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Brigham Young UniversityBYU ScholarsArchive
Engineering Applications of GIS - LaboratoryExercises Civil and Environmental Engineering
2017
Laboratory Exercise: Calculating Hobble Creek100 Year Flow Using ArcMapMicklane Farmer
Saul Ramirez
Riley Vane
Follow this and additional works at: https://scholarsarchive.byu.edu/gislabs
Part of the Civil Engineering Commons, and the Geographic Information Sciences Commons
This Article is brought to you for free and open access by the Civil and Environmental Engineering at BYU ScholarsArchive. It has been accepted forinclusion in Engineering Applications of GIS - Laboratory Exercises by an authorized administrator of BYU ScholarsArchive. For more information,please contact [email protected], [email protected].
Recommended CitationFarmer, Micklane; Ramirez, Saul; and Vane, Riley, "Laboratory Exercise: Calculating Hobble Creek 100 Year Flow Using ArcMap"(2017). Engineering Applications of GIS - Laboratory Exercises. 6.https://scholarsarchive.byu.edu/gislabs/6
Laboratory Exercise: Calculating Hobble Creek 100
Year Flow Using ArcMap
Background
GIS is used extensively in watershed analysis. In previous labs, students have determined a
watershed from DEM data. Calculating flow can be very useful in many engineering aspects.
With the correct data, GIS can be used to develop a hydrologic model to calculate the flow from
a watershed. The purpose of this lab is to develop a model to calculate the flow for the Hobble
Creek 100 year event.
Problem Statement
Hydrology is an important part of any civil engineering project. No matter what type of project it
is, the engineer will deal with water. In any city, a 100 year event would need to be calculated
for insurance, building, and other purposes. There are applications out there that give a rough
estimate for flows in particular areas. Using Utah’s rational method, one can more accurately
calculate the flow for a 100 year event using area specific equations that have been calibrated
using stream gauge data.
Let’s assume that you need to identify the 100 year flow for Hobble Creek Canyon. Your goal is
to calculate a value using the provided data and ModelBuilder tools to come up with a more
accurate value for the 100 year flow for the city.
The rational method should be used to compute peak flows for small watersheds. This method
is centered on the idea that the peak flow can be obtained by multiplying the rainfall intensity by
the watershed area, and the unit less runoff coefficient. The rational method equation can be
written as:
Q = CIA
Q = Peak discharge
C = runoff coefficient
I = Rainfall intensity
A = Area of watershed
This method has been developed using stream gauges to obtain rainfall intensity values for
different regions throughout the state, for more accurate results. The article Methods for
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Estimating Magnitude and Frequency of Peak Flows for Small Watersheds in Utah, published
by UDOT, was used to obtain the information needed to determine which coefficient to use.
Data
● https://streamstats.usgs.gov.ss : Download the Hobble Creek drainage basin shapefile
and estimated 100 year flows. Step 1 goes through the procedure to do this.
● https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html: Download Csv file, procedure
found in Step 1.
● Arcgis.utah.gov: Download the 10 m DEM for Utah County
ModelBuilder Tools
In this exercise, you may use previous tools and the following new tools.
● Fill- Fills in areas that are imperfections in elevation data.
● Clip- Use this tool to cut out certain portions of a feature class.
● Zonal Geometry- Calculates certain geometry parameters. In this lab it is used to
calculate area.
● Raster Calculator- Builds and executes equations using a calculator like setup.
● Flow Direction- Creates a raster of flow for each cell based off of slope.
● Flow Length- Calculates the upstream or downstream distance for in a flowpath for each
cell.
● Zonal Statistics- A basic tool that can calculate things such as maximum or minimum
values.
● Feature to Point- Creates a feature class containing points.
● Raster to Point- Converts a raster dataset to points.
● Merge- Combines multiple data sets into one dataset.
● Points to Line- Creates line features from points.
● Polyline to Raster- Creates raster features from polylines.
● Create Constant Raster- Creates a raster with a constant value within an analysis
window.
● Slope- Calculates the slope based on raster values from file.
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3
Example Model
Model 1 - Time of Concentration
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PeakFlow
5
Model 2 - Peak Flow
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6
Step by Step Solution
Step 1
Navigate to the Utah ArcGIS data website and download the 10 m DEM for Utah County
https://gis.utah.gov/data/elevation-and-terrain/
Next, go to https://streamstats.usgs.gov.ss to retrieve the Hobble Creek drainage basin
shapefile and estimated 100 year flows to compare to your final calculated flow. Follow prompts
to delineate the basin just above the pond and to down load the shapefile. This is the shapefile
we will use to clip the DEM in ArcMap.
