Streams and Flooding

Hydrosphere

Hydrosphere

Where all ground and surface water contained, is essentially constant.

Total volume of water on & in Earth: $1.39$ billion $k{m}^3=1.39x10^{21}$ liters Equivalent to $1/1000^{th}$ of Earth’s volume.

Reservoirs:

• Oceans $97.5\%$
• Glaciers $1.81\%$
• Groundwater $0.63\%$
• Lakes/streams $0.016\%$
• Soil moisture $0.005\%$
• Atmosphere $0.001\%$

Hydrologic Cycle

The hydrologic cycle represents the movement (circulation) of water across reservoirs of hydrosphere.

Processes: Evaporation, Precipitation, Condensation, Transpiration, Runoff, Infiltration, Percolation

Why do we care about streams?

• Streams: any body of flowing water confined within a channel
• Water shapes the earth’s surface
  • Streams are the dynamic agent of erosion, sediment transport and deposition
• Streams play a major role in human affairs
  • Source of water for drinking, agriculture, industry
  • Used for navigation, commerce, producing power, recreation
  • Source of fresh water for all types of life, habitats, fisheries
• Floods are the most widely experienced catastrophic geologic hazard
  • Need to understand controlling factors
  • Determine ways to reduce hazard and impacts

Flooding

Definitely the costliest of natural hazards: https://globalnews.ca/news/8388250/bc-floods-damage-cost-repairs-insurance-most-expensive-natural-disaster-canadian-history/

Two biggest flood drivers:

1. Population growth and urban development
2. Climate Change

Updates on flood insurance and potential relocation: Government page Public Safety Canada.

Streams

Streams

• Drainage basin (watershed): region from which stream draws water
• The channel carved by a stream is related to volume of water it accommodates
• More or less water over time means geography will change
• Tributaries are streams that feed into other streams, creating a network
• Discharge: a volume of water flowing past a given point (or through a given cross section in a specified length of time)
• $Q=A\times V$

  • $Q$, discharge $m^3/sec$

  • $A$, area of cross section $m^2$

  • $V$, velocity of water $m/sec$

Sediment Transport

Bed Load:

• Heavy particles are rolled, dragged or pushed along the bottom of stream ⇒ traction load
• Intermediate size material is carried along in hops or skips ⇒ saltation

Suspended Load: Material that is light and fine enough to be carried along in the water ⇒ muddy appearance

Dissolved Load: Material dissolved in water ⇒ salt

Stream Load: Bed + Suspended + Dissolved loads

Capacity: total load of material a stream can move

Longitudinal Profile - sketch from source to mouth Velocity is related to discharge and
• Gradient ⇒ difference in elevation between two points divided by horizontal distance between points

• Gradients tend to decrease downstream
• Velocity often decreases but depends on additional water from tributaries, friction at stream bed, and geometry of channel
• Near the mouth, stream approaches base level ⇒ lowest elevation to which the stream can erode, usually elevation of body of water they flow into

Sediment Sorting and Deposition

• Stream velocity dictates sediment sorting
  • Slow-moving water only carries fine-grained sediments
  • Fast-moving water carries a wider range of grain sizes
• Sediments are commonly well sorted by size and density • ⇒ like-sized grains deposited in same area
• Depositional features of a stream
  • Delta, a large, fan-shaped pile of sediment in still waters created by a stream
  • Alluvial fan, a fan-shaped pile of sediment in a larger stream or a region between mountains and a plain formed by a small tributary stream

Deltas in lakes or oceans:

Alluvial Fans on land or into larger stream

Channels

• Streams do not flow in straight lines, and they change shape over time
• Meanders: bends in the stream formed by flowing water
• Cut bank: outside of meander; faster and deeper water; erosional bank
• Point bar: inside of meander; slower, shallow water; depositional bank

Floodplain Development

• Meanders slowly migrate downstream over time
• Meanders move laterally quickly! Usually <10 m/y but up to hundreds of m/y possible.

Flood Characteristics

• Higher velocity and discharge during flood increases mass of flowing water ⇒ can lead to destruction
• Stream stage – elevation of stream at any point
• Flood stage – when stream stage exceeds bank height
• Crest – when maximum stage is reached
• Flooding can occur at different times along a stream depending on how long the water takes to reach those points

Types of floods:

• Upstream floods – small, localized areas flooded; typically, caused by sudden, localized storms; shorter as downstream regions accommodate excess water
• Flash floods – type of upstream flood; very rapid rise of a large volume of water in a confined area (no floodplain); steep-sided canyons or valleys and highway underpasses; can be extremely dangerous
• Downstream floods – larger drainage areas; caused by prolonged heavy rains or snowmelt; longer because whole system is at capacity

Stream Hydrograph

• Hydrograph – a plot of stream discharge at a particular location over time
  • Show fluctuations in discharge or stream height over time
  • Useful tool to monitor stream behaviour remotely
  • Important information for detecting and predicting floods
• Create a Hydrograph - plot discharge/stage on vertical axis; plot time on horizontal axis
• Can see short and rapid peaks in small upstream areas
• Longer, broader peaks in larger downstream areas

Flood-Frequency Curves

• Displays frequency of flood events
  • Discharge as a function of recurrence interval or probability
• Recurrence interval–how frequently a flood of that severity occurs
• Probability–inverse of recurrence interval
  • Fifty-year flood has a 2% chance of occurring (=1/50)

Development in Floodplains

Based on what you know about how floodplains work, would you live in one?

• Fertile land
• Water for power and transportation
• No idea where 100-year flood line is

• Peak lag time – time lag/delay between rain event and peak flood discharge
• Urbanization increases risk of flooding by decreasing peak lag time and causing a higher peak discharge in a shorter amount of time

• Urbanization impacts surface runoff proportion and rate
  • Asphalt and concrete - reduce infiltration
  • Buildings - replaces water volume, raises stream height
  • Filling in floodplain land - reduces volume (destroying wetlands)
  • Storm drains - rapid delivery of storm water to streams
  • Tile drainage farming – rapid delivery of subsurface water to streams
  • Vegetation loss – farming and urbanization remove natural vegetation and expose the soil
    • Vegetation helps soil soak up water and prevent soil erosion
    • Streams can silt up – decrease volume and reduce capacity to carry away water

Reducing Flood Hazards

• Restrictive Zoning: Careful mapping and placing features strategically (e.g., parks or pasture in flood zones, not residences); Building on stilts

• Retention Pond: Build structure to trap some surface water runoff

• Diversion Channel: Redirects some water into other safer places when levels rise

• Channelization: Various modifications of the stream channel itself to increase the velocity of water flow, the volume of the channel, or both Recall Discharge, Q = Velocity x Area ⇒ increases discharge

  • Widen, deepen, or straighten channel
  • Use of concrete channels was common
  • Constant maintenance often required
  • Ecological impacts on wetlands, fish habitat, etc.

• Levees: Ancient technique of raising banks along a stream channel

  • Same as channelization, water is shunted downstream faster rather than onto floodplain
  • Levees can be breached, and water is stuck
  • Sedimentation raises stream stage, levees too low

• Flood Control Dams and Reservoirs: dams built along stream to hold back water in a reservoir (an artificial lake)

  • Benefits: irrigation water; recreation; hydropower

• Cons: disruptions to navigation of people and animals; flooding of human and animal habitat

• Silting-up of reservoir – can fill completely; needs constant dredging

• Downstream impacts on vegetation and habitats

• Earthquakes! Added pressure on rocks and faults from dam and reservoir

• Dam failure does happen

• Overuse of dam for multiple purposes with contrasting needs

Do Quiz 5!