Benefits of Establishing Floodplains. Two Stage Ditch Approach. D Ambrosio The Ohio State University

Transcription

1 Benefits of Establishing Floodplains in Agricultural Ditches: Two Stage Ditch Approach Andy Ward, Jon Witter Jessica D Ambrosio The Ohio State University

2 STREAMS Website Streams.osu.edu

3 How to use this CD: Open MAIN- MENU.pdf with Adobe Reader (free at Stream Restoration Design Helping People Help the Land USDA Natural Resources Conservation Service National Engineering i Handbook Part 654 August 2007 (NEH-654)

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5 To Build a Better Ditch

6 What is Dynamic Equilibrium? i Dynamic equilibrium is a self-maintaining state that balances stream power with the discharge of bed material sediment. The stream system transports its sediment load without aggrading or degrading while maintaining its dimensions, meander pattern, and profile.

7 Equilibrium States of Stream Systems A stream can be failing, recovering, in a quasi- state of equilibrium, or in dynamic equilibrium. Failing streams down-cut or get wider or both.

8 Equilibrium State Recovering Streams Recovering streams build features such as point bars, benches, and floodplains. Often they will also be getting narrower.

9 What are Channel-Forming Discharges? Channel-forming discharges are the range of flows that shape the channel, bars, benches, and the active floodplain elevation associated with dynamic equilibrium.

10 Incised Channels

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12 A Natural Stable Stream vs A Designed Stream

13 Main Channel Meandering And A Bench Forming

14 Why Is Bench Formation Associated with Low Energy Channel Forming Discharges? There is an available supply of fine material at the bottom of the ditch Sediment from bank instability The dominant flow is subsurface drainage Most of the flow entering the ditch contains very little sediment Grass rapidly stabilizes the benches

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16 Maintenance Often Removes Fluvial Benches That Will Rebuild Again Material Commonly removed during cleanout

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18 Two-Stage Channel Design

19 Jin-Mei Creek, Taiwan Natural Floodplain within a 200 year flood levee Constructed Terrace within a 200 year flood levee

20 Sizing Two-Stage Channels 1. Project Identification 2. Data Collection 3. Data Analysis 4. Channel Sizing 5. Hydrologic Evaluation 6. Project Assessment 7. Final Sizing and Design 8. Construction 9. Monitoring i and Performance Evaluation

21 1. Project Identification Problem Identification Project Situation Watershed conditions Channel Failures Bank instability Cut banks Sediment deposition Restricted drainage outlets Restricted drainage outlets Inadequate subsurface drainage Insufficient capacity

22 2. Data Collection

23 3. Data Analysis Width Depth Cross Sectional Area A (ft or squ uare ment, W, D, ft) Measure A = 6.95DA 0.73 R2 = 0.93 D = 1.30DA R2 = 0.81 W= 5.34DA 0.45 R2 = Drainage Area, DA (square miles) Figure 9 A regional curve for the St Joseph Figure 9. A regional curve for the St. Joseph watershed developed by the project team.

24 4. Channel Sizing

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26 6. Project Assessment A final project assessment and design presentation should occur with all stakeholders participating

27 8. Construction

28 9. Monitoring Pre-Construction Anticipated Final Dimensions

29 Fast Road, Wood County

30 Klase Ditch, Ohio

31 Crommer Ditch, Michigan TNC constructed two- stage ditch (2003) Drainage area of 4.5 mi 2

32 Creel Ditch, Indiana Pre-construction Post-construction

33 Two-Stage Ditch in Minnesota A B A. Prior to construction a sandy point bar at a bend. B. During construction ti a stabilizing i blanket was placed on the benches and bank

34 Things do not always work as planned!

35 Benefits of Two-Stage Systems Ecological Benefits? Lower Shear Stresses on Bed and Bank Peak Flow Reduction Nitrate Nitrate-Nitrogen Nitrogen Reduction Other Water Quality Benefits?

36 Ecological Benefits of Alternatives Ecological Benefits Shape variable & hard to quantify Ecology Viability Constructed Natural Channel Viability Incised Two-Stage Self-Forming Natural

37 Benefits of Benches

38 Peak Flow Reduction

39 Little Schenk: Relative Bedload Transport vs Floodplain Width Ratio Relat tive Bed dload Tra ansport BDF = 0 BDF = 8 BDF = Floodplain Width Ratio

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41 Nitrate-Nitrogen Nitrogen Reduction

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44 Potential Processing of Nitrogen in Benches vs. Grass Buffer Strips Benches high potential Benches receive NO 3- via tile drains and stream flooding Frequent inundation during storms Close to the water table (more saturation = increased anoxia) Grass Buffer Strips low poten NO 3- bypasses buffer via tile drains and deep ground water Rare inundation (~ 100 year flood) Well-drained, further from the water table More bench surface area = more N removal

45 Two-Stage at Shatto Ditch Etna Green, IN (Kosciusko County) Tile drain inputs Watershed > 80% row crop agriculture Historically, conventional ditch maintenance

46 Experimental Design Downstream Treatment Reach Upstream Control Reach flow Before After Control Impact (BACI) 1 year of pre-construction data collection (Sept 2006 Nov 2007) ~2 years of planned post- construction data collection (Nov 2007 Nov 2009) 2007 google map

47 What Do We Expect on the Benches? Denitrification occurs at depth in benches due to increased nitrate penetration Inundation stimulates bench denitrification by creating anoxic conditions in bench N N N 2 2 N 2 N 2 2 N 2 NO 3

48 Flood Response December 2007 You can see the edge of the bench hby the grass tussocks in the water

49 Scaling Bench Denitrification to the Reach Conservative estimate based on average December denitrification in dry, new bench Scaling denitrification rate to reach Area (m 2 ) Before After bench stream % bench to stream 50% 252% N removed via DN g N d Ratio of bench area to stream Take-home message: Adding bench surface area with two-stage t construction ti increased N-removal by 500%

50 Summary Two-stage channels, based on geomorphic principles, are an alternative to traditional trapezoidal channels Inset channels are more self-flushing, flushing, and have coarser bed materials

51 Floodplain Recommendations (low gradient channel systems) Minimum Rules for Floodplains 2 5 bankfull widths (rural) 5 10 bankfull widths (urban) Target Condition 10+ bankfull widths

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