Figure 1- Delineating the area of interest
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Figure 2- Outlining the watershed boundary
Continue to follow prompts to build a report and get estimated 100 year peak flow statistics. The
100 year peak flow should approximate 1,000 cfs. This is a rough, ball park estimate, we will
compare our results to this number for accuracy.
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Figure 3- StreamStats data and possible flows for the watershed
Now go to the NOAA Atlas 14 for storm event intensity data for the basin. Go to
https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html and navigate to your estimate of the
centroid of the shapefile you just downloaded from StreamStats. Change the Data Type from
Precipitation Depth to Precipitation Intensity.
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Figure 4- NOAA Rainfall data for the area of interest
Download the csv at the bottom of the page to reference later on. The table should resemble
this when opened in excel.
Table 1- Sample precipitation data for area
PRECIPITATION FREQUENCY ESTIMATES (INTENSITY) FOR HOBBLE CREEK DRAINAGE BASIN
by duration for ARI (years): 1 2 5 10 25 50 100 200 500 1000
5-min: 1.63 2.09 2.88 3.58 4.67 5.65 6.8 8.11 10.2 12.1
10-min: 1.24 1.59 2.19 2.72 3.55 4.3 5.17 6.17 7.79 9.24
15-min: 1.03 1.32 1.81 2.25 2.94 3.55 4.28 5.1 6.44 7.64
30-min: 0.692 0.886 1.22 1.51 1.98 2.39 2.88 3.44 4.33 5.14
60-min: 0.428 0.548 0.754 0.937 1.22 1.48 1.78 2.13 2.68 3.18
2-hr: 0.272 0.342 0.451 0.549 0.709 0.85 1.02 1.21 1.52 1.8
3-hr: 0.214 0.268 0.341 0.409 0.516 0.608 0.717 0.844 1.05 1.24
6-hr: 0.146 0.18 0.221 0.256 0.308 0.353 0.403 0.46 0.562 0.654
12-hr: 0.096 0.118 0.143 0.165 0.195 0.22 0.246 0.276 0.322 0.363
24-hr: 0.06 0.074 0.09 0.103 0.12 0.134 0.149 0.163 0.183 0.198
2-day: 0.036 0.044 0.054 0.062 0.073 0.082 0.091 0.101 0.114 0.125
3-day: 0.027 0.033 0.041 0.047 0.056 0.063 0.071 0.078 0.089 0.097
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4-day: 0.023 0.028 0.035 0.04 0.048 0.054 0.06 0.067 0.076 0.083
7-day: 0.016 0.02 0.024 0.028 0.033 0.037 0.042 0.046 0.052 0.057
10-day: 0.013 0.016 0.019 0.022 0.026 0.029 0.032 0.035 0.039 0.043
20-day: 0.009 0.011 0.013 0.015 0.017 0.019 0.021 0.022 0.025 0.026
30-day: 0.007 0.009 0.011 0.012 0.014 0.015 0.017 0.018 0.02 0.022
45-day: 0.006 0.007 0.009 0.01 0.011 0.013 0.014 0.015 0.016 0.017
60-day: 0.005 0.007 0.008 0.009 0.01 0.011 0.012 0.013 0.014 0.015
Step 2 - Time of Concentration
The actual flow will be calculated using 2 separate models. Because the equations used in the
UDOT design manual are highly empirical, you will be forced to translate all lengths and areas
to feet, and square miles.
The first model will calculate the time of concentration, which will then be used to calculate the
intensity of the rainfall event. The model will follow the UDOT provided equation of:
𝑡𝑐 = 2.4 ∗ 𝐴0.1 ∗ 𝐿0.25 ∗ 𝐿𝑐𝑎0.25 ∗ 𝑆−0.2
A = Area of watershed [sq mi]
L = Length of longest flow path [mi]
Lca = Length of watershed from drainage output, through shape centroid. (We will estimate this
distance by multiplying the distance of the output to the centroid by 2) [mi]
S = Average slope of longest flow path L [ft/mi]
Clip and fill the DEM with the Hobble Creek Basin.
Figure 5- Setting up the Time of concentration model
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Calculate the area (A) of the basin using Zonal Geometry. Use a cell size of 10. Convert to
square miles using a Raster Calculator. (2,589,988 square meters per 1 square mile)
Calculate the longest flow length using methods that have been done in previous labs (L).
