Question

A small community is expecting to expand the capacity of its water distribution system and associated components. Specifically, the community will need to consider the design of its groundwater wellfield, conveyance structures, and the high-lift pumps for its pump station. To begin the analysis necessary for that planned expansion, (A) estimate the following flows for this community: average, maximum daily, max weekly, maximum monthly, and maximum hourly. B. Determine the required capacities of those expanding structures assuming (1) arithmetic growth and (2) geometric growth trends. Compare your answers for these assumed growth conditions accordingly. You may assume the following population and water usage patterns: current population is 120,000 people; population estimates for 10 years out are 142,500 people; water usage over the last year totaled 2.096e7 m3; water usage reached a maximum of 92.4e6 L on April 3rd of that year; and the pump station holds 3 pumps plus one reserve unit.

Answer 1

Solution –

1. Given Data,

Current population = 120,000

Water usage over last year = 2.096 * 10⁷ m³ = 2.096 * 10¹⁰ litres

                                             = 5,537,046,217.43 gallons

                                             = 5537.05 million gallons

Average Daily consumption (q) = Total yearly water usage / (365)

q = 5537.05/ (365)

q = 15.17 MGD

(MGD = million gallons per day)

For determining other average flows, we use Goodrich formula

P = 180 * t^-0.1

Where,

P = Factor multiplied by average daily to determine the maximum of that period.

t = time in days.

Maximum daily consumption = 1.8 * q

                                                            = 1.8 * 15.17

                                                            = 27.306 MGD

Maximum hourly consumption = 1.5 * [1.8 * q/24]

                                                            = 2.7 * (q/24)

                                                            = 1.71 MGD

Average weekly = Total yearly water usage / (52)

                              = 0.29 Million gallons per week

Average monthly = Total yearly water usage / (12)

                              = 1.26 million gallons per month

Maximum weekly consumption = 1.48 * average weekly

                                                            = 1.48 * 0.29

                                                            = 0.43 million gallons per week

Maximum monthly consumption = 1.28 * average monthly

                                                            = 1.28 * 1.26

                                                            = 1.62 million gallons per month

2. Current population = 120,000

After 10 years, population = 142,500

By arithmetic method,

Annual increment = (142500 – 120000) / 10

                              = 2,200

After 10 more years,

Population will be = 142500 + 2200*10

                              = 164,500

After 1 year,

Population will be = 142500 + 2200

                              = 144,700

By geometric method,

Percentage increase = 2200 / 120000

                                             = 1.83 % per year

After 10 years

Population will be = 142500 (1 + 0.0183)¹⁰

                                             = 170,331

After 1 year

Population will be = 142500 (1+0.0183)

                                             = 145,108

Increase in population in geometric method is more as compared to increase in population from arithmetic method.