The first article in our series of the I/I Cycle discussed the four major steps involved in establishing a successful and sustainable inflow & infiltration (I&I) reduction program.  The second article covered the first of four steps, Flow Monitoring. Now we move on to step two.

Once you have had the monitors in the ground for at least three months and hopefully captured data from several good rain events (the 1’-2” variety), what do you do with that data?

Step two in the four step I/I Cycle is to Prioritize Mini Basins.  Mini Basins are generally broken down into areas upstream of major lift stations or trunk sewers, and typically range from 15,000 to 25,000 feet in size.

Inflow vs. Infiltration

Inflow is extraneous stormwater that enters into the sanitary system through roof drains, defective or missing cleanout caps, sump pumps and cross connections between storm sewers and sanitary sewers. On the hydrograph, inflow can be seen instantly and has typically subsided within 48 hours of a rain event.

Infiltration is water that enters a sewer system from the ground through defective pipe joints, deeper manhole defects, broken sewer mains or service line to main connections. Infiltration is primarily related to high levels of groundwater, and the effects of high groundwater infiltration are part of the baseline hydrograph. Rain events which raise the groundwater level may result in an increase in flow for weeks after the rain event.

Measuring (RI I/I)

Inflow, termed Rain Induced Inflow/Infiltration (RI I/I) for quantifying, is measured in gallons per 1,000 feet.  The gallons are everything under the curve of the initial hydrograph spike above the baseline. This is typically captured in the first 24 hours. For example, a 25,000 feet mini system that measured 250,000 gallons of RI/II, would have a number for ranking of 10 gallons per 1,000 feet.

Measuring (HGWI)

Infiltration, termed High Groundwater Infiltration (HGWI) for quantifying, is measured in gallons per day per inch mile (GPD/in-mile). Inch mile is the distance multiplied by the pipe diameter. This method accounts for the potential surface area exposed to the high groundwater conditions.

HGWI is seen on the hydrograph as that quantity of flow measured above the baseline after the initial spike and until the system has recovered or stabilized.

Using the mini system from the previous example and assuming the entire system is 8” pipe, and the extra flow measured from the end of the spike to system recovery was 100,000 gallons over four days, the HGWI calculation for GPD/in-mile would be:

(100,000gals/4days) ÷ (8”x25,000ft÷ 5,280ft/mile)

Ranking = 660 GPD/in-mile


Like all things funding is a critical component. To focus your efforts in the place where the greatest return on your time and finances is possible use the 80/20 rule. The premise is that 80 percent of your issues likely can be found in 20 percent of the system.

Using the calculated scores, take the highest 20 percent scores in for RI/II and the highest 20 percent score for HGWI. Use the following to assign priorities:

  • Priority 1 Mini System Ranked Top 20% Both
  • Priority 2 Mini System ranked Top 20% Either
  • Priority 3 All Other

Tips for success

Try to get three good 1” rain events for this step. After ranking, focus the efforts for SSES & Rehab on the Priority 1 Mini Systems. We will discuss the SSES process in the next article. Remember, it is a never-ending cycle.