Sediment Deposition in Culvert and Bridge Systems Due to Channel Expansions

Final Project Report

CE 394K.3: GIS in Water Resources

by Marla Sykora

marlasykora@hotmail.com

Table of Contents

Background

   The design of culvert and bridge systems is based on allowing the natural and storm flows to pass through the system, while also maintaining some minimum freeboard upstream.  Sometimes, the design criteria forces the culvert or bridge geometry to be wider than the natural width of the channel.  In these cases, an artificial channel expansion is required.  Artificial expansions disturb the natural flow of the channel, and these disturbances can lead to sediment deposition within the culvert or bridge system.  Sediment deposition within a culvert or bridge system may cause significant problems to the hydraulic performance of the system in the event of a large storm event.  In addition, the deposition also poses a maintenance problem that must be addressed.  For these reasons, the Texas Department of Transportation is currently funding a study to better understand the effects of channel expansions on culvert and bridge design. 

   In attempting to correlate this research effort with my GIS project, several possible project objectives have been considered.  The chosen project objective is to use the ArcView extension, HEC-GeoRAS, to extract geometric data from a Triangulated Irregular Network (TIN) for use in  HEC-RAS.  HEC-RAS is a river analysis system developed by the Hydrologic Engineering Center.  After entering geometric data and flow data for a particular river reach, HEC-RAS can simulate those conditions and calculate a variety of parameters for each cross section.  The parameter of interest in this project is bed shear stress.    

    Several forces acting upon a bed particle may potentially cause bed movement.  Those forces include differences in pressure forces, lift forces, and shear forces.  The shear force is, as expected, dependent partially upon the shear stress acting upon a particle.  Various people have investigated sediment transport, and several methods for analyzing sediment bed load transport exist.  These methods vary in complexity, as well as in data requirements.  The Einstein-Brown model appears to be the most comprehensive for the issue of sediment transport and deposition within culvert systems.  This model is based on several parameters, including settling velocity, volume of grains crossing a particular cross section and shear stress.  So, while it is clear that shear stress alone does not determine the complete picture, the shear stress can give some indication of a particle's tendency to move.  For this project, the shear stress has be determined along a portion of a river that is just upstream of a bridge.  Three flow conditions representing periods of high, medium, and low flows have been simulated.  Since a complete bed load analysis is somewhat complex and all the needed parameters are not known, only the bead shear stress has been determined.  However, determining this information is a valuable step along the way to a complete analysis.  In addition, simply looking at the shear stress may give some indication of a particle's tendency to move.

Study Area

   The study area for this project is Pecan Bayou, which is located near Brownwood, Texas in the Pecan Bayou Watershed.  The Pecan Bayou Watershed is located in Central Texas and covers an area of 6503.6 acres spread out over seven counties.  Figure 1 indicates where the watershed is located within Texas, and Figure 2 relates the general shape of the watershed.  Figures 1 and 2 were obtained from the Environmental Protection Agency (EPA) website.

                                 Figure 1 - Location of Watershed                                                        Figure 2 - Pecan Bayou Watershed

For this project, only a small portion of Pecan Bayou has been considered.  Near Brownwood, Pecan Bayou passes under a bridge, and this location seems to be the most logical choice to conduct this study.  So, a portion of the river just upstream of the bridge has been analyzed.  To simplify the project somewhat, the small tributaries merging into Pecan Bayou in this area have been neglected.  The study area has been circled in Figure 3 below.  This figure clearly indicates where Pecan Bayou passes under the bridge, just east of Brownwood.

                                            Figure 3 - Location of Study Area East of Brownwood, Texas

Methodology

                 Data requirements for this project included obtaining a TIN of the project area and discharge data.  A TIN for this area was used during thesis research work by former student David Anderson.  Ryan Murdock had obtained a copy of this TIN, and he was kind enough to share this file with me.

                 There are no gage stations that record discharge data located in the project area.  However, there is a gage station located along Pecan Bayou, near the town of Mullen, that does record discharge data.  The gage station is approximately 25 miles downstream of the study area.  The discharge data from this station (USGS Station 08143600) was used.  The data was obtained from the United States Geological Survey (USGS) website.  The USGS provides historical flow data and historical peak flow data.  I obtained historical peak flows for a period of five years.  By averaging that data, I determined that the average peak flow was 7,594.6 cfs.  This value was used in the project as the peak flow.  Also, I obtained historical flow data for a period of one year.  By averaging that data, I determined an average flow condition of 210 cfs.  This value was used as the medium flowrate in the simulations.  Finally, the low flowrate of 1.1 cfs was determined by simply reviewing the data.

                 In order to link ArcView and HEC-RAS, certain shapefiles must be developed.  In order to perform the necessary steps, the HECGeoRAS, 3D Analyst, and Spatial Analyst extensions must be loaded.  First, a new shapefile of Pecan Bayou was created by going to View - New Theme and then, while in edit mode, using the    button to trace along the river path as depicted by the TIN.  As shown below, only a portion of the Bayou located just upstream of the bridge crossing was digitized.  Although using the TIN as a guide is acceptable, a better alternative would be to use a visual data source, such as a Digital Orthophoto Quadrangle, if one is available.  Using a visual data source would provide better accuracy. 

