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Addressing habitat failures through
gravel augmentation:
assessment for adaptive management
Peter W. Downs
& thanks to
Joh...
NRC 1992
1. Problems of water quantity and flow-
mistiming
2. Morphological modifications of the
channel and riparian zone...
1. Water quantity &
flow mis-timing
2. Morphological
modifications
channel / riparian
zone (3. Excessive
erosion/
sediment...
•gravel quality (permeability)
•gravel mobility (redd scour)
•redd dewatering
•spawning habitat availability
•spawning hab...
Non-structural preservation and natural recovery
Improving network connectivity: restore flow and
sediment processes
Impro...
The Aim: Prompt river towards ecological integrity
The Goal: Add a supply that is mobile (but not too mobile)
to achieve s...
Problem: definitive data (historical / direct monitoring) are rare
Solution: develop a corroboratory approach…
Technique A...
Upton
Pulham
Withiel Florey
Paucity of alluvial material, subtle morphological response, shallow
depth to bedrock, very coarse sediments, reduced disc...
The current Q1.5 peak discharge is ~50–65% of pre-dam magnitude
using FEH area-based methods; flow duration statistics dep...
Reach
Area
regulated
Pre-dam
Q1.5-Q2
Post-dam
Q1.5-2
Overtopping
flood
Post-dam
flow
contained
% m3s-1 m3s-1 m3s-1 RI
Just...
Wolman bed surface samples:
 Bed is coarser below the dam,
suggests loss of finer material
Shear stress estimates :
 Sed...
Present potential for
morphological
adjustment
(Schmidt & Wilcock 2008)
Sediment
Deficit
Potential
Incision
Potential
Narr...
BAGS software: Pitlick et al. 2009: potential transport rates 5–8–times higher
than present day. Below dam and lowest reac...
Impacts are very different to dammed alluvial
systems with finer sediments…
Below dam: mild erosion of bed and banks, loss...
Transport simulation allows optimization of (S, D50, n):
1. Salmonid spawning preferences (16-64 mm)
2. Offset sediment lo...
Adaptive management: “Actively learning through experience in
systems characterised by uncertainty”
 Little known about d...
 Analysis of contemporary sediment supply and transport dynamics
in a historical context and future projections
 Multipl...
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WRT CaBA/CRF Conference 02/12/14 - Peter Downs

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In December 2014 WRT held a Catchment Based Approach and Catchment Restoration Fund Conference in Exeter. The University of Plymouth's Peter Down gave a presentation on his work studying the hydromorphology of rivers, especially the effect of reservoirs on river substrates.

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WRT CaBA/CRF Conference 02/12/14 - Peter Downs

