Ways To Identify Background Verses Accelerated Erosion

Transcription

1 Ways To Identify Background Verses Accelerated Erosion Establish Background Condition From Old Ground Photos, Aerial Photos, and Maps Compare Rate Over Time At the Same Location, or for Reaches

2 Channel and Bar Width and Active Erosion Length (aerial photo and cross-section) section) BAR RIVER Length DON. ft. ft. ft. ft. ft. ft. WILSON BARKER WILSON TANN. N. WILSON DILL KILCHIS L. GOM KILCHIS M. GOM KILCHIS

3 Average Erosion Rates BAR RATE RATE RATE RATE RATE ft./yr. ft./yr. ft./yr. ft./yr. ft./yr. DONALD BARKER DILL L. GMS M. GMS

4 Accelerated Cont. Recognizing Stages II, III and IV in CEM Headcuts

5 Echo Creek, UT Stage II after Straightening to accommodate Highway

6 Echo Creek, UT End of Bridge Apron

7 Echo Creek UT Downstream at Stage III to IV Trying to Meander & Develop F. P.

8 Accelerated Erosion Cont. Scour Under Tree Roots Increases Significantly from Year to Year Trees Actively Slumping Into Stream No Sediment Deposits (scour regime) Vertical Side Slope Sloughed Material Readily Removed Slumping Trees FIASC

9 Vertical Side Slope and No Sediment at Base of Slope Miami River OR

10 Accelerated Erosion Cont. Cultural Features Exposed (later) Ground Photograph Comparison (later) Bank Pins (later) Will Stream Transport Sediment Load (transport limited and easier to erode streambank than transport coarse sediment load)

11 What are the Reach Specific Mass Failure and Fluvial Causes of Streambank Erosion These Tend to Work in Combination with Each-Other

12 Mass Failure-Flow Flow Causes of Streambank Erosion Bank Materials (composition and stratigraphy), Height, Overhang, High Pore Pressure, Seepage Forces, Liquefaction, Upwelling Flow, Rotational and Planer Failure. Boundary Shear Stress or Tractive Stress (especially washing out soft layers or matrix material in gravels)

13 How are Rates of Streambank Erosion Determined Aerial Photograph Comparison

14 South Fork Stillaguamish 1963 &2003

15

16 Measurement of bank profiles. Real-time validation of bank loss (recession). Considered validation. Add representative streambank distance on profile to derive volume. If representation is in question, add more bank profiles!

17 Bank Pins

18 South Fork Stillaguamish Bank Loss Pins and Cultural Features

19 Rates Cont. Reference to cultural or known features and or ground photographs comparison.

20 Cultural Features S.F. Stillagaumish

21 Cultural Features Stillaguamish

22 Cultural Features Little Washougal, WA

23 Cultural Feature Right of Way Fence Stage III

24 VARIOUS RATING PROCEDURES ROSGEN (Bank Erosion Hazard Index, see Section Six in Binder) RECKENDORF (Erosion Rating Index) RECKENDORF RAPFAHRS (Rapid( Assessment Procedure For Aquatic Habitat, Riparian and Stream Bank see Section Six in Binder) RAPFAHRS (example in Walla Walla Basin, WA)

25 Data Needs FLUVIAL-GEOMORPHIC DATA ARE NEEDED TO EVALUATE: STREAMBANK EROSION PROBLEMS, AND SOLUTIONS

26 Fluvial-Geomorphic Data Needed Erosion Rate Establish Accelerated Erosion Bankfull Discharge Gage Data (peak verses mean daily) Regional Curves (Leopold Example) Manning Equation Continuity Equation Regional Equat.. like USGS, & ORWRD Establish Bankfull indicators to Determine Width, Depth and Area, and Estimate Velocity

27 Manning s equation v n 1 2 Where: r s A r = = WP s = ft ft ft ft 2 hydraulic radius rise divided by run n = mannings roughness coefficient n, which is dimensionless Q = a v Where: a = area in ft 2 v = velocity in fps

28 What Is Bankfull By definition, the flow in a stream goes over bank and floods when it exceeds bankfull. Leopold defined as the first flat depositional surface formed by the stream and flooded by the stream at a frequent interval The recurrence interval for bankfull flow is very frequent, and has been reported in the literature to have between a 1.0 and 3.0 year average recurrence interval.

