Transcript ENVI 412 GROUNDWATER

Introduction to
Rainfall & Streamflow
Philip B. Bedient
January, 2006
Watershed - Elevation Contours
Water flows at right angles to elevation contours
and from higher to lower elevations
Texas River Basins
Red
Trinity
Colorado
Rio Grande
Brazos
San Jacinto
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
Major Thunderstorm
Thunderstorm cell with lightning
Characterized by updrafts and downdrafts
Strong convergence and divergence
Most intense rainfall possible
 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 Ivan’s 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.
Hurricane Ivan
September, 2004
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
Lifting Mechanisms
Fronts and Low Pressure
• Cold/Warm Front
• Lifting/Condensation
• High and Low Pres
• Rainfall Zone
• Circulation Issues
• Main weather makers
Warm
Cold
Winds increase as cold front approaches
Major Storm Damages
• Intense Rainfalls
• 8 to 15 inches
• Severe Flooding
• Billion$ damages
• Lost productivity
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
Hurricane Katrina in the
Gulf - 8/29/2005
Katrina 08-28-2005 at 17:00 UTC
Measuring Rainfall - Tipping
Bucket
• Recording gage
• Collector and Funnel
• Bucket and Recorder
• Accurate to .01 ft
• Telemetry- computer
• HCOEM website
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
Monthly Rainfall Distribution
Average Annual Precipitation
9-Hour Total Rainfall - TS Allison
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
Mass Curves &
Rainfall Hyetographs
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
Rainfall Analysis
Center of Mass of
rainfall in time
T = S ti Pi/ S Pi
Avg intensity
I = S Pi / n
Six Hour Rainfall
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
Rainfall Averaging Methods
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
P = S (Ai*Pi) / AT
WS
by lines and
boundary
II. STREAMFLOW
Brays Bayou - Main St
Typical Streamflow Gage
High Flow
Brays Bayou - Low Flow
TS Allison level reached 41.8 ft MSL
TMC is at 44 ft & Rice Univ is at 50 ft
Brays Flooding at Loop 610 - 1983
Main Channel
Overbank
Bull Creek, Austin - CEVE 412
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)
Instantaneous Hydrograph
• Rainfall falls over the basin
Ii
• Intensity I reaches the outlet response based on travel time
Small Basin
• Produces a total storm
response hydrograph as shown
• Little delay and no storage
Qi = Ii A
• The above only occurs in
small urban basins or parking
lots
Unit Hydrograph (UH) Method
• Linear transform method
Pi
• Converts complex rainfall
Uj
to streamflow at outlet
• Produces a total storm
hydrograph from given
UH
• Used in complex watersheds
Q
T
• Each subarea is unique
• Storage effects considered
Qn = Pn U1 + Pn-1 U2 + Pn-2 U3 + … +P1 Uj
Time-Area
Method
• Watershed travel times
• Time Area Graph
• Rainfall Intensities
• Add and Lag Method
• Resulting Hydrograph
Time Area Hydrograph
Peak Flow at Q2
• Q1 = P1 * A1
• Q2 = P2*A1 + P1*A2
• Q3 =P3*A1 + P2*A2
+
P1*A3
• And So Forth
At time step 2, P2 over A1
arrive at the same time -
and P1 over A2
Similar for time step 3 with 3 contributors
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)
Hydrograph - Watershed Flow
Response to Rainfall
 Peak Flow and time to
Peak Flow
Outflow
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
Hydrograph
RF
Volume of Runoff
DRO
Time Base
Time
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)
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**
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
Hydrograph Flood Routing to
Next Downstream Location
Crest
1
Rising
Limb
Falling
Limb
2
Recession
Time Base of Hydrograph
Flood wave is lagged and attenuated as it moves downstream
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Excess
flows
cause
major
damage in
New
Orleans
Brays Bayou
High Flow
Kissimee River - The Everglades
I-45 over Clear Creek - 1979
California - Temecula
Hurricane Katrina - Most
Damaging Storm in U.S. History
Mississippi
New Orleans
$100 billion loss