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Engineered Log Jams

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					Engineered Log Jams

                Instructor:

       David T. Williams, Ph.D., P.E.
David T. Williams and Associates, Engineers

          david@dtwassoc.com
              What are Engineered
               Log Jams (ELJs)?

   Also known as Large Woody Debris (LWD) structures


   Used classic spur design techniques


   Deflects erosive flows and promote sediment deposition
    at the base of eroding banks


   Large wood (LW) includes whole trees with rootwad and
    limbs attached, pieces of trees with or without rootwads
    and limbs, and cut logs 36-48” diameter
              What are Engineered
               Log Jams (ELJs)?
   Smaller wood in streams decays rapidly and is more
    transient than LW

   Installed into the banks to act like spurs or hardpoints

   Can also be installed in the channel to divert flow

   Permeable to flow

   Wood provides habitat to fish (salmonids) and
    macroinvertebrates.
                   Pros of ELJs

   May be more inexpensive than traditional
    practices


   Create more habitat than traditional practices


   Finished projects resemble natural features


   Materials are usually readily available
                     Cons of ELJs
   Currently do not have codified design methodology (but this
    is changing!)


   New application = agency or client apprehension


   Requires non-traditional methods and materials


   Backwater effects can result from piling up of water upstream
    of a debris accumulation. This can raise the water surface,
    resulting in an increase in the frequency and magnitude of
    flooding in the vicinity of the log jam.
                      Cons of ELJs
   ELJs can also have significant downstream effects on the
    system. Often the thalweg of the river will be altered or re-
    directed as a result of the re-introduction of large wood.


   This transformation of the location of the high-energy flow also
    can have significant local effects.


   Log jams have also been linked to numerous avulsions in small
    streams and large rivers.


   These cons also apply to most river structures
        Risks of ELJs (WDFW, 2004)
   Safety hazards caused by the log jams or the cables that
    anchor them (can be somewhat reduced by placing warning
    signs upstream from the log jams to alert boaters),


   Blockage of culverts or bridge openings by large woody debris
    that has been dislodged from a log jam upstream,


   Unanticipated erosion across the channel or to the adjacent
    streambank,


   Increased channel roughness and constriction, and/or


   Increased flood stage.
Example of ELJ - www.tfhrc.gov
Types of ELJs (WDFW, 2004)
Types of ELJs (WDFW, 2004)
Geomorphic Effects of ELJs
    (Drury et al., 1999)
Terminology of ELJs (Shields et al, 2004)
Terminology of ELJs (courtesy of
         Scott Wright)
Forces on an ELJ (NRCS 2001)




   FB = force due to buoyancy
   FG = force due to gravity
   FF = force due to flow
   Fμ = force due to friction between LW and bed
   FL = force due to lift
   FN = force normal to LW at the tip and the rootwad
          Force Balance / Momentum

   ΣFy = 0 , FF (sin θ) + FG = FB + FL +FNT + FNRW


   ΣFx = 0 , FF (cos θ) = FμRW + FμT

   ΣMo = 0, FNT (LTcosθ+z) + FB z + FL z = (FG + BR)z + FF (2/3 dw)


   FSB = FG/FB = factor of Safety; If FSB < 1.5, add required ballast (BR)
    to obtain FSB= or greater than 1.5


   FSB = (FG + BR)/FB


   BR = ((FSB)( FB)) - FG
        Gee – Lots of Computations!

   Can use Spreadsheet developed by Scott Wright of
    River Design Group


   Spreadsheet calculates buoyancy, sliding (by resistance
    only, does not take into account mass of soil, boulders
    only), scour depth at ELJ, scour at end of ELJ (sill),
    spacing, buried tree stability, and super elevation at a
    bend, wood properties (specific gravity by species)


   Scott has given permission to use for people in this
    course – just give him credit if you use it.
DESIGN CONSIDERATIONS (NRCS, 2001)

   Location - Typically placed within a stream channel in
    locations that will enhance habitat and compliment natural
    stream processes – use a natural analog. E.g., placement of
    LW in a scour pool will increase the depth and size of the pool
    while providing cover for fish.


   Height – The relative height of LW near the stream bank (H) is
    determined by the elevation of channel-forming discharge
    (approximately a 1.5-year event for “wet” areas).


    LW below the channel-forming flow level will be saturated on
    a regular basis and will provide in-stream habitat.
      DESIGN CONSIDERATIONS

LW that is located above the channel-forming flow elevation
will trap sediment and debris and may also support
vegetation.


If the intent is to have a semi-permanent structure that
accumulates more debris, then the top of the structure must
be at or above the channel-forming flow level.


If it is below, scour over the top of the structure may occur
reducing the factor of safety, and resulting in greater
buoyancy.
            DESIGN CONSIDERATIONS
   Angle and Offset – The LW portion of a structure should be
    oriented such that the forces acting upon the LW increase its
    stability

    If a rootwad is left exposed to the flow, the bole placed into a
    streambank should be oriented downstream parallel to the flow
    direction so that the pressure on the rootwad pushes the bole into
    the streambank and bed. Wood members that are oriented
    parallel to flow are more stable than members oriented at 45 or
    90 to the flow.

    The most common mode of LW movement is for a piece oriented
    other than parallel to flow to rotate and slide until it assumes a
    position parallel to flow, and then becomes stable.
        DESIGN CONSIDERATIONS

   Profile – Structures with a lower profile will have fewer
    forces acting upon them.