  1. 1. Addressing habitat failures through gravel augmentation: assessment for adaptive management Peter W. Downs & thanks to John Hickey, Matt Healey Alastair Morriss, John Jepps Claire Bithell, Nick Jackson, Josh Moore, Sarah Mortimore, Greg Rushby, Sarah Twohig AERIP SHRIMP
  2. 2. NRC 1992 1. Problems of water quantity and flow- mistiming 2. Morphological modifications of the channel and riparian zone 3. Excessive erosion and sedimentation 4. Deterioration of sediment quality 5. Deterioration of water quality (chemical and physical) 6. Introduction and invasive spread of alien species
  3. 3. 1. Water quantity & flow mis-timing 2. Morphological modifications channel / riparian zone (3. Excessive erosion/ sedimentation) 4. Sediment quality deterioration 5. Water quality deterioration
  4. 4. •gravel quality (permeability) •gravel mobility (redd scour) •redd dewatering •spawning habitat availability •spawning habitat quality •in migration flows •physical migration barriers •migration hazards •juvenile habitat availability •proximity of fry rearing habitat to spawning areas •flows: stranding or displacement •food availability •water quality •out migration flows •predation •diversion hazards •loss of estuarine rearing habitat •water quality •harvest •ocean conditions The salmonid challenge: Limiting factors are often linked to bed sediment
  5. 5. Non-structural preservation and natural recovery Improving network connectivity: restore flow and sediment processes Improve habitat diversity by prompted recovery Improve habitat diversity by reconstructing channel & floodplain Sustainable? Less Sustainable?
  6. 6. The Aim: Prompt river towards ecological integrity The Goal: Add a supply that is mobile (but not too mobile) to achieve spawning / rearing parameters Severity of issue – balance between:  lifecycle requirements of valued species (‘known’)  magnitude of habitat alteration (‘unknown’) Magnitude of alteration - reach-scale changes:  in channel morphology  in sediment supply and transport dynamics Facilitates  understanding of specific requirements  need for complementary measures  evaluation and learning
  7. 7. Problem: definitive data (historical / direct monitoring) are rare Solution: develop a corroboratory approach… Technique Assessment Field reconnaissance interpret channel conditions Hydrology analysis flow regime changes Channel surveys extent of morphological adjustment Estimate of metrics severity of sediment deficit, available spawning area Shear stress analysis mobility of bed sediments Numerical modelling volumes of sediment transported
  8. 8. Upton Pulham Withiel Florey
  9. 9. Paucity of alluvial material, subtle morphological response, shallow depth to bedrock, very coarse sediments, reduced discharges, greatly reduced sediment supply Brandt 2000
  10. 10. The current Q1.5 peak discharge is ~50–65% of pre-dam magnitude using FEH area-based methods; flow duration statistics depend on position within the catchment… Just below dam Below unregulated tributary Reservoir only holds 75% of average annual runoff – spills frequently in wet years…
  11. 11. Reach Area regulated Pre-dam Q1.5-Q2 Post-dam Q1.5-2 Overtopping flood Post-dam flow contained % m3s-1 m3s-1 m3s-1 RI Just below dam 98.1 9.0 – 11.3 5.0 – 5.7 6.3 – 6.8 2.5 – 3.0 In mid-catchment 57.5 14.7 – 18.4 8.7 – 10.9 21.6 – 26.9 40 – 100+ Lower catchment 49.7 16.6 – 20.7 10.6 – 13.2 11.5 – 16.3 1.75 – 4 Morphological response is reach- differentiated, but indicates significant enlargement of mid- valley cross-sections
  12. 12. Wolman bed surface samples:  Bed is coarser below the dam, suggests loss of finer material Shear stress estimates :  Sediment was more mobile, pre- dam, but  Current Q1.5 capable of moving D50 in all reaches  Suggests selective winnowing of finer material, esp. below dam
  13. 13. Present potential for morphological adjustment (Schmidt & Wilcock 2008) Sediment Deficit Potential Incision Potential Narrowing Threshold <1 >0.4 <0.4 Below dam 0.27 0.16 0.50 Mid-valley 0.72 0.12 0.59 Lower valley 0.58 0.08 0.64 Empirical studies (Atlantic salmon and brown trout) - preferred D50 = 30 mm  Haddeo D50 = 57-68 mm ~50% bed in excess of preferred size Spawning area impact (Riebe et al 2014)… Below dam reach Useable Area Fish size 400 mm 600 mm Pre-dam 74 86 Post-dam 48 69
  14. 14. BAGS software: Pitlick et al. 2009: potential transport rates 5–8–times higher than present day. Below dam and lowest reaches the volumetric potential is negligible, except during peak flows of moderate floods and larger 446 3 5 193 133 150 31 1 Wilcock-Crowe equations Pre-dam Post-dam
  15. 15. Impacts are very different to dammed alluvial systems with finer sediments… Below dam: mild erosion of bed and banks, loss of bedforms, spills permit bed mobility, coarsening bed increases roughness, loss of alluvial material, significant impact for smaller spawners, slow rate of further change Mid-valley: significant channel capacity increases, mostly by width, adjusting to new sediment regime, possible loss of alluvial material, stabilisation of deposits as islands, future coarsening during contained large floods Lower valley: channel may be adjusting to changed conditions due to material supply from upstream and alluvial floodplain
  16. 16. Transport simulation allows optimization of (S, D50, n): 1. Salmonid spawning preferences (16-64 mm) 2. Offset sediment loss since dam closure (finer than required) 3. Feasible volumes of annual augmentation (50-100 t a-1) 4. Relatively stable during incubation (barely mobile) Reach Sediment mix Roughness Transport D50 D84 ‘n’ t a-1 Below dam 1 36 (68) 62 (112) 0.060 (0.051) 30-94 Below dam 2 48 (68) 81 (112) 0.051 23-57 Mid-valley 36 (59) 62 (107) 0.060 (0.043) 45-137 Lower valley 29 (57) 56 (96) 0.041 19-75 Challenge is often flushing flows; here is to retain sediment, use logs / boulders  wet years capable of moving hundreds of tonnes of sediment
  17. 17. Adaptive management: “Actively learning through experience in systems characterised by uncertainty”  Little known about dispersal dynamics of augmented sediment, esp. in upland channel types: distances travelled, contribution to building functional meso-habitats - monitor and evaluate
  18. 18.  Analysis of contemporary sediment supply and transport dynamics in a historical context and future projections  Multiple lines of evidence offset common data deficiencies  Allows WHAT IF scenario setting  Amenable to integrated monitoring and evaluation as basis for improving future practice…a contribution to sustainable practices Allows ‘complexity’: a strategy of bravery lying on the boundary between order (cowardice) and chaos (recklessness) (Geldof, 1995) A rapid, robust approach to augmentation planning:

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