Figure 6- Model for calculating the longest flow length
Use the max flow length in combination to with the maximum and minimum elevations
(converted to miles) to get the slope of the longest flow path.
This will be done using separate Zonal Statistics for both maximum and minimum elevation
values from the clipped DEM. The Raster Calculator would then have the following equation to
calculate the slope of the flow path (S).
3.28084*("%Max_Ele%" - "%Min_Ele%")/"%Max_Flow_Length_mi%"
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Figure 7- Calculating the slope of the longest flow path
The distance from the basin output point through centroid and extending to the far side of the
watershed can be difficult to calculate using Modelbuilder. This may be approximated by
multiplying the distance from the output point to the centroid by 2. Start by using the same raster
containing the value of the minimum elevation used in the prior step to locate the drainage
output. This is done using a Raster Calculator.
Figure 8- Calculating the distance minimum elevation point
The Raster Calculator should isolate cell that has the minimum elevation. Use this equation:
Con("%HC_Basin_DEM%"=="%Min_Ele%","%HC_Basin_DEM%")
Extracting the centroid of the Hobble Creek watershed can be easily done with a single tool,
Feature to Point
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Figure 9- Calculating the centroid of the watershed
Before the distance between the 2 points can be calculated, the points need to be merged into a
single raster.
Figure 10- Merging the centroid to the centroid
Use the Points to Line tool, followed by the Polyline to Raster tool to get a distance. Populate
the Polyline to Raster tool in the following manner.
Figure 11- Getting the distance between the exit of our watershed to the centroid
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The Raster Calculator should then calculate the distance, in miles, of the line. Assuming the
watershed has a general shape of an ellipse, we will multiply this distance by 2 to estimate the
distance across the watershed (Lca) in the final Raster Calculator. (1609.34 meters per 1 mile)
"%centroid_length_m%" / 1609.34
You are now ready to combine the 4 values into a single Raster Calculator to find the time of
concentration.
Figure 12- Calculating the time of concentration
Use a Raster Calculator with the following equation (the same as the equation stated at the
beginning of Step 2)
2.4*"%HC_Area_sqmi%"**0.1*"%Max_Flow_Length_mi%"**0.25*(2*"%centroid_length_mi
%")**0.25*"%HC_slope_ft_per_mi%"**-0.2
The calculated Time of Concentration should approximate 5 hours.
Step 3 - Interpolate Intensity
The intensity value can be interpolated using the data obtained from NOAA in step 1. The table
from the spreadsheet is shown below. Use 100 years for the duration. Using the calculated time
of concentration from step 2 (hours), you will need to interpolate an intensity value. The
equation is shown below.
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General equation: Intensity=(Calculated time of concentration-lower time bound)/(Upper time
bound-Lower Time Bound)*(Lower Time Intensity - Upper Time Intensity)+(Lower Time
Intensity)
In our case:
Intensity= (Calculated time of concentration-3)/(6-3)*(.403-.717)+(.717)
The calculated intensity should be approximately 0.5.
by duration for ARI (years): 1 2 5 10 25 50 100 200 500 1000
5-min: 1.63 2.09 2.88 3.58 4.67 5.65 6.8 8.11 10.2 12.1
10-min: 1.24 1.59 2.19 2.72 3.55 4.3 5.17 6.17 7.79 9.24
15-min: 1.03 1.32 1.81 2.25 2.94 3.55 4.28 5.1 6.44 7.64
30-min: 0.692 0.886 1.22 1.51 1.98 2.39 2.88 3.44 4.33 5.14
60-min: 0.428 0.548 0.754 0.937 1.22 1.48 1.78 2.13 2.68 3.18
2-hr: 0.272 0.342 0.451 0.549 0.709 0.85 `1.02 1.21 1.52 1.8
3-hr: 0.214 0.268 0.341 0.409 0.516 0.608 0.717 0.844 1.05 1.24
6-hr: 0.146 0.18 0.221 0.256 0.308 0.353 0.403 0.46 0.562 0.654
12-hr: 0.096 0.118 0.143 0.165 0.195 0.22 0.246 0.276 0.322 0.363
24-hr: 0.06 0.074 0.09 0.103 0.12 0.134 0.149 0.163 0.183 0.198
2-day: 0.036 0.044 0.054 0.062 0.073 0.082 0.091 0.101 0.114 0.125
3-day: 0.027 0.033 0.041 0.047 0.056 0.063 0.071 0.078 0.089 0.097
4-day: 0.023 0.028 0.035 0.04 0.048 0.054 0.06 0.067 0.076 0.083
7-day: 0.016 0.02 0.024 0.028 0.033 0.037 0.042 0.046 0.052 0.057
10-day: 0.013 0.016 0.019 0.022 0.026 0.029 0.032 0.035 0.039 0.043
20-day: 0.009 0.011 0.013 0.015 0.017 0.019 0.021 0.022 0.025 0.026
30-day: 0.007 0.009 0.011 0.012 0.014 0.015 0.017 0.018 0.02 0.022
45-day: 0.006 0.007 0.009 0.01 0.011 0.013 0.014 0.015 0.016 0.017
60-day: 0.005 0.007 0.008 0.009 0.01 0.011 0.012 0.013 0.014 0.015
Step 4 - Peak Flow
The key to calculating the flow, and the beauty of the UDOT technical manual, is developing a
good runoff coefficient. The technical manual describes the equations to develop accurate
runoff coefficients based on 7 separate regions, and the event in question. For the 100 year
event in Hobble Creek (Region 4), the following equation is prescribed.
