Rivers: Lecture 14-15

Question 1

Define what a catchment is and describe some of its components

Answer 1

A DRAINAGE BASIN (or Catchment or Watershed) is a well-defined network of branching stream channels and adjacent slopes having a distinct order
Adjacent drainage basins (dendritic branches) are separated by divides

Question 2

What are some of the different types of drainage basin patterns and the controls that make them different?

Answer 2

Dendritic, parallel, trellis, rectangular, radial. Controls include topography, surface material, geologic rock structure and geometry of the basin.

Question 3

Describe the laws of drainage basin composition

Answer 3

1. You will always have a lot more first-order streams than higher-order streams
2. Lower-order streams are also shorter and are on steeper slopes

Question 4

How do changes in base level affect the behaviour of river channels?

Answer 4

Base levels change (e.g. sea level), so the longitudinal profile will change as it changes
If sea levels rise, river backs up, positive feedback as less sediment is delivered to the ocean

Question 5

Why do meandering and braided river channels form?

Answer 5

Meandering: large and constant discharge, low gradient (high suspended load)
Braided: freq. variation in discharge, coarse sediments, steep gradients

Question 6

Describe the evolution of a series of fluvial terraces

Answer 6

1. Initial floodplain
2. Tectonic uplift or base level change
3. First terrace is formed, abandoned floodplain becomes fluvial teeace and river stabilised at lower elevation forming new floodplain.
4. Repeats, eventually stabilises

Question 7

What are some of the ways that we can quantify river channels?

Answer 7

Velocity x Cross-sectional area (depth x width) = stream discharge (Q). Also note wetted perimeter, hydraulic radius, bankfull discharge and river stage

Question 8

Explain the spatial variability in river flow velocity

Answer 8

Flow velocity at a given point in a river/stream varies with distance above the bottom (bed) and across the stream. Measurements of flow velocity taken at 0.6d are assumed to be representative values.

Question 9

What factors can impact the amount of river discharge?

Answer 9

Stream width, depth and velocity

Question 10

How do we describe the types and behaviour of river flow?

Answer 10

1. Laminar flow: usually occurs in pipes and smooth surfaces, little friction and hydraulic shear
2. Turbulent flow: friction and hydraulic shear causes swirls, flow velocity greatest in middle

Question 11

What are the modes of sediment transport associated with river flow?

Answer 11

i) BEDLOAD (grain-grain)
→ rolling, sliding
ii) SALTATION (grain-fluid-grain)
→ hopping
iii) SUSPENSION (fluid-grain-fluid)
iv) SOLUTION (dissolved load)

Question 12

Describe some of the causes of floods

Answer 12

1. Increased rainfall
2. Snowmelt
3. Storm sugre (tsunamis)
4. Flashfloods

Question 13

Describe ways that we can prepare for and manage the impact of floods

Answer 13

1. Tracking occurance (recurrance interval)
2. Levees: raised embankment (natural, built up by humans)
3. Adding rougher material on the bed to dissipate energy <- gabion baskets
4. Weir <- can be manually opened and closed