Topic 9.1: Flooding

hydrosphere

all water at or near Earth's surface

components of hydrologic cycle

evaporation, condensation, precipitation, runoff, infiltration, percolation

stream

body of flowing water confined to a channel

river

reserved for major branches of a stream system

drainage basin

region where stream draws its water supply. size/area affects stream size

stream discharge

volume of water passing a given point in a given time through a channel of a certain width and depth. measured in m3/s

formula to calculate discharge

water cross section (width X depth) X velocity (distance/time)

laminar flow

straight or gently curved streamlines that run parallel without mixing, characteristic of slow moving rivers or along edges of fast moving rivers

turbulent flow

streamlines mix, forming eddies. characteristic of fast flow

capacity

total sediment load carried

competence

ability to carry material of a given size

graded stream

average of neither erosion nor sedimentation, low spots filled in and high spots eroded

equilibrium

state of graded stream

gradient

steepness of stream channel, affects velocity

base level

lowest elevation to which a stream can flow, can be sea level if the stream flows there

major stream types

meandering and braided

meandering stream

occurs on gentle gradients, generally erode weak bedrock and unconsolidated sediment, carry fine sediments. path may shift significantly with erosion and deposition

braided stream

many channels that variably diverge and merge. typical of steep gradients and high sediment loads, combined with frequent variations in discharge

natural levees

ridges of coarse material that confine the stream within its banks between floods, even when water levels are high

where does the speed of flowing water drop most quickly

stream margins

stream hydrograph

plot of stream discharge/height over time, records fluctuations. discharge/stage on vertical axis, time on horizontal

flood factors

excessive rainfall, snowmelt, severe storms, hazardous blockage

stage

water elevation

crest

maximum stage reached

upstream flood

occurs in a small, localized, upper part of a basin, affect localized areas

downstream flood

occurs in a larger, lower part of a drainage basin, affect larger river systems, tend to be longer in duration

flash floods

type of upstream flood characterized by rapid rise of stage

upstream flood hydrograph characteristics

discharge rises most steeply and creates a sharp peak, post-flood discharge drops rapidly

downstream flood hydrograph characteristics

smaller changes in discharge, lower peaks. broader peaks of longer duration

influencing factors of surface runoff

ground cover, topography, vegetation, climate

flood-frequency curve

a plot of annual flood discharges and their recurrence intervals

formula for flood-frequency curve

R = (N+1)/M

R - recurrence interval

N - number of years

M - annual maxima

Flood Hazard Reduction Strategies

\-Restrictive Zoning

\-Constructed or raised levees

\-Flood control dams and reservoirs

\- retention ponds

\- diversion channels