Infiltration is groundwater that enters sewer pipes (interceptors, collectors, manholes, or side sewers) through holes, breaks, joint failures, or connection failures. When it comes to distributing dry weather ground water infiltration (GWI) in a sewer model, there are a number of things to consider: 

Gathering the data – capturing enough flow data to be able to first determine a representative low flow rate for the contributing area, at enough locations to facilitate the distribution of GWI among the various basins and sub-basins of your collection system.

Processing the data – once you’ve gathered data, how do you determine the volume of representative low flow for the contributing area? Do you determine a true numerical minimum, or review the entire period and then manually select a representative low flow?

Applying the data – there will typically always be some flow from sewer customers in the system. This means that the low flow identified in the step above is a combination of this customer flow plus GWI. Typical ranges for calculating GWI are 60-90% of the typical low flow. Once the GWI has been calculated, the next step is to distribute it throughout the model. Because infiltration is related to the amount of piping in the ground, it is frequently distributed in a model in proportion to the length of pipe (per foot) in the area served, or in terms of both the lengths and diameters of sewer pipe (per foot-inch). In models for some systems, a constant GWI load is applied to every manhole. 

I’d be really interested in hearing about other opinions and approaches to evaluating, quantifying, and distributing GWI in a hydraulic model.