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DISASTERS INVOLVING THE HYDROSPHERE. FLOODING. Going into this discussion, we need to remind ourselves of two fundamental facts pertaining to the Hydrosphere:

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Presentation Transcript
Going into this discussion, we need to remind ourselves of two fundamental facts pertaining to the Hydrosphere:

(1) the Hydrosphere and the events occurring within it are functionally related (systematically related) to other subsystems of the overall Earth system

(2) Biological and physical processes are intimately tied to water – specifically to quantities of water of sufficient quality

--- this is the root of the old sage about water

Too much, Too little, Too bad

Here we are looking at the “Too much” of water

Flooding can be very diverse in:

origin: single sharp event – to – building over a season

pattern: very infrequent – to – annual event

type: flash – to – widespread regionally

impacts: none – to – total devastation

Potential to occur in most any climate or landscape

--- there is however a strong correlation with patterns/types/impacts

[I can attest to this having been in radically different floods in: Phoenix; Glenville, WV; Virginia/North Carolina]

The common dominator of all floods is that an abnormal and uncontrollable amount of water is encountered that damages – or completely destroys – the land and structures and many drown thousands of people

Floods are historical companions of Man, having been recorded throughout human record

… look at the record of the Nile; the Huang He; the Yantze

Interesting: “Floods, like earthquakes and volcanic eruptions, appear to be on the rise”

--- this is very likely a function of more comprehensive reporting and the greater extent to which populations are concentrating on riverine and coastal floodplains

[the second point is particularly pertinent as eiver areas provide the essentials for development and offer the amenity aspects that populations increasingly desire]

--- it is estimated that 15% of U.S. urban land is located in floodplain

It can then be argued that floods are becoming more frequent because:

(1) more people are venturing into flood-prone areas

(2) human development activities make flood-prone aread more vulnerable to flooding

In addition, there are perceptual considerations to the human presence in flood-prone areas:

(1) human water control engineering frequently gives us a false sense of security against floods

(2) humans have a short memory span about many natural disasters – including flood incidents

[complicated by differing perception / idea that it’s a “nuisance]

how rivers and streams work
How Rivers and Streams Work

In the formation and action of a river / stream, several correlations of flow gradient are present – and gradient correlates to the volume of flow and capacity to move objects:

(1) the greatest gradient is associated with headwater flows

--- greater runoff potential

--- steeper slopes associated with this portion of watershed results in greater potential for erosion

(2) eroded material is carried downstream and precipitated out, decreasing the stream gradient --- defining a base level – the level at which a stream / river will no longer erode downward

[in land it will be at confluences; coastal it is the ocean]

the equilibrium stream
The Equilibrium Stream

Worldwide, streams exhibit longitudinal (gradient cross sections) patterns. This implies that equilibrium processes are at work

A riverine system will establish a dynamic equilibrium where change causes compensation within the parameters of its physical environment

(it is systemic and passes through stages of fluvial- geomorphic development)

… here is where man-river problems emerge

Volumes of water, over time, that enters a stream course are a result of many factors. What your text terms key variables:

(1) discharge

(2) available sediment

(3) gradient

(4) sinuosity

Where discharge and available sediment are independent system variables

Additional influences in flood potential include:

topography – steepness, length, orientation;


vegetation condition

Bottom line, the most important factor must be the amount of water that a given stream system is expected to accommodate

--- If input of water, for one reason or another, exceeds a river’s capacity, the channel will overflow


When we looked at the 1993 Mississippi Flood we saw the combination of conditions that spawn a “monstrous” flood

(1) persistent heavy rains over an extended period of time… 150% or more of normal

(2) soil becomes saturated and resists additional infiltration… results in rapid runoff and elevated river flow levels

(3) abnormally low pressure off the Pacific and into the western U.S.

(4) almost stationary jet stream… rapid W-E flow

flood styles the nature of floods
Flood Styles (The Nature of Floods)

Minor flooding is frequent

Unusually large ones: (1) are much less frequent and

(2) get the press

We designate major flood events through expected frequency as measured in years: a “10-year flood”;

a “50-year flood”

Floods can be categorized according to cause (snowmelt; heavy rainfall; ice-jam flooding; etc) and magnitude (extend of flow input and elevation of flow-level)

The most common river flood is the single-event flood

… produced by intensive precipitation over a number of hours / days; over a given drainage basin

… 1” of rainfall on 1 acre of land = 26,000 gal

The surface runoff from an intense storm is relatively rapid and usually does not have far to travel before entering a stream… small streams respond quickly

As large amounts of water from the upper basin finds its way down downstream, a flood crest (flood wave) builds and works its way downstream

… and may be described in terms of single or multiple peaks; speed of development; behavior within their flood plain

The nature of mid-latitude storm paths in the northern mid-latitudes results in “trains” of storm systems and frequently result in multiple flood peaks

In upper river / stream reaches strong hydraulic action “plucks” and “scours” material from the side and bottom of the channel

Transported into the middle and lower channel:

(1) the height of the crest will decline

(2) the length of the wave will increase

(3) the duration of the wave passing a particular location will increase… and water levels will fall more slowly that they rose

In many flood episodes, factors creating the flood build up over a period of hours or days, allowing preparation and / or evacuation… in the case of a flash flood event, this is not possible

Typically, a flash flood is the result of a stationary or slow moving thunderstorm where persistent precipitation exceeds the soil’s ability to absorb it

Flash flooding may occur anywhere in the world – mountain regions are the most frequent sites

--- as a seeming paradox, arid regions are some of the most unpredictable sources for flooding

[I can attest that Phoenix, AZ floods pretty bad]

Additionally, some arid soils may be hydrophobic, resisting infiltration until sufficiently wet,

then soil pore spaces may become “plugged”, further reducing infiltration

Though by definition rare events, sharp desert thunderstorms can turn a dry creek rain-swollen very quickly

ex: 7/31/76 Big Thompson River Canyon Flood (p. 381)

Societal Responses to Flood Hazard

(1) dams

(2) levees

(3) sandbagging

(4) forecasting

(5) zoning and land use

(6) insurance

Urbanization and Urban Flooding

Humans have significantly modified the surface of the Earth and created cultural landscapes

… true for both urban and rural landscapes

Flooding potential is generally increased by human activities:

(1) removal of vegetation

(2) compaction of soil

(3) covering extensive areas with impervious materials

(4) storm drains designed to removal water quickly…but having a limited total capacity…to stream systems