Information Management for County Infrastructure

Hydraulic Data Development
The second step in the modeling of the watershed is to take the flows from the hydrologic model and put them into a hydraulic model. In this case, the hydraulic model used is the U.S. Army Corps of Engineers Hydrologic Engineering Center. HEC-RAS. The HEC- RAS model takes the geometry of the watershed and combines it with the flows extracted from the HEC1 model to determine areas of flooding. This model works in conjunction with the GIS very well and has a long reputation as being dependable.

The data that HEC-RAS requires is basically just a 3-D representation of the watershed area and flow values. In order to get this geometry information, a package of arc macro language (AML) macros has been developed by ESRI and HEC that extracts this data. The package is called HEC-GeoRAS and the version used in this study operated within Arc/INFOTM. The HEC-GeoRAS package uses a digital terrain model and Arc/INFOTM coverages to create a text file that is formatted such that the HEC-RAS modeling software can import it. A lot of time and effort is saved by using GIS tools to generate the geometry files instead of doing it all by hand and this method allows the model to be reproduced at any time because it can be extracted from the information in the enterprise database.

A detailed discussion of the HEC-GeoRAS package will not be included here, but rather a quick overview of what was needed to run the package. If a more detailed discussion of the HEC-GeoRAS package is desired the software and documentation are available at the HEC website ( http://www.wrc-hec.usace.army.mil/ ). The three pieces of data that are mandatory when generating a geometry file are the digital terrain model, stream centerline and the cross section cut lines. In addition to these three things the overbank locations and the flow path locations may also be added into the geometry file.

For this study the hydraulic modeling will be done on a sub-watershed basis just as the HEC1 modeling was done. Upon completion of all of the sub-watersheds they will be combined into one larger file that will incorporate the entire Mill Creek watershed. Each of the sub-watersheds has a set of coverages that are used for HEC-GeoRAS. The coverages are developed through a combined effort of the GIS team and the hydrologic engineers on the project. The cross section cut lines, over bank locations and flow paths are all heads up digitized in ArvViewTM and then converted to Arc/INFOTM coverages for use in the HEC-GeoRAS package.

Mapping the new flood plain
The culmination of all the modeling efforts is the mapping of the 100 year flood plain for the watershed. The study has not reached this stage currently but will rely on the HEC- GeoRAS software package once again, to provide the tools necessary to map the new flood surface. Once the new flood plain has been mapped it will be submitted to FEMA for review and eventually, update of the current flood plain maps. These new flood plain boundaries will also be added into the county's information management system and will be available for many different planning applications.

Conclusion
The primary goal of this watershed study is the mapping of the 100 year flood plain but the real benefits of the study are much greater than just the mapping of the flood plain. Granted, whether the information management aspect of this project is present or not the flood plain analysis would still be done but the county would be short changing themselves because of all the opportunity they left out on the table. Johnson county has chosen to grab every opportunity they can to work towards a future goal of enterprise information management for all systems in the county. They may not be where they want to be right now but are using opportunities like this watershed study to work towards their goal. A little for thought in the mid 1980's and looking at all the opportunities in each project they need done has set the county up with a information management system capable of aiding in any number of activities.