Streams, floods and droughts

Take away ideas and understandings:

• The water cycle is characterized by extreme events in magnitude and length.
• The main characteristic of a river is its hydrograph, the discharge rate (or stage) as a function of time.
• The magnitude and frequency of extreme events can be predicted in a probabilistic way.
• To some extent, the past can be the key to the future, with lots of caveats.

Introduction

• 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

The nature and causes of floods (and droughts)

• extreme floods occur when a river overtops its banks and flows across the floodplain
• for a hydrologist, a flood is a discharge rate that execeeds some threshold value
• 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
• floods cause the biggest natural hazard damage in the US, example: Mississippi flood, 1993
• 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
• droughts mostly related to abberations in the weather

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*stage^4.1), 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.

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
• dV/dt = I-O
• we will  explore the case of reservoir routing (Fig5.7), see Explore5.doc excercise
• we can adjust the dimensions of the reservoir (Fig5.8) and the initial conditions and look at the relationship between inflow and outflow (Fig5.10)

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)
• normal distribution works often well with precipitation data, ln normal for discharge
• problems: not deterministic, based usually on non-adequate data, climate and terrestrial environment is variable

Teleconnections

• 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, possibly sunspot cycles, some of these are quite controversial

Resources

SAST Home Page and Geographic Information Systems (GIS) Server

Mississippi River At St Louis Mo

NOAA/OGP El Niño-Southern Oscillation Page

Mississippi River Flood, lecture by David McConnell

US Army Corps of Engineers 1993 Flood Home Page

Landsat TM Color Composite Images: Flood

Lower Mississippi River Forecast Center - river flood forecasts rainfall data navigation

The Great USA Flood of 1993