CONVERTING A NEXRAD MAP TO A FLOODPLAIN MAP Oscar Robayo, Tim Whiteaker, and David Maidment* ABSTRACT: Using ArcGIS 9.0 ArcObjects and the new ModelBuilder environment, a methodology for converting a NEXRAD rainfall map time series to a floodplain map has been developed. The methodology integrates HEC-HMS and HEC-RAS modeling systems through flow change points in the stream network georeferenced in the Arc Hydro data model for water resources. The coupling of these external modeling systems via a common geographic framework enables dynamic and seamless end-to-end simulations for real time and planning operations. The integrating approach has been implemented in the San Antonio River basin, Texas, as part of a regional watershed modeling system being developed by the San Antonio River Authority (SARA), the City of San Antonio, and Bexar County. The Salado and Rosillo creeks were used to test a prototype version of the modeling framework which has been posted at SARA s metadada server and made available for public access and download. KEY TERMS: GIS; flood inundation maps; floodplain modeling; hydrologic and hydraulic modeling. INTRODUCTION Water resources planning and management relies heavily on engineering models. For a timely and systematic operation of this kind of management operations, an information technology framework is needed to streamline the modeling process. Connectivity of all modeling resources represents the main bottleneck to be addressed in order to achieve a continuous and operational workflow. The diverse nature of water resources models, file formats and data structures makes this a complex task only possible through new technological advances. A solution for reaching this integration is by using a spatial framework as the common ground for connectivity. In this light, a Geographically Integrated Hydrologic Modeling System has been devised and a preliminary definition, design, and implementation of a Regional Watershed Modeling System for the San Antonio River Basin Modeling Project have been accomplished. The spatial linkage of all modeling resources was implemented on the pilot basins of the project, The Salado and Rosillo Creeks, as a prototype work that will be later extended to the remaining watersheds of the basin. This paper describes the components needed to achieve an operational end-to-end integrated and dynamic modeling system under the ArcGIS ModelBuilder framework, the status of the on-going regional watershed modeling project, and the conclusions reached so far. STUDY AREA This research is being developed for the San Antonio River Basin (4,000 sq. miles) in Texas, which comprises a diverse basin including regulated subbasins and complex urban drainage systems that flow south-east bound towards the Gulf of Mexico. As part of the San Antonio River, the Salado Creek (222 sq. miles) and its tributary, the Rosillo Creek (29 sq. miles) were used as the pilot basins due to the availability of 2-foot contour lines from land surveys to characterize the channel cross sections, available HEC-HMS (USACE, 2000) and HEC-RAS (USACE, 1997) hydrologic and hydraulic modeling systems respectively (as provided by the city of San Antonio), and for representing complex urban settings at the south-east part of the city of San Antonio. *Respectively, Graduate Student, Graduate Student, Professor, University of Texas at Austin, Department of Civil Engineering, Center for Research in Water Resources, 10100 Burnet Rd. Bldg. 119, Austin, TX 78758-4445, Phone: (512) 471-0073, Fax: (512) 471-0072, E-Mail: oskatrin@mail.utexas.edu. 1
METHODOLOGY In order to be able to transform a rainfall map time series into a flood inundation map, the following workflow of connected processes was defined. In a broad sense, the proposed system should be able to acquire NEXRAD map time series to drive hydrologic simulations in HEC-HMS whose streamflow outcome will feed the hydraulic model HEC-RAS that provides the needed water surface elevations to perform the final floodplain delineation to get the flood inundation map. Figure 1 below provides the schematic of the conceptual workflow for the aforementioned steps. Figure 1. Conceptual Workflow to Convert a NEXRAD Map to a Floodplain Map The needed elements to support the full integration, the Arc Hydro data model (Maidment, 2002) for the study area, HEC-RAS and HEC-HMS model configurations, and previously obtained NEXRAD precipitation estimates, and all the components included in the conceptual workflow were generated. The main list of these components is shown below: 1. Reading Time Series into Arc Hydro data model 2. Transferring Time Series to corresponding modeling features 3. Input hyetographs into existing HEC-DSS for HEC-HMS 4. Executing HEC-HMS 5. Transferring flow values into Arc Hydro 6. Generating HEC-DSS file for HEC-RAS and updating related input files 7. Executing HEC-RAS 8. Transferring water surface elevations into Arc Hydro 9. Executing floodplain delineation processes From the above 9 main components, 6 of them deal with the transfer of time series records between systems having very different formats. The modeling systems of the Hydrologic Engineering Center (HEC) use the HEC Data Storage System for time series, HEC-DSS (USACE, 1995), a data base specifically designed for water resources applications that uses a block of time sequential data, called pathnames, as the basic unit of storage and that stores the records in binary format files for access efficiency. A good handle on the HEC-DSS system was critical in this implementation to make available relevant time series records from the geodatabase time series tables to the HEC-HMS and HEC-RAS models and to allow the transfer of records from HEC-DSS to geodatabase format to enable the needed geospatial manipulations. To automate the needed time series transfer processes between the involved independent systems (i.e., geodatabase and HEC-DSS), custom DLLs were developed based on HEC public domain access libraries for HEC-DSS (USACE, 1987, 1991). 2
