Flood susceptibility for the kennebecasis watershed
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The document discusses analyzing restoration sites in the Kennebecasis watershed for their susceptibility to flood damage. It describes the Kennebecasis Watershed Restoration Committee, which aims to restore the watershed. Various data factors are considered to assess flood susceptibility, including soil, land use, and satellite imagery. Methods used include delineating sub-watersheds and river buffers. Next steps include further site visits, finalizing the analysis method, and incorporating existing site data.
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Flood susceptibility for the kennebecasis watershed
- 1. RESTORATION SITE ANALYSIS: SUSCEPTIBILITY TO FLOOD DAMAGE Clio Marsh Nikias
- 2. KENNEBECASIS WATERSHED RESTORATION COMMITTEE Sussex, New Brunswick Non-for profit restoration group Partners Mandate Primary activity Kennebecasis Watershed Restoration Committee
- 3. FLOOD SUSCEPTIBILITY AND RESTORATION SITES Concept Factors Purpose Application Data
- 4. RESTORATION SITE ON TROUT CREEK
- 5. THE WATERSHED Currently 5 sub- watersheds Hammond’s Plains just added
- 9. RIVER BUFFERS Combined polyline and polygon 60 metre buffer
- 10. VEGETATION DATA
- 11. SOIL DATA
- 12. LAND USE DATA
- 13. EXAMPLE OF LANDSAT RESULTS
- 17. SAMPLE OF UNION
- 18. MILL BROOK
- 19. WHAT’S NEXT Visit client in person Decide symbology for KML files Tweak any issues with the sub-watershed outlines Apply the Delphi Method to rank and weight the factors Perform the site selection and assess the results Incorporate existing site data in to database
- 20. THE END.
Editor's Notes
- The KWRC is a non-for profit restoration group operating in Sussex, New Brunswick since 1994 They work with a large group of partners including Sussex Fish and Game Association, various branches within the Government of New Brunswick, the various towns in their watershed, Natural Resources Canada, Fundy Model Forest to name a few. The KWRC work year round to improve the health of the Kennebecasis river watershed. Their work focuses on three primary categories: Restoration of river banks and the riparian zone Monitoring and assessing the river at specific sites year after year. As well as assessing the conditions of roads and culverts. In addition they also focus on community involvement and education. Recently their primary focus has been river bank stabilization and increasing the strength of the riparian zone. The present locations of concern are where clear-cutting or farm land extends right up to the edge of the river or creek. These areas are weakened by the removal of vegetation and are susceptible to major changes during flooding events. Currently they do not use GIS software but are looking to make this change in the future
- The concept behind the flood susceptibility is to find the areas within a certain distance (a buffer) of the river that could be prone to damage from flooding based on several main factors. The main factors currently include: Change detection within riparian zone buffer Vegetation type Soil type Land use In order to select a restoration site there needs to be further understanding of its composition. Factors such as soil, vegetation, and land use can help to assess the cause and scale of the damage after a flooding event. It is important to determine whether the changes are caused by human impact or are natural changes that occurred before. Once a site is selected understanding of these factors will also affect what restoration methods may be selected.
- This is an example of an existing restoration site. This site is on agricultural land where the majority of shrubs and trees had been removed near the river. During a flood the channel blew out and never fully returned back to the oringial channel. This not only frustrated the farmer but also made the river shallower and warmer, creating a less hospitable habitat for fish. Replanting the vegetation has made the river more stable and reduced the temperature.
- Currently the KWRC looks after an area of approximately 1346 square kilometres, with this addition of the newest sub-watershed Hammond’s Plains, they will now cover an area of 2147 square kilometres. Three of the sub-watersheds, or secondary watersheds, are fed by the main source, the Kennebecasis River. The other three secondary watersheds are fed by secondary rivers, Millstream River, Trout Creek, and Smiths Creek.
