Analysis of flood levels at 10, 50, 100 and 500 year events and their impact on potable, waste, and storm water systems in Alsea, Oregon, USA.
The objective of this research is to:
- Investigate whether the existing Geographic Information Systems (GIS) information is complete, or needs to have more data added to represent the current condition of the water systems.
- Work with other individuals to update the GIS information so that aspects of the systems which are lacking can be repaired.
- Determine the cause of the sinkhole that has developed next to the Alsea School District’s playground, and under the sidewalk that accesses the school grounds along the western edge next to Route 34.
- Survey the historical flood effects and where the impacts would be detrimental to the town.
- Create boundaries of the 10, 50, 100 and 500 year floods to determine properties and systems potentially impacted.
- Develop plans to help the Alsea Emergency Preparedness Committee visualize the areas of greatest concern.
- Distribute the findings in the Alsea Valley Voice. Upon final grading and review by the Geographic Information Systems & Science (GIScience) Department at Oregon State University (OSU).
The Alsea Watershed and Benton County Department of Public Works:
Most studies investigating the spatial scale of the watershed have focused on the town and the local roads. Multiple types of storm-drain pipes from concrete to poly (vinyl chloride), also called PVC, have been used over the years. Connecting to these makes repairs difficult. The alternative is to replace the entire line.
Analyzing what is needed to ensure proper water distribution and drainage will contribute to the town’s knowledge of where the water will travel, and how the storm water system may conflict with the existing man-made pathways.
The existing data for this analysis is sourced from public records through Doug Sachinger, the GIS Coordinator from Benton County Public Works. He provided all of Benton County’s existing data for Alsea. This is because the town of Alsea does not have a public works department, and is unincorporated. GIS data was missing throughout the town. Stub-outs for water meters appear on the map without any connecting system to feed them, and no water meter data exists yet to verify that any particular stub out is providing potable water to a location.
Alsea Potable, Sewer and Storm Water Systems
This is an example of the incomplete GIS data that the town of Alsea currently has on file with Benton County. There are stub-out locations that do not appear to be connected to the potable water system, and houses that are currently being serviced by the public water system which do not have their stub-outs shown.
Alsea Stub out not connected to the water system
The size of homes and the tax evaluation to run a hot spot analysis were shaded darker red, indicating the property locations made the lots more valuable.
Regional Hotspot data
It is interesting to note that the areas of highest value are located between the inlet of the potable water from the Alsea River and the outlet of the waste water treatment facility downstream into the Alsea River.
Alsea Hot Spot for property values
EXISTING SYSTEMS IN ALSEA
Since the town of Alsea is an unincorporated township, Benton County is the authority on Public Works. In March, 2015 the old town storm water overflow was permeating into town members’ crawlspaces. To remedy this, the county added three surface catchments and rerouted the water to a storm water ditch. This added to the volume of water exiting through the school’s playground.
Temporary Stabilization Concern
When evaluating the potential cause of the sinkhole, water does not constantly flow from the opening, so it is less likely to be from a potable water line break. When there is an active rain/storm water event or soon after, water floods the playground area and travels into the school’s paved pick up and drop off area.
Sinkhole pond that develops when it rains
Alsea’s current water systems need additional mapping. Sadi Stouder, a Civil Engineering graduate student contacted me. She will be adding the necessary information, and is interested is using my on-site assistance. The objective is to gather the additional GIS data. Bob Miller, from Benton County Public Works, records the town’s water meter usage, and may be willing to provide meter base locations. This will help to document stub out locations and meter bases locations in order to connect them on GIS to the potable water system.
The raster file is now at 10 foot contour intervals. This will help determine where the water will travel through town.
10ft Contour Interval
10ft Contour at the sinkhole by the Alsea School
Close-up where gravel and a metal panel have been added for safety
Facing South toward Alsechool and pathway of the storm water drainage pipe
Permanent Stabilization Practices
A few examples of permanent stabilization of areas with open soil profiles are buildings, paved areas and seeding of all non paved areas. Surface roughening, planting and re-vegetation have decreased erosion around the Alsea School District (Pre-K through 12th grade). Individuals are functioning as Storm Water Pollution Prevention (SWPP) leaders in the community. The horticulture group is also working on gardens and selling plants to raise money. Erosion control works hand in hand with reducing the impacts of flood waters in small communities.
Land use will reduce the effects of flooding and the weather conditions that occur at the same time. By planting trees as clusters or groups, the wind is less likely to topple over a few vs. just one.