A unique identifier (model codification) to support the connectivity between features in each model was needed to spatially relate features across the models. For HEC-HMS this code corresponds to a text string that is used as a feature identifier in the basin file (i.e., The ID of the outlet-nodes over which the streamflow hydrograph is computed) and which is also located in the "B Part" of the Pathnames in the HEC-DSS time series file catalog. This hydrologic identifier is hereby called the HMSCode and populated as an attribute field inside the geographic dataset (Arc Hydro). In the same manner, in HEC-RAS, the connectivity is accomplished by knowing which cross section is associated to the previous hydrologic features (tagged with HMSCodes). This implies knowing which river, reach and stationing the crosssection lies on, e.g. Rosillo, Upper, 93528.56 representing a sequence of comma-delimited identifiers defined as the RASCode in the geographic dataset of the study area. Thus, the geographic locations where the models are to exchange hydrologic information, "Flow change location points", to allow communication between models, were related by means of a geographic association (relationship class). The spatial character of this integration requirement is what makes a geographically integrated modeling system an attractive solution for model connectivity. After the modeling features were populated with corresponding model codes (HMSCode and RASCode in this case), a relationship class was needed between them to create a 1 to 1 association between. Not all cross sections in the hydraulic model represent points of information exchange, and each outlet node will have a unique downstream cross section over which transfer of streamflow information will take place. In the Geodatabase each of these flow change points were represented as a HydroJunction on the Arc Hydro network and the HydroID of this junction was stored as an attribute of the corresponding HMSCode and RASCode features that are connected to it (i.e., a Schematic Node for a HEC-HMS Junction; a Cross Section line for a HEC-RAS crosssection). Formal relationship classes between these features and HydroJunctions were created in Arc Catalog to make them permanent inside Arc Hydro. This association of features creates the mechanism HMSCode of HMSFeature to HydroID of HydroJunction and to RASCode of CrossSection that was needed to connect the HEC-HMS and HEC-RAS elements to support the exchange of information between them. In an effort to automate and batch multiple and sequential geoprocessing tasks of datasets using scripts in ArcGIS 9, ESRI has made available a new scripting environment called ModelBuilder. This environment can not only be used with standard ESRI geoprocesses but also with scripts referenced to custom DLLs that perform user-defined tasks. By generating and connecting all of the components in Figure 1, together under the new ArcGIS ModelBuilder framework through a combination of custom DLLs and standard tools, the envisioned end-to-end integrated system can take an arbitrary HEC-HMS model simulation output and use it to drive the computation of water surface elevations in a geographically connected hydraulic model. In this light, a geographically integrated modeling system is now a suitable and viable solution to accomplish the needed integration of modeling resources, datasets and stand-alone applications. In Addition to the main components in the workflow, two interface data models (IDM), one for HEC-HMS and one for HEC-RAS are being developed. This interfaces will give support to new spatially-based model configurations (reflecting what if scenarios or new basin and floodplain developments). Thus, the HEC-HMS IDM (Obenour, 2004) and HEC-RAS IDM, as represented in Figure 1, will allow for straightforward storage, setup, and update of new modeling scenarios to be included in under this integration scheme. RESULTS The prototype system representing the end-to-end integrated solution for the automatic generation of flood inundation maps has been tested on Rosillo Creek, a 29 square mile basin with a main stem channel of about 20 miles length having 223 cross sections for the hydraulic model. A description and graphical display of the resulting main phases of the procedure (as implemented on Rosillo Creek) is given below to show the progression of the proposed workflow needed to obtain the final flood inundation maps. Reading Precipitation Records and Mapping Time Series to Watersheds NEXRAD radar precipitation data for the Hydrologic Rainfall Analysis Project (HRAP) grid (a 4x4 Km square-celled map grid) is read from a series of text files containing hourly data for a storm event on July 1st, 2002 from 4:00 AM to midnight. The text files are read and the time series transferred to the geodatabase content under Arc Hydro format (i.e., to TimeSeries and TSType tables) as well as a HEC-Time Series Type table corresponding to a mirror image of the HEC-DSS Catalog to store the HEC-DSS descriptors of the data and to enable the storage of this data back into the HEC-DSS system. The Time Series records associated to NEXRAD cells are mapped to the watersheds to generate a hyetograph for each hydro response unit to provide input for the rainfall-runoff transformations in HEC-HMS. The time series transfer operation is done by means of a previously created intersection layer generated from the NEXRAD and the watershed polygons. By computing the NEXRAD cell area inside a given watershed polygon the tool estimates the average rainfall over the watershed (i.e., aerially weighted hyetographs). The graphical representation of the previous 2 steps is shown in Figure 2 below. 3
