Erosion Study Released

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.”

According to the study, our 680-acre lake varies just 2.5 acres, or a 0.36% volume change between the suggested 9.0 and 9.53 lake levels. The study states that erosion stress on the shoreline around Devils Lake is primarily caused by natural and human-induced wave action. Natural waves are caused by wind while human-induced are assumed to result primarily from boat waves.

The report illustrates that the greatest shoreline impact is caused by winter wind events when the lake level is not controlled by the district.  More specifically 90% of the shoreline exposed to wave energies ≥ 2.0 are generated by wintertime wind waves while 10% of shoreline exposed to wave energies ≥ 2.0 are generated by summertime wind waves.

The report references studies performed by other, to access the impact of recreational boats and the wakes that they can create.  Based on this analysis, the energy expended on the shoreline associated with the boat waves averages 40 to 45 percent of the wind energy (or 28 percent to 31 percent of the total wave energy along the shoreline) on an annual basis, but varies widely depending on location.

During the fieldwork, a photographic record was created of the entire shoreline. According to those observations, approximately 65% of the shoreline has some measure of existing protection, consisting primarily of either concrete or wooden retaining walls or rock riprap, and the remaining 35% is unprotected from manmade structures. Approximately half of the unprotected shoreline occurs in undeveloped areas owned by the State, City or Siletz Tribe.

The inventory also indicates that about 16% of the shoreline is experiencing at least some level of erosion, most of which (~72%) occurs in unprotected areas, although some (~28%) also occurs in areas where existing protection has been damaged or does not extend vertically below the lowest lake level.

Where a suitable material protects the shoreline, the wave energy is absorbed by the protection measures and erosion should not be a factor. There are, however, locations where existing bank stabilization is present and appears to effectively protect the upper part of the bank; however, at least a portion of the bottom of the wall is above the area that is impacted by the waves, and the wall is being undercut.

The study provided an extensive collection of historical aerial photographs; twelve sets ranging from 1939 to 2007.  The study did not draw any conclusions related to historical migration of shoreline. Interested parties can make their own comparisons from the material provided.  The study did not make any suggestions related to erosion mitigation techniques beyond the shoreline protection already in place.

We will update this article after attending the presentation at the December DLWID board meeting should any new information surface.

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1 Comment

Filed under DLWID, Lake Level

One response to “Erosion Study Released

  1. jack strayer

    I have reviewed the “Devils Lake Shoreline Erosion Study” by Tetra Tech and have a few questions for the board and Tetra Tech.

    The Tetra Tech study concluded in its Executive summary p2, “ . . 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.”and,

    “p33 ‘. . . Using natural and recreation water levels, this study did not observe damage that can be directly attributed to changes in lake level.”

    This tells me that lake erosion is impacted by the lake bed slope and bank protection– rather than changes in lake level.

    Board Question #1: I recall that Mitchell Moore and Douglas Pirie reported lake level did not impact erosion. Mr Pirie gave both written and oral testimony that lake level was not a contributor to erosion. Douglas Pirie , a former coastal engineer with the US Army Corps of Engineer, has more wave analysis credentials, experience and more analytical abilities than the entire Tetra Tech team. What did that board learn from the Tetra Tech report beyond what Moore, Pirie and others had testified to earlier?

    Tetra Tech Question #1: On page 26 the wave energy is estimated for different boat speeds. I realize that you were not able to take actual measurements on the lake but some additional data should be analyzed and noted in your report. A recent board Vice Chair was very proud that he could surf the boat wave behind his wake boat traveling at 7 ½ mph with full ballast tanks and at least two people sitting on the stern. What would the 1] maximum waver height, 2]peak wave period and 3] the maximum energy generated [Kw/ft] for his approximately 20’ boat?

    Tetra Tech Question #2: Tetra Tech noted that,

    “the small change in fetch length results in an insignificant difference in wave energy at the shoreline; however, different still-water levels in the lake causes small (~0.5-foot) changes in the vertical location at which the erosional energy is focused. Although this difference will have little impact on the overall tendency for erosion around the lake, it could cause noticeable impacts in specific areas where existing protection measures are of limited vertical height. This is particularly true for areas without adequate bank stabilization in areas with highly erodible soils.”

    ————————————————————–

    In areas of high erosion potential, would the introduction of native aquatic plants in the littoral zone significantly reduced residual erosional impacts and increase bank stabilization? Could you point out the plants that would help reduce erosion below the mean high water level?

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