| Title How Many Residences Were Built in the 1950 Flood Zone? | |||||||||||||||||||||||||||||||
|
Author Jason Smith American River College, Geography 350: Data Acquisition in GIS; Fall 2009 jasonksmith@mac.com | |||||||||||||||||||||||||||||||
|
Abstract This project looks at the residential property data to determine how many residential properties have been built within the 1950 American River Flood zone after the flood occurred in 1950. A table of containing the year built was joined to the county parcel map. The properties intersecting the flood zone were chosen and then queried to determine the build date. The properties were mapped for each decade between 1950 and the present to see the progression of residential properties built within the flood zone with time. | |||||||||||||||||||||||||||||||
|
Introduction The Sacramento Valley has been subject to flooding throughout recorded history. Even before California was established as a state, the native people were aware of periodic flooding in the area that now comprises part of Sacramento County. In 1950, there was a severe flood event in Northern California and subsequent floods in 1955, 1986, 1996 and 1997. In Room 150 of the Ethan Way Center of ARC, there is a map of the flood area in 1950 in Sacramento County. Looking at this map, I have been curious about this question: How many new homes have been built within the flood area since the event occurred in 1950? My proposal is to use GIS to answer this question and also to document the number of new homes in the flood area every 10 years starting in 1950 and ending in the present time. | |||||||||||||||||||||||||||||||
|
Background The 1950 American River Flood map was created by Nathan Jennings. The map contains rectified aerial photography from the American River Flood, November 21, 1950. The imagery was rectified to 2006 digital orthophotography by Nathan Jennings' student assistant Fui Fang Thong. At the time of the flood,Folsom Dam was not built yet (completed 1955) and Nimbus Dam was not yet completed (1952 - 1953). The peak estimated flow rate was 180,000 cfs (cubic feet per second). The PDF version of the map can be found on Nathan Jennings' web site: 1950 American River Flood |
 |
||||||||||||||||||||||||||||||
|
Methods Project Needs To perform the analysis, I needed flood area boundaries, residential home locations and build dates for Sacramento County. For the flood area, I used a PDF file of the map that was created by Nathan Jennings and is displayed in ARC Ethan Way Center Room 150. For build dates, I used County of Sacramento Assessor's data in the form of a table that includes parcel number, year built and Assessor's Land Use code. I also used a parcel map of Sacramento County to determine the parcel locations. Finally, I used 2009 NAIP aerial imaging to georeference the flood zone map and create a shapefile of the flood zone. Model for Analysis I used the flood area boundaries and the parcel map as shapefiles in ArcMap. I joined the table containing the build date and land use information to the parcel map using the parcel numbers as the key field. I then queried to find the number of new homes built within the flood area boundary from 1950 to 1959 and then the number of new homes built from 1960 to 1969, and so on until the present year. I created maps for each time period to show the increase of residential homes within the flood area. To create the shapefile for the 1950 American River Flood zone, I downloaded the PDF file from Nathan Jennings' website. I converted the PDF image into a TIF file. I then georeferenced the TIF image to a 2009 NAIP image of Sacramento County. Finally, I digitized the flood zone boundary to create a shapefile of the flood zone area. | |||||||||||||||||||||||||||||||
|
Results There were residential buildings (houses, apartments, motels and mobile homes) built on 8790 parcels within the flood zone area. Of the 8790 buildings, 537 have an unknown build date (6.1%). Of the 8790 buildings, 266 existed prior to the 1950 Flood. 7987 residential buildings were built within the flood zone area since 1950 (91%). | |||||||||||||||||||||||||||||||
|
| ||||||||||||||||||||||||||||||
|
Figures and Maps The following maps show the
progression of residential buildings by decade starting with the existing parcels
in 1950 and ending with the current residential parcels. An additional map with the
2009 NAIP aerial photography is shown for reference. | |||||||||||||||||||||||||||||||
 |
|||||||||||||||||||||||||||||||
 |
|||||||||||||||||||||||||||||||
 |
|||||||||||||||||||||||||||||||
 |
|||||||||||||||||||||||||||||||
 |
|||||||||||||||||||||||||||||||
 |
|||||||||||||||||||||||||||||||
 |
|||||||||||||||||||||||||||||||
 |
|||||||||||||||||||||||||||||||
|
Analysis Issues and LimitationsThe table for parcel number, build date and land use codes contains entries that do not have a build date. The explanation given was that the build date is unknown for those properties. This was approximately 6 percent of the residential properties within the 1950 flood zone. The original 1950 American River Flood map boundaries are given as approximate and are meant for historical use. The extra process of georeferencing and digitizing the boundary to create the shapefile may have introduced some additional uncertainty in the location of the flood zone boundary. The RMS error from the georeferencing process was 7.3 feet. The additional error is probably not significant with respect to changing the number of parcels intersecting the flood area. The use of intersection in the spatial selection process meant that some parcels which are only partially within the flood zone are also included in the analysis. I believe this is a reasonable method; if half of the parcel is underwater, it is not a desirable event for a property owner. Overall the analysis was straightforward to perform and did answer the original question asked; at least 91% of the residential properties within the 1950 flood zone were built after 1950. | |||||||||||||||||||||||||||||||
|
Conclusions This analysis was successful at answering the original question postulated. This analysis could easily be extended to ask other questions such as: How many apartment buildings have been constructed within the flood zone? How many mobile homes are within the flood zone? Additionally, the analysis could be extended by adding other flood boundaries, such as the FEMA 100 year flood zone. In that case we could find out how many residential buildings are within the Natomas 100 year flood zone. How many have have been constructed within the last 10 years? | |||||||||||||||||||||||||||||||
|
Acknowlegements I want to thank John Solie at the Sacramento County Assessor's office for providing the data and information for the Land Use codes and year built. I also want to thank Nathan Jennings and his student Fui Fang Thong who created the 1950 American River Flood Zone map. And finally, thanks to Paul Veisze for providing the 2009 NAIP Aerial Image of Sacramento County. | |||||||||||||||||||||||||||||||