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Author  A view of the Delta Decker Island is one of the many islands located in California’s Delta. The Delta covers over 1,100 square miles, includes 700 miles of waterways and spreads across six counties: Alameda, Contra Costa, Sacramento, San Joaquin, Solano and Yolo county. There are approximately 70 major islands and tracts with inhabitants on many of these islands. It is also and island where erosion is occurring. This is one of the ongoing concerns in the Delta because delta levees protect human life, existing structures, define channels used for commercial navigation, protects the upland habitat, and protects our water quality. Maintaining and improving the delta levees is vital to our economy and the Delta's ecosystem. 
Photography By: Joel Dudas
Decker Island is a 470-acre tract located in Solano County. The northern tip of the island and the area of this study are 33.2-acres owned by the California Department of Fish and Game. Originally marshland, this island was formed when the Sacramento River was dredged and straightened at Horseshoe Bend between the years 1917-1937. The elevation of the island is approximately 20 feet above sea level from the dredged spoils that were deposited. A two-phase project developed 26 acres of fish and wildlife habitat at the northern tip of the island. The Decker Island Habitat Development/Levee Improvement Project recreated habitat that existed prior to the dredging. Phase I was completed in December 2000 and created approximately 13.5 acres of habitat. Phase 2 was constructed in 2004 and created 12 additional acres of similar habitat. After the Decker Island Habitat Development/Levee Improvement Project was completed, noticeable erosion had taken place on the northwest side of the island. The Department of Water Resources historic aerial photography of Decker Island dating from 1975 through 2006 was used to assess the change occurring from erosion. The study helped in the discussion and decision making process of the erosion taking place and what if anything that should be done to reduce its effect.  Constructed levees are the key physical element which create and maintain the Delta as we know it today. System geotechnical studies and assessments are needed for evaluating levee reliability concerning erosion, stability, seepage, and under-seepage issues in order to determine the level of safety afforded any area protected by a levee. Most delta levees are 10 to 12 feet above sea level, and often protect island interiors that are feet below sea level. Permeable lenses in the levee and foundation, together with historic relics, such as abandoned pipes, and constant burrowing by various mammals commonly result in seepage, distress and internal erosion. Erosion occurs in all rivers from the wearing away of the river banks, river bed, and channel. The movement of water is very powerful and carries a lot of energy as it travels downstream to its final destination of a lake, reservoir, or an ocean. Erosion from water creates features like waterfalls and rapids and can wear away the river bed forming a V-shaped valley. It can also make the river wider, deeper and can change its course over time. These photographs were taken to document and show the erosion taking place on Decker Island using a Ricoh 500SE GPS Digital Camera. The photos were then processed using GPS-Photo Link software where point shapefiles were generated and brought into ArcMap for future use. Date of Photography: May 7, 2008  RIMG0258.JPG  RIMG0260.JPG Date of Photography: August 27, 2008  RIMG0658.JPG  RIMG0661.JPG  RIMG0663.JPG The first thing that was done in conducting this study was to locate aerial photography of Decker Island using The Department of Water Resources historic photography. Thirteen temporal images were used to digitize the bluff and shoreline for each period using ArcMap. The dates of photography that were used are 1975, 1977, 1986, 1993, 1998, 2000, March 2001, October 2001, 2002, 2004, March 2005, June 2005, and 2006.
After the areas were digitized, the island was broken down into six distinct zones. The divisions of zones were done so that any area of erosion or area of non-erosion fell completely into a zone of its own. The data generated in ArcMap of the six zones included a field for the area of each zones bluff and shoreline. These data were then used in Excel to create graphs showing the change for that zone by total area of bluff and shoreline, the change in bluff and shoreline since the previous sampling, and the change in bluff and shoreline since 1975, in acres. The image below shows the six zones that were created for use in the comparison.
The Department of Water Resources historic photography used to digitize the bluff and shoreline are shown below. The images of the island were created at 1:5000 meters and a 30 meter square close up of the same area on each image is outlined in blue and were created at 1:250 meters. The varying quality of photography and pixel size can be clearly seen.
