Dam Removal Analysis Guidelines for Sediment

Transcription

1 A review of: Dam Removal Analysis Guidelines for Sediment Joe Rathbun (Retired)

2 Some Potential Sediment Issues Reservoir restoration Downstream water quality Downstream deposition Burial of water intakes Chemical contaminants Downstream coastal deltas & beaches Downstream reservoirs

3 Variables Affecting Sediment Impacts Reservoir sediment volume, grain size, contamination, & spatial distribution Extent & rate of sediment erosion Tolerance & adaptability of sensitive species Infrastructure vulnerability

4 Creating the Guidelines Department of Interior s Subcommittee on Sedimentation convened dam removal national experts from government, academia, & NGOs Held 2 workshops (OR, PA) to collect information, discuss issues & make writing assignments

5 Still a draft HRWC will send you the link to the final document

6 Guideline Objectives Assist with determining the level of sediment data collection analysis modeling necessary to plan & implement dam removal projects using a risk-based approach

7 10 Guideline Steps 1. Identify sediment concerns 2. Collect reservoir & river data 3. Evaluate sediment contamination 4. Determine relative reservoir sediment volume & probability of impact 5. Refine potential sediment consequences, & estimate risk

8 10 Guideline Steps, continued 6. Develop dam removal & sediment management plans 7. Conduct sediment risk analysis 8. Assess uncertainty of data & predictions 9. Determine if sediment impacts are acceptable 10. Develop monitoring & adaptive management plans

9 Other guidelines also useful (Wildman & MacBroom, 2011)

10 Emphasis of this talk: Step 4. Determine relative reservoir sediment volume & probability of impact Step 7. Conduct sediment risk analysis

11 1a. Identify sediment concerns Rather conceptual & qualitative Degree & rate of erosion Excessive incision upstream Exposure of submerged infrastructure Downstream aggradation Elevated turbidity

12 1b. Identify sediment benefits Not all sediment is bad! New riverine habitat Increase particle size diversity Increase instream habitat diversity Reconnect floodplains Replenish coastal deltas & beaches Restore nutrient & wood transport

13 2. Collect reservoir & river data Dam construction & operation Reservoir sediment deposit dimensions; longitudinal & lateral Probing survey & geomorph surveys Sediment particle size distribution Reservoir wood & other debris Old infrastructure in reservoir

14 2. Collect reservoir & river data Downstream reaches of concern Depositional reaches Floodplain dimensions Tributary junctions Bridges, water intakes, outfalls Stream hydrology Stream sediment characteristics

15 3. Evaluate sediment contamination Due diligence to assess whether contaminants are likely to be a concern If so, collect samples & analyze for appropriate contaminants Compare to appropriate standards Survey design = target silt deposits & use stratified random sampling

16 3. Evaluate sediment contamination Number of samples: Follow regulatory guidance, or Geostatistical calculations Elipgrid recommended

17 Elipgrid Example Canals off Lake St. Clair 21,700 m 2 (~ 6 football fields) Assume square grid, 95% confidence of detecting a circular hot spot Hot spot radius (m) Required # of samples ,787

18 Michigan s guidance First, sieve analysis If < 90% sand... 6 cores from 1 st 10,000 cubic yards 1 additional core from each additional 10,000 cubic yards Total PCBs Suite of PAHs Suite of metals Total organic carbon Pesticides, if suspected

19 4. Determine relative sediment volume Estimate reservoir sediment volume (Vs) Probing survey, coring, long pro, etc. Estimate average annual sediment load (Qs) sediment-discharge rating curves, sediment yield calculations, sediment trap efficiency calculations

20 Sediment Yield Plot for Michigan Creech et al, 2010

21 Magnitude of relative sediment volume Reservoir sediment volume (Vs) vs. Average annual sediment load (Qs) Vs/Qs Negligible Small Medium Large (Separate fine and coarse sediment?)