Flood of 2
rains that began overnight Tuesday and poured steadily on Wednesday triggered floods throughout western Oregon on Thursday, with high water
Quote:
January 20, 2012 5:00 am Corvallis Gazette-Times
“Waters rise: Floodwaters posed a threat throughout Benton County on Thursday as the Marys River reached a record flood stage of 21.41 feet. Not as bad as 1996, but a strong reminder. Heavy rains that began overnight Tuesday and poured steadily on Wednesday triggered floods throughout western Oregon on Thursday, with high water pouring over roads throughout Benton County and soggy ground helping to create landslides, downed trees and other weather mayhem….Area schools and universities were taking no chances, though: Oregon State University’s Corvallis campus will be closed today, as will schools in the Corvallis School District and Alsea… In the 24 hours ending at 8 a.m. Thursday, 4.02 inches of rain fell at Hyslop Farm outside of Corvallis – easily breaking the previous record for Jan. 19, 2.25 inches, a mark set in 1911. In fact, Wednesday went into the record books as the third-rainiest day in 101 years, said Kathie Dello of the Oregon Climate Service at OSU. The only other days that saw more than 4 inches of rain in 24 hours were Nov. 19, 1996, when 4.45 inches fell, triggering massive flooding that cut off access to south Corvallis for several days, and Jan. 28, 1965, when 4.28 inches fell. The effect of the rain was most evident at the Marys River. It reached 21.41 feet Thursday morning, breaking the old record of 20.9 feet, and flooded parts of Corvallis and Philomath…. The Alsea School District closed schools and its playing fields and some buildings were flooded. ”
The quote above explains the history of flooding in the Willamette Valley and Coastal Range. The next flood event may be in 10 years or it might be the 500 year event but the true extent of such an event is unknown as the data has not yet been recorded for 500 years.
The Federal Emergency Management Agency (FEMA) website allows public access to download data associated with flooding. This information is not in GIS format or even in a data table to convert. The process then was to find the Alsea River locations that were used as reference points and determine the elevation of the flood plains.
The categories are:
- 2 % likelihood i.e. not very often (500yrs)
- 1% more often (100yrs)
- 2% chance of flood is within a lifetime (50 yrs)
- 10% probability of flood levels every (10yrs)r (Federal Emergency Management Agency)
Even though it seems odd that FEMA is working on a document that is called Flood Insurance Study, maybe it had the people’s best interests at heart.
The purpose is to examine the real vs. perceived risks of flood events and determine the actual differences between a 10 year flood and a 500 year flood.O
regon 
Special Flood Hazard Area
There is only one map for the 10, 50, 100 and 500 year events. This map looks like the 10yr flood event.
The elevations on the graph provide the values of flood plains during each of the 10, 50, 100 and 500 year events.
A number of controls have been put into place by the Emergency Preparedness Group in Alsea. One control is lift stations, or pump stations. These help ensure that high surface water levels don’t cause wastewater or storm water to back flow into people’s homes.
Additionally, litter, debris, and construction chemicals that could be exposed to storm water should be prevented from becoming a pollutant source in storm water discharges. (10) Year Flood Event
accrossTen Year Flood Event Extent
The 10 year flood event appears in Alsea as:
Ten Year Flood Event in AlseaFifty Year Flood Event Extent
The 50 year flood event appears in Alsea as:
The 100 year flood event appears in Alsea as:
The 500 year flood event appears in Alsea as:
The overlays from the Benton County Public Works GIS Department show Alsea potable, waste and storm water systems, yet did not have the ArcGIS flood data. This next level of research was found through the FEMA website and adapted into the existing map. Once the flood data could be viewed with the existing layers, flood elevations were determined and placed into Table 2.
The base map was then edited through the toolbox within the customize tab. When selected, the locations A-E were added as reference points. Select the contour lines available. If 10 foot then convert to 2 ft so it will provide the results with better quality results and details. rocedural ArcMap GIS example in Alsea
When we think of floods, the worst comes to mind. Many in this are have known the effects first hand. This report is for those that haven’t experienced the high waters and to recognize the areas of concern. This will allow the development of other methods to reduce the negative impacts on our community, and our personal items in our basement.
The map below shows the 10 year flood in blue and then the additional areas affected by the 50 year flood in pink, the 100 year flood in purple, and then the 500 year flood in greenish-yellow. If it’s in your home, the couple foot difference is not minimal, but in comparison the changes in area are slight. These are also estimated and could be higher or lower depending on rain, snow loads in the mountains, water table, vegetation and many other factors.
Regional 10, 50,100 and 500 year Flood Extents
Alsea 10, 50,100 and 500 year Flood Extents
RE
FERENCES
Federal Emergency Management Agency:
https://msc.fema.gov/portal/search?AddressQuery=alsea%2C%20oregon
US Department of Fish and Wildlife – www.fws.gov
Doug Sachinger, GIS Coordinator, Benton County, Oregon, Public Works
(Staff Reporter) 2012,January 20, “The Flood” Corvallis Gazette-Times
http://heresmeme.com/AVV/Alsea%20Flood%202012/New/page_01.htm
