TECHNICAL NOTES

U.S. DEPARTMENT OF AGRICULTURE NATURAL RESOURCES CONSERVATION SERVICE

PORTLAND, OREGON June 1, 2001

ENGINEERING -- NO. 25

**INCORPORATION OF LARGE WOOD**

**INTO ENGINEERING STRUCTURES**

**DESCRIPTION**

Large wood (LW) can be incorporated into a structure or used as a
structure to (1) disperse flow energy (Buffington and Montgomery, 1999)
(2) stabilize channel banks and bed forms (Bilby 1984), (3) increase
aquatic habitat, (Bryant and Sedell, 1995) (4) narrow a stream and
reduce the width to depth ratio (Sedell and Froggatt 1984), (5) cause
localized deposition (Keller et al. 1985), (6) form pools, (Bilby and
Ward 1989) (7) route flood water, (Ellis 1999) and (8) decrease costs by
using on-site materials (Booth, Montgomery, and Bethel 1996). Also, LW
components in engineering structures are often necessary to meet permit
guidelines.

Most in-stream work with LW to date has been experiential.
Unfortunately, this has resulted in very little documentation of design
procedures, stability analyses, or even structure function or success
(Frissell and Nawa 1992). The following guidelines provide a procedure
for analyzing the stability of LW components in engineering structures.
This technical note is not intended to serve as a design template for
Engineered Log Jams (ELJ) (Abbe 1999).

**[Types of Streams]{.underline}**

Not all streams, or locations in a single stream, are appropriate for LW
placement. It is necessary to evaluate the hydraulic geometry,
floodplain connectivity, channel planform, bed and bank material, and
the associated riparian plant community of a given stream to determine
the appropriateness of large wood placement. Even when a stream system
seems to function without the presence of wood, there may actually be a
LW component missing that would provide energy dissipation, sediment
storage, and aquatic habitat. Since LW was being actively removed from
streams before 1900, the perception of \"that stream never had any wood
in it\" may be prevalent.

Potentially appropriate stream types are listed below for the Rosgen
(1996) and Montgomery and Buffington (1993) stream classification
systems and for the Channel Evolution Model (Schumm, Harvey, and Watson
1984).

+-----------------+-------------+----------------------+---------------+
| **Suitability** | **Rosgen**  | **Montgomery &**     | **CEM**       |
|                 |             |                      |               |
|                 |             | **Buffington**       |               |
+-----------------+-------------+----------------------+---------------+
| **Suitable      | C (1 - 4)   | Regime               | I             |
| \--**           |             |                      |               |
| appropriate for | E (1 - 4)   | Pool-Riffle          | IV            |
| all areas where |             |                      |               |
| LW is a natural | Bc (1 - 4)  | Plane Bed (\<2%)     | V             |
| component of    |             |                      |               |
| the geomorphic  | If BHR      |                      | If BHR \< 1.2 |
| landscape.      | \<1.2       |                      |               |
+-----------------+-------------+----------------------+---------------+
| **Marginally    | C (5, 6)    | Cascade              | I             |
| suitable** \--  |             |                      |               |
| careful         | E (5, 6)    | Step-Pool            | IV            |
| evaluation of   |             |                      |               |
| the particular  | Bc (5, 6)   | Plane Bed (\>2%)     | V             |
| stream and      |             |                      |               |
| reach is        | Or if BHR   |                      | If BHR \> 1.2 |
| required. These | \> 1.2      |                      |               |
| stream types    |             |                      |               |
| tend to be      | B           |                      |               |
| higher gradient |             |                      |               |
| and have        | A           |                      |               |
| greater stream  |             |                      |               |
| power.          |             |                      |               |
+-----------------+-------------+----------------------+---------------+
| **Not           | G           | N/A                  | II            |
| suitable**      |             |                      |               |
|                 | F           | The Montgomery and   | III           |
| **\--** these   |             | Buffington system    |               |
| are typically   | D (consider | primarily evaluates  |               |
| incised streams | ELJ)        | streams that are LW  |               |
| with very high  |             | dependent            |               |
| banks or        | Any stream  |                      |               |
| unstable        | with BHR \> |                      |               |
| braided stream  | 1.4         |                      |               |
| systems.        |             |                      |               |
+-----------------+-------------+----------------------+---------------+

Notes: BHR = Bank Height Ratio, total bank height/bankfull flow height
(Rosgen 1996)

ELJ = Engineered Log Jam (Abbe 1997)

A very important aspect in determining LW placement in streams is the
presence and relative size of the floodplain. Adding roughness in the
form of LW to a stream system is fairly low risk in areas that are
undeveloped and have a floodplain that is connected to the stream
channel. If the channel is confined by valley walls, terraces, roads, or
dikes, then the relative shear stress will be higher and the water will
be deeper. This scenario would require a larger structure to ensure
stability, and larger structures may raise the water surface elevation
causing additional flooding. Gippel (1995) found that during low flow
conditions in streams with large wood, average velocity was reduced on
the order of 35 percent, however, during moderate and high flows, there
is no difference in velocity.

