Watershed Comparison Using

ArcGIS for Flood Plain Mapping

 

 

 

 

 

Prepared by

 

Kameron Ballentine

Jeremy Jensen

 

 

 

 

 

 

 

 

Prepared for

 

Dr. Neale

CEE 6440

Utah State University

12/7/2007

TABLE OF CONTENTS

 

 

Page #

INTRODUCTION………………………………………….…………………………………………….1

WATERSHED COMPARISON OBJECTIVES………………………………………………………...…1 

WATERSHED COMPARISON METHODOLOGY………………………………………………………2 

WATERSHED COMPARISON DATA………………………………………………..………………….6

WATERSHED COMPARISON RESULTS…………………………………………………………….…6

FLOOD PLAIN DELINEATION OBJECTIVES………………………………………………………….7

FLOOD PLAIN DELINEATION METHODOLOGY………………………………………………….…7

FLOOD PLAIN DELINEATION RESULTS………………………………………………………….…14

SUMMARY AND CONCLUSIONS……………………………………………………………………..15

RECOMMENDATIONS……………………………………………………………………………....….16

 

 

 

 

 

 

 

 

 

 

 

 

 

INTRODUCTION

 

 

On June 3, 2005 a debris flow started in Cedar Canyon and traveled down the canyon towards Cedar City.  This debris flow damaged state highway SR-14 (Giraud & Lund, 2005) which comes into Cedar City from the east.  There was a state of emergency declared in Iron County due to this debris flow and other flooding taking place from “spring storms and other rapid snow melt.”  From this, concerns about the flood capacity of local rivers and the accuracy of flood maps arose.  With the national flood rate insurance map (FIRM) project underway, the state of Utah decided to verify the capacities of the rivers and remap the floodplains in Iron County.  Two of the rivers that had their floodplains remapped were Coal Creek and Parowan Creek.

 

Coal Creek and Parowan Creek are located in neighboring watersheds named Coal Creek Watershed and Parowan Watershed, respectively.  About 75 years of historical flow data is available at the mouth of the Coal Creek where it enters Cedar City, but less than ten years of data were available for the Parowan Watershed. From the historical Coal Creek flow data, the flow rate for a 100 year flood can be determined and used in the flood plain analysis, but there is not sufficient gauge data to produce an accurate flood frequency curve for Parowan Creek. Without this information, the flow rate for the 100 year flood could not be determined with the same methodology.  However, since the two watersheds neighbor each other, their characteristics should be similar.  A relationship based on watershed drainage area was developed to determine the 100 year flood for Parowan Creek. 

WATERSHED COMPARISON OBJECTIVES

 

 

The original scope of this project included comparing rainfall data, slope, and mean elevations of the two watersheds. However, it was found that these were previously compared in a study performed by Bowen, Collins, and Associates.  As a result of this finding, a new scope was developed to further analyze the hydrologic similarity between the Coal Creek and Parowan Watershed. The new scope includes the following comparisons:

 

·         Watershed Size

·         Drainage Density

·         Available Flow Data

·         Spring Density

·         Soils Data

 

These comparisons were chosen on the basis of their availability and relationship to the hydrology of the watersheds. Snow pack was also going to be used to compare the two watersheds, but no Snotel data was found to exist within the project boundaries or close enough to the project boundaries to be deemed accurate for the Coal Creek and Parowan Watersheds.

 

WATERSHED COMPARISON METHODOLOGY

 

 

In order to perform an accurate analysis of the watershed, a 10-meter Digital Elevation Model (DEM) for Iron County was obtained from the AGRC website. A hill shade of the area was produced using the spatial analyst toolbar so that the terrain could be more easily visualized as shown in Figure 1. The two watershed boundaries are shown also shown in the figure. From this figure, it can be seen that the two watersheds are touching each other and that they are located in very mountainous terrain.

