Lecture 5 - Restoration of LWD

Question 1

What size does in-stream wood have to be to be considered large woody debris?

Answer 1

> 10 cm in diameter and > 2 m long

Question 2

How does large wood get recruited to a channel? (4)

Answer 2

Stream bank undercutting
Wind throw
Slope failures
Tree decay

Question 3

What are the best trees for large woody debris? (3)

Answer 3

Cedar because they can last for ~200 years

Spruce and fir are next best

Question 4

Why should large woody debris be a certain size?

Answer 4

The larger the wood, the more stable it is in the stream channel (long pieces can catch on the banks if moved and heavier pieces require higher flows for mobilization)

Question 5

How does large wood influence habitat diversity and complexity? (6)

Answer 5

Provide slow back water areas during high flows and floods (prevents juveniles from being swept downstream to dangerous larger rivers)

Provides cover from predators and temperature

Large wood forms pools in lower order streams

Retains spawning gravels

Provides substrate for invertebrate prey

Helps retain nutrients in the stream (eg. Spawned salmon and OM)

Question 6

What percent of wood is lost from streams per decade in the PNW?

Answer 6

10% per decade

Question 7

When did it become illegal to log streams to their banks in B.C.? (2)

Answer 7

1988 on the Coast and 1995 in the Interior

Question 8

How long will it take until B.C.’s rivers will return to large supplies of wood?

Answer 8

75-150 years

Question 9

What are the consequences of decreased large wood? (5)

Answer 9

Loss of cover and structural complexity

Reduced cover protection from predators

Reduced varieties of current velocities and other hydraulic features

Less trapping of gravel and nutrients

Less habitat for salmonids and invertebrates

Question 10

Why do younger forests has a higher rate of loss of large wood? (4)

Answer 10

May be second growth - trees are predominantly maple and alder which decay more rapidly

More trees might be recruited but the overall volume of wood is smaller because the trees aren’t as big

This makes them faster to decay (even decay-resistant wood) and highly mobile

Question 11

How long would it take for large wood to recover to pre-logging levels?

Answer 11

> 250 years

Question 12

What is the ideal situation rather than habitat restoration?

Answer 12

Habitat protection!

Question 13

How is buried large wood important?

Answer 13

Buried large wood that has accumulated below the gravel surface provides important structure and nutrient supply to the subsurface hyporheic zone

Question 14

How does large woody debris differ from small streams to larger rivers? (4)

Answer 14

In small (<10 m) and intermediate (10-20 m) streams LWD may be found in complex arrangements or individual pieces scattered along the channel

In larger streams (3rd to 5th order), LWD has less continuous accumulations but may have “drift” jams that block out the flow

These can redirect the stream course and create floodplain channel features that can become important overwintering habitat for juvenile salmonids

Rivers (5+ order) are characterized by infrequent, but occasionally massive accumulations of LWD

Question 15

How do interior rivers serve in a reverse way to coastal rivers?

Answer 15

Off-channels are a refuge from high flows on the coast but the main channel acts as a refuge from freeze up in the winter in the interior

Question 16

What are some negatives of too much large wood in a stream? (3)

Answer 16

When the presence of debris jams completely blocks upstream spawning migrations

Possible impairment of water quality

Presence of floatable debris pieces that can move during storms and pose threats to life, property, or aquatic habitat downstream

Question 17

What stream orders have the most frequent disturbance regimes and thus the greatest habitat complexity?

Answer 17

Stream orders 3-5 have the greatest disturbance regimes and thus the highest effects of large wood and highest habitat complexity

Question 18

When is suitable habitat generally limiting in streams? (3)

Answer 18

At the parr stage for species that spend more than one summer in freshwater

This is because fry defend much smaller areas (eg. 2%) than parr

Territory size required is proportional to fish length and body weight

Question 19

What are some simple large wood designs for streams <20 m? (4)

Answer 19

Single root wads in the bank

Paired trees with root wad in between

Single log in-channel

Single log bank protection

Question 20

What are some advanced designs of large wood? (4)

Answer 20

Full-spanning log jams (small streams only)

Lateral log-rootwad-boulder complexes

Woody debris catchers (eg. Triangular debris catcher)

Woody debris reefs

Question 21

Triangular debris catcher (4)

Answer 21

Triangular debris catcher emulates nature by creating scour holes and collecting smaller pieces of large woody debris

Design uses the force of the river to load the structure against the stream bank, ensuring long-term longevity

Consists of 2 logs attached to trees or stumps on the bank, both ballasted by a common anchor boulder in the stream

More stable than single log

Question 22

Where should advanced structures be placed? (2)

Answer 22

In geomorphic settings where there is either:

Historical evidence of log-jam structures
Where a systematic survey and experience says an area is suitable

Question 23

What are the key forces acting on large woody debris in a stream? (3)

Answer 23

Water velocity
Drag
Buoyancy

Question 24

What is the safety factor that had been built into design charts for large woody debris placement? (2)

Answer 24

Safety factor of 2.0 for the single log jam, and safety factor of 1.25 for the triangular log jams (because they are inherently more stable)

Question 25

What does the mass of the required ballast rock depend on for a single lateral log jam? (3)

Answer 25

Diameter of the log

Velocity of the river flow

Effective length of the large woody debris

Question 26

What does the mass of the required ballast rock depend on for a triangular lateral log jam? (3)

Answer 26

The diameter of the logs and rootwads used in the complex

The effective length

It isn’t greatly affected by flow velocity like in singular log jams

Question 27

What is the Chezy equation? (3)

Answer 27

V = 20 sqrt(H*S)

Where:

V = design velocity in m/s
H = height of floodplain in m
S = slope of the channel in m/m

This can then be looked up in a table to determine the ballast requirements

Question 28

Where should the largest boulder be placed?

Answer 28

On the upstream end of a single-log jam to avoid flipping over during high discharge events

Question 29

Design velocity (3)

Answer 29

For most streams, design velocity will be within 2-4 m/s

If it is less than 2 m/s, there is a “design curve” that can be used

If it is more than 4 m/s, it is best to walk away from this altogether

Question 30

Helicopter safety tips (6)

Answer 30

Never walk under the tail boom or go anywhere near the tail rotor

Always maintain eye contact with the pilot when entering and leaving

Exit to the side and forward of the helicopter

Never ever walk uphill away from or downhill towards a moving main rotor

Do not slam the door or let the seatbelt hang outside the door

Insist on wearing a flight helmet and fly on an empty stomach