**ANALYSIS**

The difference between a traditional engineering structure and LW
placement is primarily in the lack of control of material dimension and
quality and the complex dimensions of a natural tree with an attached
root wad. The size of wood necessary to provide absolute stability may
not be available, practical, or it may simply not exist.

Analysis for stability of wood members in any structure requires a
freebody diagram, which incorporates all forces acting upon the wood
member (Figure 1). If several pieces of wood are used in conjunction,
additional analyses will be required to evaluate the interaction.
between members. The following analysis allows for evaluation of a
single wood member, LW with additional rock for ballast, and wood
incorporated into structures.

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##### Required Calculations

### Example calculations are located in the appendix

**Table 1: Friction Angles φ\***

  --------------- ------------- ---------------- ---------------- ------------
  **Sediment**    **Grain Size  **Dry Bulk       **Saturated Bulk **Friction
                  (mm)**        Density,         Density,         Angles**
                                kg/m^3^**        kg/m^3^**        

  Coarse gravel,  \>192         2100-2200        1300-1400        43-46
  boulders                                                        

  Coarse gravel   36-144        2000-2100        1250-1300        42-45

  Medium gravel   18-36         1952-1975        1175-12225       41-44

  Fine gravel     9-18          1750-1800        1100-1150        40-42

  Gravelly sand   4.5-9.0       1700-1750        1050-1100        38-41

  Coarse sand     1.0-4.5       1600-1650        1000-1050        33-39

  Medium sand     0.54-1.0      1550-1600        950-1000         29-35

  Fine sand       0.06-0.54     1450-1500        900-950          22-30

  \*From Abbe and                                                 
  others, 2000                                                    
  --------------- ------------- ---------------- ---------------- ------------

![](media/image1.wmf)**Figure 2:**

**Table 2: Densities of air-dried timber\***

+-------------------------------------------------------+--------------+
| **Species**                                           | **ρ~w~**     |
|                                                       | #/ft^3^      |
+-------------------------------------------------------+--------------+
| Cedar                                                 | 22.5         |
|                                                       |              |
| Spruce (Sitka, White and Engleman)                    | 26.8         |
|                                                       |              |
| Hemlock, Pine (Jack and Lodgepole), Spruce (Black)    | 30.0         |
|                                                       |              |
| Pine (Ponderosa)                                      | 31.8         |
|                                                       |              |
| Fir (Douglas)                                         | 33.7         |
+-------------------------------------------------------+--------------+
| **\*** Modified from D'Aoust and Millar 1999          |              |
+-------------------------------------------------------+--------------+

**All projects utilizing LW require a momentum analysis**.

The factor of safety of a structure with LW changes with flow depth and
time. If the structure is designed to catch more LW, this must be
accounted for in the initial design. The structure may initially have a
porosity of 50%; after several high flow events, the porosity may be
reduced to 20% due to sediment deposition and additional LW recruitment.
Recognize that several different conditions exist that may alter the
level of stability and factors of safety. These include the condition
just after construction, the structure after several high-flow events,
the seasoned structure which may be several years old, and the
deteriorating structure that is nearing the end of its life span.

Bed friction conditions can change with time. Trees can embed into
stream sediments increasing soil resistance. Values in Table 1 should
provide a conservative analysis. Significantly higher unit densities
than those shown in Table 2 have been documented in moist wood. The
designer should make every effort to document realistic densities as
this one variable has a large impact on the design.

If ice and debris are of concern, a higher factor of safety is
necessary. In general, a stream with a lower width to depth ratio will
have less likelihood of ice formation than a wide shallow channel.
Upstream conditions are critical since ice formation above the structure
will have the most significant impact. Ultimately, the selection of a
reasonable factor of safety for different loading conditions is up to
the designer who will take responsibility for the design and
installation of the structure.

**GENERAL MATERIAL SPECIFICATIONS**

LW for instream structures should be relatively durable and of suitable
quality to assure permanence for the design life of the LW component of
the engineering structure (this is variable from a few years to decades)
in the climate in which it is to be used.

Decay rates of wood and burial and/or exposure must also be considered
for the life span of the LW. Alternate wetting and drying, or constant
dampness, cause the highest rates of decay. Fully saturated wood has
much greater longevity.

