Part 1 of this lab was on Delineation of Watersheds. A watershed or drainage basin describes the land from which all surface waters converge at a single point. This point is the exit of the watershed in the form of a stream or lake. (https://water.usgs.gov/edu/watershed.html) This lab had students analyzing Adirondack Park in New York. Adirondack Park is located in New York and covers six million acres of land. Two million of that land is owned by New York. Within the park are one-hundred-five towns and villages.(http://visitadirondacks.com/about/adirondack-park) As this park has a lot of different uses, recreation, agriculture, and forestry, it is important to understand the flow of water and sources within the park. This lab consisted of three major components data collection, processing the data, and watershed delineation.
Methods:
Data collection
Data collection was the first step. The data collected was the Adirondack Park boundary shapefile from the New York State GIS Clearinghouse at http://gis.ny.gov. The next data collected was a hydrology shapefile from Cornell University's Geospatial Information Repository from http://cugir.mannlib.cornell.edu/index.jsp. The final data collected as a 30 arc second DEM of North America from the Esri data set through ArcMap
Processing the data
There were several processes that took place to analyze the data. The first step in processing the data was using the ArcToolbox by going to Analysis Tools-Proximity-Buffer. After that a 20 km buffer was added to the Adirondack Park boundary file making sure that the Dissolve was set to all. Once the Dissolve was set, the next step was to clip the streams feature class. Figure 1 show the results of clipping the streams to the Adirondack Park boundary.
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| Figure 1: Step 2 results |
Watershed Delineation
The first step in Watershed Delineation was to calculate flow direction for each cell in the DEM by using the Spatial Analyst hydrology tool. Once this was done, the next step was analyzing sinks that represent depressions by Spatial Analyst Tools-Hydrology-Fill. Finally, Flow direction was used to show water flows from high values to low values as shown in Figure 2.
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| Figure 2: Watershed sinks representing depressions, |
The next step in Watershed Delineation was to determine where water accumulates and creates channels by using the Spatial Analyst Tool-Hydrology-Flow Accumulation. After this step, the next step was creating a source raster that required choosing a threshold of a minimum number of cells flowing into any cell before it was designated a stream cell. In order to do this, the first step done was Spatial Analyst Tools-Conditional-Con. Flow_acc was set as the input conditional raster layer where value>50000 was put in the expression box and 1 was the true raster value. Following this step, a streamlink spatial analyst tool was used. After the streamlink was used, the final step in determine where water accumulates and creates channels was using the hydrology watershed tool again. Figure 3 represents the results of a 20 km park buffer zone with 30 m cell size with a threshold value of 50,000 for a minimum number of cells that flow into a stream resulting in 388 unique watersheds.
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| Figure 3: number of cells that flow into a stream resulting in 388 unique watersheds |
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| Figure 4: Watershed Analysis |
Lab 2 Part 2: Flooding Analysis in Denmark
Introduction
On July 2, 2011 136 millimeters of rain fell in downtown Copenhagen, Denmark. Due the the amount of rain, the sewer system backed up which caused thousands of basements to flood not only with rainwater, but wastewater as well. This weather event is called a Cloudburst. A Cloudburst is where more then 15 millimeters of rain falls in a 30 minute period. (https://learn.arcgis.com/en/projects/find-areas-at-risk-of-flooding-in-a-cloudburst/lessons/explore-the-cloudburst-issue.htm) Due to Cloudbursts happening more frequently, the Danish government looked for ways to determine the most at risk areas for flooding. Gentofte is a low lying area and prone to flooding, thus; it became a place for Danish officials to study bluespots. Bluespots are areas where rain is more likely to accumulate during cloudbursts. Due to LiDar and GPS technology, finding bluespots has become easier. This lab consisted of finding bluespots and affected buildings that are at flood risks.
Methods
The first step was to use the Fill tool. The Fill tool was used several times, The first Fill tool was used for sinks in the DEM raster and the second Fill tool was used for small sinks. The next step was to find which buildings were in more danger due to bluespots. In order for that step to be accomplished. The next step was doing a select by location analysis to find all buildings that either touched or were within the boundary of a bluespot. Figure 1 shows the result of this.
Figure 1 shows the result of at risk buildings. The steps to do this were using a spatial analysis tool, Flow Direction. The tool was used on small sinks fill raster which analyzed where the water flows. The next step was to figure out how much water capacity a bluespot has.
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| Figure 2 |
The next step in the lab was to assess flood risk buildings by calculating how much rainfall is needed. To do this, the attribute table for the bluespots Touching Buildings was opened, The fillup field was sorted into ascending order. Next, the symbol selector for the Municipal Boundary was edited to Gray 60%. After that step, importing symbology dialog box was the following step. The final result is figure 2.
Conclusion
It is important to be able to distinguish the areas that are at a heavier risk for blue spots. Especially those in low-lying areas such as Gentofte. Being able to use GIS to analysis flood risk buildings is helpful to local governments, residents, and emergency personal.
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