How Is Rainwater Harvesting Civil Design Done in the Caribbean?

The yield calculation is straightforward but unforgiving of bad inputs. The engineer measures the true roof catchment, deducts areas that cannot drain to the cistern, applies a runoff coefficient reflecting the roofing material, and multiplies by the design rainfall, usually analyzed month by month to capture the wet and dry seasons. Demand is profiled the same way: fixture counts and irrigation areas become gallons per day, with seasonal variation where it matters. The cistern volume is then set by the cumulative deficit through the design dry period. Reliability targets drive the final size. A system meant to carry a household through the typical dry season needs a different volume than one meant to survive a record drought, and the engineer presents the tradeoff explicitly: storage volume against cost, footprint, and the probability of running dry. First-flush diverters are sized to the catchment area, discarding the initial runoff that carries roof dust, debris, and contaminants before the clean flow reaches the tank. Conveyance, gutters, downspouts, and piping, is sized for the design storm intensity so the system captures water instead of overwhelming the inlets.

Treatment is designed backwards from the tap. Non-potable uses get screening, first-flush diversion, and basic sediment control; potable systems add staged filtration down to fine micron ratings plus disinfection with the contact time or dosage the equipment requires. The engineer specifies the components, their maintenance intervals, and the water-quality testing that verifies performance, because a treatment train that is not maintained is a false sense of security. All piping is labeled and separated by use, with reduced-pressure backflow assemblies wherever harvested water meets the municipal system. The cistern's structural design treats it as the water-retaining structure it is. Concrete and ferrocement tanks are designed for full hydrostatic pressure, crack control to limit leakage, and buoyancy uplift when groundwater is high and the tank is empty. Manufactured tanks need engineered foundations and anchorage for wind and seismic loads. The permit drawings show the tank details, the plumbing riser diagram with backflow devices, and the overflow connection to the stormwater system. The AHJ reviews the plumbing, structural, and water-quality aspects under the applicable codes, and site inspections verify the tank construction and the cross-connection controls before the system goes live.