San Diego River Management: History, Floods and Urban Impact

1. ENVI 485 02/20/07 • STEAMS AND FLOODING (cont.) • MASS WASTING

2. 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.

3. 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.

4. 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.

12. Rainiest years in San Diego history • 1. 1883-84    25.97 • 2. 1940-41   24.74 • 3. 1977-78   18.71 • 4. 1921-22   18.65 • 5. 2004-05    22.81

19. Meandering River, showing forms and processes

20. Meander on the Colorado River

21. Erosion

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

23. Show animation

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

29. Floods In The US

31. 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

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

36. 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”

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

40. Mean annual flood: RI = 2.23

43. Effect of dam on erosion

44. 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

46. Floodplain without and with levees

47. 07_28b Placing riprap to defend the bank

48. Natural vs. channelized stream

49. Concrete channel in LA

50. 07_28a Urban stream restoration by controlling erosion and deposition