29 Bankfull Cont. Recent studies in the Northwest indicate that the bankfull frequency is between 1.0 and 1.5 years in average recurrence interval. At the 1.5 year average recurrence interval, a stream gets out of bank two out of three years, or with a 67% chance that the stream will get out of bank in any one year.

30 Bankfull Cont. The bankfull surface of a stream is not always is not always the top of a streams bank, especially where there are multiple flood plains and terrace surfaces (to be illustrated later)

31 Bankfull Cont. Do to natural and anthropogenic caused downcutting and widening the stream can become entrenched and contain flood events within its banks. In rare situations (do to deteriorated watershed conditions) streams can become entrenched because of excessive sedimentation of the flood plain (see Trimble, 1981, Southwestern Wisconsin).

32 Six Mile Crk, Fritz Durst Farm, Dane Co, WI (38 mi 2 ) Bankfull indicators above water surface: lowest encroachment of woody vegetation & washed

33 Six Mile Creek, Fritz Durst Farm, Dane Co, WI (38 mi 2 ) Data From Two Different Classes Average bankfull channel dimensions: W = 23 D = 2.3 W/D = 10 ER >2.2 Rosgen E4/5 & CEM Stage I W = 24, D = 2.4 W/D = 10 ER >2.2 Rosgen E4/5 & CEM Stage I

34 Yahara River, Deforest, Dane Co, WI (47 mi 2 ) slope break & 1st flat, depositional surface = bankfull surface 10/16/01

35 Yahara River, Deforest, Dane Co, WI (47 mi 2 ) Twice Max. Bankfull Depth upland surface (#4) abandoned floodplain surface (#3) flood prone surface (#2) bankfull surface (Surface #1)

36 Hydraulic Geometry (Bankfull Channel Dimensions) Madison, WI Vicinity (based on field measurements made 10/16-18 and 10/23-25/01) Bankfull Q (Mannings) y = x 0.52 R 2 = Width, Depth, Area, Bankfull Q (feet, feet, square feet, cfs) Cross Section Area y = x 0.55 R 2 = y = x 0.22 R 2 = Depth Width y = x 0.32 R 2 = Drainage Area (square miles) Sample Regional Curve for Streams Near Madison, WI

37 Bankfull Discharge Leopold Regional Curve

38 Regional Bankfull Dimensions Leopold (1978)

39 Regional Curve WI Hydraulic Geometry (Bankfull Channel Dimensions) Madison, WI Vicinity (based on fi 10/16-18 and 10/23-25/01) Bankfull Q (Mann y = x R 2 = Cross Section A y = x R 2 = Widt y = x R 2 = y = x R 2 = Dept Drainage Area (square

40 Regional Curve in Arizona

41 BANKFULL FIELD EVIDENCE OR INDICATORS 1. The first flat depositional surface above the active channel. 2. The lowest extent of woody vegetation along the edge of the streambank. 3. The top of the zone of washed roots (roots exposed in the bank)

42 BANKFULL INDICATORS CONTINUED 4. A break in the slope angle of the streambank. 5. The tops of the point bars or other sediment deposits (This is typically considered the lowest elevation to be considered as potentially the bankfull surface.) 6. The average elevation of the highest surface of the channel bars.

43 BANKFULL INDICATORS CONTINUED 7. The elevation of the upper limit of sand sized particles in the boundary sediment. 8. The elevation at which the width/depth ratio of the cross-section section is a minimum (requires plotting cross-section). section). 9. The elevation corresponding to the change in the relation of cross-sectional sectional area to top width. (from graph) 10. The elevation of float debris if used in combination with other features. 11. The area of washed rock if used in combination with the six other features.