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countyhillshade.bmp

Figure 1: Hill Shade for Iron County

 

As mentioned previously, the slope and aspect of these two watersheds were compared in a previous study. In order to perform a full comparison of any two watersheds, however, these can be found using the spatial analysis tools built into ArcMap. The Elevations of the watersheds can also be compared just using the original DEMs.

 

The entire Iron County DEM was around 500 Megabytes (MB), which was difficult to work with and slowed down Arc-Map considerably.  The function “Extract by mask” in the ArcToolbox was used to cut the DEM to the size and shape of the watersheds. This made them smaller and much more manageable since the clipped DEMs were only about 20 MB each.  The clipped DEMs can be seen below in Figure 2.

 

2DEMs.bmp

Figure 2: DEM Cut to Watershed Sizes

 

The NHDPlus and USGS gage data were also much larger than the area of the Coal Creek and Parowan Watersheds. The NHDPlus data was obtained for the entire area of Region 16 and the USGS gage data was obtained for the whole of the United States. These were clipped down to size using the “Clip” function in the ArcToolbox. The hill shade for the clipped DEMs along with the clipped NHDPlus data can be seen in Figure 3.

 

NHD flowlines and hillshade.bmp

Figure 3: Hill Shades and NHDflow Lines for Each Watershed

 

After the DEMs and other data were clipped to a manageable size, they were prepared to be pre-processed. The pre-processing was done using the ArcHydro toolbar which can be downloaded from the ESRI website. First, the DEMs had to be reconditioned using the NHD Flowlines for the two watersheds. This process “burns” the streams into the DEM so that their locations area shown properly throughout the rest of the process.

 

The rest of the preprocessing prepared the data for the comparisons that were the goal of this section of the report. The commands used to preprocess the data are listed under the Terrain Preprocessing tab on the ArcHydro toolbar. The order and description of each step are as follows:

 

1.      Fill Sinks – This removes low points in the DEM that would prevent water from flowing downhill

2.      Flow Direction – This step computes the flow direction for each cell in the DEM. This is why it is so important to fill the sinks first. Without filling the sinks, there could be cells with no flow direction right in the middle of the river.

3.      Flow Accumulation – This step calculates how many other cells will flow into any given stream cell by counting all the stream cells flowing into the given cell. In this way, it shows how large the river will be at any point.

4.      Stream Definition – This digitizes the streams in the watershed by using the flow accumulation grid.

5.      Stream Segmentation – This groups the pixels from the stream definition into sections with the same cell count.

6.      Catchment Grid Delineation – This delineates the catchments for each stream segment and calculates the area of each catchment.

7.      Catchment Polygon Processing – This converts the catchment grids into polygons and assigns a HydroID, GridID, and NextDownID to each catchment.

8.      Drainage Line Processing – This converts the stream segments from raster format into lines and assigns a HydroID and GridID. It also calculates the points upstream and downstream of each stream segment. The drainage lines were used to find the total stream length for each watershed.

9.      Adjoint Catchment Processing – This calculates the adjoint catchments from the catchments and drainage lines and assigns a HydroID and GridID. The difference between the catchment and adjoint catchment is that each adjoint catchment has its own drainage point whereas catchments can share a drainage point.

10.  Drainage Point Processing – This calculates the drainage points for each catchment and assigns a HydroID, GridID, and DrainID.

 

The results of the above named steps can be seen in Figure 4. Notice that the thin strip directed towards Cedar City was lost in the process. It was determined that this lost area was negligible and made little difference in the results of the comparison.

 

After completing the preprocessing for each watershed, the watersheds were delineated using the Batch Watershed Delineation tool in the Watershed Processing tab in the ArcHydro toolbar. The watersheds shown along with the USGS gages, NHD Points, and NHD Flowlines delineated with the MAFLOWU (mean annual flow) attribute in Figure 5. Notice that one of the catchments in the Parowan Watershed was actually outside the watershed and changed the preliminary assumption of the watershed size.

