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Transcript of Drainage Basins
Water is an effective agent of sediment mobilization through raindrop impact.
Most raindrops impact the surface at terminal velocity - about 9 m/s, which can dislodge surface particles (effects depend on cohesion and soil texture).
Precipitation vs infiltration capacity - what happens when the soil reaches saturation?
Stream response to rainfall
1. Drainage density 2. Bifurcation ratio
Surface flow moves water and sediment downhill
Ingrid Luffman, Department of Geosciences,
East Tennessee State University
Images retrieved from Ritter et al. (2011), Process Geomorphology unless otherwise indicated
Describing your watershed - basin morphometry
1. discharge: expressed in m^3/second or stage (elevation)
2. profile: a cross-sectional view of stream channel elevation along its length
3. stream order: reflects the number of upstream tributaries, and the degree of stream branching
Describing your stream - channel morphometry
* streams with no tributaries are first order
* where two first order streams join, they form a second order stream, and so on
* magnitude increases only when two streams of the same order meet
Subsurface flow involves water only, under the influence of both gravity and pressure
Surface water infiltrates and percolates downward to water table
Sources of contribution to streamflow
Flood hydrograph from Hurricane Agnes flood June 1972 on Conestoga River, PA.
Typical hydrographs are asymmetrical, the water recedes more slowly than it rises because of contributions from bank storage and groundwater as stream stage drops.
The seepage of groundwater along the bank face can cause erosion, which encourages more groundwater flow, forcing flow to converge, initiating gully formation
Ratio of the number of streams of a given order, to the number of streams in the next higher order
Total length of streams per unit area. Shows impact of both climate and geology. Explain.
Soil loss can be estimated using the universal soil loss equation (USLE), developed by the Soil Conservation Service (now NRCS). USLE is a deterministic model to estimate soil movement from a plot of land given land surface conditions, precipitation amounts, and practices:
A = RK(LS)CP where A is soil loss
R is rainfall factor
K is erodibility
LS is steepness
C is cropping (agricultural) factor
P is conservation practices factor.
I+ET= interception and evapotranspiration
amount of sediment mobilized (movement initiated)
amount of sediment reaching channel
actual sediment discharged from the basin
net sediment retained in channel
3. Drainage Patterns
Food for thought
Richmond is somewhere downstream >>>
Tree-like pattern indicates homogenous subsurface
Associated with ridge and valley topography – controlled by bedrock
Tributary streams meet main channel at right angles
Streams radiate outward from a central point
Conical hill or volcano
right angled turns in the stream channel
Associated with fractures, joints and faults.
Drainage networks exercise