Flood Plain Mapping on the Yellowstone River

 

 

By: Clay Woods

 

 

CEE 6440, GIS in Water Resources, 2008

 

 

 

 

 

 

Introduction

The Yellowstone River is the longest un-dammed river in the United States and flows through most of Montana and parts of Wyoming.  It eventually joins the Missouri River and drains most of Eastern Montana.  

Eastern Montana is mostly rural, farming communities and farmers near the Yellowstone River use its water for irrigation.  Eastern Montana receives a lot of precipitation in the form of snow during the winter, which causes large amounts of runoff in the spring.  The summer weather in Eastern Montana provides a potential for local flash flooding due to intense thunderstorms.  Many of the farming communities in Eastern Montana are located along the Yellowstone River, in order to utilize the water.  Problems with flooding exist anywhere people develop communities along a river.  One way of managing this flooding is to develop flood maps that will predict which areas will be flooded during a flood of a given magnitude. 

GIS can be used with HEC-RAS using the tools available in HEC-geoRAS to develop a model of a river and predict the floodplain of that river.  This project will develop a floodplain map for the city of Glendive, Montana.  This is a small city near where I grew up and is located right on the Yellowstone River in Eastern Montana.  A google map image of Glendive is shown in figure 1.  The flood map will be generated using GIS and HEC-RAS.

 

Figure 1: View of Glendive, MT.

 

Objectives

The objectives of this project are to:

  • Gather data for the Glendive area including DEM, NHDPlus data, information about past floods, and real time flow data for the Yellowstone River.
  • Develop a base map of the study area in GIS using the data collected
  • Develop a HEC-RAS model of the Yellowstone River at Glendive using data imported from my GIS model
  • Export the results of the HEC-RAS model into GIS to develop a layer showing the floodplain for different flood conditions

Data

The first part of this project was gathering data to use in the GIS model.  NHDPlus data was downloaded for the region including Glendive.  This dataset included flowlines, a digital elevation model, basin and catchment boundaries, and information from the USGS stations on the Yellowstone River.  An aerial photograph was obtained from the Montana government website.  This photo was used to delineate the normal river from the flood plain.  Real time river flow data was obtained from the USGS website.  Data for the Yellowstone River only goes back to 2002, so the project will use a small set of flow data.

The NHDPlus data sets that were downloaded were very large and covered a large region.  This data was trimmed to contain only Dawson County, where Glendive is located.  Figure 2 shows a picture Dawson County with flowlines and where Glendive is located in the county.  The DEM for the region was also trimmed using the mask feature in Arcmap.  The trimmed DEM is shown in figure 3.  The flowline information was not actually used very much for this project.  It was useful verify that the satellite imagery and DEM matched with what the NHDPlus flowlines were showing, but was not needed for the floodplain analysis.

Figure 2:  Data trimmed to only Dawson County

 

Figure 3:  DEM for Dawson County

 

Software

The software required for this project caused a major problem.  The interface between Arcmap and HEC-RAS is called HEC-geoRAS and the current version of this software is not compatible with version 9.3 of Arcmap.  The interface was designed to work with version 9.1 or Arcmap, but version 9.2 also works based on the experience of the author.  This created a problem of finding the older version of Arcmap in order to continue with the project.  The HEC-RAS program can be downloaded for free from the U.S. Army Corps of Engineers along with the HEC-geoRAS interface files.  Each of these programs also has a users manual that can be obtained from the same website. 

Analysis using HEC-geoRas

The purpose of HEC-geoRAS is to assist in preparing data for import into HEC-RAS.  HEC-RAS then uses the geospacial data to calculate water surface elevations for specified flowrates.  The HEC-geoRAS program starts with a digital terrain model that can be either a grid or a TIN.  The downloaded DEM was used for this model, so a grid format was selected. 

The next step in preparing the export file is to create RAS layers that show the stream centerline, flowpaths, banks, levees, obstructions, bridges, and storage areas.  Some of these layers are optional, so this project utilized the centerline, banks, and flowpaths.  The new layers are created in a personal geodatabase and the lines are added using the editing tool.  The lines for stream centerline and banks were developed using the aerial picture for this project, however they can also be downloaded from existing files. 

