--- title: "Is reservoir necessary: village of 800, spring 0.5 L/s" subject: "Water Supply Engineering" chapter: 7 key: "semester_5/water_supply_engineering:res-b81" source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/res-b81 site: BCT Engineering --- # Is reservoir necessary: village of 800, spring 0.5 L/s Water Supply Engineering · Chapter 7: Reservoirs and Distribution System - Asked in: 2081 Bhadra, 8 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/res-b81 - Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-7 ## Question A village has a population of 800, with a per capita water demand of 45 liters per day. The demand is to be met by a continuous supply system from a spring source with a safe yield of 0.5 liters per second. The consumption pattern is | Time | 00-04 | 04-08 | 08-12 | 12-16 | 16-20 | 20-24 | |---|---|---|---|---|---|---| | Demand (million liters) | 5 % | 30 % | 25 % | 10 % | 20 % | 10 % | Is reservoir necessary? Calculate its capacity if necessary. ## Short answer Daily demand 36 m3, spring yield 43.2 m3/day. Peak demand exceeds inflow, so balancing tank needed. Capacity by mass curve method is 7.2 m3 (adopt 8 m3). ## Answer #### Given data - Population 800, demand 45 lpcd: daily demand = 800 × 45 = 36,000 L = 36 m³/day - Spring yield = 0.5 lps = 0.5 × 3600 = 1800 L/h = 1.8 m³/h = 43.2 m³/day - Demand pattern (% of the daily demand) for six 4-hour periods (the table gives % although the heading says million litres) #### Check of source and need of storage The yield of 43.2 m³/day is more than the demand of 36 m³/day, so the source is enough for the whole day. But in 4–8 h the demand is 30% × 36 = 10.8 m³ in 4 h = 2.7 m³/h, and in 8–12 h it is 2.25 m³/h; both exceed the inflow of 1.8 m³/h. Therefore a reservoir **is necessary**. #### Mass-curve table (analytical method) Inflow: 1.8 m³/h (spring yield). Time is counted from the start of the first demand period; a positive cumulative value means surplus in the tank. | Time | Demand (m³) | Inflow (m³) | Inflow − demand | Cumulative | |---|---|---|---|---| | 00:00–04:00 | 1.80 | 7.20 | +5.40 | +5.40 | | 04:00–08:00 | 10.80 | 7.20 | -3.60 | +1.80 | | 08:00–12:00 | 9.00 | 7.20 | -1.80 | 0.00 | | 12:00–16:00 | 3.60 | 7.20 | +3.60 | +3.60 | | 16:00–20:00 | 7.20 | 7.20 | 0.00 | +3.60 | | 20:00–00:00 | 3.60 | 7.20 | +3.60 | +7.20 | Total demand = 36.00 m³; total inflow = 43.20 m³. #### Capacity - Maximum cumulative surplus = 7.20 m³ - Maximum cumulative deficit = 0.00 m³ - Balancing capacity = max. surplus + max. deficit = 7.20 + 0.00 = **7.20 m³** - Because the inflow in a day (43.20 m³) is more than the demand, the extra water simply overflows once the tank is full. The minimum storage that still meets every demand is the largest draw-down, 5.40 m³ (from +5.40 at 04:00 to 0.00 at 12:00). The textbook value of 7.20 m³ is on the safe side. **Answer: A reservoir is necessary; balancing capacity ≈ 7.2 m³ (adopt 8 m³).** ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the daily demand of 800 people at 45 lpcd? A. 3.6 m3 B. 36 m3 C. 360 m3 D. 45 m3 Answer: B. 800 x 45 = 36,000 L = 36 m3. (Key point: Daily demand) 2. Why is a reservoir needed although the spring yields 43.2 m3/day (more than 36 m3/day)? A. The yield is too low B. Demand in 4-8 h (2.7 m3/h) and 8-12 h (2.25 m3/h) exceeds the inflow of 1.8 m3/h C. The village is large D. Yield is zero at night Answer: B. Peak-period demand rates exceed inflow, so storage must balance them. (Key point: Need of reservoir) 3. What volume is demanded between 04:00 and 08:00 (30%)? A. 10.8 m3 B. 7.2 m3 C. 3.6 m3 D. 1.8 m3 Answer: A. 30% of 36 m3 is 10.8 m3. (Key point: Demand in 4-8 h) 4. What balancing capacity is found? A. 5.4 m3 B. 7.2 m3 C. 3.6 m3 D. 43.2 m3 Answer: B. Max surplus 7.2 + max deficit 0 = 7.2 m3 (adopt 8 m3). 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