 


Preprocessing Results.bmp

                                    Figure 4: Preprocessing Results

 

Gages Seeps and Flowlines.bmp

                                    Figure 5: Watershed Delineation with Gages and NHD Flowlines

 

 

Also as part of the comparison, the NHD Points shown in Figure 5 were also analyzed. It was found in the attribute table that the points shown were spring seeps, which are the probable sources of baseflow in each watershed. The soils data obtained from the USBR Soil Data Mart can be seen in Figure 6. Due to the fact that the data only covered about half of each watershed, it was decided that the soils data could not be used for accurate comparison due to the lack of data.

 

                                                Figure 6: USBR Soils Data

 

Table 1 shows the results of the preprocessing and data collected from .

 

Table 1

 

Coal Creek Watershed

Parowan Watershed

Area (from delineated watershed)

83.0 mi2

58.9 mi2

Stream Length

61.5 mi

56.12 mi

MAFLOWU

2.56 cfs

1.89 cfs

Spring Count

30

12

 

WATERSHED COMPARISON DATA

 

 

Several sources were used to gather the data used in comparing the two watersheds. Data sources included the AGRC website to obtain the DEM, USGS NWIS website for streamflow data, NHDPlus website for stream and watershed data, and USBR Soil Data Mart for soils data. The Snotel website was also searched, but no data was found for the Coal Creek or Parowan Watersheds.

WATERSHED COMPARISON RESULTS

 

 

After all the data was gathered and manipulated, the two Coal Creek and Parowan Watersheds could be compared. The results of these comparisons are shown in Table 2 where the percentage was found by dividing the Parowan Watershed value by the Coal Creek Watershed value. As stated earlier, the soils data was not included in this comparison due to lack of data.

 

 

Table 2: Watershed Comparisons

Comparison

Coal Creek Watershed

Parowan Watershed

Percentage

Size (from delineated watershed)

83.0 mi2

58.9 mi2

71.0 %

Drainage Density

0.741 mi/mi2

0.950 mi/mi2

128 %

Mean Annual Flow

2.56 cfs

1.89 cfs

73.8 %

Spring Density

0.361 springs/mi2

0.204 springs/mi2

56.5 %

 

It should also be noted that the Parowan Watershed is much denser than the Coal Creek Watershed, but has a much lower spring density. This illustrates the fact that springs most likely provide the baseflow for both of these watersheds. By multiplying the drainage density and spring density percentages, a 72.3 percent difference is obtained which is very close to the mean annual flow. As a result of these comparisons, it was found that the two watersheds are hydrologically similar. It is suggested that a scaling factor of 75 percent be used to apply the Coal Creek flood frequency curve to Parowan Creek. The study previously done by Bowen, Collins, and Associates found that the 100 year flood for Coal Creek was 5,500 cfs. Using this flow and the scaling factor, the 100 year flood for Parowan Creek can be projected at 4,125 cfs.

 

Flood Plain Delineation Objectives

With the 100 year flood-rate obtained for Parowan Creek, the process of creating a 100 year flood plain can begin.  The delineation of the 100 year flood plain will be carried out for Coal Creek.  The objective of this aspect of the project will be to create a floodplain from the USGS gauging site then downstream of the USGS gauging site for approximately 4.5 miles.  The floodplain will be determined using the following three programs: Arc-GIS to create a river profile and to draw the floodplain, HEC-RAS will be used to model the river, and Hec-Geo Ras will be used to interface between the two programs. 

 

Flood Plain Delineation Methodology

Before beginning the process of creating a floodplain, an aerial photo of the area is needed.  The photo that was downloaded was obtained from the AGRC website is referred to as a Mr. Sid county mosaic.  It is an aerial photo in the form of a sid file that covers most of Iron County.  From this county mosaic Coal Creek, its riparian area, and the Coal Creek watershed can be seen.  The DEM also needs to be altered before proceeding to create the floodplain.  Using the raster calculator, the DEM for Iron County was converted to measure elevation in feet.  This is because the projected coordinate system that was used in this project measures distances in feet. 

