THE RECURRENCE INTERVAL

Size: px

Start display at page:

Download "THE RECURRENCE INTERVAL"

Morris Dean
5 years ago
Views:

1 Floods GEOG 366

3 THE RECURRENCE INTERVAL How often can a storm of a given magnitude be expected to recur? Calculate as follows: Take list of annual peak flows (largest flow each year) and rank them. R.I. = (N + 1)/M R.I. is recurrence interval; N is years of record, an M is the rank of the individual flow

6 Probability The probability of a flood of a given recurrence interval occurring in any given year: P = 1/R So, the probability that the 10-year flood will occur in any one year is 1/10 or 0.1 (10 percent). The probability that the 100-year flood will occur is 1/100 = 0.01 or 1 percent.

7 100 year flood 100-year flood commonly is used as the design flood for bridges and other structures along a stream, as the basis for delineating flood plains for flood-insurance purposes, and as the basis for zoning ordinances.

8 Overlay of FEMA s 100-year flood plain

9 Importance of 100-year floodplain An inaccurate determination of this discharge, if too high, could result in the overdesign of a structure and a resulting waste of construction funds; If too low, a structure could be repeatedly damaged or destroyed

10 BUT the 100-year flood plain changes through time with human development.

11 FACTORS AFFECTING THE MAGNITUDE AND VARIABILITY OF ANNUAL FLOODS

12 A. DRAINAGE AREA Drainage area has been found to be the most significant factor affecting the magnitude of floods.

13 B. PRECIPITATION Generally, the greater the amount of precipitation or the higher its intensity, the greater the flood magnitude. A storm of a given total amount and intensity will produce floods of different magnitudes if the storm travels up the basin, down the basin, or across the basin.

14 Though the total precipitation may be the same in two storms, the temporal distribution during the storm period can cause differences in flood peaks. When greater amounts fall near the end of a storm, the peak is apt to be greater than when they fall near the onset of the storm. Why?

15 Very intense rainfall can exceed infiltration rates and surface depression storage capacity. The ground does not have to be saturated for this condition to exist. Intense rainfall

16 Precipitation is usually heavy and intense and often prolonged. Where only the fringe of a hurricane strikes land, severe flooding can occur locally over a relatively small area. Hurricane floods have occurred many times from Texas to Massachusetts in the United States. Hurricanes

Hurricane Floyd, Sept 1999 15-20 inches of rain fell on the eastern half of North Carolina and every stream and river flooded 57

17 Hurricane Floyd, Sept inches of rain fell on the eastern half of North Carolina and every stream and river flooded 57 deaths from flooding Many rivers set new records Thousands of homes lost; extensive crop damage; some communities underwater for weeks

18 Hurricane Ernesto, Aug-Sept 2006 Winds not a major problem Chief damage was from flooding Flooding in Virginia and N. Carolina inches of precip. in Virginia

19 Flooding conditions are worse if 1. There is orographic uplift (the Appalachians) 2. The storm remains stationary over land Ernesto, Sep. 1, 2006

20 Hurricane Camille

21 C. ANTECEDENT CONDITIONS The condition of the basin prior to the storms producing the floods. Whether: The ground is near saturation from previous rainfall or is dry, The ground is frozen or not, vegetation has been removed or is natural, there is a snowpack or not, and an alluvial channel was wetted by recent flood runoff.

22 D. SNOWMELT Copious, warm rain on an existing snowpack can melt a lot of it. The conversion of water stored as snow to runoff, augmenting that from rainfall, has caused some major floods.

23 Areas subject to snowmelt flooding The Sierra Nevadas of California, usually in November or December; the plains of central North America; the Atlantic provinces of New Brunswick, Nova Scotia, and Newfoundland, as well as in southern Ontario and southern Quebec in the early spring; the northern Rocky Mountains in early summer.

24 Flooding in southern Canada coincides with the spring snowmelt.

25 E. BASIN GEOLOGY: Affects- Infiltration rates and amounts, stream patterns, basin shape, land slopes, natural basin storage, resultant vegetation.