The other required layer is cross sections.  The cross sections are where the terrain is exported into HEC-RAS for the channel information.  Care must be taken to use enough cross sections and to ensure the cross sections are long enough to go across the floodplain.  These cross sections can be previewed and edited if needed.  Figure 4 shows the study area with the created cross sections, banks, and centerlines.  To improve the speed of the model for this project only six cross sections were used.  The HEC-RAS model will be more accurate if more cross sections are used.

Figure 4:  Study area to be exported

Processing in HEC-RAS

 

The information developed in HEC-geoRAS is imported into the geometry portion of HEC-RAS.  This portion is what creates the river channel with its associated floodplain in HEC-RAS.  This imported geometry is shown in figure 5 and a sample of the imported cross sections is shown in figure 6.  The cross sections can be edited after importing if the banks that were drawn in HEC-geoRAS do not match up with the banks based on the DEM..

 

Figure 5:  Imported geometry for the study area

Figure 6:  Imported cross section

 

Flow data must be provided for HEC-RAS to run.  The flows for this project were obtained from the USGS gaging station in Glendive.  The maximum recorded flow was 118,000 cfs and the average maximum flow during spring runoff is 54,000 cfs.  A large flow of double the maximum recorded was also modeled for this project.  Manning’s n values must be given for both the river channel and overbank areas.  For this project the channel’s n value was assumed at 0.04 which is based on a bed of sand to large gravel with some vegetation.  The overbank areas were assigned a value of .055 based on small shrubs and agricultural land.  The flow in the river was assumed to be subcritical and a slope of 0.0027 was used for the HEC-RAS calculation of normal depth.  This slope was determined by getting elevation and distance information from the DEM and calculating the slope. 

 

The next step in the project was to run the HEC-RAS model for the various flows.  Information generated by running the model can be useful without exporting the data back into Arcmap.  Each cross section shows the water surface in relation to the cross section elevation, so it can be seen where the banks are overtopping.  A cross section is shown in Figure 7 representing the normal flows seen during runoff.  Figure 8 shows the same cross section with the maximum recorded flow selected to mode. 

 

Figure 7:  Water surface at a cross section

Figure 8: Max flow cross section

 

Back to Arcmap

 

After the HEC-RAS model has been run, the information is exported back to Arcmap in order to create an inundation map of the study area.  This is done by incorporating the water surface levels developed in HEC-RAS with the elevations of the DEM in Arcmap.  This process creates an inundation polygon that shows which land areas will be flooded by the flow modeled in HEC-RAS.  The inundation for the maximum flow is shown in figure 9. 

 

Figure 9: Inundation for max recorded flood

 

Conclusions

 

Because of the terrain around Glendive, an example of which can be seen in figure 7, floods of different sizes are not likely.  The ground is very flat along the banks of the river and then the terrain rises very quickly once out of the river valley.  As seen in the profiles in HEC-RAS, once the water overtops the banks of the Yellowstone, the flood has to get much worse before any more land area is inundated.  The inundated land around Glendive is all agricultural land with little development, so the impacts of flooding would be minimal. 

 

The quality of this flood map could be improved with more cross sections modeled in HEC-RAS and an investigation of actual land cover and river channel conditions in order to more accurately input values of Manning’s n.  The bridges and low flow areas of the river could also have been modeled in HEC-RAS, however the final results would be similar to the ones received from a simpler mode.  A higher quality DEM would also improve the flood map by giving more accurate elevation data.  In spite of all of the improvements that could be made to the models, the terrain around Glendive basically controls any flooding and limits the flood to land mostly used for agriculture.

 

References and Data Sources

 

Montana Geographic Information Clearing House.  http://mris.mt.gov/gis/

National Hydrography Dataset.  http://www.horizon-systems.com/nhdplus/

Google Earth.  www.google.com

HEC-RAS website.  www.hec.usace.army.mil/software/hec-ras/.