                                    Figure 4 - Creation of the Pecan Bayou Shapefile

                The next step was the creation of the stream centerline shapefile.  This file was easily created by selecting the Create Stream Centerlines command under the Pre-RAS menu, while the digitized stream was in the view.  The features from the digitized stream shapefile were copied to the new stream centerline file.  The river reaches then had to be identified using the River ID tool.  With that, the stream centerline file was created.  Since it looks identical to the first stream file created, the digitized stream can be deleted from the view, leaving only the centerline theme.  Arrows were added to the theme in the legend editor in order to assure that the flow was shown in the correct direction.

                                    Figure 5 - Stream Centerline Creation

                Next, the stream banks file had to be created.  This was done using a similar technique to the one described earlier for digitizing the stream.  Selecting the Create Banks command under the Pre-RAS menu creates a new theme.  In edit mode, the same digitizing procedure was used.  Two bank lines must be created, one for the right bank and one for the left bank.  

                                    Figure 6 - Banks Creation

             The flow path centerlines theme must be created next.  This theme is needed to identify the hydraulic flow path in the left overbank, right overbank, and main channel.  This theme was created by choosing the  Create Flowpaths command under the Pre-RAS menu.  Since the stream centerlines have already been defined, these shapes can simply be copied into the flowpath theme.  At this point, the flowpaths must be digitized much in the same way as previously described for the banks. 

                                    Figure 7 - Flowpath Creation

                The next step was the creation of the cross section cut lines theme, which was done using the Create XS Cut Lines command.  For this study area, I decided to use 18 cross sections.  Of course, the more cross sections that are used, the better the final results.  However, there are some rules that must be followed in the creation of the cross sections, and the meandering nature of this river made it difficult to add more cross sections without breaking any of those rules.  For example, the cut lines should be perpendicular to the flow, but they should not cross.  In the southwest corner of the study area, following those two rules was particularly difficult.  Another rule states that cut lines must be drawn from the left bank to the right bank.  Arrows were added in the legend editor to confirm that this was done.

                                    Figure 8 - Cross Section Creation

            The final steps within the pre-process area are the extraction of the spatial data from the polyline themes.  To begin, the theme attributes were identified by selecting the Theme Setup command under the PreRAS menu.  This identified the RAS GIS Import file as rasinput.geo.

                                                                                                    Figure 9 - Theme Setup

            Next, by selecting the Centerline Completion command under the Pre-RAS menu, the spatial data was extracted from the TIN and added to the attribute table.  Finally, by choosing XS Attributing under the Pre-RAS menu, the stream/reach names, stationing, bank stations, and reach lengths were assigned.  After checking the attribute table to confirm that the data was added correctly, the pre-processing of the data was complete.  The HEC-RAS import file was ready for creation.  By clicking on the button, the import file was created!

                The GIS file was imported into HEC-RAS by going to File - Import Geometry Data - GIS Format and then choosing the input file.   By clicking on Edit - Geometric Data, I could verify that the data had been imported correctly.

                                                                            Figure 10 - Imported Geometric Data 

            At this point, the flow values must be entered.  This is done by going to Edit - Steady Flow Data.  The first simulation was performed using the average peak flowrate.  After completing that simulation, I went back and changed the flow to the medium flowrate obtained from the USGS data.  Finally, I performed a simulation using the low flow condition.  

            As stated before, HEC-RAS calculates several parameters.  A cross section output table can be viewed to obtain the data for each cross section.  Below is the Cross Section Output for the peak flow condition at Station 2037.183 ft.  The shear stress data for each cross section during each of the three flowrates was recorded into an excel spreadsheet.

                                                                               Figure 11 - Cross Section Output

Results

    For each case, I was able to determine the shear stress at each cross section along Pecan Bayou.  As stated earlier, this data was copied into an excel spreadsheet and plotted.  The following graphs are the results.

                                                         Figure 12 - Shear Stress Data for Peak Flowrate

                                                          Figure 13 - Shear Stress Data for Average Flowrate

                                                          Figure 14 - Shear Stress Data for Low Flowrate

        These graphs are a great visual tool for getting an idea of areas where sediment transport would be more likely to occur.  In addition, we can see how the shear stress changes as the flow deceases.  Also, it appears that the shear stress is greater along the right bank than the left bank.  Finally, these graphs indicate that the shear stress is the greatest just before the bridge is reached.  This result likely has little to do with the bridge itself, since the bridge was ignored in the analysis.  One possible conclusion is that the shear stress is exaggerated because this is the end point in the analysis, and HEC-RAS is unaware that the river actually continues.

Conclusion

   Sediment transport and eventual deposition within culvert and bridge systems can be a significant problem.  In an effort to predict bed load sediment transport, several models have been developed, all requiring slightly different data.  However, shear stress is always linked with determination of sediment transport.  This project has shown that ArcView and HEC-RAS can be used to calculate shear stress.  This data can then be incorporated into whichever model is chosen to predict the bed load sediment transport.   Another useful aspect is that HEC-RAS calculates parameters not only along the channel, but also on the banks.  Knowing which side of the bank is more likely to erode could be of great import.  In addition, HEC-RAS calculates numerous other parameters that can also be used in these models, including the wetted perimeter and the stream power.  

    This area of GIS application could potentially be of great use.  In order to perform a complete bed load analysis, more data would need to be compiled, and using a data source such as a Digital Orthophoto Quadrangle to digitize the steam and bank lines would provide more accuracy.  Certainly, though, this application will prove useful in the future for determining parameters necessary to complete a bed load analysis.

References