Ch 07 - Rivers and Flooding

Learning Objectives

After reading and studying this chapter, students should
* Understand basic river processes
* Understand the nature and extent of the flood hazard
* Understand the effects of urbanization on flooding in small drainage basins
* Know the major adjustments to flooding and which are environmentally preferable
* Know the potential adverse environmental effects of channelization and the benefits of channel restoration

Chapter Ch 07 Summary
This chapter focuses on river processes and flooding. The first portion of the chapter focuses on the purpose rivers serve in the hydrologic and rock cycles, and on the physical processes of drainage basins and river channels, of sediment transport, and of erosion and deposition. A subsequent section addresses the effects of land-use changes on rivers. The remainder of the chapter focuses on aspects of flooding including channel patterns and floodplain formation, causes and consequences of flooding, the impacts of urbanization on flooding, and human adjustments to and perceptions of flooding.

Chapter 07 Outline
I. Rivers: Historical use
A. Americans have lived and worked on floodplains for more than 200 years
1. enticed by rich alluvial soil, water supply, ease of waste disposal, proximity to river commerce
B. Floodplain
1. the flat surface adjacent to the river channel that is periodically inundated
2. produced by processes of flooding
C. Two parallel trends in floodplain settlement
1. accelerating program to control floods
2. even greater growth of flood damages
II. Streams and rivers
A. Hydrology: study of water transport in natural systems
1. runoff
2. drainage basin
B. River slope or gradient
1. drop of channel over horizontal distance
C. Base level
1. theoretical lowest level to which a river may erode
2. usually sea level
3. base level may be temporary, as with a lake

III. Sediment in rivers
A. Sediment load
1. total load
2. bed load
3. suspended load
4. dissolved load
IV. River velocity, discharge, erosion and sediment deposition
A. Rivers are a transportation system involving erosion and deposition of sediments
1. velocity varies along course, affecting erosion and deposition of sediment
B. Discharge
1. volume of water moving past a particular location in a river (in a unit of time)
2. continuity equation: Q=WxDxV
C. Faster flowing river transports has greater erosional capacity than does slower moving river
1. alluvial fans
2. delta
3. distributary channels
D. Reasons for erosion or deposition correlated to physical properties of the river
1. change in channel geometry
2. composition of channel bed and banks
3. vegetation
4. land use
E. Sediment transport measures
1. competency
2. capacity
V. Effects of land-use changes
A. Dynamic equilibrium
1. steady state between sediment transported and sediment delivered to stream
2. changes in amount of water or sediment brings changes in channel slope or cross-section
B. Land use change from forest to agriculture
1. increased soil erosion
2. increase in sediment load
3. increasing channel slope
4. new dynamic equilibrium may be reached
C. Dam construction
1. water slows at head of reservoir, causing deposition
2. below dam, water released has minimal sediment
3. channel erosion will predominate below dam
VI. Channel patterns and floodplain formation
A. Configuration of channel as seen in aerial view
1. braided
2. meandering
B. Meandering stream features
1. riverine environment: channel and floodplain
2. pools
3. riffles
VII. Flooding
A. Definitions
1. flooding is the natural process of overbank flow
2. flood discharge
3. stage
4. flood stage
B. Upstream and downstream floods
1. upstream floods: caused by intense rainfall of short duration over relatively small area
2. downstream floods: caused by storms of long duration over large area
VIII. Urbanization and flooding
A. Human use of urban environments
1. has increased magnitude and frequency of floods in small drainage basins
2. percent impervious cover and percent of area served by storm sewers are a measure of the degree of urbanization
B. Urbanization causes many changes in rainfall-runoff relations
1. increased runoff
2. flooding from blocked storm drains
3. influences limited to smaller floods of greater frequency; larger, less frequent floods are not significantly affected
IX. The nature and extent of flood hazards
A. Flooding is one of most universally experienced natural hazards
1. number one type of disaster in U.S. in 20th century
B. Factors that cause flood damage
1. land use on floodplain
2. magnitude of flood
3. rate of rise and duration of flooding
4. season
5. sediment load deposited
6. effectiveness of forecasting, warning, and emergency systems
C. Effects of flooding
1. primary: caused directly by the flood
2. secondary: caused by disruption and malfunction of services and systems
X. Adjustments to flood hazards
A. Historically, humans have attempted to prevent flooding
1. creating physical barriers
2. straightening stream
3. widening stream
4. deepening stream
B. Recent years, alternative adjustments
1. flood insurance
2. controlling land use on floodplains
C. The structural approach
1. physical barriers, such as levees and floodwalls
2. reservoirs
3. on-site retention basins
4. some engineered structures actually increase the flood hazard
D. Channelization
1. straightening, deepening, widening, clearing, or lining existing stream channels
2. adverse effects of channelization
E. Channel restoration: alternative to channelization
1. cleaning urban waste from channel
2. allowing stream to flow freely
3. protecting existing banks
4. Kissimmee River, Florida restoration
F. Flood insurance
1. U.S. National Flood Insurance Program
2. Special Flood Hazard Areas
3. National Flood Insurance Reform Act of 1994
G. Flood-proofing
1. raising building foundation
2. constructing floodwalls or earth berms around buildings
3. waterproofed doors, walls, windows
4. improved drains with pumps
H. Floodplain regulation
1. objective: obtain most beneficial use of floodplains while minimizing flood damage and costs of protection
2. need to recognize that the floodplain belongs to the river system
3. flood hazard mapping
4. floodplain zoning
I. Relocating people from floodplains
1. Rocky Mount, NC, after Hurricane Floyd
2. Churchs Ferry, ND, rise of Devils Lake
J. Personal adjustment: What to do and what not to do
1. see Table 7.2
XI. Perception of flooding
A. People are tremendously variable in their knowledge of flooding, anticipation of future flooding, and willingness to accept adjustments
B. At institutional level, progress includes flood hazard mapping