WATER DISTRIBUTION SYSTEMS DESIGN

INTRODUCTION

The main source of water is the hydrologic cycle, a continuous recycling of naturally occurring water through the processes of evaporation, precipitation, transportation, infiltration, percolation and runoff. As population increases, the total water requirement of any community or settlement increases also. According to the United Nations Population Reference Bureau, the overall rate of increase in population may be taken as about 0.35 percent per year.  In our traditional communities, shallow wells (ground water storage) were a regular source of water. The growing demand for urbanization has increased the water demand per capita and also the need for more complex and extensive ground water production, storage, distribution and operation requirements.

The importance of water to a community cannot be overemphasized and it therefore must be produced in an efficient, reliable and economic manner. Bearing in mind that plumbing fixtures in today’s buildings do not function properly on pressures below 20 psi (14m), a reliable water supply system must not only provide sufficient quantities of water, it should be provided at acceptable operational pressures.

SYSTEM DESIGN REQUIREMENTS

The water requirement of any community largely results from

·         The population

·         Existing climatic condition

·         The mode of living and habitation

·         Type of plumbing facilities in use

·         Type of sewerage system available

·         Rate of industrialization

·         Taxation rate (water tax)

Considering the above, a typical water consumption table for different uses for residential designs is given in Table 1 below. In some instances where large areas are covered in the design, the figures shown in Table 2. Can be applied. It is however note worthy that these figures will vary for different locations. It is therefore necessary to consider the location you are designing for.

A water distribution system is a link between the water supply source and the water consuming end use. The primary design factors for any system are based on the calculated total water requirement and the peak flow rate that must be delivered. In designing this distribution system, consideration should be given to the frictional losses that occure in transmission channels (conduits), which is closely related to the flow in the conduit, size of conduit, number, type and size of fittings.

A network of pipes is used to distribute water to a community and this is achieved through patterns such as the branching pattern, grid pattern or a combination of the two. Whatever system we choose, the pressure and flow requirements must be met. For residential areas having houses three to four stories high, the pressure in the pipes should range between 18.6 m to 28m of water whereas for business areas, the requirement is about 50 m of water. In any case, the water velocity in the pipes should not exceed 0.6 to 1.2m/sec.

Relevant codes and standards should be checked to ensure conformity to minimum size requirements and minimum pressure requirements for service stubs, distribution mains and arterial or feeder mains.

Flow Considerations in design: A typical design criteria used in some water systems design follows thus:

1. Design Flows - The water main system must be able to meet the following flow requirements:

A. Peak day demands plus fire flow demands.

B. Instantaneous peak demands for water mains from source treatment and/or storage facilities.  Peak day demands plus fire flow demands must also be met.

2. Peak Day Demands

A.            General

The peak day demand is the average rate of consumption on the maximum day. The maximum day is the 24-hour period during which the highest consumption total is recorded in the latest 3-year period. High consumption that will not occur again due to changes in the system, or that was caused by unusual operations, will not be considered. When no actual figure for maximum daily consumption is available it will

be estimated on the basis of consumption in other similar service areas.

Such estimates will be at least 2.0 times greater than the average daily demand for more than 500 people and 2.5 times greater for 500 people or less. When a system is in two or more service levels, consider the total maximum daily consumption that must pass through the service level being reviewed.

B.            Average Day Demand (Minimum)

Average Daily Demand = Area x Area Density x Rate =

                                          =Number of Units x Unit Density x Rate

3. Instantaneous Peak Demands

Based on the assumption that the instantaneous peak flows for water supply should be greater than the extreme peak wastewater flow the following has been set as the Instantaneous Peaking Factor:

A. 220 people or less = Average day demand (gpm) x 9.0.

B. more than 220 people = Average day demand (gpm) x 7/P0.167

P = design year population in thousands.

If major water users exist in the system, the peak may be greater than those

listed above.

4. Fire Flows

Fire flows shall be in accordance with Uniform Fire Code or International Fire Code.

 Hydrant Distribution: This shall be in accordance with Uniform Fire Code or   

International Fire Code.

To be continued

The subsequent part of this paper discusses the water distribution design for a 51.22 hectare community development partitioned into 308 residential and commercial plots, public and green spaces.  The final design  drawings are also available. The discussion will also cover hydraulic modeling and analysis of the distribution network.

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