22 Negligible sediment volume Vs < 0.1 Qs Alternative criteria Wres / Wch 1.5 Dam height bankfull height No sediment found by visual observation or probing Longitudinal profile = no sediment wedge Vs < volume of a sand or gravel bar Bar volume = Wbf 2 x Dbf

23 Magnitude of relative sediment volume How much is too much? Depends on sensitivity of downstream resources Negligible = never a problem Large = always a problem Confer with DEQ, DNR, other stakeholders

24 5. Refine estimate of potential sediment consequences, & estimate risk Consequences depend on: Particle size Fines = water quality & siltation; Coarse = channel morphology Spatial extent of deposition Deposition greatest near dam? Duration of turbidity or deposition Hours to decades

25 5. Refine potential sediment consequences, & estimate risk Risk = Probability of impact x Consequences of impact Consequence Probability Small Medium Large Small Low risk Low risk Moderate risk Medium Low risk Moderate risk High risk Large Moderate risk High risk High risk

26 6. Develop dam removal & sediment management plans Rapid or staged removal Partial or complete removal Certain season or flows Downstream channel degradation? Non-erodible materials in reservoir? Contaminant issues? Sensitive biota?

27 6. Develop dam removal & sediment management plans Options for sediment management Allow natural erosion Mechanical removal Sediment & channel stabilization Sediment isolation and/or capping Combination

28 7. Conduct sediment risk analysis 2 basic questions: Fate & effects of reservoir sediment? Appearance of new reservoir landscape? Usually similar to upstream channel, minus mature trees

29 Some reservoir sediment transport rules of thumb: New channel will form by headcut(s) & incision, and channel widening/floodplain formation 1. Rapid erosion (incision) due to increased energy gradient 2. Slower erosion (widening) due to hydrologic events (storms, etc.)

30 More reservoir sediment transport rules of thumb: Incision continues until pre-dam surface is reached Presence of pre-dam thalweg = new channel will probably occupy the old stream bed Incision will not extend past upstream end of reservoir delta unless the downstream channel has degraded

31 Conceptual model of sediment erosion & channel formation (Doyle et al, 2003)

32 More reservoir sediment transport rules of thumb: Colonizing vegetation can stabilize sediment, but only if root depth sediment thickness Tributaries to reservoir are wild cards Headcuts = unexpected upstream incision Sediment deltas = affect channel alignment

33 More reservoir sediment transport rules of thumb: Fine sediments will transport until they reach a deposition zone Coarse sediments will travel downstream in long, low-amplitude waves (with maximum deposition just d/s of the dam?)

34 Tools for sediment analysis, by risk category

35 Step 7: Numerical Sediment Transport Models Good at: Vertical channel incision 1-D transport & deposition with time & distance d/s Bad at: Lateral channel erosion & deposition Meander formation Headcut formation & migration

36 Step 7: Numerical Sediment Transport Models Model Type Distance Time Lateral sediment erosion 1-D 10s of miles Years to decades 2-D < 10 miles Days to months No Yes 3-D < 1 mile < 1 week Yes 1D = HEC-6 2D = SRH-W 3D = U2RANS

37 Also contains modeling information (Papanicolaou & Barkdoll, eds., 2011)

38 8. Assess uncertainty of data & predictions Uncertainty always present, but hard to quantify Assess by monitoring during removal (Step 10), and return to Guidelines if necessary Iterative

39 Some Uncertainty ( U ) Rules of Thumb Annual sediment load (Qs): U depends on measurement method used Reservoir sediment volume (Vs): U depends on method used & sediment thickness

40 Some Uncertainty ( U ) Rules of Thumb Particle sizes & contaminants: U depends on number & distribution of samples Sed transport & deposition: U depends on input data quality, plus model choice & skill of modeler

41 9. Determine if sediment impacts are acceptable A qualitative judgment, informed by stakeholders & regulators If acceptable, proceed to Step 10 If not acceptable revise removal & sediment management plans (Step 6) and add sediment mitigation steps Phased removal Removal timing Remove or stabilize sediment prior to removing dam Leave a portion of dam in place

42 10. Develop monitoring & adaptive management plans Monitoring performed to: Confirm permit compliance (necessary) Identify adaptive management actions (necessary if problems arise) Assess environmental effectiveness - success story (not always needed) See Collins et al, 2007 for recommendations

43 Collins et al, 2007

44 Thanks! Any questions?