Hydrology & Stream Stability

1. Hydrology & Stream Stability Hydrologic Studies Unit Land and Water Management

2. OR Why is that Streambank Eroding? Hydrologic Studies Unit Land and Water Management

3. Causes of Streambank Erosion • Natural river dynamics • Sparse vegetative cover due to too much animal or human traffic • Concentrated runoff adjacent to the streambank, i.e. gullies, seepage • An infrequent event, such as an ice jam or low probability flood • Unusually large wave action cont.

4. Causes of streambank erosion • A significant change in the hydrology of the watershed • A change in the stream form impacting adjacent portions of the stream, i.e. dredging, channelization Either of these two causes could produce an unstable stream.

5. Assessing Stream Stability A comprehensive assessment of potential causes of erosion may be necessary so that the proposed solutions will be permanent and do not move the erosion problem to another location. Gabion baskets damaged by high flows

6. Stream Stability is no net change in channel shape and dimensions. Some sediment movement and streambank erosion is natural.

7. Stream Instability causes excessive erosion at many locations throughout a stream reach.

8. Stream Stability A stream's channel morphology - its plan form, dimensions, and profile - develops in response to flood flows. Relatively modest flows, because of their higher frequency, have more effect on channel morphology than extreme flood flows. Flows with a one to two year recurrence interval are generally the dominant channel-forming flows in stable streams. Hydrologic changes that increase these flows can cause the stream to become unstable.

9. Channel-Forming or Effective Discharge

10. The effective discharge is the product of the transport rate of individual storm events and the statistical frequency of each event. For a stable stream, the dominant channel forming flow is the effective discharge and occurs, on average, about every 1.5 years.

11. Changes within a watershed can increase peak flows, increase total runoff volume, and reduce the lag time. Note: Volume changes are not indicated in this illustration.

12. Stream Stability • Increasing discharge generally increases both velocity and depth, and both increase energy. Energy increases with the square of velocity and linearly with depth. • If the velocity is doubled, water can carry particles 64 times as large. Discharge = Velocity x Depth x Width Energy = V2/2g +p/g + h

13. Stability Indicators • Field survey • evaluate extent of erosion • compare historical land use to current conditions • check for other causes (foot traffic, boat wakes) • anecdotal information • Comparison of aerial photos • land use changes • stream channel movement • Gage Analysis • Hydrologic Study

14. Gage Analysis Peak flows since 1930 that exceed the 67% (1.5 year) flow. 4712 cfs 67% (1.5-year), 6368 cfs 50% (2-year), 10397 cfs 10% (10-year)

15. Gage Analysis

16. What is a Hydrologic Study? 1. Analysis of possible changes in the parameters that determine the volume, rate, and timing of surface runoff. Estimate values for applicable parameters. 2. Calculate the impact of identified changes. Modeling may be helpful. 3. Evaluate the meaning of the results.

17. Parameters That Affect Discharge • Antecedent moisture • Snow melt • Frozen ground • Spatial extent of storm • Watershed size (delineation) • Ease of water movement (Time of concentration) • Soils • Land use

18. Watershed Delineation • Doesn’t usually change • But ...

19. Ryerson Creek Initial Delineation Final Delineation ~15% area increase

20. Time of Concentration • Time for runoff (wave) to travel from the hydraulically most distant point of the watershed • Decreases with channelization, addition of drains, pavement

21. Soils • Don’t usually change - possible exceptions: clay caps, significant excavations, or fills

22. Land Use • The most likely cause of hydrologic change. • In 1954, SCS developed the runoff curve number technique to evaluate surface runoff based on land use and soils information. It is the procedure most frequently used by hydrologists nationwide to estimate surface runoff from ungaged watersheds.

23. Selected Curve Numbers

24. Curve Numbers (cont.) Curve numbers are not a runoff percentage. SRO = (P-0.2S)2/(P+0.8S) S = (1000/CN) - 10

25. Internet address for more information.

26. Modeling Purposes: • To estimate changes in discharge volumes, peaks, and timing due to changing hydrology • To estimate the effectiveness or size of added detention • Cannot demonstrate river stability, although may indicate instability

27. HMS Modeling - Data Needed • Soils • Land use: historical, current, future • Energy slope of river reaches (can be estimated) • Detention storage-discharge relationship

28. Model, Detention Added

29. Sample of model results. 100-Year Storm at C&O, No Detention

30. Sample of model results. 100-Year Storm at C&O, No Detention compared to 2360 Acre-Feet of Detention

31. Examples

32. Pine River Sometimes the cause of the erosion is obvious. No further analysis was needed in this case.

33. This outlet of this detention pond did not detain water. No hydrologic study was required since the detention pond was sized for the development. Schoolhouse Creek

34. Plaster Creek A larger detention pond was proposed for this site. Field observation of existing land use indicated that land use in the watershed had not changed in the past 22 years. No further hydrologic study was required.

35. Sprong Lake Inlet This stream flows through a culvert under a road, makes a right angle turn, and then flows into the lake a few hundred feet downstream. Homes could be threatened by continued erosion at this bend. Field observations indicated stable land use in the watershed. Stabilization of this streambank will protect nearby homes. No further hydrologic study was required.

36. Ryerson Creek Land use comparison

37. Ryerson Creek, Holland Drain: Projected Peak Flows (cfs) 1978 1997 Build-out 50% (2-Year) 26 35 84 10% (10-Year) 67 82 149 1% (100-Year) 143 164 250 Dramatic increases in peak flows are predicted for the upper watershed unless appropriate BMP’s are utilized to compensate for continuing development.

38. This is an unstable stream with extensive erosion all along the banks, which was not caused by a low frequency flood. The erosion is worse in some areas due to heavy foot traffic. A hydrologic study, incorporating modeling was conducted to help select the appropriate remediation techniques. Hager Creek

39. Hager Creek The detention areas due not appear to detain the 50% flows.

40. Pine River Tributary This stream may be impacted by increased runoff from new development along the edge of a city, as well as loss of floodplain due to filling. Further hydrologic analysis would be helpful to ascertain this.

41. Bear Creek The property owner stated that 30 feet of stream bank has eroded. Anecdotal observations can be valuable.

42. Bear Creek This erosion may be caused by flow diverting around debris or ice periodically piling against the former bridge supports. Nearby streambanks are stable. Removal of the former supports may eliminate the cause of the erosion at this site.

43. East Branch AuGres River This erosion is caused by the diversion of approximately fourteen miles of natural stream through three miles of straight channel. A limited hydrologic study was conducted.

44. Photo by John McColgan