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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

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introduction to hydrologic processes rainfall streamflow 2004

Introduction toHydrologic Processes - Rainfall & Streamflow - 2004

Dr. Philip B. Bedient

Civil and Environmental Eng

Rice University

slide2

Watershed Shapes

  • Important hydrologic characteristic
  • Elongated Shape
  • Concentrated Shape
  • Affects Timing and Peak Flow
  • Determined by geo - morphology of stream
slide3

Watershed - Elevation Contours

Water flows at right angles to elevation contours and from higher to lower elevations

texas river basins
Texas River Basins

Red

Trinity

Colorado

Brazos

Rio Grande

San Jacinto

Hydrologic features with several different types of flow processes

slide6

Precipitation

Water on Surface

Overland Flow

Channel

Flow

Reservoir

Ground Water

Ground

Water

Flow

Ocean

The Hydrologic Cycle

slide7

Sources of Rainfall

  • Severe Storms - Convective Cells
  • Low Pressure Systems - Hurricanes
  • Frontal Systems - Cold or Warm
  • Dew and Fog
  • Hail and Ice Storms
  • Condensation
slide8

Thunderstorm cell with lightning

  • Characterized by updrafts and downdrafts
  • Strong convergence and divergence
slide9

Causes of Precipitation

Orographic lifting over mountain ranges

Convective heating at or near surface - summer

Frontal systems and buoyancy effects - winter

slide10

Fronts and Low Pressure

  • Cold/Warm Front
  • Lifting/Condensation
  • High and Low P
  • Rainfall Zone
  • Circulation Issues
  • Main weather    makers
slide11

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
the hyetograph
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

slide15

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
slide16

Tipping Bucket Rain Gage

  • Recording gage
  • Collector and Funnel
  • Bucket and Recorder
  • Accurate to .01 ft
  • Telemetry- computer
  • HCOEM website
slide18

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 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
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.

slide21

T.S. ALLISON RADAR RAINFALL

OVER BRAYS BAYOU WATERSHED

12 HOUR TOTALS BY SUBAREA

slide23

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
slide24

Gage Averaging

Methods

  • Arithmetic
  • Thiessen Polygon
  • Isohyetal Contours
slide25

Horton’s 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 > Silt > Clay

slide26

Horton’s 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

slide28

STREAMFLOW

Brays Bayou - Main St

slide31

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

slide32

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
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
Small Basin Response
  • Rainfall falls over the basin
  • Rainfallreaches 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

rainfall and runoff response
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)

slide36

Time-Area

Method

  • Watershed travel times
  • Time Area Graph
  • Rainfall Intensities
  • Add and Lag Method
  • Resulting Hydrograph
slide37

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
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

unit hydrograph uh method
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

uh for a complex rainfall
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

slide41

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

slide43

Snyder’s Method

5 to 7 points

slide44

Hydrograph Convolution

Add and Lag Method

1

2

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
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