Figure 2. Ingesting Precipitation Data Executing the Hydrologic and Hydraulic Model A previously setup HEC-HMS hydrologic model for the Rosillo Creek basin is called with reference to the precipitation records now stored in the HEC-DSS system. The execution of HEC-HMS is done through a batch file based on the existing project file and the proper RUNID name. For the hydrologic-hydraulic connectivity to take place, the HEC-HMS outlet nodes must pass its output hydrographs to the geographically related HEC-RAS cross sections to appropriately perform the hydraulic routing of flows. This model elements (outlet nodes and cross sections) represent flow change points that deliver or receive information between consecutive models to allow for integration. To georeferenced and relate the needed flow change points, the Arc Hydro data model was used as the geographic integrator (as shown in Fig 3 below). A previously setup HEC-RAS model for the river system is called with reference to flow change location records from the previous hydrologic model. The execution of HEC-RAS is done through the HEC-RAS object library that exposes some execution and writing functionalities. The graphical representation of the previous 2 steps is shown in Figure 3 below. Figure 3. Floodplain Modeling Component 4
Floodplain Delineation Process The basic steps used to obtain the flood inundation polygon are similar to the processes used in the latest version of HEC-GeoRAS (ESRI, 2003) as follows: the simulated water surface elevation values are read from cross sections and used to create a water surface TIN. The water surface elevation TIN is converted to raster format, and an intersection is done between the water surface raster and the land surface raster to obtain a water surface depth raster. A polygon feature class is generated based on the generated water surface depth raster which represents the inundated area. The above main Post-processing steps for Floodplain delineation are graphically shown in Figure 4 below: Figure 4. Floodplain Delineation steps The final implementation of the automated system for transforming a NEXRAD map into a floodplain map resulted in the use of 10 custom designed dynamic link libraries (DLLs) and 9 standard ESRI tools tightly connected together to make the integration possible under the Arc GIS 9.0 ModelBuilder framework. The final ModelBuilder layout containing the 10 custom design scripts and 9 standard ESRI tools is shown in Figure 5 below. 5
Start End Figure 5. Integrating Workflow in ArcGIS ModelBuilder A historic storm event recorded by NEXRAD radar on July 1st, 2002 from 4:00 AM to midnight has been fed into the system to provide the primary input for the rainfall-runoff hydrologic and routing transformations. The computational time of the aforementioned integrated process (as shown in Figure 5 above) for the July 2002 storm event is only 2 minutes on an Intel XEON CPU with 2.20 GHz and 1 GB RAM. CONCLUSIONS A spatial linkage scheme was hereby outlined, designed, and tested which uses a GIS representation of the stream network and water features of any given region as a reference frame to link the operation of a series of engineering models, HEC-HMS and HEC-RAS. In linking HEC-HMS and HEC-RAS through the Arc Hydro data model, the inherent expertise of each model is exploited by allowing one model to de termine streamflow, and the next to determine water surface elevations and extents as a function of given streamflow obtained from previous simulations. Critical to the success of this integrating effort is the transfer of time series between linked model features and the wrapping geographic framework. This central transfer of time series was done at three levels, from the HEC-DSS system to the Geodatabase, within the Geodatabase, and between the Geodatabase and HEC-DSS system. The proposed integrated modeling system can be further expanded with flood information and impact analysis modules to assist in flood mitigation planning, and floodplain management. By streamlining the mapping of flood inundation maps it is now possible to loop through the system multiple what if scenarios for planning purposes and for optimality studies as well as to empower the possibility of incorporating the system into real-time forecasting operations based on National Weather Service products like NEXRAD estimates. The implementation of standard data models and the higher level of customization provided by the new ArcGIS software through COM-compliant programming languages has provided the needed integrating platform that glues together diverse and external modeling resources sharing the same interconnection protocol, empowering GIS to connect stand-alone engineering models. ACKNOWLEDGMENTS This on-going study is being developed in close partnership with the following agencies and consulting firms which have provided key datasets, model configurations, and technical support: San Antonio River Authority, City of San Antonio, Bexar County, PBS&J Austin, and the Water Resources team at ESRI. The authors would also like to express their appreciation to those who reviewed this paper for AWRA. 6
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