Real time inflow and infiltration analysis - a new tool to focus your renewal spend

Rainfall derived inflow and infiltration (RDII) in sewers has long been an important aspect in the operation and design of wastewater collection systems. Although wastewater collection systems are generally designed to accommodate RDII these flows often exceed design allowances and cause operation problems. Over the past 30 years monitoring rainfall and flows in wastewater collection systems has become common practice for many water authorities for assessing and evaluating the level of RDII in the system. The collected data can then be used to identify problematic areas within the collection system and calibrate hydraulic models so that operational changes, rehabilitation work, renewal work, and expansions can be planned in a rational and comprehensive manner.

Recent developments in GPRS telemetry and server based cloud computing provide the ability/opportunity to auto process raw monitoring data in near real time and present estimated RDII results on line to any interested stakeholder. This allows the interested stakeholder to view current and historic RDII data under the same context and provide the technical framework which reduction programs can be built.

Essential the tool is constantly keep track of the catchments antecedent wetness and calculating an average dry day for each day of the week. The average dry day is then plotted under the actual data and volume and peak variations from the average dry day are calculated and tabulated. The calculated differences are then normalized based on upstream catchment area and pipe length. The user has the ability to look at any period wet or dry to get a better understanding of the flow in that period compared to the average dry day. RDII values over the entire monitoring period or a group of user selected events is then used to compute an average RDII measurement which can then be compared to other monitored catchments in the area or across the region/country.

This presentation outlines how real time RDII calculation works through example in several recent flow gauging studies undertaken within New Zealand. It discusses the issues that were confronted during the trial such as subtraction catchments and timing between catchments. It also highlights the advantages to the real time approach for operational staff, and planning engineers, and consultants all working on the same catchment.