.

In some instances an economic Eskom power supply may not be available, in which case alternative power sources for driving pumps must be considered. There are many factors to be considered in these instances and it will be up to the engineer to investigate these factors. Some of the factors to be considered are:

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  Amount of power required.
  
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  Can this be provided by solar means - usually for small power requirements.
  
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  Can the pumps be directly driven by a diesel engine.
  
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  Can a diesel generator be provided in anticipation of later conversion to Eskom power supply.
  
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  Are facilities for operation and maintenance of diesel engines available,
  
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  The comparable cost of each solution.
  

5.5Guidelines for economic optimisation of pump - pipeline sizing

Two or three of the preferred layouts should be compared in terms of cost, technical feasibility, economic viability, social and environmental impacts.

5.5.1Economic analyses

At the preliminary design stage, economic analyses will normally only be undertaken for surface water pumping mains. The purpose is to optimise the pump and pipeline sizes by comparing the net present value of alternative configurations.

These analyses are not usually relevant for small village pumping mains from localised boreholes.
Usually the analysis compares net present values over a planning horizon of 30 years.
The recommended criteria to be used for economic analysis of water supply infrastructure are as follows:
(i) Infrastructure Lifetime:
Small pumps and motors 15 years

Electric installation 15 years

Structures and buildings 30 years

Pipelines 30 years

(ii) Power Costs: Eskom Tariff A
(iii) Operation and Maintenance Costs:
Pump station: 0,5 % per annum (p.a.) of total pipeline costs

0,25 % p.a. of pump station civil costs

4 % p.a. of pump station mechanical and electrical costs
Pipeline: 0,5 % p.a. of total costs
Civil Works: 0.25 % p.a. of total costs
(iv) Discount Rates:
8% p.a. or the official Government discount rate as revised from time to time (but sensitivity analysis also done to test for 6 % and 10 %)

5.5.2Pump and pipe sizing for small borehole schemes

In most cases the boreholes for small schemes have yields of less then 5l/sec and only require a pumping main of one to two kilometres to the village storage or reticulation. Typically, pipe diameters are less than 100mm. Pumps for borehole schemes are also usually small, typically less than 10 kW.

Sizing of pipes, and therefore pumps, can normally be made using simple engineering calculations without the need for undertaking optimisation analyses.
Simple guidelines for this type of scheme are:
(i) Pipeline velocities of between 0,8 and 1,0 m/s generally result in optimised pump and pipeline size.
(ii) For diesel engine powered pumps the optimum pump and pipeline should be governed by the option with the lowest recurrent diesel fuel consumption.

6DESIGN REPORT, DRAWINGS AND SPECIFICATIONS

6.1Introduction

This section deals with the design report and the design drawings.

A typical water supply scheme comprises the following components:

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  A source, which could be boreholes or a dam or a weir;
  

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  A water treatment plans;
  
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  A pumps station;
  
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  Pipelines;
  
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  Reservoirs; and
  
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  Consumer connections.
  

The design of all of these components are discussed in following sections.

A section has also been included on VIP Pit Latrines.

6.2Design report

Design reports should generally contain the following information:

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  Cover page (Region/Province’s name, District Council/Water Services Authority, Consultant’s name, Scheme name, CWSS No. Locality number, File No., Date, and report's status, i.e. draft 1, draft 2 etc.).
  
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  Index/Contents page.
  
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  Executive summary.
  
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  Description of the project. Refer to locality and scheme layout plans, preferably in A3 size.
  
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  Historical background – for existing supply.
  
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  Statistical data.
  
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  Design philosophy.
  
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  Water source - to be correctly and thoroughly described as described below.
  
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  Design assumptions.
  
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  Sizing of components/infrastructure (litres per second or m³) - clearly indicate how sized. Peak factors should be motivated.
  
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  Discussion of alternatives and choice of preferred solution.
  
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  Design standards – SANS, BS, etc. standards used to design each component.
  
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  Special problems and their solution.
  
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  Methods of analysis.
  
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  Geology.
  
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  Foundation treatment.
  
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  Diversion.
  
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  Instrumentation/ Scheme’s control system.
  
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  Quantities.
  
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  Costs including fees, contingencies and VAT and indicate schemes budgeted amount.
  
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  Programme – bar chart in weeks/months.
  
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  Recommendations.
  
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  References - White papers, feasibility reports, previous and / or other phases design reports, etc.
  
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  Annexures - not greater than A3 size containing a locality plan.
  

Usually 5 copies of each report will be required.

An expanded framework for the design report is included in the design folder.