> TECHNICAL NOTES
>
> U.S. DEPARTMENT OF AGRICULTURE NATURAL RESOURCES CONSERVATION SERVICE
>
> [PORTLAND, OREGON January 1, 2000]{.underline}
>
> ENGINEERING - NO. 24
>
> **DESIGN OF ROCK WEIRS**
>
> **DESCRIPTION**

Porous and solid rock weirs are channel spanning rock structures that
are installed to (1) center, and sometimes create, a stream thalweg; (2)
protect streambanks by redirecting stream flow; (3) establish and
maintain a lower width to depth ratio; (4) provide fish passage by
concentrating low flows in flat-bottomed channels into narrower, deeper
channels; (5) increase sedimentation along streambanks; (6) control flow
direction and therefore minimize meandering; (7) raise water surface
elevations to provide water to diversions and channel alcoves; (8)
stabilize stream gradient; (9) provide energy dissipation; (10) create
pool habitat; and (11) buttress the bole of a rootwad for aquatic
habitat cover.

Rock weirs are very low structures that should be completely overtopped
during channel-forming flow events (approximately a 1.5-year flow
event). Channel-forming flow, or bankfull discharge, is defined as the
flow that transports the greatest amount of sediment over a long period
of time and controls the channel geometry. Porous weirs have spaces
between the exposed rock near the middle of the channel to further
accommodate fish passage for some species, while solid weirs are
continuous across the channel. Both porous and solid rock weirs
are-designed in a \'V\' or \'U\' shape with the trough oriented upstream
(see attached figures).

Each stream channel and project site is unique. Geomorphic
characteristics, such as meander pattern, width/depth ratio, radius of
curvature, particle size distribution, channel gradient, and pool/riffle
spacing, all impact the effectiveness of rock weirs. Onsite evaluation
of the appropriateness and utility of rock weirs is necessary. They are
most effective in gravel- and cobble-bed streams with slopes less than
three percent. These structures should NOT be used in sand-bed streams.
For streams with a channel-forming flow width greater than 100-feet,
\'V\' or \'U\' shaped weirs are not recommended; \'W\' shaped weirs are
more effective in very wide streams but are more complex to design and
build and are not covered in this technical note.

Rock weirs redirect stream flow to the center of the stream channel and
disrupt the velocity gradient in the near-bank region. They utilize a
low weir section pointed upstream to force water flowing over the weir
into a hydraulic jump. Flowing water turns to an angle perpendicular to
the downstream weir face causing the flow to be directed away from the
streambank. The weir effect continues to influence the bottom currents
even when submerged by flows greater than the channel-forming discharge.
The length of the thalweg created downstream varies with slope and
radius of curvature, but is typically 100 to 200­

> *Prepared by Janine Castro, Geomorphologist, USDA NRCS*
>
> feet. The disruption of the velocity gradient reduces channel bed
> shear stress and interrupts sediment transport in the near bank
> region.

Using rock weirs in conjunction with bioengineering methods is the most
favorable combination. The weirs provide direct streambank protection
from flow and vegetation provides for energy dissipation and sediment
deposition.

> **GENERAL MATERIAL SPECIFICATIONS**

Rock for weirs shall be durable and of suitable quality to assure
permanence in the climate in which it is to be used. The rock shall be
sound and dense, free from cracks, seams, and other defects that would
tend to increase deterioration from weathering, freezing and thawing, or
other natural causes. The rock fragments shall be angular to sub-rounded
in shape. The least dimension of an individual rock fragment shall not
be less than one-third the greatest dimension of the fragment. Rock will
have a minimum specific gravity of 2.5.

> Depending on availability, large rock (generally greater than 3-feet
> in diameter) can be less expensive by weight and can take less time to
> install. If large rock is not available or is not preferred, follow
> the rock sizing criteria listed below.

Material sizing should follow standard riprap sizing criteria for
turbulent flow (Far West States-Lane Method) for the design flow and be
modified with the following formulas (Chapter 16, Engineering Handbook):

> D~50-barb~ = 2 x D~50-riprap~
>
> D~100-barb~ = 2 x D~50-barb~
>
> D ~minimum~ = 0.75 x D~50-riprap~

Note that the Far West States-Lane method gives the riprap D~75~ and not
the D~50~ \-- a gradation is required to obtain the riprap D~50~. Once
the riprap D~50~ is obtained, use the

gradation listed above. When the ratio of curve radius to channel width
is less than six, rock sizes become extremely large, and result in a
very conservative design.

> *Rock in the weir should be well graded in the D~50~ to D~100~ range
> for the weir section (the*
>
> smaller material may be incorporated into the bank key). The largest
> rocks should be used in the exposed weir [[section.
> DO]{.underline}](http://section.DO/) NOT use the Isbash Curve when
> sizing rock for rock weirs as it results in sizes too small for this
> application.

Rock sizing depends on the size of the stream, maximum depth of flow,
planform, entrenchment, and ice and debris loading. Adjustments may be
necessary for your local area.

**GENERAL DESING GUIDANCE**

See attached figures for reference

> \(1\) **[Location]{.underline} -** Rock weirs are typically placed in
> straight reaches of a stream channel near the downstream end of a
> riffle section. The upstream tip of the weir should be
>
> NRCS, OR
>
> Version 1.3
>
> January 2000
>
> 2
>
> located at one to two channel widths downstream of the crossover. They
> should be placed in areas where pools would naturally form. Rock weirs
> will not protect banks that are eroding due to rapid drawdown or mass
> slope failure; other techniques should be employed.

\(2\) **[Height]{.underline} -** The height of the weir section 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 elevation is not necessarily the top of
the bank; for most streams, the channel-forming elevation is equal to or
slightly above average annual peak flow.

