Introduction%20to%20Rainfall%20

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Introduction toRainfall Streamflow

• Philip B. Bedient
• January, 2007

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Watershed - Elevation Contours
Water flows at right angles to elevation contours
and from higher to lower elevations
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Texas River Basins
Red
Trinity
Brazos
Colorado
Rio Grande
San Jacinto
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I. Mechanisms of Rainfall

• Convective Storms - Radiational
• Low Pressure Systems - tropical       storms
  and hurricanes
• Frontal Systems - Cold or Warm
• Dew and Fog
• Hail and Ice Storms
• Condensation

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Major Thunderstorm
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• Thunderstorm cell with lightning
• Characterized by updrafts and downdrafts
• Strong convergence and divergence
• Most intense rainfall possible

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Hurricane IvanSeptember, 2004

• Ivan spawned tornadoes from Florida into Alabama
  with deaths reported near Panama City and
  Tallahassee.
• Waves as high as 50 feet were measured 75 miles
  south of Dauphin Island. Ivan steadily moved
  northward through Alabama.
• Hurricane Ivans devastating march was precisely
  predicted because a ridge of high pressure around
  the Bahamas steered it
• The combination of slow speed and no clear zones
  of low and high pressure created the potential
  disaster zone of more than 350 miles across.

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Formation of Precipitation

1. Source of moisture
2. Lifting mechanism (orographic or heating)
3. Phase change from vapor to water - Energy
4. Small nuclei or dust for droplet formation
5. Droplets must grow as they fall to earth

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Lifting Mechanisms
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Fronts and Low Pressure

• Cold/Warm Front
• Lifting/Condensation
• High and Low Pres
• Rainfall Zone
• Circulation Issues
• Main weather makers

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Warm
Cold
Winds increase as cold front approaches
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Major Storm Damages

• Intense Rainfalls
• 8 to 15 inches
• Severe Flooding
• Billion damages
• Lost productivity

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Hurricane Andrew -1992

• Formed in the Atlantic
• Moved to Florida coast
• Winds in excess of 150
• Major damage to Florida
• Moved over Gulf and    strengthened and hit LA
• Most damaging until 2005

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Hurricane Katrina in the Gulf - 8/29/2005
Katrina 08-28-2005 at 1700 UTC
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Measuring Rainfall - Tipping Bucket

• Recording gage
• Collector and Funnel
• Bucket and Recorder
• Accurate to .01 ft
• Telemetry- computer
• HCOEM website

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Largest One Day U.S. Total Rainfall

• Alvin, Texas
• 43 inches in 24 hours
• Measured in one gage
• Associated with T.S. Claudette in July 1979
• Texas accounts for 12 world rainfall records

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Monthly Rainfall Distribution
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Average Annual Precipitation

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9-Hour Total Rainfall - TS Allison
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The Hyetograph

• Graph of Rainfall Rate (in/hr) vs Time (hr) at a
  single gage location
• Usually plotted as a bar chart of gross RF
• Net Rainfall is found by subtracting infiltration
• Integration of Net Rainfall over time
•   Direct RO Vol (DRO) in inches over a Watershed

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Mass Curves Rainfall Hyetographs
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Design Rainfalls

• Design Storm from HCFCD and NWS
• Based on Statistical Analysis of Data
• 5, 10, 25, 50, 100 Year Events
• Various Durations of 6 to 24 hours

Six Hour Rainfall
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Rainfall Analysis

• Center of Mass of rainfall in time
• T S ti Pi/ S Pi
• Avg intensity
• I S Pi / n

Six Hour Rainfall
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Intensity-Duration-Frequency

• IDF design curves
• All major cities
• Based on NWS data
• Various return periods    durations
• Used for drainage   design of pipes roads
• Used for floodplain   designs - watersheds

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Rainfall Averaging Methods
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Thiessen Polygons - Areal Average Rainfall from
Gages

• Connect gages with lines
• Form triangles as shown
• Create perpendicular      bisectors of the
  triangles
• Each polygon is formed      by lines and WS
       boundary

P S (AiPi) / AT
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II. STREAMFLOW Brays Bayou - Main St
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Typical Streamflow Gage
High Flow
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Brays Bayou - Low Flow
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TS Allison level reached 41.8 ft MSL TMC is at 44
ft Rice Univ is at 50 ft
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Brays Flooding at Loop 610 - 1983
Main Channel
Overbank
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Brays Bayou High Flow
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Bull Creek, Austin - CEVE 412
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Stream Cross-Section for Q

• Measure V (anemometer) at 0.2 and 0.8 of depth
• Average V and multiply by (D width depth)
• Sum up across stream to get total FLOW
• Q S (Vi Di DWi)

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Excess flows cause major damage in New Orleans
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Kissimee River - The Everglades
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I-45 over Clear Creek - 1979
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California - Temecula
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Hurricane Katrina - Most Damaging Storm in U.S.
History
Mississippi
New Orleans 100 billion loss