Inversely 84% of Shoreline is Not Eroding and
Some of the Erosion is Indeed Quite Slow
In the recently re-released erosion study, it was noted that there were areas of observed erosion. These included areas where with differing bank stabilization measures created a discontinuity that led to erosion near the interface, as well as areas that are dominated by reed canary grass, where undercutting was commonly observed. While vegetated, this was also considered to be eroding. It was stated that differentiating “eroding” from “non-eroding” parcels proved to be more challenging than expected because the degree of erosion was highly variable.
Based on their observations, about 16% of the shoreline, or approximately 11,000 linear feet, is exhibiting at least some level of erosion, most of which (~72%) occurs in unarmored areas, although some (~28%) also occurs in areas where existing armoring has been damaged or is of limited vertical extent. Inversely 84% of the shoreline, or approximately
In an attempt to chart the advance erosion on a historical basis the District asked that an animation be made of the shoreline from aerial photographs. Observing these it struck us that there really was not much movement of the shoreline from 1939 to 2007; especially in the natural areas owned by the State. We noticed that the northern shoreline designated as eroding did not change whatsoever; this is clearly observable in the animation without enhancement. 58,000 linear feet, is exhibiting no signs of erosion. Over half, nearly 6,000 feet of the unarmored areas identified as eroding are located in the western end of the lake in the land owned by the State; a section of the shoreline adjoining the campground as well as the marsh on the southern shore where Rock Creek enters the lake.
In an attempt to verify that the shoreline did not see much change, we extracted images from each year and drew a line along the shore. In doing so, we discovered that there were two distinct water level groups; those primarily prior to 1992 where the resting height was somewhat lower than today and those primarily after 1992 where the height was about where it is today. Of course, the water levels vary at the time of the photo each year impacting the location of the shoreline. If one looks at each year individually you will see each line vary slightly toward and away from the lake but generally they stay about the same within the two groups.
To simplify this we have shown a comparison of the two sets; low water years and years with water about the same as now with just the oldest and most recent lines. The low water years include 1939 and 1992; in this image, one can see that the water level is lower in 1992 but the shoreline shelf representing the higher watermark is in the same location as in the 1939 image. It does however appear that a lobe of shoreline near the entrance of Rock Creek disappears by 1992 when the last meander of Rock Creek breaks through the shoreline. Perhaps more important to today’s discussion is the second image because it encompasses the years the dam has been in operation. This image displays a shoreline in 1977 that is almost a perfect overlay of the shoreline in 2007 indicating the shore did not move in a significant way during the 30-year period.
Many variables would prevent us from making absolute claims related to shoreline advance. The angle of the image, the height of the lake, the quality of the image, all make this inexact; but from all appearances better than half the shoreline the District claims to be eroding is not doing it in a way significantly observable way over a 68 year history. Take a close look at the animation video at each section of the lake and while you will see some man-made changes in the shoreline, you will not see major sections of shoreline disappearing from erosion. The study does not quantify the rate of erosion so this video is all we have in the place of scientific measurements. There are locations that are eroding but this does not seem to be an issue of epic proportions that justify the corrective measures being proposed by the District.
Delay of Erosion Study May Delay Dam Placement
The Devils Lake Water Improvement District signed a contract with Tetra Tech, Inc. for a shoreline erosion study. This contract was issued on December 29, 2010 for $28,840. The District has received a second iteration of the erosion report and returned it with additional comments to Tetra Tech. The New Erosion Study is now available and its contents will be presented at this month’s meeting. We encourage all to review the report. It is not clear how this report will impact the operation of the dam this summer, however the excerpt below from this month’s staff report indicates that due to the late arrival of the report the dam placement would be delayed. It also seems to suggest that lake level will be on the agenda in the May meeting. Again, please review the report and come to your own conclusions.