 

The next step is to create the layers that will be exported to Hec-Ras.  The Hec-Geo Ras toolbar is used for this process.  A personal database is created by the Hec-Geo Ras interface in which layers for the river, river cross sections, flowlines, and structures are created.  Once these layers are created and measured by Arc-Map, they can be exported to Hec-Ras.  Hec-Ras is used to model the river and the 100 year flood.

Text Box: Area of InterestCounty Mosaic

Figure 7: Iron County Mosaic

 

The river representing Coal Creek is drawn by hand, using the edit toolbar.  The line must be drawn from upstream to downstream, and must be one continuous line.  The river can be easily followed and navigated with the county mosaic in place, as shown in Figures 8 and 9.  The figures 8 through 11 only show a small portion of the river, to make the layers easier to see.

 

WithoutRiver   WithRiver.bmp

         Figure 8: Area Without Coal Creek Drawn                       Figure 9: Area With Coal Creek Drawn

 

With the river layer drawn and stored in the database, the flowpaths layer can be cut into the database.  The flowpaths are created by taking the river layer and using the edit command “copy parallel.”  They are the layer which represent the center line of the flow that will travel outside the river banks.

 

Flowpaths.bmp

Figure 10: Flowpaths Delineated

 

The layer that is used to represent bridges and culverts is referred to as the bridge layer.  They are drawn by hand, like the other layers.  They are used in Hec-Ras to show the exact location of the structures so they can be modeled correctly.

 

 

Bridges.bmp

Figure 11: Bridges and Culvert Cross Sections

 

The final cross layer that is drawn in Arc-Map is the river cross sections, which are labeled as XScutlines.  These are the cross sections that represent the river in Hec-Ras.  They define the shape and area for the river and the floodplain.  They need to be created so they are large enough to contain the 100 year flow rete.  Once the cross sections are drawn some of them are surveyed.  This is to ensure that the information that is input into the model will be as accurate as possible.  However, not all the cross sections are surveyed, only a portion of them.

 

 

crosssections.bmp

Figure 12: River Cross Sections

 

With all the layers drawn, the Hec-Geo Ras tool can prepare the database to be sent to Hec-Ras with the needed information.  With the Iron County DEM used for measuring elevations, and a projected coordinate system used for measuring distance in the X and Y directions, stationing can be assigned for each of the layers from a reference point.  The reference point, or zero station, in this situation is Rush Lake, where Coal Creek ends.  The river and flowpaths are assigned stationing at every point of inflection along their path.  The elevation associated with every one of those stations is also assigned and populated in their respective attribute tables.  The bridge cross sections and river cross sections have stationing assigned from left to right, relative to the downstream direction of the river, at every point of inflection.  The left side of the cross section is station zero, and the station at the right side of the cross section is the length, in feet, of the cross section.  Then elevations are assigned for each one of those stations.  These cross sections are also given a station in relation to the river. This station is assigned based on the point where the cross section intersects the river layer.  Figure 13 shows the toolbar command that assigns stationing and elevations for the cross sections, and Figure 14 the station that is assigned with each cross section that is related to the river.

Figure 13: Screenshot of the Hec-Geo Ras Toolbar

 

.PreRasExport.bmp

Figure 14: Cross Sections Stationing

 

With all the information assigned to each of the layers the information can be exported out of Arc-GIS and then imported into Hec-Ras.  When the information is imported into Hec-Ras it comes in with all the stationing and elevations that were assigned in by Arc-Map.  Figure 15 shows the river and cross sections as they appear in Hec-Ras.

 

Figure 15: Overview of the Hec-Ras Model

 

Once the information is imported into Hec-Ras the cross sections that have survey data available are updated.  The survey data for the cross sections needs to be input by hand.  The survey data for the structures is also input by hand.  Manning’s friction values are also input into the model, as are the boundary conditions.   The flow that was calculated as the 100 year flood rate earlier in this report is also input into Hec-Ras.  Once all the values are input, the model is ready to run and create a water surface profile.  From the water surface profile, water surface elevations and top widths are assigned for each cross section.  These top widths and water surface elevations are then exported from Hec-Ras in a spatial data (sdf) format file.

