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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. Brazos. Colorado. Rio Grande. San Jacinto.

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Introduction to rainfall streamflow

Introduction toRainfall & Streamflow

Philip B. Bedient

January, 2006


Introduction to rainfall streamflow

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

Brazos

Colorado

Rio Grande

San Jacinto


Introduction to rainfall streamflow

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



Introduction to rainfall streamflow

Thunderstorm cell with lightning

Characterized by updrafts and downdrafts

Strong convergence and divergence

Most intense rainfall possible


Hurricane ivan september 2004
Hurricane IvanSeptember, 2004

  • 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.


Introduction to rainfall streamflow

Formation of Precipitation

Source of moisture

Lifting mechanism (orographic or heating)

Phase change from vapor to water - Energy

Small nuclei or dust for droplet formation

Droplets must grow as they fall to earth



Introduction to rainfall streamflow

Fronts and Low Pressure

  • Cold/Warm Front

  • Lifting/Condensation

  • High and Low Pres

  • Rainfall Zone

  • Circulation Issues

  • Main weather makers


Introduction to rainfall streamflow

Warm

Cold

Winds increase as cold front approaches


Introduction to rainfall streamflow

Major Storm Damages

  • Intense Rainfalls

  • 8 to 15 inches

  • Severe Flooding

  • Billion$ damages

  • Lost productivity


Introduction to rainfall streamflow

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


Introduction to rainfall streamflow

Hurricane Katrina in the Gulf - 8/29/2005

Katrina 08-28-2005 at 17:00 UTC


Introduction to rainfall streamflow

Measuring Rainfall - Tipping

Bucket

  • Recording gage

  • Collector and Funnel

  • Bucket and Recorder

  • Accurate to .01 ft

  • Telemetry- computer

  • HCOEM website


Introduction to rainfall streamflow

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





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



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


Rainfall analysis
Rainfall Analysis

Center of Mass of rainfall in time

T = S ti Pi/ S Pi

Avg intensity

I = S Pi /n

Six Hour Rainfall


Introduction to rainfall streamflow

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



Introduction to rainfall streamflow

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      by lines and WS      boundary

P = S (Ai*Pi) / AT


Introduction to rainfall streamflow

II. STREAMFLOW

Brays Bayou - Main St




Introduction to rainfall streamflow

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
Brays Flooding at Loop 610 - 1983

Main Channel

Overbank



Introduction to rainfall streamflow

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
Instantaneous Hydrograph

  • Rainfall falls over the basin

  • Intensity I reaches the outlet -      response based on travel time

  • Produces a total storm response   hydrograph as shown

  • Little delay and no storage

  • The above only occurs in small   urban basins or parking lots

Ii

Small Basin

Qi = Ii A


Unit hydrograph uh method
Unit Hydrograph (UH) Method

  • 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 unique

  • Storage effects considered

Pi

Uj

Q

T

Qn = Pn U1 + Pn-1 U2 + Pn-2 U3 + … +P1 Uj


Introduction to rainfall streamflow

Time-Area

Method

  • Watershed travel times

  • Time Area Graph

  • Rainfall Intensities

  • Add and Lag Method

  • Resulting Hydrograph


Introduction to rainfall streamflow

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 and P1 over A2 arrive at the same time -

Similar for time step 3 with 3 contributors


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)


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


Rainfall and runoff response
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
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
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
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







Hurricane katrina most damaging storm in u s history
Hurricane Katrina - Most Damaging Storm in U.S. History

Mississippi

New Orleans

$100 billion loss