1. ENVI 485 02/15/07 INTRO. TO NATURAL HAZARDS & DISASTERS STEAMS AND FLOODING Case Study 1

2. Phase 1: Emergency (days) Normal activities stop Search and rescue Emergency shelter/feeding Capital damaged or destroyed

3. Phase 2: Restoration (weeks-months) Normal activities return but at minimal levels Restoration of urban services Return of refugees Capital patched

4. Phases 3 & 4: Reconstruction I & II (months-years) Normal activities return to predisaster levels Capital rebuilt (replaced) Activities improved and developed Capital improved beyond predisaster levels Disaster preparedness & response improved

6. Effects of Selected Hazards in the U.S.

8. Rivers and floods Stream/river –any body of water that flows in a channel Floods –any high flow of surface waters that overtops normal confinements or covers land normally dry –most devastating of all geologic agents -in loss of life

9. Extent of a floodplain

12. Longitudinal Profile & Gadient

13. Drainage basin and cross sections at the headwater and near base level

14. Sediment in Rivers Stream total load  Bed load: Coarse particles moving along the bottom of river channel  Suspended load: Accounts for about 90% of its total load  Dissolved load: Carried in chemical solutions

15. Show animation

16. Factors in Stream flow Several basic factors control the way a stream behaves: –Gradient [ h/x ] (expressed in meters per kilometers) –Stream-cross-sectional area [A] (width * average depth, expressed in square meters) –Average velocity of water flow [v] (expressed in meters per second) –Discharge [Q] (expressed in cubic meters per second) –Load (expressed in kilograms per cubic meter) Dissolved matter generally does not affect stream behavior

17. Discharge, Velocity, & Channel Shape The relationship of discharge, velocity, and channel shape for a stream can be expressed by the equation: Q = A*V Discharge Cross-sectional Average (m 3 /s) area of stream velocity (width x average (m/s) depth) (m 2 )

19. Flood stage The elevation (in meters, feet, etc.) of the water above normal at the highest point in the flood

20. Stream Gage USGS: 7000 nationwide Measures water level (gage height) every 15 minutes Sends data via satellite every 4 hours “rating curve” converts gage height to discharge Rating curve modifed after technition measures streamflow on site every 6 weeks

21. Measuring Stream Velocity

22. Calculating Discharge from a Stream Gage Velocity X Area = Q

23. Rating curve

24. USGS National Streamflow Program http://water.usgs.gov http://waterdata.usgs.gov/ca/nwis/uv?11023000

27. San Diego River Watershed Two USGS stream gages Fashion Valley: 429 sq. mi

28. Santee Stream Gage

29. 482 Stream gages in CA

36. 1927 Flood Photo taken on February 2, 1927 shows the Old Town railroad bridge washed out by the flood. This rail right- of-way still exists - you can see it looking east from I-5; Friars Rd. runs underneath it.

37. San Diego River 1852 - Since San Diego Bay was a deeper harbor, and the San Diego River carried heavy silt deposits, it was decided to deflect the San Diego River into False Bay (Mission Bay) The project was completed in two years by Indian laborers who reportedly hauled building materials in baskets. The Darby dike washed out one year after its completion and the San Diego River returned to its old course.

38. San Diego River 1862 – Possibly the largest flood in the history of the San Diego River occurred (almost 100,000 cfs). 1875- New dike constructed (cobblestone face two to three feet thick). A small channel was constructed on the north side of the dike that the river was diverted into the eastern part of Mission Bay.

39. River Erosion Erosion types  Abrasion by sediments transported by river  Hydraulic action of moving water  Chemical corrosion Erosion location  Down cutting  Lateral: Concentrating on the outer bends  Headward erosion

40. Meandering River, showing forms and processes

41. Meander on the Colorado River

42. Erosion

43. Koyakuk River, Alaska, showing meander bends, point bar, and cut bank

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46. Braided channels in Granada, southern Spain with multiple channels, steep gradient, and coarse gravel

48. Effects of Land-Use Changes Changes in infiltration rate: Change of the amount of water flowing into a river Soil erosion: Change in the amount of sediments in a river Amount of water and sediments in river: Changes in the velocity of water flow Changes in river’s velocity: Leading the change in river dynamics

49. Effects of Land-Use Changes Forest to farmland  Increases soil erosion, stream deposition  Increases gradient and velocity  Increases river-channel erosion Urban build-up  Increases impervious cover  Increases certain flood frequency  Reduces the lag time of flood

50. Floods In The US

51. Flooding Flooding: Overbank flow condition, discharge greater than channel’s holding capacity Stage: The height of the water level in a river at a given location at a given time Hydrograph: a graph that plots stream discharge (Q) against time (t) Lag time: The amount of time between the occurrence of peak rainfall and the onset of flooding

52. Flood magnitude Recurrence interval –Discharge (Q) on a stream is measured over a period of time (N) –Each flood is ranked (highest discharge = 1) (M) –Recurrence interval: (N + 1)/M Probability of a flood of a given magnitude in a year is 1/recurrence interval

55. Example of a discharge-frequency curve for Patrick River

57. Urban development and flooding Flooding usually increased by urban development –Affected by impervious cover –Storm sewers More water reaches stream Water reaches stream faster Affects the relationship between rainfall-runoff –Reduced lag time = “flashy discharge”

60. Smaller floods are more affected by urbanization than larger floods

61. Mean annual flood: RI = 2.23

64. Effect of dam on erosion

67. Regulation of the Floodplain Floodplain belongs to the river system and the river WILL reoccupy it. Flood hazard mapping –Floodway & floodway fringe district Area of the floodplain covered by a 100 year flood O.k. for some uses

68. Adjustments to Flood Hazards The structural approach  Engineering barriers: Levee augmentation  Channelization  River-channel restoration Flood insurance Flood-proofing

69. Floodplain without and with levees

70. 07_28b Placing riprap to defend the bank

71. Natural vs. channelized stream

72. Concrete channel in LA

73. 07_28a Urban stream restoration by controlling erosion and deposition