- The main watershed was created using the hydrology toolset in ArcMap. Initially sinks needed to filled to level out any extreme drop offs. Flow direction and accumulation tools when then run on the DEM, this helps to identify where the limits of the watersheds. At that point the Watershed tool can be run and the output something similar to the image above. At this point, Mark Hebert mentioned that the province may have some of this data already so I found a layer from the province of New Brunswick with watersheds and conveniently there was the Kennebecasis watershed. I compared the two and sent a KML to my client who confirmed that the extent appeared to be as he expected. Then to create the sub-watersheds I used pour-points in to subdivide the primary watershed. This wasn’t fool proof, the three sub-watersheds that are fed by secondary rivers were easier to outline. It didn’t work as well for the other three. The client did however have a jpeg map of the sub-watersheds, so I georeferenced that to the primary watershed, it didn’t line up exactly, but that help guide any small corrections to the output. The newest sub-watershed was easy to outline as it was the remainder of the primary watershed.
- For restoration site analysis I need to create an area of interest around the rivers and creeks. Generally the KWRC would ideally like to have a 30 m riparian zone buffer, but in this case I created both 30 m and 60 m. This is for a couple reasons. The first is to get more data in the analysis of the sites and the second if because of the format of the data. The hydrology layers from the province come in both polygon and polyline files. The polygons represent the true area of the larger rivers and lakes, whereas the polyline features do not. Above is an example. The features that are only represented by polyline are generally not very wide (possibly 1 to 5m, maybe a little wider) therefore if the desired area is 30 m, having a 60m buffer allows room for error. Another issue is that the buffers should ultimately be one feature to simplify the process and the rivers are represented by many features, as seen above. So once the buffer is made, I merged them into one singlepart feature as opposed to multipart.
- This is the results of the polygon buffers.
- This is the results of the polygon and polyline buffers merged together. You can see here the polyline feature for the Kennebecasis river would not have sufficed for a true 60 m buffer since the river at this point can be over 2 km wide.
- This shows a the change from two landsat images. The first image was from July 2008, the second from August 2014. A value of 1 is the most change and a value of 0 indicates no change.
- Delphi method: the client and their partners (experts) will be asked to assess the rank and weights for each of the factors. For this method they will individually weight each element and then the mean of all participants will be calculated. Once the group has reached a consensus those values will be applied to the data. An example of this method will be done using arbitrary ranks and weightings for Trout Creek sub-watershed focusing on the Mill Brook.
- In order for this process to work the input factors need to be polygon features. To convert a raster to polygon the values needed to be integers so the landsat results were reclassfied in to 6 equal groups, these would represent their ranks. For the example the ranks were given a weight of 0.5.
- Here are the attribute tables for soil and land use. I have arbitrarily ranked and weighted them to show how the process works. The vegetation change layer is already ranked so it only needed to be weighted, I have it a weight of 0.5. The soil was weighted at 0.2and land use at 0.3. Then the weighted rank is calculated.
- The figure on the left is the result of the first union between soil and land use. The second map is the results of a union with the first output and vegetation change added. The attribute table is then updated with a total sum of the weighted ranks, adding all the weighted ranks together and providing a value between 1 and 6 for each polygon within the 60 m riparian buffer zone.
- The first figure on the left is an example of the results of the sum of weighted ranks. The green highlights areas that least susceptible to damage during a flood, where the red are the most susceptible. The map the right shows all the areas within 60 m of the Mill Brook that have a value great than 4, the highest value being 6. You can see the area in the centre would be likely have the least damage and could be avoided for now.
- The kml files a are not going to be used for analysis but instead to make the information accessible for the affected members of the community, who are interested but have software limitations. They also have a lot of data on their current restoration sites and culvert assessments and would like to include these data sets in this database, so going forward that will be added A road network and property boundary layers were also added to this database to facilitate creating a restoration after an ideal location has been identified.
- Cananda land inventory data http://sis.agr.gc.ca/cansis/nsdb/cli/classdesc.html#classes