Transcript ENVI 412 GROUNDWATER

Introduction to
Hydrologic Processes Rainfall & Streamflow - 2004
Dr. Philip B. Bedient
Civil and Environmental Eng
Rice University
Watershed Shapes
• Important hydrologic
characteristic
• Elongated Shape
• Concentrated Shape
• Affects Timing and
Peak Flow
• Determined by geo morphology of stream
Watershed - Elevation Contours
Water flows at right angles to elevation contours
and from higher to lower elevations
Subareas - divided according
to topography and hydrology
Sub A
Outlet
Texas River Basins
Red
Trinity
Colorado
Brazos
Rio Grande
San Jacinto
Hydrologic features with several different types of flow processes
Precipitation
Water on Surface
Overland Flow
Channel
Flow
Ground Water
Reservoir
Ground
Water
Flow
Ocean
The Hydrologic Cycle
Sources of Rainfall
• Severe Storms - Convective Cells
•
•
•
•
•
Low Pressure Systems - Hurricanes
Frontal Systems - Cold or Warm
Dew and Fog
Hail and Ice Storms
Condensation
•
Thunderstorm cell with lightning
•
Characterized by updrafts and downdrafts
•
Strong convergence and divergence
Causes of Precipitation
1. Orographic lifting over mountain ranges
2. Convective heating at or near surface - summer
3. Frontal systems and buoyancy effects - winter
Fronts and Low Pressure
• Cold/Warm Front
• Lifting/Condensation
• High and Low P
• Rainfall Zone
• Circulation Issues
• Main
weather
makers
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
hit LA
strengthened and
Average Annual Precipitation
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
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
Tipping Bucket Rain Gage
• Recording gage
• Collector and Funnel
• Bucket and Recorder
• Accurate to .01 ft
• Telemetry- computer
• HCOEM website
9-Hour Total Rainfall - TS Allison
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
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
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.
T.S. ALLISON RADAR RAINFALL
OVER BRAYS BAYOU WATERSHED
12 HOUR TOTALS BY SUBAREA
T.S. Allison Storm Total
June 8-9, 2001
,.-
45
26.6 in
Bayous
Counties
Highways
Drainage
Ú
Ê
TMC
Storm Total (in)
.,-
10
0.01 - 0.25
0.25 - 0.5
Ú
Ê
0.5 - 1
1-2
2-4
4-6
6-8
8 - 10
10 - 12
12 - 14
,.-
14 - 16
16 - 18
59
18 - 20
20 - 22
N
22 - 25
> 25
0
5
10 Miles
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 (Ai*Pi) / A
Gage Averaging
Methods
• Arithmetic
• Thiessen Polygon
• Isohyetal Contours
Horton’s Infiltration Capacity
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 > Silt > Clay
Horton’s Infiltration Concep
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
Horton’s Eqn
Vol  Area  fdt  A  [ f c  ( f 0  f c )ekt ]dt
F

STREAMFLOW
Brays Bayou - Main St
Typical Streamflow Gage
High Flow
Brays Bayou Flooding at Loop 610
Main Channel
Overbank
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
1.49
Q
AR2 / 3 S1/ 2
n
Where R  A / P and S  Slope
n  Manning' s roughness coeff
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)
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)
Small Basin Response
• Rainfall falls over the basin
Ii
• Rainfall reaches the outlet response based on travel time
Small Basin
• Produces a total storm
response hydrograph as shown
• Some delay and little storage
Qi = CIi A
• The above only occurs in
small urban basins or parking
lots
Rainfall and Runoff Response
2000
Flow Measured
from USGS Gage 403
Inside Harris Gully
0.60
1800
0.50
1600
1400
1200
1000
0.30
800
0.20
600
Rainfall Measured
from USGS Gage 400
at Harris Gully Outlet
400
0.10
200
0
6:00
8:00
10:00
12:00
14:00
16:00
18:00
20:00
22:00
0.00
0:00
Time
February 12, 1997 on Harris Gully
Net Rainfall * Area = integration of direct runoff hydrograph
Vol under blue bars * Area = Volume under red line (hydrograph)
Rainfall (in)
Outflow (cfs)
0.40
Time-Area
Method
• Watershed travel times
• Time Area Graph
• Rainfall Intensities
• Add and Lag Method
• Resulting Hydrograph
Time Area Hydrograph
Peak Flow at Q3
• Q1 = P1 * A1
• Q2 = P2*A1 + P1*A2
• Q3 =P3*A1 + P2*A2
+
P1*A3
• And So Forth
Each area contributes according to its
time of travel and rainfall intensity
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
Unit Hydrograph (UH) Method
• 1 Inch of net
Pi
rainfall spread uniformly
Uj
over the basin
• Response is unique
for that basin and duration
D
• UH - from measurements
Q
T
• UH - Synthetic equations
• Still used today for
most
U.S.
watershed studies in
UH for a Complex Rainfall
• 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 uniform
• Storage effects considered
Qn = Pn U1 + Pn-1 U2 + Pn-2 U3 + … +P1 Uj
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
Snyder’s UH Method
t p  Ct (LLc )
0.3
Qp  640C p (A /t p )
TB  3 to 5 tim es t p
Duration D  t p /5.5
Snyder’s Method
5 to 7 points
Hydrograph Convolution
1
2
Add and Lag Method
0.5
1
2
STORM
HYDRO
0.5
Add up the ordinates of all three to produce storm hydrograph
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
Flood Flows Cause Major
Damage
The End