Flooding
Some people are concerned about the possibility of flooding along the wetlands. This raised my interest. So here are some rough notes about flooding.
Flooding on February 14, 2019. This is the most water that I have seen.
The above photo is a composite of to two photos taken from my deck. Because of the way the two photos were joined things like the arborvitae seem to bend when they are actually very straight. Most of the wetlands are flooded. But notice that the wetland is divided into two parts by a ridge of land. This ridge is the man-made berm that was formed when the current channel of Bronson Creek was dug. The channel is on the near-side of the berm. The land closer to the rock wall is higher and remains dry.
The distance from our rock wall to the wall on the far side of the wetlands is about 400 feet. This was measured using Google maps. This distance will vary from place to place but my impression is that all along the Claremont Greens the width of the valley floor is about 400 feet.
The Oregon Department of Geology and Mineral Industries provides a LIDAR tool for measuring elevations. You can access this tool at https://gis.dogami.oregon.gov/maps/lidarviewer/ The following image of the wetlands near my deck was made using the "Canopy Heights Hillshade" layer of this tool.
At the top of the photo are some of the Claremont Greens Townhomes. Right below the townhomes is a rock wall where the ground drops to the river valley bottom. Further down the photo are the trees that are along the current channel of Bronson Creek. Near the bottom of the image are trees on the other side of the wetland. At the very bottom we start to see the houses on the other side.
LIDAR is a method for determining ranges (variable distance) by targeting an object with a laser and measuring the time for the reflected light to return to the receiver. In the image on the left the highest object is being shown.
This image is of the same area but the "Bare Earth" layer is being shown. The trees have vanished. Amazingly, the houses also have vanished. The circular lake on the right side of the image is easy to see because the water is very flat.
When this layer has been selected the tool allows you to measure elevation. It reports the elevation to a fraction of a foot. For the lake surface it generally reports the same elevation to within an inch.
By taking multiple elevation readings, from the top to the bottom of this image, I used this tool to profile the elevation of the valley floor. A large part of the valley has an elevation of 217 feet with a variation of less than a foot. However, once you get near the retaining walls the elevation starts to increase.
On the left we see the profile of the valley at three different places south of my house. Because I only measured the elevation every 25 feet this profile has poor horizontal resolution.
At the left, at an elevation of 240 feet, we have the pavement of Aberdeen Drive. As we move south we go down a bank and arrive at the rock wall. The rock wall abruptly drops about 10 feet to an elevation of about 221 feet. Over the next 25 (or so) feet the elevation drops to about 217 feet and remains at about 217 feet for most of the valley floor.
Bronson Creek, being less than 25 feet wide, does not show up with this horizontal resolution.
Perhaps the most amazing feature of this valley profile is how flat the floor happens to be. Did people bring earth movers down here to make the floor level? It is common for streams to have a "V" profile as the stream continues to erode the center of the valley.
Flat areas are common when there are lakes that eventually fill with sediment. (Sediment raises the bottom of the lake, not the level of the water.) Examples of very flat areas are the Alvord Desert, the Great Salt Lake and Death Valley. The most likely reason for our flat profile is that, long ago, beavers constructed a series of lakes thru this valley. Sediment in each lake eventually made the valley floor flat. In around 1830 the British decided that if they trapped all of the beavers in the Oregon area no natural resources would remain to attract Americans to move west. The beavers were eradicated but Americans came anyway.
What is the Flood Level for the Claremont Greens?
My grandparents lived on the Rogue River, which occasionally flooded. First, the water would come over the bank and flood the lowest terrace. Then it would reach the first wall and climb to the top to flood the second terrace. Then it would climb the second wall and get within feet of the top, never making it to their home. After the water receded the walls were still there, the terraces were a mess but soon things looked just like they once did.
We only have a problem after the water gets to Aberdeen Drive. Our homes are all above Aberdeen Drive. So, we are OK until the water goes over 240 feet of elevation. Where my home is, the water level can rise 23 feet without serious problems.
To consider how this area might flood we need to look downstream for something that will block water from leaving this area. The obvious thing to examine is the culverts under Bethany Boulevard. There are two culverts. They frighten me. Around them the bank is steep. I wondered how to measure their size. Eventually, I decided to place a yardstick near one of the culverts and then photograph the culvert from a distance. The yard stick in the photograph would allow me to estimate the size of the culvert.
The yardstick plan was good. However, I took my telephoto camera thinking that I would get a photo from some distance. Upon arriving I was not able to photograph the culvert from a distance. So this photo is a composite of many high resolution photos. The yardstick that you see in the culvert has been moved to this location with photoshop. I taped it to a tree above the culvert. From this position you can see some light at the end of the tunnel.
I have estimated that the culvert is about 13.4 feet wide and about 9.6 feet tall. It has a cross section of 400 square feet. The water level here has an elevation of about 213 feet. There is a second culvert to the left this is harder to photograph. Together, the two culverts have a cross section area of 800 square feet. Since our valley floor is 400 feet wide we could expect the water level to rise two feet. This is a very crude calculation.
This section of Bethany Boulevard is relatively new. The culverts were engineered to get the maximum flood to the other side of the road. The size of the Bronson Creek watershed is well known. So, if the road engineers are right they only expect the water level to rise by 6 feet at the culverts. Based on this it looks like our homes are safe.
We can now combine the valley floor profile and the cross section of the culverts to see how the culverts will block the water flowing thru the wetlands.
This is a very crowded diagram so let us go thru it step by step. The red, blue and yellow lines show the profile of the Bronson Creek valley. Starting at the left we have (at 240 feet) Aberdeen Drive. Our homes are above this level. As we go south we come to the Rock Wall. The approximate position and height of the Rock Wall is shown in the diagram. Further to the right we come to the valley floor at an elevation of about 217 feet.
In the lower left corner are two culverts, shown to scale and at the correct elevation. Engineers have determined that these culverts can handle our floods. The culverts are shown narrow and tall because of the vertical and horizontal scales used in this diagram.
Perhaps the engineers made a mistake. Or, perhaps the culverts get plugged up with dead trees and other stuff. In this case water will go over Bethany Boulevard. The elevation of the top of Bethany Boulevard is shown with a green line. By the time our homes start to flood, the water will be about 20 feet over the top of Bethany Boulevard. That seems very unlikely.
I have added a cross section of the pipe from a beaver pond leveler to the above schematic diagram so that we can see how it fits in with the other components involved with flooding. The pond leveler is insignificant.
Flooding remains a vanishingly small possibility. Studies show that people have trouble estimating the likelihood of many events that are very unlikely. Typically, it is the car accident that will get you. We are comfortable with hopping in the a car and driving around even though we know that driving is a high risk. For us, flooding is a very low risk. A rational course of action is to buy flood insurance and be worried when you are in a car.
Air Photos of the Wetlands
1998 -- Claremont houses on the right have no yards. Horses are in fields below townhomes. Images from https://wcgis1.co.washington.or.us/Html5Viewer/index.html?viewer=Intermap
2008 -- Small trees have started growing in the upper left corner of the wetlands.
2012 -- The subdivision at the bottom of the photo is being developed.
2016 -- Shrubs have been planted above Bronson Creek. They appear as faint dots occasionally in rows.
2019 -- Planted shrubs and trees below the townhomes are easier to see. Bronson Creek occasionally flows thru the former pasture starting the change into a wetland.
2022 -- Bronson Creek has mostly shifted to flow thru former pastures. More native plants have been planted near the townhomes. Planted shrubs are growing together. This image from Google maps.