The density of the available material has a large impact on the analysis
and on the ultimate stability of the project. Table 2 values are
conservative in that they are for air-dried wood. If wood is moist, or
can be soaked prior to installation, this will impact the wood density
and the stability of the installation.

Cottonwood and alder have the highest decay rates with an estimated
design life of 5 to 10-years. Maple is more durable, but even under
saturated conditions survival is only 10 to 20-years. Conifers are
generally more desirable because of slower decay rates, however, hemlock
is the least desirable of the conifers. Sitka spruce is durable but is
limited in availability because of its distribution and high value as
lumber. Douglas fir is the most available conifer and is very durable
surviving from 25 to 50-years. Western red cedar is the most desirable
because it lasts twice as long as Douglas fir, has natural rot-resistant
properties, and is extremely durable (Johnson and Stypula 1993).

Trees with attached rootwads are the most desirable for incorporation
into engineering structures because the rootwad can be exposed to stream
flow while the bole is buried and used as a natural anchor. Rootwads
also elevate the center of gravity of the tree, which are then more
stable because it takes a much greater depth of water to float the tree.
Trees without rootwads often result in scour beneath the log (Booth,
Montgomery, and Bethel 1996).

LW sizing depends on the size of the stream, maximum depth of flow,
planform, entrenchment, ice and debris loading, and available tree
species. Adjustments will be necessary for your local area. If there is
a limit on the size of material that you have available or the material
has already been identified, use the actual wood size in your analysis
to determine the amount of ballast that will be required for structure
stability.

**DESIGN CONSIDERATIONS**

*see attached figures for reference*

(1) **[Location]{.underline}** -- LW is typically placed within a stream
    channel in locations that will enhance habitat and compliment
    natural stream processes -- use a natural analog. For instance,
    placement of LW in a scour pool will increase the depth and size of
    the pool while providing cover for fish.

(2) **[Height]{.underline}** -- The relative height of LW near the
    stream bank (H) is generally determined by the elevation of
    channel-forming discharge (approximately a 1.5-year event). For
    ungaged streams, channel-forming discharge can be determined using
    field indicators such as bed features and the presence or absence of
    vegetation. The channel-forming flow elevation is not necessarily
    the top of the bank; for most streams, the channel-forming flow
    elevation is equal to or slightly above average annual peak flow. LW
    below the channel-forming flow level will be saturated on a regular
    basis and will provide in-stream habitat. 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.

(3) **[Angle and Offset]{.underline}** -- 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
    (Braudrick and Grant 2000).

(4) **[Profile]{.underline}** -- 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.

(5) **[Anchoring and Ballast]{.underline}** -- Anchoring is a major
    concern when designing a structure with LW. Determine whether you
    need anchoring based upon calculations in the analysis section of
    this technical note. Anchoring is not always necessary.

> Maintaining flexibility is the primary concern for structural
> stability in a stream channel. 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 then be comprised of
> several individual members rather than one large structure that is
> cabled into place.
>
> 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 (D\'Aoust and Millar 2000).
>
> When using cable to anchor LW, keep in mind that 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.
>
> For a bole with attached rootwad, bury the bole end in the downstream
> direction with channel gravel or cobble. There is some evidence that
> this ballast and subsequent low velocity zone stabilizes the LW
> (Braudrick and Grant 2000).
>
> Buried cut-off logs or rocks can be used in conjunction with a
> streambank structure to reduce the risk of flanking. The buried log or
> rock should be oriented perpendicular to the direction of flood flows.
> Left over rock, or rock that is too small for the instream portion of
> the structure can be used in the cut-off trench.

\(6\) **[Depth of the Bed Key]{.underline}** \-- 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.

In lieu of a scour analysis, scour depth can be estimated using the
following:

Expected scour depth for gravel or cobble bed streams can be estimated
by:

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.*

\(7\) **[Construction]{.underline}** -- Construction should occur during
low flow conditions to minimize instream disturbances. LW should be
placed with the proper equipment to insure that the wood is interlocked
and stable. It is CRITICAL that the designer or an inspector experienced
in these structures be present during installation.

**EXAMPLE ANALYSES**

Attached are two specific examples (see Appendix), which illustrate how
LW stability is evaluated using the calculations presented earlier in
this technical note.

1.  Single tree with rootwad -- fully submerged

2.  Single tree with rootwad -- partially submerged

3.  Single boulder -- fully submerged

**REFERENCES**

Abbe, T.B., and D.R. Montgomery. 1996. Large woody debris jams, channel
hydraulics and habitat formation in large rivers. In: [Regulated Rivers:
Research and Management]{.underline}, 12:201-221.

