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 Rio Grande Colorado San Jacinto

4. I. Mechanisms of Rainfall Convective Storms - Radiational Convective Storms - Radiational Low Pressure Systems - tropical storms and hurricanes Low Pressure Systems - tropical storms and hurricanes Frontal Systems - Cold or Warm Frontal Systems - Cold or Warm Dew and Fog Dew and Fog Hail and Ice Storms Hail and Ice Storms Condensation 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.

8. 1. 1. Source of moisture 2. 2. Lifting mechanism (orographic or heating) 3. 3. Phase change from vapor to water - Energy 4. 4. Small nuclei or dust for droplet formation 5. 5. Droplets must grow as they fall to earth Formation of Precipitation

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. Cold Warm 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 08-28-2005 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. 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 Largest One Day U.S. Total Rainfall

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 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 =  t i P i /  P i Avg intensity I =  P i / 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. Connect gages with lines Form triangles as shown Create perpendicular bisectors of the triangles Each polygon is formed by lines and WS boundary Thiessen Polygons - Areal Average Rainfall from Gages P =  (A i *P i ) / A T

27. II. STREAMFLOW Brays Bayou - Main St

28. Typical Streamflow Gage High Flow

29. Brays Bayou - Low Flow

30. 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. 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 FLOW Q =  (V i D i  W i ) Stream Cross-Section for Q

35. Excess flows cause major damage in New Orleans

36. Kissimee River - The Everglades

37. I-45 over Clear Creek - 1979

38. California - Temecula

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