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Introduction to Hydrologic Processes Rainfall

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Rice University. Important hydrologic characteristic. Elongated Shape ... Formed in the Atlantic. Moved directly to Florida. Winds in excess of 150 mph ... – PowerPoint PPT presentation

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Title: Introduction to Hydrologic Processes Rainfall


1
Introduction to Hydrologic Processes - Rainfall
Streamflow - 2004
  • Dr. Philip B. Bedient
  • Civil and Environmental Eng
  • Rice University

2
Watershed Shapes
  • Important hydrologic characteristic
  • Elongated Shape
  • Concentrated Shape
  • Affects Timing and Peak Flow
  • Determined by geo - morphology of stream

3
Watershed - Elevation Contours
Water flows at right angles to elevation contours
and from higher to lower elevations
4
Subareas - divided according to topography and
hydrology
Sub A
Outlet
5
Texas River Basins
Red
Trinity
Colorado
Brazos
Rio Grande
San Jacinto
Hydrologic features with several different types
of flow processes
6
Precipitation
Water on Surface
Overland Flow
Channel Flow
Reservoir
Ground Water
Ground Water Flow
Ocean
The Hydrologic Cycle
7
Sources of Rainfall
  • Severe Storms - Convective Cells
  • Low Pressure Systems - Hurricanes
  • Frontal Systems - Cold or Warm
  • Dew and Fog
  • Hail and Ice Storms
  • Condensation

8
  • Thunderstorm cell with lightning
  • Characterized by updrafts and downdrafts
  • Strong convergence and divergence

9
Causes of Precipitation
  • Orographic lifting over mountain ranges
  • Convective heating at or near surface - summer
  • Frontal systems and buoyancy effects - winter

10
Fronts and Low Pressure
  • Cold/Warm Front
  • Lifting/Condensation
  • High and Low P
  • Rainfall Zone
  • Circulation Issues
  • Main weather    makers

11
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

12
Average Annual Precipitation

13
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

14
Mass Curves Rainfall Hyetographs
15
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

16
Tipping Bucket Rain Gage
  • Recording gage
  • Collector and Funnel
  • Bucket and Recorder
  • Accurate to .01 ft
  • Telemetry- computer
  • HCOEM website

17
9-Hour Total Rainfall - TS Allison
18
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

19
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
20
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.
21
T.S. ALLISON RADAR RAINFALL OVER BRAYS BAYOU
WATERSHED 12 HOUR TOTALS BY SUBAREA
22
T.S. Allison Storm Total June 8-9, 2001
23
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

24
Gage Averaging Methods
  • Arithmetic
  • Thiessen Polygon
  • Isohyetal Contours

25
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
26
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
27
Hortons Eqn
F
28
STREAMFLOW Brays Bayou - Main St
29
Typical Streamflow Gage
High Flow
30
Brays Bayou Flooding at Loop 610
Main Channel
Overbank
31
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
32
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)

33
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)

34
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
35
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)
36
Time-Area Method
  • Watershed travel times
  • Time Area Graph
  • Rainfall Intensities
  • Add and Lag Method
  • Resulting Hydrograph

37
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
38
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
39
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
40
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
41
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
42
Snyders UH Method
43
Snyders Method
5 to 7 points
44
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
45
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
46
Flood Flows Cause Major Damage
47
The End
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