Readings
Significant Floods in the US in the 20th Century _describes
types of floods
http://ks.water.usgs.gov/Kansas/pubs/fact-sheets/fs.024-00.html
http://www.ncdc.noaa.gov/oa/reports/billionz.html#narrative
Billion Dollar weather disasters
http://www.ncdc.noaa.gov/oa/climate/severeweather/rainfall.html
Rainfall events
Big Thompson Canyon flood
A USGS poster presentation
Retrospective
analysis
1993 Mississippi River Flood
Other floods we will examine:
2002 flooding in Central Texas and 1998 floods in the Guadalupe
basin (Why such big and frequent floods in central Texas?)
2002 flooding in northern Minnesota (and general floods in the
Red River Valley)
2001 floods Houston area
Try to find out something about these, why did they occur
Other reading:
Preface and Overview to Flood Geomorphology (handout)
Easterbrook Pg 109-114
Easterbrook pg 381, Fig 14-19
Drainage basin morphometry and floods - Peter Patton (handout)
Geomorphic measurements after a flood - Williams and Costa (handout)
Lundgren - Chapter 7 (covers some basic hydrology, Miss River
1993 flood, hazard mapping and forecasting
http://www.fema.gov/
Find out how to get a map, what is Q3Digital data (what formats),
what is the status of the map modernization efforts?
What is the National Flood Insurance Program? How does it work?
How many Texas towns are in the flood insurance program? Is your
hometown?
What is the Flood Disaster Protection Act of 1973 and the National
Flood Insurance Reform Act (NFIRA)
SHARING THE CHALLENGE: Floodplain Management into the 21st
Century
Read the Executive summary (on the SAST page from above)
http://edc.usgs.gov/sast/
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Floods
Flooding, particularly related to urbanization is becoming more of a problem. Small creeks can cause major problems with the large increase in paved surfaces.
Floods account for 1/4 to 1/3 of the average annual dollar
loss from geologic hazards and 80% of the annual loss of life
from geologic hazards
30 year average - 143 fatalities
Flooding has caused the deaths of more than 10,000 people since
1900.
Property damage from flooding now totals over $1 billion each
year in the United States.
| 1998 data | Fatalities | Injuries | Property (M$) | Crop damage (M$) |
| damage | ||||
| Flash flood | 118 | 4578 | 909.1 | 140.9 |
| River flood | 14 | 1856 | 1400.9 | 169.2 |
| Stream/Urban flood | 4 | 6 | 14.8 | 8 |
From 1978 to present, there has been 70 or so flood events that have led to severe losses. (A significant event is one with 1,500 or more paid losses, or occasionally one added for other reasons. FEMA, NFIP). At least a dozen of these impacted Texas. The biggest loss has been caused by Tropical Storm Allison in 2001.
What factors govern damage?
Types of Floods
Regional Floods
Flash Floods
Ice-Jam Floods
Storm-Surge Floods
Dam- and Levee-Failure Floods
Debris, Landslide, and Mudflow Floods
So what affects flooding. It is a natural process, but have we exacerbated the problem? We will discuss factors in class.
Human Effects on Floods (we will discuss benefits and drawbacks, think about hydrograph changes)
Channelization
Leveeing
Dams
Urbanization
Agriculture
Tiling/Storm sewers
Can we relate geomorphic variables and flood potential?
