1. Introduction to Rainfall & Streamflow
Philip B. Bedient
January, 2007

2. Watershed - Elevation Contours
Water flows at right angles to elevation contours and from higher to lower elevations

3. Texas River Basins
Red
Trinity
Brazos
Colorado
Rio Grande
San Jacinto

4. 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

5. Major Thunderstorm

6. Thunderstorm cell with lightning
Characterized by updrafts and downdrafts
Strong convergence and divergence
Most intense rainfall possible

7. Hurricane Ivan September, 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.
Hurricane Ivan September, 2004

8. 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

9. Lifting Mechanisms

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

11. Warm
Cold
Winds increase as cold front approaches

12. Major Storm Damages Intense Rainfalls 8 to 15 inches Severe Flooding
Billion$ damages
Lost productivity

13. 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

14. Hurricane Katrina in the Gulf - 8/29/2005
Katrina at 17:00 UTC

15. Measuring Rainfall - Tipping
Bucket
Recording gage
Collector and Funnel
Bucket and Recorder
Accurate to .01 ft
Telemetry- computer
HCOEM website

16. 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 world rainfall records

17. Monthly Rainfall Distribution

18. Average Annual Precipitation

19. 9-Hour Total Rainfall - TS Allison

20. 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

21. Mass Curves & Rainfall Hyetographs

22. 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

23. Rainfall Analysis Center of Mass of rainfall in time T = S ti Pi/ S Pi
Avg intensity
I = S Pi / n
Six Hour Rainfall

24. 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

25. Rainfall Averaging Methods

26. 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

27. II. STREAMFLOW
Brays Bayou - Main St

28. Typical Streamflow Gage
High Flow

29. Brays Bayou - Low Flow

30. TMC is at 44 ft & Rice Univ is at 50 ft
TS Allison level reached 41.8 ft MSL
TMC is at 44 ft & Rice Univ is at 50 ft

31. Brays Flooding at Loop 610 - 1983
Main Channel
Overbank

32. Brays Bayou High Flow

33. Bull Creek, Austin - CEVE 412

34. 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)

35. Excess flows cause major damage in New Orleans

36. Kissimee River - The Everglades

37. I-45 over Clear Creek

38. California - Temecula

39. Hurricane Katrina - Most Damaging Storm in U.S. History
Mississippi
New Orleans
$100 billion loss