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. Bull Creek, Austin - CEVE 412

33. 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)

34. 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

35. 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

36. Time-Area Method Watershed travel times Time Area Graph
Rainfall Intensities
Add and Lag Method
Resulting Hydrograph

37. Time Area Hydrograph Q1 = P1 * A1 Q2 = P2*A1 + P1*A2
Peak Flow at Q2
Q1 = P1 * A1
Q2 = P2*A1 + P1*A2
Q3 =P3*A1 + P2*A    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

38. 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)

39. 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

40. 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)

41. 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**

42. 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

43. 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

44. Excess flows cause major damage in New Orleans

45. Brays Bayou High Flow

46. Kissimee River - The Everglades

47. I-45 over Clear Creek

48. California - Temecula

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