    Design flow should be the channel-forming flow level (1.5-
    year flow).


    Banks that are frequently overtopped will require a more
    extensive key that extends further back into the bank.


    Bank material will also need to be considered when
    designing the dimensions of the key.
         DESIGN CONSIDERATIONS
   Anchoring and Ballast – Anchoring is not always necessary.
    Maintaining flexibility is the primary concern .

    The less flexible the structure, the more potential damage if a
    failure occurs. Instead of traditional anchoring, such as cable
    and deadmen, consider increasing the mass of the LW.

    This can be accomplished by anchoring two or more logs
    together or by using rebar or cable to attach large rock to the
    log near its center of gravity.

    The structure will be comprised of several individual members
    rather than one large structure that is cabled into place.
     DESIGN CONSIDERATIONS
When using ballast to increase the mass of the structure, use
the stability analysis to determine how much material is
needed.

A factor of safety of 2 is recommended for all structures
incorporating LW.

For streams that are entrenched (Rosgen types F, G, A, and
potentially B), or for streams with very low width to depth
ratios (<12) an additional 60% ballast weight may be
necessary due to greater flow depths and higher velocities.

The factor of safety for ballast should be a minimum of 1.5.
     DESIGN CONSIDERATIONS

When using cable to anchor LW, disturbing the channel
banks can lead to a rapid failure if the cable comes loose.


Vibration of the cable due to flowing water and movement
of the LW can cause bank destabilization and failure.


When anchoring with cable, consider anchoring into the
streambed or at a 45-degree angle into the bed and bank.
This will reduce damages if there is a failure.
     DESIGN CONSIDERATIONS
For a bole with attached rootwad, bury the bole end in the
downstream direction with channel gravel or cobble.


Buried cut-off logs or rocks can be used in conjunction with a
streambank structure to reduce the risk of flanking.


Buried log or rock should be oriented perpendicular to the
direction of flood flows.


Leftover rock, or rock that is too small for the instream portion
of the structure can be used in the cut-off trench.
         DESIGN CONSIDERATIONS

   Depth of the Bed Key - The bed key depth should be
    determined by calculating expected scour hole depth
    downstream of a proposed structure.


    Note that scour depth will likely exceed the depth of the
    thalweg (deepest part of the channel).


    Scour depths will be greater in streams that are relatively
    deep or have higher gradients.
          DESIGN CONSIDERATIONS

   In lieu of a scour analysis, scour depth can be estimated using
    the following for gravel or cobble bed streams:


    Scour = 2.5*h; Where h = height of exposed rootwad to bed
      elevation.


    For sand, use 3 to 3.5*h
       To reduce scour depths, decrease the structure height.
       Higher structures cause greater flow convergence, and
       thus greater scour depths. The use of LW can help reduce
       scour depth by dissipating energy, replacing the need for a
       downstream apron.
      Post Construction Activities

Due to the experimental nature of ELJs, a commitment to
inspection monitoring and maintenance is a highly
recommended criterion for proceeding with ELJ
implementation at sites where infrastructure is situated
within the fluvial environment (e.g., original channel
migration zone).
      Post Construction Activities
   Due to the experimental nature of ELJs, a commitment to
    inspection monitoring and maintenance is a highly
    recommended criterion for proceeding with ELJ
    implementation at sites where infrastructure is situated
    within the fluvial environment (e.g., original channel
    migration zone).


   Should be done on a regular schedule and after major events


   See references for monitoring programs
The Hoh River Bank Protection
 Project (Hall and Moler, 2006)
                              References
   Abbe, T.B., D.R. Montgomery, and C. Petroff. 1997. Design of stable in-
    channel wood debris structures for bank protection and habitat restoration: an
    example from the Cowlitz River, WA. Proceedings of the Conference on
    Management of Landscapes Disturbed by Channel Incision. Univ. of
    Mississippi. Oxford, MI. pp. 809-814.


   Drury, T., Petroff, C., Abbe, T B, Montgomery, D.R., and Pess, G. R. 1999.
    Evaluation of engineered log jams as a soft bank stabilization technique: North
    Fork Stillaguamish River, Washington, Proceedings of the ASCE conference on
    Stream Restoration


   Hall, M. and Moler, S. (2006 January/February), Mimicking Mother Nature,
    Public Roads, 69(4),35-39.


   NRCS, 2001. Incorporation of Large Wood into Engineering Structures.
    Engineering Technical Note No. 15, Boise, ID
                            References
   NRCS, 2007. Stream Restoration Design of the National Engineering
    Handbook (NEH) Part 654 (Chapter 14)


   Shields, F. D., Jr., et. al., 2004. Large Woody Debris Structures for Sand-
    Bed Channels. Journal of Hydraulic Engineering, vol. 130, no. 3.


   Wallerstein, N., C.R. Thorne, and M.W. Doyle. 1997. Spatial distribution
    and impact of large woody debris in Northern Mississippi. In Wang, C.C.,
    E.J. Langendoen, and F.D. Shields (Editors). Proceedings of the
    Conference on Management of Landscapes Disturbed by Channel Incision.


   Washington Department of Fish and Wildlife (WDFW), and US Fish and
    Wildlife Service, Washington Department of Ecology, 2004. Stream
    Habitat Restoration Guidelines

				
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