Topic 8: Reservoir Sizing
Description
Reservoir sizing is one of the most important tasks in drinking water storage planning. The selected storage volume directly influences supply reliability, operational flexibility, water quality, construction costs, and long-term system performance.
A reservoir must be large enough to balance fluctuations in demand, provide emergency reserves, and support maintenance activities. At the same time, it should not be so large that water remains in storage for excessive periods.
Finding the correct balance between storage capacity and water turnover is therefore a key objective of reservoir design.
Contrary to popular belief, the largest reservoir is not necessarily the best reservoir. An optimally sized facility provides sufficient operational security while maintaining good hydraulic performance and high water quality.
Why Reservoir Sizing Matters
Water demand varies continuously throughout the day, week, and year.
During some periods, water consumption is low and reservoirs are filled. During other periods, especially during peak demand, stored water is released into the distribution network.
The storage volume determines how effectively the reservoir can:
- balance production and consumption
- stabilize network operation
- provide emergency reserves
- support firefighting requirements
- maintain supply during maintenance activities
- improve operational flexibility
An undersized reservoir may struggle to meet demand, while an oversized reservoir may create water quality challenges.
Factors to Consider
Successful reservoir sizing requires consideration of several technical, operational, and future planning factors.
Average Daily Demand
The starting point for sizing calculations is the average daily water demand.
This value represents the typical amount of water consumed by the service population under normal operating conditions.
Understanding average demand helps determine the baseline storage requirements and provides the foundation for all further calculations.
Peak Demand
Water consumption is rarely constant throughout the day.
Morning and evening periods often generate significantly higher demand than the daily average. Seasonal variations may also increase consumption in certain climates.
The reservoir must be capable of supplying these temporary demand peaks without compromising system pressure or service quality.
Peak demand analysis helps determine the active balancing volume required within the reservoir.
Emergency Reserve
Every drinking water system should maintain a reserve for unexpected situations.
Potential emergency scenarios include:
- pump failures
- well outages
- treatment plant interruptions
- power failures
- transmission pipeline breaks
- natural disasters
Emergency storage allows the system to continue operating while corrective actions are implemented.
The required reserve volume depends on local risk assessments and the desired level of supply security.
Firefighting Requirements
In many systems, reservoirs must also provide water for firefighting purposes.
Fire events can require large water quantities over relatively short periods. Storage capacity must therefore account for these temporary but critical demands.
Where firefighting reserves are required, they often represent a significant portion of the total storage volume.
Future Population Growth
Reservoirs are typically designed to operate for several decades.
During this period, population growth, urban expansion, industrial development, and changing consumption patterns may increase water demand significantly.
A well-designed reservoir should therefore consider:
- projected population growth
- future service area expansion
- economic development
- changing water consumption patterns
Planning for future demand can reduce the need for costly infrastructure upgrades later.
The Common Mistake: Oversizing for Safety
A frequent planning mistake is the assumption that a larger reservoir automatically creates a safer and more reliable system.
As a result, some reservoirs are intentionally oversized to provide a perceived safety margin.
While additional storage may appear beneficial, excessive storage volume often introduces new operational challenges.
Consequences of Oversized Reservoirs
Oversized reservoirs commonly result in:
- excessive water retention times
- poor water turnover
- water aging
- temperature increases
- greater risk of microbial growth
- sediment accumulation
- increased cleaning requirements
- higher construction and maintenance costs
In extreme cases, water quality problems caused by excessive storage can outweigh the operational benefits of the additional volume.
The Goal: Balanced Design
The objective of reservoir sizing is not to maximize storage volume but to optimize it.
An effective reservoir should provide:
- sufficient balancing capacity for daily demand fluctuations
- adequate emergency reserves
- reliable firefighting storage where required
- flexibility for maintenance and operational activities
- good water turnover and acceptable retention times
The most successful designs achieve both operational reliability and water quality protection simultaneously.
Summary
Reservoir sizing is a critical component of drinking water storage planning. Storage volume influences system reliability, emergency preparedness, operational flexibility, and water quality.
When determining reservoir capacity, engineers must consider average demand, peak demand, emergency reserves, firefighting requirements, and future growth. At the same time, excessive storage should be avoided because it can increase retention times and negatively affect water quality.
A properly sized reservoir is not the largest possible reservoir—it is the one that provides the right balance between supply security, operational efficiency, and long-term water quality protection.