Water & Waste — I&I Simulation
Rainfall-derived inflow & infiltration by sewer basin, using the EPA SSOAP RTK unit-hydrograph method. Rain volume is split into fast direct-connection inflow and a slow groundwater infiltration tail, with a tidal term for coastal basins.
This view uses illustrative model parameters. RTK triads, tidal coefficients and capture ratios are assumed defaults until calibrated per basin from measured storms.
Net RDII reduction: 0%. About 50% of I&I is assumed to enter through private laterals, so public work alone tops out at 30% even at full effort.
What this chart is saying
The pipes are overwhelmed. At hour 12 the basin is trying to send 14.8 MGD into a system that can carry 13.6 MGD (1.1× capacity). Demand stays above the red line for 4 hours, from hour 11 to hour 14. The excess isn't conveyed: it surcharges the mains and escapes as sanitary sewer overflows (SSOs) at low manholes and backups into buildings.
The three colored bands are stacked, so the top edge of the stack is the total flow in the pipe. The dashed red line is the most the pipes and pumps can carry. Anything above that line has nowhere to go.
Base sanitary. Normal wastewater from homes and businesses. It rises and falls each day and doesn't depend on weather.
Inflow (fast). Rainwater that enters directly through vented manhole lids, roof drains and storm cross-connections. It spikes while the 3h storm is falling and drops off quickly after. The fix is to seal lids and remove illegal connections.
Infiltration (slow). Groundwater and seawater seeping in through cracked pipes and joints. It builds slowly, lasts for days and rises and falls with the tide twice a day even after the rain stops. The fix is to line the pipes.
Why the green band keeps waving: this is a coastal basin. High tide pushes seawater into submerged pipes, so infiltration crests again around hour 25, long after the rain ends. Lining these pipes does more than sealing manholes.
Try the rehabilitation slider: it lowers the inflow and infiltration bands. The goal is to bring the top of the stack under the red line.
- Base sanitary
- 8.1 MGD
- Inflow (fast)
- 3.6 MGD
- Infiltration (slow)
- 3.2 MGD
- Flow demand
- 14.8 MGD
- Pipe capacity
- 13.6 MGD
- Over capacity
- 1.2 MGD
- Volume with nowhere to go
- 0.12 MG
Engineering reality check
- The curve shows demand, not delivered flow
A system rated for the red line cannot convey the peak above it. Gravity mains surcharge to the hydraulic grade line, wet wells fill, and the roughly 0.1 MG above capacity backs up into homes or surfaces at low manholes as SSOs. The lift station only ever receives flow up to capacity, so a downstream flow meter will read lower than this curve.
- Lining mains alone won't close the gap
Sealing manholes removes the fast inflow band. CIPP lining on public mains does not reach private service laterals, which make up as many as half of the joints in the network. With seawater-saturated ground at high tide, saline head still pushes through cracked lateral tie-ins and cleanouts. Pair manhole dish inserts and raised rims with private lateral inspection, lining to the building and backflow preventers.
- The plant takes the hit too
Saline, dilute influent upsets activated-sludge biology, and the extra volume can exceed permitted secondary clarifier overflow rates and injection-well capacity at the receiving district plant.
Why this matters in Miami-Dade
- EPA / FDEP Consent Decree
WASD's consent decree (CMOM) requires it to find and remove high RDII before untreated sewage reaches Biscayne Bay, the Miami River or the coastal canals. 4 hours of surcharge in this scenario means reportable SSO risk and exposure to penalties.
- Miami Oolite and the Biscayne Aquifer
The basin sits on porous limestone that is hydraulically tied to the bay. At a King Tide (+2.5 to +3.0 ft NAVD88), seawater moves through the rock and submerges the pipes under pressure, so the unlined mains and the private laterals take the full head.
- Coastal salinity structures (S-29)
When the bay surges, SFWMD closes the gates at structures like S-29 on Snake Creek. Runoff can't drain out, canal stages rise, and the water table comes up to within inches of the road. The mains are squeezed between ponded stormwater above and tidal groundwater below.
Briefing takeaway: 73% of the extra volume in this scenario is slow infiltration and saline intrusion, not fast rain inflow. With demand peaking at 14.8 MGD against 13.6 MGD of conveyance, lining public street mains won't eliminate SSOs on its own. WASD also has to address private laterals, put check valves and backflow isolators on coastal outfalls, and raise pump station discharge capacity during astronomical high tides.
Method: EPA SSOAP RTK unit hydrograph. Each rainfall increment over the 4,200-acre sewershed is convolved with fast, medium and slow triangular responses; volume is conserved as acre-inches × 0.027154 MG/acre-in × ΣR. Coastal basins add a semidiurnal tidal infiltration term. Parameters are illustrative until calibrated per basin.