Sewer hydraulic models are developed to reveal and simulate problem areas in sewersystem networks. During model development precipitation input data, typically fromrain gages, are applied and system parameters are adjusted until the calibrated modeloutput matches the collected system flow meter data. The calibrated model is laterused to evaluate problem areas and identify solutions to the system’s deficiencies inorder to comply with regulatory requirements, which are met by eliminatingsurcharge and overflow locations within the sewer system.Calibration of the sewer system is only as accurate as its input rain data, so it shouldbe a priority to use the best rain data attainable. While rain gages can be accurate andprecise, they can only provide data at discrete points, resulting in low spatialresolution and ambiguity regarding how much rain has fallen in between gages.Installing denser networks of rain gages can help improve the spatial resolution of thegage data, but is expensive in terms of equipment fees, installation, and maintenance.For this reason, the City of Atlanta initiated a pilot program which uses a techniquecalled Gage-Adjusted Radar Rainfall (GARR). GARR datasets are produced bytaking regional NEXRAD radar data and calibrating them with co-located rain gages.GARR combines the high spatial and temporal resolution of radar with the volumetricaccuracy of rain gages.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 incorporatedNEXRAD radar data on a 2x2 km grid, with a 15-minute sample rate. Correlationsbetween the two precipitation measurement systems were strong. Rainfall timing waswell matched. Incorporating the gage volumes resulted in lowering the radar rainfallestimated 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 closelywhen GARR data is used as compared to the conventional rain gage data.Because rainfall is the input variable with the largest impact on modeling sewersystems, it is also essential information for determining rehabilitation efforts andsystem upsizing. By using the GARR process to enhance real time rainfall data, theCity of Atlanta will create more accurate models, attain a better representation of
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