Drainage basin morphometry and floods - Peter Patton
Drainage area- flood magnitudes Qx=aAb
Qx discharge of some recurrence interval x
Q increases at a lesser rate than drainage area, so b is usually less than 1
Relationship of drainage density to hydrologic response
Low drainage density basins dominated by infiltration and subsurface
flow
High drainage density basins are product of erosion and dissection
by overland flow
Correlation of drainage density with hydrologic phenomena
* negative correlation with baseflow in east-central US
* positive correlation with flood runoff (runoff intensity and
mean annual flood)
* positive correlation with mean annual precipitation and precipitation
intensity (Fig 2)
* inverse correlation with flow duration and hydrograph rise time
(ephemeral streams)
* weak correlation with climate
However it is still difficult to use as a measure for Q because drainage density is not sensitive to changes in the hydrologic response of the basin during an individual storm, i.e., effective drainage density changes with duration, volume, intensity. requires a dynamic model linked to dynamic precipitation model
Relationship of relief and ruggedness (DD*R)
More relief, time of concentration decreases, increases flood
peaks
Central Texas Regression formula for predicting flood magnitudes
from drainage basin morphometry
Qmax = 17,369M0.43(HD)0.54F1-0.96 R2 =
0.85 P = .001
Qmax = 36,650M0.64(RR)0.54DD-1.68 R2 = 0.85 P = .001
M = basin magnitude (Shreve ordering system), HD = ruggedness, F1 = first order frequency, DD drainage density, RR = relief ratio
Hydrogeomorphic Methods for the Regional Evaluation of Flood Hazards - Vic Baker
Central Texas
Floods controlled by climate (transient factor) and physiography
(permanent factor)
83-53 cm annual precip
Two major types of events - tropical marine air masses and convective storms
Delineating flood profiles using backwater analysis is expensive
$1000/mi (70s price)
Correlation with topographic features or soils associations ($1-4/mile)
Occasional flood method
Flood lines from aerial photos
Historic evidence
Local observations
Botanic Approach
Discuss map and species data
Soils method
Geomorphic method
Morphology
Sedimentology
Distinctive erosional features
Time sequences of channel movement/abandonment
Then add botanic, pedologic, and hydrologic
Historical trends in discharge
Longer time frame than that afforded by gaging records
Why needed? Gaging records too short for extremes.
Pecos River, Tx 27,400 m3/s 1954
Paleoflood analysis Easterbrook Pg 109-114
There are several techniques
· Estimation of hydraulic factors based on size of largest
particle transported
· Analysis of slack-water deposits
· Effects on floodplain vegetation
· Modeling
· Flood erosion
· Dimensions of former channels
· Truncation of tributary fans
Best reference is Flood Geomorphology ed. V. Baker C. Kochel, P. Patton
Bretz and the Channeled Scablands
Easterbrook pg 381, Fig 14-19
Particle Size
Equations for calculating drag, lift, and resistance at incipient
motion. It is often harder to initiate transport than it is to
keep it going - Hjulstom diagram
Related to the 6th power law r3 = kv6
Also USBuRec v=5.9d1/2
Precautions:
Maximum size available or maximum size transported
Rafting, bank erosion, mass movement
Flood Erosion
The Chezy equation v=C(RS)1/2 . estimate the height of flood on
walls etc to get R, measure slope of channel, estimate C (very
difficult)
Could also use Manning eq. if steady flow assumed
Slack Water Sediments
The highest sediments deposited represent the peak of stage height
Where do you find slackwater deposits?
Tree Ring Analysis
Reaction after a flood Williams and Costa
There has been a flood. An investigation is ordered. You are sent
out to survey the area. What do you do?
Flood forecasting
Now we have the ability to accurately forecast downstream floods
to with tenths of feet and hours of time of arrival. This is for
the big stream which are gauged. Predictions for small ungaged
is not possible.
Floodplain development
Why live on a floodplain?
Development of flood hazard maps
Organization in charge of floodplain maps - FEMA
Find out how to get a map, what is Q3Digital data (what formats),
what is the status of the map modernization efforts?
http://www.floodmaps.fema.gov/fhm/ot_main.shtm
http://water.usgs.gov/pubs/fs/fs-022-01/
The National Flood-Frequency Program—Methods for Estimating
Flood Magnitude and Frequency for Natural Basins in Texas, 2001
Zoning, floodplain regulation
What is the National Flood Insurance Program? How does it work?
How many Texas towns are in the CRS? Is your hometown?
SHARING THE CHALLENGE: Floodplain Management into the 21st
Century
Read the Executive summary. What do you think? How should we deal
with the problem of floodplain management? What do you think of
the NFIP?