26 Infiltration Granite Low inherent permeability (e.g., granitic basins of the Sierras and Rockies). But moderate to high infiltration can occur in highly fractured rocks.

27 Alluvium High infiltration rates and stores large quantities of water. In southern California, some streams leave canyons and spill flood waters over the apexes of alluvial fans. Unless the flood is a large one, the water infiltrates into the alluvium and may disappear entirely.

28 Limestone and dolomite In karst areas, water is lost into sinkholes and underground drainage channels, thus reducing both the peak discharge and flood volume. The areas of dolomitic rocks in the Ozarks province of southeastern Missouri have many springs that modify flood runoff patterns.

29 Tributaries Because tributary slopes are usually steeper than those of the main stream and tributary lengths are shorter, most floods on tributaries are of shorter duration with more rapid rises and recessions. Tributary peak discharges or stages seldom are concurrent with those on the main stream.

30 Natural basin storage The last ice age left numerous depressions in north-central north America. Small depressions, sometimes called "prairie potholes" provide some natural storage of flood runoff, thus reducing the peak discharges.

31 The reclamation of wetlands by government agencies or developers is thought to have increased the magnitude and frequency of floods.

32 Vegetation Vegetation and the ground litter it generates can intercept rainfall, thereby delaying runoff and reducing the amount of water available for runoff. Vegetated areas have more porous soils. Vegetation in an stream channel or along the banks increases the resistance to flood flows, which in turn increases the flood stage and the time of travel of the flood peak along the channel.

33 TRANSIENT EFFECTS ice jams volcanic eruptions landslides glacier-damned lakes man's activities such as the failure of a dam or deforestation.

34 1. Ice Jams Ice jams may occur during the spring breakup when the ice cover disintegrates into chunks of ice, which jam at a natural constriction in the stream or at a bridge crossing.

35 City workers attempting to remove ice jam with dynamite

36 In many northern rivers, ice-related flooding produces the highest water levels. No particular region in Canada is immune from an occasional ice-jam flood.

37 2. Volcanoes Intense heat melts the snow and ice cover, and the water transports mud, ash, sediment, blocks of ice, and other debris. Mount St. Helens, May caused outstanding floods on several streams, but principally in the Toutle River basin, Washington. Some material carried 120 km downstream.

3. Landslides 1959 Hebgen Lake earthquake, near Yellowstone, occurred after a M 7.5 Montana earthquake. 28 campers killed and a landslide blocked the Madison R.

39 3. Landslides 1959 Hebgen Lake earthquake, near Yellowstone, occurred after a M 7.5 Montana earthquake. 28 campers killed and a landslide blocked the Madison R., forming a lake. Landslides dam a stream temporarily. The threat of a flood caused by breaching of the slide material at some unknown future time is very real.

40 Gros Ventre Landslide A landslide occurred in June 23, 1925 on the Gros Ventre River in northwestern Wyoming, impounding about 200 million m 3. About 60 million m 3 was released 23 months later on May 18, 1927, when seepage and erosion of the slide material became sufficient to cause the failure of the "dam." The flood wave was about 5 m high near Kelley, 6 km downstream.

41 Gros Ventre landslide

42 4. Jokulhlaup A glacially related flood. Mount Rainier in Washington is a source of glacier outburst floods. Floods in this area occurred at least seven times between 1926 and The flood flows included rocks, boulders, trees, and other debris.

43 1996, Iceland

44 5. Failure of man-made dams Failures of man-made dams usually occur in a relatively short time. The resulting flood wave can destroy most anything in its path for many kilometers downstream. The high velocity of the floodwater is the main destructive force, but submersion can cause serious damage.

UGRC 144 Science and Technology in Our Lives/Geohazards

UGRC 144 Science and Technology in Our Lives/Geohazards Flood and Flood Hazards Dr. Patrick Asamoah Sakyi Department of Earth Science, UG, Legon College of Education School of Continuing and Distance Education