Hydrology BC ENV 3025
Streams and floods
- extremes in hydrology take many forms,
from extremes in magnitude and length
- by definition, an extreme event must
be rare and unusual
- magnitude and frequency of extreme
events can be estimated from ordinary, not as severe events
- floods are very important issues all
over the world
- current examples: Venezuela,
Mozambique, Hurricane Floyd, Katrina, Thailand
The nature and causes of floods (and
droughts)
- for a hydrologist, a flood is a
discharge rate that exceeds some threshold value, generally floods occur when a river over tops its banks
and flows across the floodplain
- the principal causes of floods in the
Eastern United States and the Gulf Coast are hurricanes and
storms
- the principal causes of floods in the
Western United States are snowmelt and rainstorms
- in rivers, floods and low flows are
expressions of the temporal variability in rainfall or
snowmelt interacting with river basin characteristics (basin
form, hillslope properties, channel network properties)
- flooding may also be the result of
sudden release of water from dams or lakes, ice jams
- land use changes can affect the severity
and effects of floods
- climate cycles
- most techniques described above tend
to treat floods or droughts as random events in a stationary
series. However, climate and riverflow are clearly
non-stationary and follow trends and cycles
- example: The 1922 Colorado Compact
apportioned water rights on the basis of the average
discharge from 1896-1930 (21 billion m3/y) while
in 1931-65, the discharge rate was 16 billion m3/y
- examples for teleconnections: El
Nino/ENSO, possibly sunspot cycles, some of these are
quite controversial
- long term drought in western US as
reconstructed from tree rings (Cook et al., 2004) (Fig)
- floods cause the biggest natural
hazard damage in the US, example: Mississippi flood, 1993
- damages
caused
by
floods
in the US
- More than half of all fatalities
during floods are auto related, usually the result
of drivers misjudging the depth of water on a road and the
force of moving
water. A car can float in just a few inches of water.
- definition of a drought even more
difficult than the definition of floods
- British Rainfall Organization:
absolute drought = 15 consecutive days with less than
0.25mm/day on any day, partial drought: at least 29
consecutive days with a mean rainfall less than
0.25mm/day
- agricultural droughts: 'at least a
partial crop failure'
- hydrological drought: actual flow in
the rivers is of most concern; length and extremeness of
flow below a certain level are important ; e.g.: low flows
of 10-day duration that occur no more than 5% of the time
- various drought
indices, e.g. the Palmer drought severity index (PDSI)
- PDSI is calculated based on
precipitation and temperature data, as well as the local
Available Water Content of the soil
- NOAA
monitoring
system
- current situaton (Fig)
The hydrograph
- a graph of river stage or discharge
versus time at a point is called a stage or discharge
hydrograph (Fig5.1)
- there are about 6000 gaging stations
in the US that typically measure stage, which needs to be
converted into discharge rate using a calibration or rating
curve (Fig5.3,
Snake River in Colorado, Q = 76.5*stage4.1),
dimensions!
- a typical hyetograph and hydrograph of
a creek in VA (Fig5.4)
- peaks in the hydrograph are called floods,
background discharge between peaks is called baseflow
- differences in hydrographs of three
streams (Fig5.5)
Flood prediction
- Student Excercise: How can we
predict extreme events? Download the daily discharge rate data
for the Mississippi at St
Louis, MO, (St_Louis_daily_70s.tsv).
Convert
the
file
into
an EXCEL spreadsheet and plot the timeseries of the discharge.
Consider overlaying the data for all 10 water years. Also look
at a map that shows the Mississippi River Basin (Fig).
Can you see any patterns? When do flood occur and why? How
could you predict floods? This Fig shows the
discharge rate for 4 years in the 1970s.
- USGS
Streamflow Information Program
Flood routing
- movement of flood waves, complicated
as a result of many factors, which?
- example: Flooding in
Central Virginia, June, 1995
- flood warning and flood mitigation
depends on how quickly a flood crest travels downstream and
how high it gets; example: nested basins of the Potomac river
(Fig5.6)
- flood routing: prediction of
downstream hydrograph, if the upstream hydrograph is known
- flood routing in rivers and by
reservoirs (Fig5.10)
- dV/dt = I-O
Flood frequency analysis
- simplest approach: use worst event on
record
- past record key for the future? Statistical
techniques
use the following approach:
- highest discharges recorded in each
year are listed
- the floods are ranked according to
magnitude, the largest flood is assigned a rank 1, the
second largest rank 2, etc
- The flood statistics are estimated
graphically by plotting on normal probability paper the
logarithm of discharge for each flood in the annual series
against the fraction of floods greater than or equal to that
flood; this fraction is given by r/(n+1), where r is the
rank of the particular flood (Fig5.13)
- r/(n+1) is the exceedence probability
for this particular event
- the return period, the average span
of time between any flood and one equaling or exceeding it,
is calculated as Treturn = 1/(exceedance probability).
- the 100 year flood can then be
estimated from the graph
- example: Holiday Creek, VA
(Fig5.13)
- another way of estimating flood
frequencies (as discussed in precipitation lecture):
- list the maximum annual discharge
values (Qmax)
- calculate the logarithm of the
discharge rates and make a histogram of the log (Qmax) data
- if log (Qmax) is normally
distributed, determine average and standard deviation
- use EXCEL's NORMINV function to
determine the the log(Qmax) corresponding to the exceedence
probability of interest (e.g. 0.01 for the 100 year flood)
- transform log(Qmax) back to Qmax
- normal distribution works often well
with precipitation data, log-normal for discharge
- look
at
the
following
two rivers, plot a timeseries of the peak discharge, make a
histogram and determine the 100 and 1000 year flood
- problems: not deterministic, based
usually on non-adequate data, climate and terrestrial
environment is variable
Resources
USGS
fact sheet
Global
Effects
of ENSO: Temperature and Precipitation
Drought indices
National Drought Mitigation
Center
NOAA
monitoring
system
Cook, E.R. et al.: Long-Term Aridity Changes in the Western
United States, Science, Vol. 306, No. 5698, pp. 1015-1018, 5
November 2004