Channel Initiation

1
Channel Initiation

• What are the processes whereby water movement
  becomes sufficiently concentrated to cut a
  definable channel?
• Linked to modes of hillslope water movement and
  erosion
• Under what conditions is that initial cut
  maintained and enlarged to initially ensure a
  permanent channel?
• Linked to network development
• Its all about whats a hillslope and whats a
  valley

2
Channel Initiation

• Channel head the upstream limit of concentrated
  water flow between banks (Dietrich and Dunne,
  1993)
• a major boundary between hillslopes and channels
• pivot point in sediment transport between
  diffusive process and incisive process
• transition to incisive processes from diffusive
  processes
• an erosional threshold is exceeded enabling
  channel-forming processes to become effective
• Two views
• topological transition from convex to concave
  profile, or divergent to convergent topography
• threshold based driving forces (erosion
  manifested through high hyd. gradient, high
  porewater pressure) exceeds resisting forces
  (soil cohesion, vegetation)

3
Mechanisms of Channel Initiation

• Channel initiation requires runoff
• Channel initiation occurs by (driving forces)
• Horton overland flow
• Saturation overland flow
• Subsurface flow
• seepage erosion
• shallow landsliding
• Resisting forces
• Infilling rate, vegetation types, soil cohesion
• Often multiple processes at work

4
Channel Head Location and Topography
Montgomery and Dietrich, 1989
5
Channel Initiation and Basin Morphometry

• Process model for channel initiation by shallow
  landsliding
• convergent topography causes colluvium eroded
  from adjacent hillslopes to accumulate
• at critical threshold, landsliding occurs
  exposing underlying material
• erosion of underlying material by saturation
  overland flow initiates channel
• Channel heads controlled by hillslope process
  rather than network extension
• Inverse of source basin length drainage density

6
Networks

• A stream or river network is composed and
  characterized by
• A set of nodes classified into (a) outlet node,
  (b) exterior node, (c) interior node.
• Exterior nodes occur in headwater basins.
• Interior nodes include all inner basins with
  multiple tributaries. The outlet node is the root
  of the network.
• A set of links or reaches classified into (a)
  interior links or (b) exterior links.
• A numbering system to order the stream links
  and nodes
• Horton (1945) developed a numbering system
• Strahler (1957) improved upon Hortons
  classification system

7
Network Ordering

• Channels originating at a source are first order
  (headwater) streams
• For Horton-Strahler,
• When stream of same order join, add 1 to the
  stream order
• When stream of different order confluence, assign
  highest order
• For Shreve,
• When streams of similar order join, add them to
  get d/s order

http//www.ees.nmt.edu/vivoni/mst/lectures/Lecture
18.pdf
8
Limitations of Horton-Strahler ordering (Mike
Kirkbys slide)

• Order does not express the intuitive size of a
  catchment very well

9
Network Development Deterministic vs.
Probabilistic

• Uniform energy expenditure and minimum energy in
  any link of the network were tested with respect
  to the development of network topology
  Rodriguez-Iturbe and Rinaldo (1992).
• They calculated power, P, applying these
  conditions, at each link in several hypothetical
  networks, and then summed power in all the links
• The network that most closely resembles a
  natural network had the smallest rate of energy
  expenditure shows the tendency towards an optimal
  state exists for network topology

P574
P303
Different networks and rate of energy
expenditure, P, for a set of equally spaced
points connected to a common outlet. From
Rodriguez-Iturbe and Rinaldo (1992).
P151
10
Watershed Networks

• Watershed network comprised of
• headwater and network systems
• First and second order streams often comprise 70
  of the stream network (Benda et al, 1992)
• High ecological value
• Stream networks defined by
• Natl Hydrography Dataset (1100,00)
• Terrain analysis
• (area, area-slope area-length thresholds)

11
Effects of low order channels on downstream
reaches in the network

• Synchronous (or asynchronous) inflows of water,
  sediment, nutrients, and organic matter create a
  variety of channel conditions and biological
  assemblages
• Connectivity of headwater systems to downstream
  reaches affects the cumulative and dispersed
  nature of material transport processes
• Gomi, et al, Understanding processes and
  downstream linkages of headwater systems,
  BioScience, Oct. 2002, vol. 52, no. 10