1. Introduction to Hydrologic Processes - Rainfall & Streamflow - 2004 Dr. Philip B. Bedient Civil and Environmental Eng Rice University

2. Important hydrologic characteristic Elongated Shape Concentrated Shape Affects Timing and Peak Flow Determined by geo - morphology of stream Watershed Shapes

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

4. Subareas - divided according to topography and hydrology Outlet Sub A

5. Texas River Basins Hydrologic features with several different types of flow processes Red Trinity Brazos Rio Grande Colorado San Jacinto

6. Precipitation Water on Surface Overland Flow Channel Flow The Hydrologic Cycle Ground Water Ground Water Flow Ocean Reservoir

7. Sources of Rainfall Severe Storms - Convective Cells Severe Storms - Convective Cells Low Pressure Systems - Hurricanes Low Pressure Systems - 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

8. Thunderstorm cell with lightning Thunderstorm cell with lightning Characterized by updrafts and downdraftsCharacterized by updrafts and downdrafts Strong convergence and divergenceStrong convergence and divergence

9. 1. 1. Orographic lifting over mountain ranges 2. 2. Convective heating at or near surface - summer 3. 3. Frontal systems and buoyancy effects - winter Causes of Precipitation

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

11. 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

12. Average Annual Precipitation

13. 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

14. Mass Curves & Rainfall Hyetographs

15. 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

16. Tipping Bucket Rain Gage Recording gage Collector and Funnel Bucket and Recorder Accurate to.01 ft Telemetry- computer HCOEM website

17. 9-Hour Total Rainfall - TS Allison

18. 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

19. 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

20. T.S. Allison – Radar Data 1 a.m. 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

21. T.S. ALLISON RADAR RAINFALL OVER BRAYS BAYOU WATERSHED 12 HOUR TOTALS BY SUBAREA

22. June 8-9, 2001 T.S. Allison Storm Total June 8-9, 2001 26.6 in

23. Connect gages with lines Form triangles as shown Create perpendicular bisectors of the triangles Each polygon is formed by lines and WS boundary P =  (A i *P i ) / A Thiessen Polygons - Avg P

24. Gage Averaging Methods Arithmetic Thiessen Polygon Isohyetal Contours

25. 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

26. Horton’s Infiltration Concept f(t) = Rate of water loss into soil f = f c + (f o - f c ) exp (-kt) f c = final rate value f o = initial rate value K = decay rate Can integrate to get F(t) = Vol of infiltration

27. Horton’s Eqn F

28. STREAMFLOW Brays Bayou - Main St

29. Typical Streamflow Gage High Flow

30. Brays Bayou Flooding at Loop 610 Main Channel Overbank

31. 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

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

33. 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)

34. Small Basin Response Rainfall falls over the basin Rainfall reaches 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 IiIi Q i = CI i A Small Basin

35. Rainfall and Runoff Response Rainfall Measured from USGS Gage 400 at Harris Gully Outlet Rainfall Measured from USGS Gage 400 at Harris Gully Outlet February 12, 1997 on Harris Gully Flow Measured from USGS Gage 403 Inside Harris Gully Flow Measured from USGS Gage 403 Inside Harris Gully Net Rainfall * Area = integration of direct runoff hydrograph Vol under blue bars * Area = Volume under red line (hydrograph)

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

37. Time Area Hydrograph Q 1 = P 1 * A 1 Q 2 = P 2 *A 1 + P 1 *A 2 Q 3 =P 3 *A 1 + P 2 *A 2 + P 1 *A 3 And So Forth Peak Flow at Q 3 Each area contributes according to its time of travel and rainfall intensity

38. 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 Hydrograph Volume of Runoff DRO Outflow Time Time Base Peak Flow Lag or time to peak RF

39. Unit Hydrograph (UH) Method T Q 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. PiPi UjUj

40. UH for a Complex Rainfall T Q 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 Q n = P n U 1 + P n-1 U 2 + P n-2 U 3 + … +P 1 U j PiPi UjUj

41. Synthetic UH Methods Methods to characterize ungaged basins - 1938 Use data and relationships developed from gages Variety of approaches but most based on t p and Q p, Where t p = lag time (hr) and Q p = peak flow in cfs

42. Snyder’s UH Method

43. Snyder’s Method 5 to 7 points

44. Hydrograph Convolution 1 2 0.5 12 Add up the ordinates of all three to produce storm hydrograph STORM HYDRO Add and Lag Method

45. Hydrograph Flood Routing to Next Downstream Location Recession Rising Limb Crest Falling Limb Time Base of Hydrograph 1 2 Flood wave is lagged and attenuated as it moves downstream

46. Flood Flows Cause Major Damage

47. The End