FLOOD01296

• County: Statewide
• Title: Method to Estimate Effects of Flow-Induced Vegetation Changes on Channel Conveyances of Streams in Central Arizona
• Date: 9/16/1998
• Prepared For: Central Arizona
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

SleD 3a

Vflex values are 8.85 ft-Ib and 63.2 ft-Ib for the
brush and willow, respectively, for all flows
(table 10). Go to STEP 4.

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SteD 3b

Vjlex values remain constant for all flows. Go to
STEP 4 (Slep 4b).

SteD 3c

Vflex values remain constant for all flows. Go to
STEP 4 (Step 4c).

STEP 4. Detennine the orientation of the
vegetation (either oriented randomly or parallel to
flow) as well as the cross-section area of flow
blocked by vegetation, and the ratio of hydraulic
radius to average vegetation height for the selected
discharge. Detennine the vegetation-blocking
coefficient, Cblocking' the vegetation-distribution
coefficient, Cd/st' and the flow-depth coefficient,
Cdeptb (tables 4, 5, and 6).

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SteD 4a

Cblock/ng is equal to 4.0 for both vegetation
types, Cdist is equal to 1.0 for both vegetation
types and, for the 2-year flow, Cdeptb is equal to
5 and 60 for the brush and willow, respectively.
Go to STEP 5.

SteD 4b

Cblocking and Cdist values remain constant. For
the IO-year flow, however, Cdeptb is equal to 1.0
and 20 for the brush and willow, respectively.
Go to STEP 5 (Step 5b).

SteD 4c

Cblocking and Cd/st values are assumed to remain
constant for willow. For the 50-year flow,
however, Cdepth is equal to 5.0 for willow. Go to
STEP 5 (Step 5c).

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STEP 5. Compute the vegetation-susceptibility
index for the selected discharge using equation 3.

SleD Sa

For the 2-year flow, the vegetation-
susceptibility indices are 177 ft-Ib and
15,200 ft-Ib for the brush and willow,
respectively (table 12). Go to STEP 6.

SteD 5b

For the IO-year flow, the vegetation-
susceptibility indices are 35.4 ft-Ib and
5,060 ft-Ib for the brush and willow,
respectively (table 12). Go to STEP 6
(Step 6b).

SteD 5c

For the 50-year flow, the vegetation-
susceptibility index is 1,260 ft-Ib for willow
(table 12). Go to STEP 6 (Step 6c).

STEP 6. Compute stream power for the selected
discharge using equation I.

SteD 6a

For the 2-year flow, computed stream power is
1.78 ft-lb/s/ft2 (table 12). Go to STEP 7.

SteD 6b

For the I O-year flow, computed stream power is
11.6 ft-lb/s/ft2 (table 12). Go to STEP 7
(Step 7b).

SteD 6c

For the 50-year flow, computed stream power is
28.0 ft-lb/s/ft2 (table 12). Go to STEP 7
(Step 7c).

STEP 7. Plot the values for the vegetation-
susceptibility index and stream power for each
type of vegetation present in the channel (figure 9).
If the values plot below the vegetation-
susceptibility threshold, repeat steps I through 6
using the next largest discharge. If the values plot
above the threshold, no further steps are required.

Effects of Flow-Induced Vegelation Changes on Channel Conveyances 25