Introduction to Hydrologic Processes - Rainfall

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Introduction toHydrologic Processes - Rainfall
Streamflow - 2004

• Dr. Philip B. Bedient
• Civil and Environmental Eng
• Rice University

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Watershed Shapes

• Important hydrologic characteristic
• Elongated Shape
• Concentrated Shape
• Affects Timing and Peak Flow
• Determined by geo - morphology of stream

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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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Subareas - divided according to topography and
hydrology
Sub A
Outlet
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Texas River Basins
Red
Trinity
Colorado
Brazos
Rio Grande
San Jacinto
Hydrologic features with several different types
of flow processes
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Precipitation
Water on Surface
Overland Flow
Channel Flow
Reservoir
Ground Water
Ground Water Flow
Ocean
The Hydrologic Cycle
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Sources of Rainfall

• Severe Storms - Convective Cells
• Low Pressure Systems - Hurricanes
• Frontal Systems - Cold or Warm
• Dew and Fog
• Hail and Ice Storms
• Condensation

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• Thunderstorm cell with lightning
• Characterized by updrafts and downdrafts
• Strong convergence and divergence

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Causes of Precipitation

1. Orographic lifting over mountain ranges
2. Convective heating at or near surface - summer
3. Frontal systems and buoyancy effects - winter

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Fronts and Low Pressure

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

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Track of Hurricane Andrew -1992

• Formed in the Atlantic
• Moved directly to Florida
• Winds in excess of 150    mph
• 25B damage to Florida
• Moved over Gulf and    strengthened and hit LA

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Average Annual Precipitation

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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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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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Tipping Bucket Rain Gage

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

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9-Hour Total Rainfall - TS Allison
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Intensity-Duration-Frequency

• IDF 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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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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T.S. Allison Radar Data
NEXRAD data is measured every 5 min over each
grid cell as storm advances (4 km x 4 km
cells) The radar data can be summed over an area
to provide total rainfall depths
1 a.m.
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T.S. ALLISON RADAR RAINFALL OVER BRAYS BAYOU
WATERSHED 12 HOUR TOTALS BY SUBAREA
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T.S. Allison Storm TotalJune 8-9, 2001
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Thiessen Polygons - Avg P

• 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) / A

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Gage Averaging Methods

• Arithmetic
• Thiessen Polygon
• Isohyetal Contours

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Hortons Infiltration Capacity f
Horton (1933 - 1940) studied the response of
different soils to application of water at
varying rates Rate of rainfall must exceed the
rate of infiltration and antecedent condition is
an important parameter Sand gt Silt gt Clay
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Hortons Infiltration Concept f(t) Rate of
water loss into soil
f fc (fo - fc) exp (-kt) fc final rate
value fo initial rate value K decay rate Can
integrate to get F(t) Vol of infiltration
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Hortons Eqn
F
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STREAMFLOW Brays Bayou - Main St
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Typical Streamflow Gage
High Flow
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Brays Bayou Flooding at Loop 610
Main Channel
Overbank
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Brays Bayou - T.S. Allison in June, TS TS Allison
level reached 41.8 ft MSL TMC is at 44 ft Rice
Univ is at 50 ft
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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 (width depth)
• Sum up across stream to get total Q S (Vi Di
  Wi)

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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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Small Basin Response

• Rainfall falls over the basin
• Rainfall reaches the outlet -      response
  based on travel time
• Produces a total storm response   hydrograph as
  shown
• Some delay and little storage
• The above only occurs in small   urban basins or
  parking lots

Ii
Small Basin
Qi CIi A
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Rainfall and Runoff Response
Flow Measured from USGS Gage 403 Inside Harris
Gully
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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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 Q3

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

Each area contributes according to its time of
travel and rainfall intensity
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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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Unit Hydrograph (UH) Method

• 1 Inch of net rainfall    spread uniformly over
  the    basin
• Response is unique for    that basin and
  duration D
• UH - from measurements
• UH - Synthetic equations
• Still used today for most    watershed studies
  in U.S.

Pi
Uj
Q
T
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UH for a Complex Rainfall

• 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 uniform
• Storage effects considered

Pi
Uj
Q
T
Qn Pn U1 Pn-1 U2 Pn-2 U3 P1 Uj
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Synthetic UH Methods
Methods to characterize ungaged basins - 1938
Use data and relationships developed from
gages Variety of approaches but most based on tp
and Qp, Where tp lag time (hr) and Qp peak
flow in cfs
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Snyders UH Method
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Snyders Method
5 to 7 points
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Hydrograph Convolution
Add and Lag Method
0.5
1
2
1
2
STORM HYDRO
0.5
Add up the ordinates of all three to produce
storm hydrograph
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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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Flood Flows Cause Major Damage
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The End