“Staff believes that the contract can be ruled complete and thus full payment should be issued to Tetra Tech, Inc. Further, staff recommends to have staff, the Board, and encourage the public to take the time to make an even more detailed review of the document and the deliverables over the next month in preparation for an open discussion on the dam and lake level as it relates to the Erosion Study as well as to other components previously identified such as fish and wildlife, recreation, septic systems, Harmful Algal Blooms, vegetation, wetlands, shorelines, and water quality. When the District issued the Erosion Study RFP it did so with the pledge to consider and act on the results of the study. While it was the District’s intent to have had time earlier in the year to have this discussion, the timing of the receipt of the final deliverables has made that most appropriately held in May. Given this and given the fact that the District pulses the dam, releasing water for fish passage mitigation through the end of May, and given the District has demonstrated that the full water impoundment can be achieved even if waiting until late June, Staff would recommend waiting until after the May discussion to make a decision about the use of the dam this summer and thus the installation which typically would otherwise begin around April 15.”
The News Guard
An erosion study that could play into a future decision on the water level in Devils Lake has received a rough reception from lake officials, who said the work failed to fulfill many of its objectives.
The District commissioned the study to inform its decision about whether to change the level of the D River dam, which helps keep the lake higher during the summer months. Continue reading
See Years of Lake History in Seconds
The Devils Lake Water Improvement District recently had an erosion study done to determine if and how the lake level affects erosion on properties surrounding Devils Lake. The study and related materials were released on the District website prior to the December Board meeting.
The study provided an extensive collection of historical aerial photographs; twelve sets ranging from 1939 to 2007. The report did not draw conclusions from these photographs related to the historical migration of the lake’s shoreline. The RFP for the study discussed the creation of an animation created from the historical photographs. The contractor decided to accomplish this with the use of a PDF file because of its wide availability. This format does allow the curious to click hyperlinks in the document and move from year to year for any given areas of the lake. It does provide some interesting viewing.
We thought it might be easier to see what happens to any individual section of shoreline if this animation was done using a time-lapse video. This would let you see the shoreline change as the video plays; the video progress bar could also be used to select any view in the middle of the time-lapse. Seemed like a good idea, so we figured out a way to create three time-lapse videos.
Imbedded in this article are three views of Devils Lake progressing over time. Each of these views were created by scaling and rotating aerial photographs from different times. The lake level at the time of each photograph is unknown therefore; no attempt was made to compensate for it. Given the scale of the photos the impact of this omission is negligible. It is recommended that you select full screen viewing for best results. Here are a few observations of each of the time-lapse videos. Continue reading
Study Concludes Lake Level Has Little Influence
The Devils Lake Water Improvement District recently had an erosion study done to determine if and how the lake level affects erosion on properties surrounding Devils Lake. The District specifically requested that the study consider how wave energy, both natural and man made, impact the shoreline as a result of having the lake held at 9.53’ (full exercise of water right), at 9.0’ (partial exercise of water right), and if the lake was to be allowed a natural hydrology without impoundment.
The text of the study, photographs, maps and presentation are available online by following the links at the end of this post. In general, the study concludes, “whether waves are generated by boats or wind, the highly erodible nature of the soils, the presence or absence of sufficient bank stabilization, and the bathymetric slope are likely more influential on shoreline conditions than relatively small changes in lake elevations.” Continue reading
Lake resident Mark Christie posted this comment on his Facebook page this weekend, “Wow. 29″ from my floating temporary dock to my fixed deck. Who pulled the plug out of D Lake?” Those of you who have been to the lake in recent weeks know that the lake is very, indeed it is at its lowest point all year. The unofficial gauge installed at my dock reads 8.8’ and I have exposed lakebed all the way around my seawall.
The Devils Lake Water Improvement District removed the D River dam on October 12th, at which time the lake had already dropped to 9.0’. In October there was just 2.05” of precipitation prior to the dam removal and since that time, there have been 5 days of rain, registering a total .03” of precipitation. Historically, October will provide many very nice days to enjoy but it does generally provide much more rain with a monthly average of 7.5”. Not to worry, you can double that figure for each of the months of November, December and January, so I would not bet on the lake staying this low for too much longer.
The following statement was made today on the DLWID Facebook page.
“Tetra Tech will be continuing the erosion study through next week. Dam to be replaced following completion of the study which will most likely be done by Thursday.”
The DLWID reports that the D-River dam was installed on Wednesday June 22nd.