''Let It Rain'' — Gage-Adjusted Radar Rainfall (GARR) Data for Peachtree Creek Sewer Basin Modeling

Sewer hydraulic models are developed to reveal and simulate problem areas in sewer system networks. During model development precipitation input data, typically from rain gages, are applied and system parameters are adjusted until the calibrated model output matches the collected system flow meter data. The calibrated model is later used to evaluate problem areas and identify solutions to the system's deficiencies in order to comply with regulatory requirements, which are met by eliminating surcharge and overflow locations within the sewer system. The study compared the hydraulic models that were developed using a GARR dataset, and a dataset consisting of rain gages only. Both datasets used information from 30 "tipping-bucket" rain gages from March 2001. The GARR analysis incorporated NEXRAD radar data on a 2 x 2 km grid, with a 15-minute sample rate. Correlations between the two precipitation measurement systems were strong. Rainfall timing was well matched. Incorporating the gage volumes resulted in lowering the radar rainfall estimated by 20%. The sewer hydraulic modeling results showed that the Flow (Q), Velocity (V) and Depth (d) response to the flow meter data matched more closely when GARR data is used as compared to the conventional rain gage data.