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Floods: Water as Earth's Lifeblood (Understanding Flood Recurrence…
Floods: Water as Earth's Lifeblood
Definitions
Flood
Anytime a water body overflows or when an unusual amount of water collects in dry areas
Most common and destructive natural hazards
Can be naturally occurring or human-induced factors
Watershed
An area of land from which all runoff drains, or sheds to the same river, lake, or body of water
Can also be referred to as a drainage basin or a catchment area
Tributary: a small stream flowing into a larger one
Divide: a ridge or strip of high ground that separates one drainage basin from another
Like Russian nesting dolls (watersheds contain other watersheds)
Hydrologic Cycle
Describes movement of water and water vapour from the sea to the atmosphere, to the land and back to the sea and atmosphere
Includes: precipitation, evaporation, infiltration, run off, transpiration
Streams
Concept of a stream
A body of running water that is confined in a channel and moves downhill under gravity
Stream Channel: a long narrow depression eroded by the stream into rock or sediment
Stream Banks: the sides of the channel
Stream bed: the bottom of the channel
A river is a large stream, a creek is a very small stream
Headwaters: the upper parts of the stream near its source in the mountains
Mouth: the location where a stream enters the sea, a lake, or a larger stream
Stream Discharge
The volume of water that flows past a given point in the stream per unit of time
Dishcharge = width x depth x velocity , Q = w x d x v
Usually in cubic feet per second, cubic metres per second or litres per second
Stream discharge and flooding
Torrential rainfall and snowmelt can cause discharges to be 50 to 199 times higher than normal
When the increase happens too fast, the stream cannot accommodate it and it floods
Monitoring meteorological conditions and stream discharge
Hydrograph: Graphical representation of of time versus stream discharge
Rising Limb: describes the period when stream discharge increases as a response to rainfall or snowmelt
Falling Limb: Describes the period when stream discharge decreases after the rainfall episode or snowmelt episode has ended
Lagtime: time interval from the peak of the rainfall and the peak flow
Hyetograph: graphical representation of time versus rainfall or snowmelt depth
Baseflow: amount of stream discharge fed by groundwater even in the absence of rainfall or snowmelt
Overland Flow: the volume of floodwater reaching the stream from surface runoff
Through flow: the volume of floodwater reaching the stream through the soil and underlying rock layers
Flood Mechanisms
Hydrometeorological Floods
Local thunderstorms leading to flash (upstream) floods. Only last a few hours but build and end quickly. Cause most of flood related deaths
Example: Edmonton July 2004
Rainfall over several days that causes regional (downstream) floods. they build and dissipate slowly. Few deaths but extensive damage. Occur in large river valleys with relatively flat topography
Red River Flood of 1997: more rainfall and snow than usual in combination with a quick melt
Assiniboine river basin 2014: $200 million in road/bridge repair, largest tributary to red river
1996 Saguenay Quebec Flood: $1 Billion in damages, dams breached, 280 mm rain in 36 hours
Storm surges associated with hurricanes and which can flood coastal areas
Ice-breakups on rivers that can dam up, block water flow and then fail
Floods by dam failures
Short-lived natural dams
Human-built levees and dams
Understanding Flood Recurrence Intervals
100 Year Flood: has the probability of 1 in 100 of occurring every year
Larger floods have larger recurrence times between them
Recurrence intervals are studied by constructing flood frequency curves, Y axis: flood discharge volumes. X-axis: recurrence intervals
To create a flood frequency curve:
Record peak discharge for each year
Rank the years accordingly
Calculate, for each years maximum flood, the recurrence interval using: (N + 1)/M
N is the number of years of records; M is the rank of each year
Plot the recurrence interval vs discharge for each year, and connect points as a best-fit line
Flood Control Measures
Structural measures are likened to engineering efforts
Embankments(dykes/artificial levees): mounds built on river banks with steeper slopes. They trap sediment that would've been deposited in floodplain. Won't withstand extreme flooding
Channelization: Altering a stream channel to speed up flow and prevent flooding. Includes clearing debris, dredging to widen and deepen, lining with concrete, straightening the channel
Diversion
Storage structures (Dams & Reservoirs): traps flood water and releases slowly after a storm, lowering the crest of the flood. Also used for agricultural irrigation water supply, hydroelectric power generation, recreational water pool
Other: Protective walls, floodwalls, bypasses
Non-structural are linked to sound floodplain management
Identification of high-risk areas and flood insurance
Zoning regualtions for development
Watershed management, floodplain reforestation, wetland creation and rehabilitation, drainage modification on agricultural and urban land
Advantages of Floods
Redistribution of nutrients, sediments and water, improves soil fertility, plant growth, forest health and agricultural production
Improves ecosystem health and biological production
Offers groundwater recharge
Periodic flooding and erosion and deposition bring changes to topography and physical features within these areas over time