Flooding - Flood Hazards Flashcards
this refers to when a normally dry areas of the land become inundated
flooding
what is the most common way that a flood can occur?
when excessive amounts of overland flow cause an increase in discharge, river channel can no longer hold all of the flow of water and will eventually overflow its banks
scientists use _________ and __________ to compare floods in a quantitative manner
stream discharge and height
is the height at which a river begins to overflow ots banks
flood stage
how is discharge computed?
multiplying cross sectional area of a river to its average velocity
t or f: flood stage gets higher as you go further from banks
t
the severity of flood is quantified by measuring wither __________ or _____
stream discharge or stage
refers to any level where stream has gone over its banks
flood stage
t or f: there is an inverse relationship between discharge and height
F (direct)
why are floods difficult to predict?
because they are largely dependent on specific weather events that have a high degree of natural variability
t or f: flood frequency does not tell us when a flood of a certain magnitude will occur, but rather the probability or chance of it happening
t
represents the frequency of a particular discharge value can be expected to repeat itself
recurrence interval
what does a 100-year recurrence interval mean?
on average, 100 years should pass before that same discharge will occur again
t or f: a 100-year flood could take more than 100 years to repeat, but could just as easily reoccur in less than 100 years
T
t or f: low discharge events occur frequently more than high discharge events
t
the inverse or reciprocal of recurrence interval (1/RI)
percent probability
a 10-year flood has a percent probability of?
10%
what does a 10% probability of a flood means?
10 percent probability that a flood will take place in any given year
this tell us how often we can expect floods of a certain size
recurrence interval
indicate the chances of occurring of flood of certain size
percent probabilities
what is the difference between recurrence interval and percent probabilities?
recurrence interval tell us how often we can expect a flood of certain discharge can occur, whereas percent probabilities can tell us their chances of occurring
the reliability of recurrence interval is dependent on the ___________
availability of historical discharge records
formula for recurrence interval
RI = (N+1)/M
where:
N = number of values in the record
M = rank of the discharge
what is the unit for recurrence interval?
year
the potential for flooding in a given area naturally increases as the ________ and _________ of rainfall increases
intensity and duration
rainfall events might range from _______ to ______
light to torrential rains
t or f: light rains are steady and may last for few days while torrential rains only last for a few minutes to several hours
t
another important factor associated with rainfall is the ____________
size of the area over which the rain falls
relatively small storms that tend to keep moving and may generate intense rains, but flooding is more localized
isolated thunderstorm
storms that move inland from the ocean and can drop tremendous amounts of water over large areas, thereby producing widespread flooding
large regional storms
a generic term used by meteorologists to describe a rotating, organized system of clouds and thunderstorms that originates over tropical or subtropical waters and has closed, low-level circulation
tropical cyclone
The weakest tropical cyclones are called
tropical depression
when is a tropical cyclone considered a hurricane or typhoon?
when its wind reaches 74 miles per hour or higher
where is the term “hurricane” used?
Atlantic ocean, Eastern Pacific ocean
where is the term “typhoon” used?
Northwest Pacific
where is the term “tropical cyclone” used?
South Pacific and Indian Ocean
what are the natural factors that affect flooding?
- nature of precipitation events
- ground conditions
- vegetation cover
the ability of ground to absorb water
infiltration capacity
explain how infiltration capacity plays a critical role in flooding
water that is unable to infiltrate will br eventually forced move as overland flow
the actual rate at which at which water can infiltrate is determined by the _______, _______, and ________
slope of the surface, type of ground material, and moisture content of material
t or f: as slope becomes progressively steeper, infiltration capacity decreases
t
t or f: gravel and sand-rich soils have higher infiltration capacity than clay soils
t
t or f: infiltration capacity decreases as soil becomes more saturated
t
how can vegetation cover affect flooding?
vegetation intercepts and stores a certain fraction of rain, thereby preventing it from reaching the land surface and moving as overland flow
vegetation enhances absorptive capacity, decreasing runoff and overland flow
the potential for flooding depends on:
- how much and for how long precipitation rate exceeds the infiltration rate
- size of the area of which the rain falls
- vegetation density
floods that have short duration with relatively high peak discharge
flashfloods
t or f: flashfloods generally affect only localized areas
t
t or f: flashfloods are dangerous due to their sudden nature
t
why are flashfloods dangerous?
they are sudden in nature, which means people have less time to evacuate
flashfloods are also referred to as
upstream floods
t or f: flashflooding can occur in downstream areas
t
where in downstream areas can flash floods occur?
along small tributaries
t or : flashfloods can also occur from sudden failure of dams
t
can be defined as one where the river leaves its channel farther down in its drainage basin, flowing out into floodplain and inundating large areas of the valley floor
downstream flood
what does downstream flood inundate?
valley floors
what causes downstream floods?
regional accumulations of water higher up in the drainage basin
how does downstream flood differ from flashfloods?
- lag time between the rain event and peak discharge (higher on downstream floods)
- length of time the river remains above flood stage (higher on downstream floods)