Equipment Used in Field:
In Office:
Tabular data was generated from the six polygon areas and used in Excel to create graphs of Decker Island’s total area of bluff and shoreline, the change in bluff & shoreline since the previous sampling, and the change in bluff & shoreline since 1975 in acres.
An animated web page was created as a tool in visualizing the change occurring to Decker Island from the erosion taking place. Source code was copied from the National Snow Analysis web page and modified with images containing the digitized bluff and shoreline of Decker Island and the 2006 aerial photography as a background. Link to Animation
Analysis of the graphs proves that in zones 2, 3, and 6 there is loss in area occurring due to erosion and that not enough noticeable change has occurred in zones 1, 4, and 5 to be concerned about. From the animation you can visually see the erosion that is taking place in three different areas. Because of the varying pixel resolution the digitized bluff and shoreline are more accurate in some years and less accurate in others. The imagery that had a lower resolution made it difficult in determining where the bluff and shoreline were at times so reference had to be made to the digitized polygons from images of a higher resolution. The shoreline data proved to be useless in the end because the exact dates and times of the aerial imagery could not be determined and tidal information could not be applied to account for its variations. The un-rectified aerial images from the years of 1975, 1977, 1986, 2000, March 2001, and 2004 induced more error in the digitized polygons than that of the ortho-rectified images. The use of a single image made it hard to determine where the bluff or shoreline actually was and may also have induced some error in the digitized polygons. Yet another problem that was encountered in the photography was that of having an unknown spatial reference. Determining the correct coordinate system can be challenging but it can prove to be even more challenging when working with the DWR’s historic photography because one image had an unknown spatial reference and it took awhile to figure out that it was in NAD27 UTM Zone 10.5. DWR uses NAD27 UTM Zone 10.5 on occasion because it covers the entire legal delta in one zone. Erosion is definitely occurring on Decker Island but the results can only prove to be approximate at best. Almost the same results had been achieved by visiting the island and visually seeing the erosion that was occurring. But as a GIS volunteer for the Department of Water Resources there was little if any cost involved in completing this study and proved to be great learning experience for me as well as providing more positive proof of the erosion that is occurring and where it’s occurring on island. A more thorough study and analysis might need to be considered to determine the exact amount of erosion taking place on Decker Island.
Decker Island, Flood Management - California Department of Water Resources
Land Use and Resource Management Plan for the Primary Zone of the Delta - Levees, DPC Levees - Delta Protection Commission
Appendix B Example Analyses, [PDF] Economic Analyses Guidebook - California Department of Water Resources
System Geotechnical Studies, Flood Management - California Department of Water Resources
Wikipedia, File:Resolution illustration.png
Web site on Erosion, Erosion
Introduction to Rivers, Enchanted Learning
National Weather Services, National Snow Analysis
GPS-Photo Link
Although not required as part of this project I don’t feel this paper would be complete without giving credit where credit is due to everyone that helped me achieve my goal and obtain my certificate in GIS. To Dave Montgomery, Business and Employment Specialist, Golden Sierra Job Training Agency, for all his assistance, support and putting up with all my crazy emotions at times. To Jeff Galef, Engineer, California Department of Water Resources – Delta Suisun Marsh Office, for introducing me to Joel Dudas and being a great resource in GIS when I needed it as well as a friend. To Joel Dudas, Senior Engineer, California Department of Water Resources – Delta Suisun Marsh Office, for being a great mentor and forcing me to think for myself, no matter how uncomfortable that was. To Hugh H. Howard, Ph.D., Assistant Professor, GIS/Geography/Earth Science, GIS Coordinator, American River College, who was very intense with his fast paced lectures but one of the best instructors of GIS that I had, and who taught most of the GIS courses that I took. He also advised me of what I needed to do to complete the program in the timeline I wanted. To the entire Adjunct Faculty of GIS, at American River College, who worked fulltime in the field of GIS yet took the time to share their knowledge and experience by teaching it.
To American River College for having the greatest, and most affordable, GIS Certificate Program in the United States. And last but not least to my parents Milton & Delores Uhler who are by far the best parents anyone could hope for and whom have always supported all their children in what ever endeavors they pursued! As well as all my family and friends.
Thank you ALL!!
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