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. In: S.S.Y. Wang,
E.J. Langendoen, and F.D. Shields, Jr. (eds.) Proceedings of the
Conference on Management of Landscapes Disturbed by Channel Incision.
ISBN 0-937099-05-8

Abbe, T. and four others, in review. Bank protection and habitat
enhancement using engineered log jams. Watershed Science Institute
Report \_\_\_\_. USDA -- Natural Resources Conservation Service.

Bilby, R.E. 1984. Removal of woody debris may affect stream stability.
[Journal of Forestry]{.underline}: 82:609-613.

Bilby, R. E., and Ward, J. W., 1989, Changes in characteristics and
function of woody debris with increasing size of streams in western
Washington: Transactions of the American Fisheries Society, v. 118, no.
4, p. 368-378.

Booth, D.B., D.R. Montgomery, and J.Bethel,1996,Large woody debris in
urban streams of the Pacific Northwest: *in* Roesner, L.A., ed., Effects
of watershed development and management on aquatic ecosystems:
Engineering Foundation Conference, Proceedings, Snowbird, Utah, August
4-9,1996 (invited).

Braudrick, C.A. and G.E. Grant. 2000. When do logs move in rivers?
[Water Resources Research]{.underline}, 36(2):571-583.

Bryant, M. D., and Sedell, J. R., 1995, Riparian forests, wood in the
water, and fish habitat complexity. Armantrout N.B., editor, Condition
of the world\'s aquatic habitats, proceedings of the World Fisheries
Congress: New Delhi, Oxford and IBH Publishing Co. Pvt. Ltd., p.
202-224.

Buffington, J.M. and D.R. Montgomery,1999. Effects of hydraulic
roughness on surface textures of gravel-bed rivers. Water Resources
Research, 35(11):3507-3521.

D\'Aoust, S.G., and Millar, R.G., 2000: \"Stability of ballasted woody
debris

> habitat structures.\" J. Hydr. Eng., ASCE, 126 (11), 810-817.

D\'Aoust, S.G. and R.G. Millar. 1999. Large Woody Debris Fish Habitat
Structure Performance and Ballasting Requirements. Watershed Restoration
Management Report No. 8, Watershed Restoration Program: Ministry of
Environment, Lands and Parks, and Ministry of Forests, British Columbia,
Canada.

Ellis, L. M., 1999, Floods and fire along the Rio Grande: The role of
disturbance in the riparian forest, Ph.D. Dissertation: Albuquerque, New
Mexico (USA), The University of New Mexico.

Frissell, C.A. and R.K. Nawa, 1992. Incidence and causes of physical
failure of artificial fish habitat structures in streams of western
Oregon and Washington. [North American Journal of Fisheries
Management]{.underline}: v. 12, p. 182-197.

Gippel, C.J. 1995. Environmental hydraulics of large woody debris in
streams and rivers. [Journal of Environmental Engineering]{.underline}:
121(5)388-395.

Johnson, A.W. and J.M. Stypula 1993. Guidelines for bank stabilization
projects in the riverine environment of King County. King County
Department of Public Works, Surface Water Management Division. Seattle
WA.

Keller, E. A., MacDonald, A., Tally, T., and others, 1985, Effects of
large organic debris on channel morphology and sediment storage in
selected tributaries of Redwood Creek, Geomorphic Processes and Aquatic
Habitat in the Redwood Creek Drainage Basin: Washington, D.C., USGS
Professional Paper, Washington Office.

Montgomery, D.R. and J.M. Buffington. 1993. Channel classification,
prediction of channel response, and assessment of channel condition:
Washington State Department of Natural Resources, Report TFW-SH10-002,
84 p.

Rosgen, D.L. 1998. River Assessment and Monitoring: fluvial
geomorphology short course. Pagosa Springs, CO. September 21 - 25, 1998.

Rosgen, D.L. 1999. Natural Channel Design and River Restoration: fluvial
geomorphology short course. Pagosa Springs, CO. October 5-14, 1999.

Schumm, S.A., M.D. Harvey, and C.C. Watson, 1984. Incised channels:
morphology, dynamics and control. Water Resources Publications,
Littleton, CO, 200 pp.

Sedell, J. R., and Froggatt, J. L., 1984, Importance of Streamside
Forests to Large Rivers: The Isolation of the Willamette River, Oregon,
U.S.A., from its Floodplain by Snagging and Streamside Forest Removal:
Verhandlungen. Internationale Vereinigung Fur Theoretische Und
Angewandte Limnologie, v. 22, no. 3, p. 1828-1834.

**APPENDIX**

####### Sample Calculations

Sample Calculations

Sample Calculations