> To achieve proper weir function the slope is nearly flat (slope should
> not exceed ,1V: 5H) but [MUST]{.underline} always have a [positive
> slope]{.underline} towards the center of the channel. Rock weirs that
> are constructed with flat sections may lose a few rocks in the near
> bank region resulting in a negative slope essentially forcing water
> closer to the bank.
>
> Relative height of weirs used in a series is very important. Generally
> the slope between the weir crests should not be flatter than the
> pre-project water surface slope at low flows. The center of the weir
> should be at grade with the channel bed to allow for sediment
> transport and fish passage. To reduce scour depths, decrease the weir
> height. Higher weirs cause greater flow convergence, and thus greater
> scour depths.

\(3\) **[Spacing]{.underline} -** Rock weirs are typically used in a
series if the intended purpose is fish passage or grade control with an
elevation change (headcut) greater than one-foot. For fish passage, the
spacing depends on slope, length of backwater effects created and
associated depth, and length of thaiweg created downstream. For grade
control, the rock weirs should be placed no closer than the net drop
divided by the channel slope. As an example, a one-foot high weir in a
stream with a two-percent gradient will have a minimum spacing of
50-feet (1/0.02).

\(4\) **[Angle and Offset]{.underline} -** The structure should project
upstream such that the flow is directed away from the streambank. The
angle from a tangent to the bank can vary from 20 to 60 degrees,
although a hydraulic analysis is necessary for weirs with arm angles
approaching 60 degrees. Once the arm length reaches 50 to 60-feet, it is
necessary to truncate the \'V\' and cross the channel in more of a
horseshoe shape.

\(5\) **[Profile]{.underline} -** The rock weir transitions from the
exposed weir section to the bank key on a slope of IV: 1.5H to 1V:2H.
The weir section at the streambank should not exceed the channel-forming
flow level (1.5-year flow). The bank key must be long enough and high
enough to prevent water from flowing around and eroding behind the
structure. 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.

> The exposed weir section slopes from the channel-forming flow level at
> the banks down towards the center of the channel. The center of the
> weir should be at grade. The legs of the weir can be maintained at the
> channel-forming level into the stream channel if the purpose is to
> reduce channel width and concentrate stream flow. This may be
> advantageous in streams where fish passage is a concern during low
> flow periods.
>
> \(6\) **[Width]{.underline} -** The top width of a rock weir generally
> ranges from one to three-times the D100-weir. The weir width may need
> to be increased to accommodate construction
>
> equipment in large rivers because of the rock weight and track hoe
> reach. Wider structures will result in a more uniform, stronger
> hydraulic jump. Wider structures should be used if a deep scour hole
> downstream of the weir is expected.
>
> \(7\) **[Length of Bank Key]{.underline} -** The purpose of the bank
> key is to protect the structure from flanking due to near bank
> erosion. The length of the bank key is 4 times the D100­
>
> weir and should not be less than 1.5 times the height to the top of
> the bank or eight
>
> feet (whichever is greater). Buried cut-off logs or rocks can be used
> in conjunction with the bank key. 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 barb
> can be used in the cut-off trench.
>
> \(8\) **[Depth of the Bed Key]{.underline} \--** The bed key depth
> should be determined by calculating expected scour hole depth
> downstream of the weir. The bed key is typically placed
>
> to a depth of ~D100-weir-~ However, channel excavation depth in a live
> stream is
>
> sometimes limited because of sloughing; a very large rock often works
> better than trying to place two large rocks on top of one another.
> 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.
>
> ![](media/image1.png){width="6.65625in" height="1.6229166666666666in"}
>
> Expected scour depth for gravel or cobble bed streams can be estimated
> by:
>
> Scour = 2.5\*h
>
> Where h = height of exposed rock relative to bed elevation.
>
> For sand, use 3 to 3.5\*h
>
> *To reduce scour depths, decrease the weir height. Higher weirs cause
> greater flow convergence, and thus greater scour depths.*
>
> \(9\) **[Construction]{.underline} -** Rock weirs should be
> constructed during low flow conditions to minimize instream
> disturbances. It is usually necessary to work in the stream channel
> while constructing rock weirs. [The rock should never be
> dumped.]{.underline} The rock
>
> should be placed with the proper equipment to insure that the rocks
> are interlocked and stable. It is CRITICAL that the designer or an
> inspector experienced in these structures be present during
> installation.
>
> **REFERENCES**
>
> Davinroy, Robert B. and Redington, Stephen L. 1996. Bendway Weirs on
> the Mississippi River, A Status Report. Proceedings of the Sixth
> Federal Interagency Sedimentation Conference. March 10 - 14, 1996, Las
> Vegas, Nevada. Pg. 11-32 to 11-37.

Derrick, David. 1996. Harland Creek innovative bank stabilization
demonstration project. Proceedings of the Sixth Federal Interagency
Sedimentation Conference, March 10 - 14, 1996, Las Vegas, Nevada. Pg.
II-1 to 11-5.

LaGrone, D.L. 1996. \"Bendway Weir General Guidance Memorandum\", US
Army Corps of Engineers, Omaha, NE, revised from 1995.

Reckendorf, F.R. 1996. Bear Creek, Oregon, Fish Habitat Enhancement.
Report by Reckendorf and Associates, Salem, OR for the Wallowa County
Conservation District, Enterprise, OR.

Reichmuth, D.R. 1993. Living with fluvial systems. Seminar on February
23 - 25, 1993. ODFW, Portland, Oregon. GEOMAX, Bonners Ferry, Idaho.

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.

Saele, L.M. 1994. Guidelines for installing stream barbs, Streambank
Protection and Restoration Conference, September 22 - 24, 1994. Red
Lion, Portland, Oregon. Sponsored by Oregon Water Resources Research
Institute, Corvallis, Oregon.

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