The topics covered in this chapter can be summarized as follows:
|13.1 Factors that Control Flooding
|Most flooding results from heavy rainfall. Flooding also results from rapid snow melt, and in some cases rain and snowmelt are coincident. To understand flooding we need to understand overland flow (water flowing over the surface) and base-flow (from groundwater discharge). Interpreting stream discharge hydrographs is key to understanding stream behaviour and flooding.
|13.2 Examples of Flooding Events
|The flooding of southern Texas from hurricane Harvey in August 2017 is a good example of flooding from heavy rainfall. Harvey was the wettest tropical storm on record to strike the US, but we can expect future storms to be increasingly intense and wet because of climate change. The flooding in central Alberta in June of 2013 was caused by a combination of heavy rain and rapid snowmelt. Long-term records of annual maximum discharge levels can be used to estimate the probability of specific flood levels in the future, but they do not allow us to predict flooding.
|13.3 Managing Floods and Limiting Damage
|Infiltration of water into the ground is one of the keys to reducing flood potential, but most of what we do in urban areas results in hardened surfaces that prevent infiltration. We need to change that. We can also carry out engineering projects on streams to limit flooding (e.g., dams, dykes, levees, by-pass channels), but sometimes those measures lead to negative outcomes. Work done in the area of the Red River in Minnesota, North Dakota and Manitoba provides some good examples of flood prevention measures.
|13.4 Flooding and Earth Systems
|Stream flooding contributes to Earth systems by creating highly fertile flood plains, through the deposition of sediments that later become rocks, and via turbidity flows that transport sediments far out into the ocean.
Answers for the review questions can be found in Appendix 1.
- How does a stream’s discharge from base flow correlate with the amount of groundwater storage?
- What typically happens to the velocity of stream flow when a stream first overtops its banks? Why?
- Why did Hurricane Harvey lead to such serious flooding in Texas in 2017.
- Explain how a slope failure could lead to flooding.
- Consider a forested area with a runoff coefficient of 0.14 versus an urban area with a runoff coefficient of 0.84. If both receive the same amount of rainfall over the same time period, how many times more surface runoff would you expect in the urban area than in the forest?
- How might a dam across a river be effective in reducing floods, and how might that effectiveness be reduced if the dam was also designed and operated to generate electricity?
- Why do the wing dykes on the Mississippi River increase the risk of flooding in upstream areas, and why to the levees (dykes along the sides of the river) increase the risk of flooding in downstream areas?
- In what ways has the Greenway project in Grand Forks (N. Dakota and Minnesota) reduced the risks associated with river flooding in that area?