Table of Contents
Definition of Design Period:
The design period could be defined as the number of years in the near future in which the supply will surpass the demand. Or, It is the number of years in the near future in which the given facility is obtainable to satisfy the demand.
Why Design Period Is Provided?
The main reasons behind providing Design Periods are:
 It is prodigiously strenuous or unfeasible to produce frequently.
 It is affordable to supply one mammoth unit in lieu of constructing a variety of bijou ingredients.
Also Read: 19 Different Types of Slabs in Construction  What Is a Slab  Types of Slabs
Factors Affecting Design Period:
Factors affecting the Design Period are given below. Following are some aspects that act on the Design Period of the structure:
 Availability of Funds.
 Life of the structure.
 Ease or Difficulty in Extension.
 Rate of Increment.
 Lead Time.
 The economy of Scale.
 Interest Rates.
 Quality of Material.
 First Cost.
1. Availability of Funds:
If funds are obtainable, one must hold on to a smaller design period.
2. Life of Structure:
Life of structure is the number of years in the near future in which the design period is physically & practically suitable to bestow the intended amenity.
So it always ought to be less than a lifetime of the structure.
 Pumps.
 Conduits(Type of Pipes).
Factors Affecting Design Period Type of Pipe 
Average Service Life

Steel Pipes  25 – 50 Years 
Cast Iron Pipes  100 Years 
Concrete Pipes

75 Years 
3. Ease or Difficulty in Extension:
For projects whose augmentation is unquestionably feasible, it’s generally kept low. For instance, we are capable of inaugurating new tube wells at any time since we don’t require to install all tube wells which might be required after 20
 Tubewell: Easy to drill.
 Treatment Plant: Easy to enlarge.
But for the projects whose augmentation is strenuous, their design period is kept sizeable. For instance, dams and reservoirs cannot be augmented
Adroitness and difficulty that’s likely to be faced in expansions, if undertaken at future dates. For instance, vigorous expansions mean choosing a finer value of the design period.
4. Rate of Increment:
If the speed of growth is lofty, then for that region dwarf design period is required. Anticipated rate of population growth, including feasible shifts in communities, industries, and commercial inceptions.
For instance, if the speed of increase of population is comparatively sloth, a better figure for the design period could also be chosen.
5. Lead Time:
It is the time from the outset of a project to its completion. The design period should be greater than the interval.
Lead Time = Time for Design of Project + Construction Time
6. Economy of Scale:
 The decrease in monetary value parallel to the increase in a facility is thought of as an Economy of Scale.
 If the economy of scale is little, smaller design periods are used.
 It’s economical to make an oversized structure, for an extended design period.
7. Interest Rates:
 If the charge per unit is bijou, the lofty value of the design period is also economically possible.
 The rate of interest depends on the borrowings and also the ancillary money which needs to be invested.
 Amount and availability of ancillary investment likely to be incurred for extra arrangements. For instance, if the funds aren’t available, one should keep a bijou design period.
8. Performance Time:
Structures are checked under working conditions in due course, which mustn’t be considered in the design period. During this point, it’s not providing facilities to the community.
9. Quality of Material:
 The design period mustn’t be longer than the lifetime period of the material utilized in the installation project.
 The quantity of years in which a provision is formed in designing the capacities of the varied modules of the water system scheme is recognized as the design period.
10. First Cost:
In case of High first cost and High Rate of interest, the design period should be brief, so that the aggregate cost of the project should not be disseminated to the present population e.g. Houses.
Design Period Values:
Water system projects, under a standard state of affairs, could also be designed for a design period of 30 years. This 30year period may, however, be amended in relevance to specific units of the project, depending upon.
 The useful lifetime of the component facility.
 Ease in polishing off extensions when required.
 The rate of interest is in order that expenditure, far ahead of utility, is avoided.
The design period for designing the assorted components of a facility project is provided in the ensuing table:
Item 
Design Period in Year

Storage by Dams  50 year 
Intake Works  30 year 
Pumping
1. Pump House 
30 year 15 year 
Water treatment units  15 year 
Pipe connection to the sundry treatment units and other small paraphernalia.  0 year 
Raw water and clear water transporting units  30 year 
Clearwater reservoirs at the head works, balancing tanks, and service reservoirs  15 year 
Distribution system  30 year 
What Are Design Period and Return Period for Design Conditions?
 Design life means the rock bottom span of a structure that is predicted to last.
 The longer the design life; the further the cost of a project.
 Therefore, in choosing the design life for a structure, engineers ought to inspect the design life which spawns an economical project without relinquishing the desired function.
 In a selection of return periods of particular design conditions, winds, waves, etc., one ought to consider the connotations of overexpenditure.
 In fact, there are ordinarily no paramount or maximum values of those design conditions, and its selection is anticipated on the probability of expenditure which is set forth to the return period.
 Hence, design life might not be indistinguishable from the return period of design conditions because their selections are succored on different considerations.
What Is Per Capita Demand
Per capita demand. Total consumption of water for a water supply system in a year divided by the population and the number of days in the year is called per capita demand. It can be expressed in liters per capita per day.
Design of Grit Chamber
Transition law: The design of grit chamber is based on removal of grit particles with minimum size of 0.15 mm and therefore Stoke’s law is not applicable to determine the settling velocity of grit particles for design purposes.
Per Capita Demand of Water
Per capita water demand: It is the ratio of the annual average water consumption of a community to the total population for 365 days. Hence, Per Capita Demand is the annual average amount of daily water required by one person.
Return Period Formula
Thus the return period of an event of a given magnitude may be defined as the average recurrence interval between events equal or exceeding a specified magnitude. The probability p = P(X >= x_{T}) of occurrence of the event X >= x in any observation may be related to the return period in the following way.
Flood Return Period Formula
The theoretical return period between occurrences is the inverse of the average frequency of occurrence. For example, a 10year flood has a 1/10 = 0.1 or 10% chance of being exceeded in any one year and a 50year flood has a 0.02 or 2% chance of being exceeded in any one year.
Exceedance Probability and Return Period
The exceedance probability may be formulated simply as the inverse of the return period. For example, for a twoyear return period the exceedance probability in any given year is one over two = 0.5, or 50 percent. 1 (1p)^{n} .
Return Period in Hydrology
The return period is a probabilistic concept used to measure and communicate the random occurrence of geophysical events such as floods, droughts, storm surges, tsunamis, earthquakes, landslides that may produce huge economic, social and envi ronmental damages.
What Is Return Period of Flood
Commonly referenced return periods when assessing flood risk include: 1 in 30 year (3.3% annual exceedance probability) High risk; 1 in 100 year (1% annual exceedance probability) Medium risk; and. 1 in 1000 year (0.1% annual exceedance probability) Low risk.
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