3.0 channel design

Question 1

three types of natural channels

Answer 1

threshold
alluvial
transition

Question 2

what is a threshold channel

Answer 2

forces are below threshold for movement of boundary material
fine sediment passes through as throughput or washload, (not considered part of bed)
canrt adjust geometry
morphological concepts dont apply
design methods well established (veloc, shear stress)
need to evaluate sediment deposition at low flows

Question 3

two pics of threshold channels

Answer 3

grass, bedrock

Question 4

alluvial channel?

Answer 4

bed and banks formed of material transported under present flow conditions
adjust geometry in response
bank errrosion etc are natural

Question 5

stability of alluvial channel means

Answer 5

ability to pass incoming deiment without significatn aggregation. sed tranpost analyisis required
use geomorphic principals

Question 6

alluvial pics

Answer 6

sand bed, sand gravel,

Question 7

transition

Answer 7

not clear distinctrion
armouring (threshold at low flows, alluvial at high)
one reach may be different from the other

Question 8

pics of thershold

Answer 8

bolder bed

Question 9

three types of stream flow and what are they

Answer 9

perennial-always has flow

intermittent-seasonally flow

ephemeral-only flows at flood

Question 10

bed material sediment inflow for each (sig or not)

Answer 10

insig for threshold, sig for alluvial

Question 11

deisgn goal for the two types

Answer 11

threshold, pass discharge without mobilizing banks

alluvial, pass incoming sed without sig aggregation or degradation or planform change

Question 12

design discharges for each

Answer 12

threshold: max design discharge
alluvial: channel forming discharge, flow duration curve

Question 13

four genergal design methods

Answer 13

analogy-copy
hydraulic geometry - emipircal relationships between dependant (width or slope) to discharge
analytical- uniform flow equations with sediment transport theory
hybrid-combo

Question 14

the three design methods and their recommedned determination

Answer 14

analogy - top width of channel forming discharge
hydraulic geometry - same
analytical-depth and slope

Question 15

when to use: 
allowable velocity
allowable shear
tractive power
grass lined/tractive stress
alluvial channel design techniques

Answer 15

allowable velocity : boundary smaller than sand
allowable shear: larger than sand
tractive power : material not a discrete particle
grass lined/tractive stress: no base flow
alluvial channel design techniques : significant sediment load and moveable channel boundaries

Question 16

steps for allowable velocity

Answer 16

check to see if appropriate
find Q
concentration entering
find allowable V
use with mannings and Q=VA to get R or Kd-conveyance factor

Question 17

why should safety factor be applied

Answer 17

allowable vleocity and shear based on short (a few hours) periods, longer decreases erosion resistance

Question 18

the three allowable shear stress methods and the basis behind each

Answer 18

shields: force on a single particle that initiates motion (non-cohseive) saftey factor required (for flumes and channels)

Glessler probability: probability distribution of force on a particle mixture (non-cohesive) no need saftey factor (for flumes)

Lane: force on surface area , seperate equations/charts for coarse and find non cohesive and cohesive, no need saftey. (for channels)

Question 19

limitations of both stress and veloctiy mehtods

Answer 19

aggregation of sediment
bends
difference between allowable and incipient
saftey factor allowance for turbulance