Storm Water System Analysis Integrating MMS, GIS and Cad Mapping for Decision Support

Until recently the City of Helena, Montana maintained an electronic inventory of its storm water facilities in a maintenance management system (MMS) by Hansen, Inc. (Asset Management) database and an AutoCAD™ drawing file. The Hansen database provides a means for operation and maintenance personnel to schedule and record maintenance activities of the storm water facilities as well as document the physical condition and characteristics of facilities. The CAD drawing of the storm water system was created to help operation and maintenance personnel locate facilities in the field, as well as assist engineers in documentation and analysis of storm water collection and conveyance features, such as, pipes, catch basins, and manholes. Historically, these two electronic information sources had been maintained independently.The CAD drawing elements were imported into a geographic information system (GIS), ArcGIS 8.2, developed by Environmental Systems Research, Inc. (ESRI). Each storm water feature was assigned a unique identification tag, enabling data from the Asset Management database to be linked to the storm water features in GIS. Data for the natural, open channel conveyance features, such as, ditches, gullies, and coulees, were not included in the Hansen database. High resolution digital orthophotography with two foot contours was used in the storm water system inventory to identify elevations of open channels. Textbook roughness coefficients for the channels were assumed, and then verified in the field.The relationship between the asset management and GIS databases enable efficient spatial analysis of maintenance information. For example, the defect ratings from television inspection of the storm water mains, stored in the asset management database, can be visually displayed and reported, in lieu of tabular condition assessment reports alone. A hydrologic and hydraulic model (Hydraulics and Hydrology model) of the storm water system was constructed with XP-SWMM 2000™ version 8.5, by XP Software, Inc., using data from both the asset management and GIS databases to aid in the assessment of hydraulic capacity limited portions of the storm water system. The focus of the model is on areas with high energy gradeline, an indication of uncontrolled flows and potential to exit the system through overtopping. Storm events for 5-year 2-hour, 10-year 2-hour and 100-year 2-hour recurrence intervals and hourly precipitation data from an historic flood in 1981 were used in the model. Runoff from these events was used to analyze capacity limited portions of the storm water system and compared with anecdotal evidence of areas of known flooding. The Hydraulics and Hydrology model was then used to develop recommendations for future improvements, such as regional detention facilities and increases conveyance sizing, to reduce the potential for surcharge.