Shaping Earth Surface Final

What is the Froude number and what's its significance in bedform development.

The Froude number (Fr) is the ratio of inertial forces to gravitational forces in a fluid flow. It is significant because it helps determine the flow regime and predict the types of bedforms (like ripples, dunes, or antidunes) that will develop on the riverbed.

What are the key differences between ripples and dunes as bedforms?

Ripples are small bedforms typically found in the viscous sublayer, composed entirely of bedload, and do not interact with the water surface. Dunes are larger bedforms that involve both bedload and suspended load, influence the water surface, and are limited in height by the water column.

How does a change in the Froude number influence bedform progression in a river?

As the Froude number increases, bedforms progress from lower plane bed to ripples, then dunes (potentially with superposed ripples), washed-out dunes, and finally to upper plane bed. A high Froude number can also lead to the formation of antidunes.

What are barforms and how do they differ from bedforms? Provide an example.

Barforms are large-scale sediment accumulations in a channel that are not limited by water depth but rather by channel width. Unlike bedforms, their scale is not set by the height of the water column. Point bars and free bars are examples.

What is Hack's Law and what does it describe about river systems?

Hack's Law states that the area of a watershed drainage basin (A) is related to the length of the river's longest channel (L) raised to a power (h), typically around 1.6 (A = L^h). It describes the relationship between drainage basin size and the longest river channel length within it.

Define sinuosity and explain how it is used to classify channel forms.

Sinuosity is defined as the ratio of the channel length (LC) to the valley length (LV), calculated as LC / LV. A sinuosity of less than 1.3 indicates a straight channel, while a sinuosity greater than 1.5 indicates a sinuous channel.

Describe the main forces contributing to the formation of helical flow in meandering channels.

The main forces contributing to helical flow are the centrifugal force, which pushes water towards the cut bank and increases with velocity (higher near the surface), and the gravitational force, which pushes water towards the point bar and is related to the difference in water surface height across the channel.

What are the primary causes for the development of braided rivers?

Braided rivers are typically caused by a combination of factors including weak banks due to lack of vegetation, a high sediment load that exceeds the river's transport capacity, and frequent flood events that rearrange the sediment.

Define avulsion and state the condition, in terms of slope stress, that triggers it.

Avulsion is the abrupt change in a river's course. It occurs when the stress driving flow down the natural levee slope is approximately double the stress driving flow down the existing channel slope (Levy stress ≈ 2 * River stress).

Describe the difference between a river-dominated delta and a wave-dominated delta in terms of shoreline morphology