Sizing a cistern to meet Low Impact Development (LID) rules is challenging because most jurisdictions provide limited guidance, and existing tools do not yet include Rainwater Harvesting (RWH). Sizing is best done with a continuous daily simulation model that calculates runoff captured, overflow and runoff released, domestic water savings, and required makeup water.
In terms of sizing there are basic relationships that are important to understand. There is a limit to the percentage of runoff reduction RWH can achieve. For sites where annual re-use demand is less than annual runoff, the runoff reduction limit is simply the re-use demand divided by the annual runoff volume. No matter how big the cistern is, this threshold cannot be surpassed. Even for sites where re-use demand is greater than the annual runoff volume, increasing cistern size offers diminishing returns because of the seasonality of rainfall in most locations.
It is usually better to include more re-use applications to increase re-use demand than to increase cistern size beyond standard water quality volumes typically in place today.
2. Design Life
Design life is an important design consideration and different material types offer a wide range of life expectancy. Similar to detention systems, it is imperative to select a material that will last in the local conditions for the life of the project, which can be many decades.
Unlike detention systems where the primary concern is the structural integrity, cisterns must also remain watertight during their life. For example, systems depending on liners may have a significantly shorter design life because they may be prone to leakage and cannot be repaired. While underground metal cisterns can have a design life of more than 50 years in many situations, some sites have soils that can be corrosive to metals. Other materials such as Steel Reinforced Polyethylene (SRPE) may be better suited for these situations.
Ultimately, the best solution is dependent on the desired design life and the local soil conditions.
3. Structural Capacity
Structural capacity is a crucial factor when selecting a cistern. For commercial-scale systems, cisterns can be large, like detention systems, and many times the obvious choice is to install beneath a parking lot.
Some of the smaller, entry level cisterns have limited loading capacity, so it may be better to locate them in a green space. For project sites that experience seasonal high groundwater, the cistern manufacturer should supply buoyancy calculations. Most cisterns can be equipped with anti-buoyancy devices that prevent floating.
Cisterns built from crates should be avoided in this situation because they cannot be strapped down effectively.
Installation and handling can be an overlooked design factor when choosing a cistern. Many materials — such as fiberglass — require stone backfill and are not strong enough to be backfilled with native soil. For large systems, these costs can add up.
For other materials such as SRPE, depending on the quality of the native soil, it may be possible to use a competent native material as backfill for the cistern. Using local materials as backfill can save the cost of exporting excavated soils and importing expensive stone.
In addition, some materials must be handled with care and have a low impact resistance; bumps and dings sustained during unloading and installation can be costly to repair and upset project schedules.
This is an excerpt from the Professional Development Article: Cistern Design Considerations for Large Rainwater Harvesting Systems.