 

 

 

 

 

 

 

 

 

 

 

 

 


Figure 16: Cross Section Data Within Hec-Ras

 

 

Once the information is imported back into Arc-GIS, a new layer set is created with the imported bridge and river cross sections, and their water surface elevations.  The water surface elevations are read in on the DEM and to make sure that they area accurately placed.  The top widths are also used to create the flood plain that is imported from Hec-Ras.  The floodplain from Hec-Ras is called Bounding Polygons, as shown in Figure 17.

 

exported.bmp

Figure 17: Hec-Ras Flood Plain

 

From this Hec-Ras flood plain the actual flood plain can be drawn.  Contours are created from the DEM and then used to examine the topography of the surrounding area.  Using the topography, cross sections top widths, water surface elevations and engineering judgment, the final floodplain is delineated. 

Flood Plain Delineation Results

The final flood plain looked significantly different then the flood plain exported from Hec-Ras.  This is due to the topography surrounding the river.  The west side of the river slopes downhill, so the cross sections that were cut were not able to hold the flood.  Therefore the flow was assumed to leave the river and flow away from it.  Hec-Ras does not have the capability to model this scenario with any accuracy.  Also, due to this loss of water, the flow in the river needed to be reduced at the cross section where it is lost.  Hec-Ras also has difficultly with this situation.  Both of these limitations of Hec-Ras account for the major differences in the flood plain that Hec-Ras mapped and the final flood plain.

Interstate 15

 
FinalFloodplain.bmp

Figure 18: Final Flood Plain

Interstate 15 is elevated approximately 8 feet from the surrounding area.  This causes the Interstate to act as a dam for the water that leaves the river. The flood water will not reach depths greater than 3 or 4 feet so it cannot over top the Interstate.  Most of the water flows back into the river at the culvert that travels under Interstate 15, increasing the flow in the river back to a value close to its original flow rate.  Then later downstream, the river doesn’t have capacity to contain the flow, and the water spills out of the river until it reaches the end of the floodplain study. 

SUMMARY AND CONCLUSIONS

As a result of the watershed comparisons, it was determined that the Coal Creek and Parowan Watersheds are hydrologically similar. From this knowledge, a scaling factor of 75 percent is suggested to obtain a conservative estimate for the 100 year flood for Parowan Creek.

The floodplain was created from the 100 year flood rate determined from the watershed comparison.  The river did not have capacity to hold this flow rate at a few of its cross sections, and because of the topography, the water did not flow back into the river until it reached Interstate 15.  The Interstate functioned as a dam for this scenario, and brought all the water back into the Coal Creek.  But downstream of Interstate 15, the river did not have capacity to hold the 100 year flood rate, which causes more flooding.

RECOMMENDATIONS

There are only a few cross sections in Coal Creek that did not have capacity to hold the 100 year flow rate.  These cross sections that cannot hold the flow are causing most of the flooding.  One of the recommendations that would improve the capacity of the river is to determine exactly how many cross sections are causing the flooding problem and increase their capacities.  If the few cross sections that do not have capacity have their capacities increased the floodplain could be reduced, or even eliminated for the 100 year flow rate.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

References

Army Corps of Engineers.  http://www.hec.usace.army.mil/software/hec-ras/hecras-hecras.html

Automated Geographic Reference Center  (AGRC).  Utah GIS Portal.  http://agrc.utah.gov/

National Ocean and Atmospheric Administration (NOAA).  http://www.noaa.gov/

Natural Resources Conservation Science (NRCS).  http://www.wcc.nrcs.usda.gov/snow/

United States Bureau of Reclamation (USBR).   http://www.usbr.gov/

United States Geologic Survey (USGS).  http://www.usgs.gov/