44 Bankfull Top of Washed Root Zone

45 Bankfull Flood Plain

46 Fluvial-Geomorphic Data Needs (Continued) Other Appropriate Discharges and Frequencies (Applegate River example where needed to know effects of a specific flood) Discharge Distribution HEC-RAS

47 Fluvial Geomorphic Conditions Continued Bankfull Width (see cartoon example) Bankfull Depth (see cartoon example) Entrenchment Ratio Stream Slope (see cartoon example) Stream Sinuosity (see formula and example)

48 Bankfull Width and Depth Ratio & Entrenchment Ratio

49 Stream Slope

50 Sinuosity

51 Echo Cr. Sinuosity

52 Fluvial Geomorphic Conditions Continued Stream Bed Material Class (see Section Eight in Binder, and photographs) Rosgen Stream Classification System (see Section Seven, and Key to Classification)

53 Stream Geometry a = amplitude λ = a= wavelength b = beltwidth r = radius of curvature w = bankfull width at inflection point z = arc length (stream length between successive inflection points, i

54 Bed Material Class see Sec. 8 Alternative is Gravelometer for particle class to get d50

55 Rosgen Stream Classification

56 Rosgen Stream Classification

57 CONDITIONS CONT. Streambank Materials Streambank Stratigraphy Streambank Height Stream Side-slope CEM Streambank Stability Class Describe Mass Failure Observed (planer or rotational; preferential and or pop out) Streambank Stability Sampling for Soil Mechanics Analysis (? Rare Case) Stream Planform Condition (examples)

58 Stream Planform Montgomery and Buffington

59 5 Stages of CEM (Schumm et al) (Figure 1.1 from Schumm et al, 1981)

60 M & B Planform Types

61 M & B Interpretations

62 M & B Example Interpret.

63 CONDITIONS CONT. Vegetation (native and exotic examples) Vegetation Overhang (examples) Tree Overhang Tree Throw (examples) Backchannels (example) Upstream and Downstream Sedimentation LWD in Channel and on Bars

64 Conclusions Field data used for stream classification are most useful if it is used in both evaluation and in design. This is true for both Rosgen and CEM (Schumm et al. & Simon) stream classification systems.

65 Conclusions (Continued) The Rosgen, Schumm et al. (& Simon) and Montgomery and Buffington classification systems are complimentary even though they use different terminology. All three systems are broadly applicable.

66 Conclusions (continued) The Rosgen Stream Classification System is the most practical and applicable stream classification ever developed. It has no equal in terms of communication of what a stream looks like. However people do need training in its use particularly in how to identify bankfull stage for various stream types. Regional curves can be developed for bankfull depth, width, cross-sectional sectional area and discharge. Regional bankfull curves are and excellent tool to establish depositional features and their associated bankfull indicators in the field

67 Conclusions (Continued) The Rosgen Stream Classification System reflects both morphology and process. Rosgen s stream classification system is based on geomorphic measurements and indicators taken at bankfull discharge. The term bankfull discharge is sometimes used interchangeably with the term effective discharge and/or the term channel-forming flow; yet, their meanings are distinct and should be explained as such because under specific circumstances such as channel incision, bankfull discharge and effective discharge may vary significantly.

68 Conclusion (Continued) It is believed that the discharge or flow at the bankfull is most effective to carry sediments overtime (Leopold, 1994). Though the formation process of a natural channel is complex, there are quantifiable and consistent patterns for the process, especially at the bankfull stage (Leopold et al., 1964).

69 Conclusions (Continued) When Rosgen Stream Classification System is used in concert with the Schumm et al. CEM or the Simon CEM that reflect process and trend, one has an outstanding combination available for streambank practitioners.