HW #5
1. What factors affecting the stream load?
Answer:
There are several things that can affect how much stuff a stream carries:
- The steepness of the land - Steeper land means water flows faster down the slope.
Faster water can pick up and move more materials.
- How fast the water is moving - The faster the water speed, the more load it can
transport. Slower water won't carry as much.
- How wide or big the stream is - Wider streams with more water can move larger
amounts than skinny, narrow ones.
- How much water there is - More water volume means the stream has more power
to transport rocks, soil, plants and other things.
- The kind of rocks and land around it - Different rock types affect how much erodes
into the stream. Some wash away easier than others.
2. Describe at least 2 ways how urbanization affect local flood hazard!
Answer:
1. Changing from forest to farmland
- More soil erosion - Without trees holding soil in place, plowing of farmland
causes more dirt and sediment to wash away into streams.
- More deposited in streams - This extra eroded soil then ends up
accumulating in the creek beds.
- Steeper stream - Farmland is often on slopes, so the lack of trees lets
water flow faster, making river banks steeper over time.
- More stream erosion - The increased speed of water causes more erosion
along the creek banks and bottom as it carries away more material.
2. Urban development
- Increases impervious cover - Roads, buildings, parking lots mean less
ground is absorbed by soil and plants. More areas just funnel water away
instead.
- More small floods - All the hard surfaces don't let water soak in, so even
medium downpours cause flooding where it didn't before.
- Floods happen quicker - With less vegetation and more manmade
channels, it takes almost no time at all for floodwaters from heavy rain to
reach creeks and streams.
3. Explain why landslides occur and what factors influence the slope stability?
Answer:
- A landslide happens when the force of gravity exceeds the frictional or internal
strength of the rock, soil, or silt.
- Factors influence the slope stability:
• Type of earth material
• Slope Angle and Topography
• Climate
• Vegetation
• Water
• Time
4. How can we identify areas of possible past landslides?
Answer:
Past landslide zones can be identified by conducting a meticulous analysis of the
exposed vegetation, land surface, and contours of a coastal bluff.
A hummocky land surface or terraces along the slope of a bluff serve as indicators of
previous landslides or slumps (Figure 1). Spotless slump wounds or slopes serve as
conspicuous indications of expeditious soil displacement. A bowl-shaped incline
frequently indicates the site of a previous landslide, with the most significant failure
occurring in the bowl's center. Soil expansion or previous landslides can be inferred
from the presence of deformed vegetation. Curvilinear tree trunks in proximity to their
base serve as indications of downslope displacement (Figure 2). The presence of
trees slanted at varying degrees indicates the occurrence of previous slides or erosion-
induced undermining. Destroyed trees and exposed roots can be identified as
indicators of erosion that has exacerbated the gradient of a bluff (Figure 3). A bluff that
is undercut and steepened may cause the slope to become unstable, resulting in
increased erosion and potentially a landslide. Sediment terraces or lobes in the
intertidal zone are frequently the product of previous landslides.
Figure 1. Terraces or a hummocky ground surface on a bluff's slope could indicate a previous
landslide. (Source: Maine Geological Survey)
Figure 2. Tree trunks will tilt toward the base of a slope due to soil "creep," or downslope
movement of soil on a slope. (Source: Maine Geological Survey)
Figure 3. Wave erosion at a bluff's base removes slope sediment and steepens the cliff.
(Source: Maine Geological Survey)
5. List three ways in which development on hillsides may increase the landslides
hazards?
Answer:
1. Removal of vegetation, especially plants with deep and extensive root
2. Increases impervious cover
3. Failures of retaining wall
