Introduction to Rainfall

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Introduction toRainfall Streamflow

• Philip B. Bedient
• January, 2006

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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
6

• 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

• Source of moisture
• Lifting mechanism (orographic or heating)
• Phase change from vapor to water - Energy
• Small nuclei or dust for droplet formation
• 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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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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Instantaneous Hydrograph

• Rainfall falls over the basin
• Intensity I reaches the outlet -      response
  based on travel time
• Produces a total storm response   hydrograph as
  shown
• Little delay and no storage
• The above only occurs in small   urban basins or
  parking lots

Ii
Small Basin
Qi Ii A
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Unit Hydrograph (UH) Method

• Linear transform method
• Converts complex rainfall to   streamflow at
  outlet
• Produces a total storm   hydrograph from
  given UH
• Used in complex watersheds
• Each subarea is unique
• Storage effects considered

Pi
Uj
Q
T
Qn Pn U1 Pn-1 U2 Pn-2 U3 P1 Uj
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Time-Area Method

• Watershed travel times
• Time Area Graph
• Rainfall Intensities
• Add and Lag Method
• Resulting Hydrograph

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Time Area Hydrograph
Peak Flow at Q2

• Q1 P1 A1
• Q2 P2A1 P1A2
• Q3 P3A1 P2A2    P1A3
• And So Forth

At time step 2, P2 over A1 and P1 over A2
arrive at the same time - Similar for time step
3 with 3 contributors
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The Hydrograph

• Graph of discharge vs. time at a single location
• Rising Limb, Crest Segment, Falling Limb,and
  Recession
• Base Flow is usually subtracted to yield DRO
• Peak gives the maximum flow rate for the event
• Area under curve yields volume of runoff (inches)

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Hydrograph - Watershed Flow Response to Rainfall

• Peak Flow and time to peak relate to area/shape
  of watershed
• Area under curve is the volume of DRO
• Time Base is time that flow exceeds baseflow
• Time to peak or Lag is measured from center of
  mass of rainfall pattern

Lag or time to peak
Peak Flow
Hydrograph
RF
Outflow
Volume of Runoff DRO
Time Base
Time
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Rainfall and Runoff Response
Flow Measured from USGS Gage 403 Inside Harris
Gully
RF
Q
Rainfall Measured from USGS Gage 400 at Harris
Gully Outlet
February 12, 1997 on Harris Gully
Net Rainfall Area integration of direct
runoff hydrograph Vol under blue bars Area
Volume under red line (hydrograph)
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Hydrograph Flood Routing from Point 1 to Point 2
Once overland flow arrives at a stream - becomes
channel flow Channel flow begins as low flow and
increases with accumulation Flood wave moves
downstream at a predictable velocity Subarea
flows contribute to the flood wave along the
stream Wave maximum near outlet

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Hydrograph Flood Routing from Pt 1 to Pt 4 in
the Stream
G and F to Pt 1 Pt 1 to Pt 2 D to Pt 2 Pt 2 to Pt
3 E, C, and B to Pt3 Pt 3 to Pt 4 A to Pt
4 Compute Final hydrograph
1
2
3

4
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Hydrograph Flood Routing to Next Downstream
Location
Crest
1

Falling Limb
Rising Limb
2
Recession
Time Base of Hydrograph
Flood wave is lagged and attenuated as it moves
downstream
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Excess flows cause major damage in New Orleans
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Brays Bayou High Flow
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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