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𝐶100 (𝑅𝑒𝑔𝑖𝑜𝑛 4) = 100.094 ∗ 𝐵𝑆−0.888
BS = Average basin slope [percent]
To begin the peak flow model, copy the following tools and intermediate steps from Model 1: the
clipped and filled DEM, and the raster of the area in square miles.
The first tool introduced in Model 2 is the Slope tool, used to find the slope of each raster.
Follow this tool with a Zonal Statistics to find the average slope for the basin.
Figure 13- Calculating the Slope
This is the only parameter needed for a Region 4 event. We can then use a Raster Calculator
to calculate the runoff coefficient (c). Use the following equation.
10**.943*("%HC_Mean_Slope%"/100)**-.888
Figure 14- Calculating the mean slope
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You now need to incorporate the interpolated intensity value into the model. This may be done
by the Create Constant Raster tool.
Figure 15- Creating a constant value raster to enter the interpolated intensity
First, right click the background of the model, and select Create Variable. Right click your new
variable and select Model Parameter. Bring in the Create Constant Raster Tool and establish
parameters in the following manner.
Figure 16- Setting for constant raster
Double click the created variable and enter your interpolated intensity value (approximately 0.5).
You are now ready calculate the 100-year flow.
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Figure 17- Calculating the final 100 year event
Include the following equation in your Raster Calculator
"%ConstIntensity%"*"%HC_Area_sqmi%"*"%C_Value%"
You should have an approximate 100 year flow for Hobble Creek of 970 cfs.
Deliverables
Using the data and steps listed above, create a model in ModelBuilder that will calculate the
Hobble Creek 100 year flow. Your resulting map should show the watershed shapefile of Hobble
Creek. The legend should show your shapefile along with the drain point. Prepare a report that
contains your model and the step by step process by which you obtained your results. Don’t
forget to include a copy of your map with the final results displayed. Your report should also
include the values that were calculated in this lab, comparing them to the values found in the
streamstats data.
References https://streamstats.usgs.gov.ss
https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html
Arcgis.utah.gov
Utah Department of Transportation. (2010). Methods for Estimating Magnitude and Frequency
of Peak Flows for Small Watersheds in Utah . Utah Department of Transportation Research .
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Example Map
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Rubric for Estimating Peak Flow
Item Points
Assignment Title, Author Name, Date, Course Name /5
Brief summary of the requirements of the project /5
Describe your model and code ● List each of the tools used including inputs and outputs ● Describe each input dataset including type (point, line, polygon raster)
and the source of the data ● Describe the important steps in the analysis
/10
Show your model on 8.5x11 inch size paper ● All text is readable (10 point font minimum) ● All tools and data sets are shown
/10
● Describe some of the challenges with the lab. What went right what went wrong?
● Describe why the results may differ from that of StreamStats. /5
Make a full page (8.5x11) map of your results ● Map Title: 1 point ● Neat Line: 1 point ● North Arrow: 1 point ● Scale Bar: 1 Point ● Text box with author name, date, map projection: 1 point. ● Watershed shown: 3 points ● Locator map showing where in Utah the analysis was performed: 3
points ● Visible map: 1 point ● All text is legible on the map: 3 points
/15
Bonus Task: ** Instructor’s Discretion