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.

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. 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. 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. •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. 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. 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. 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. Upton
Pulham
Withiel Florey

9. Paucity of alluvial material, subtle morphological response, shallow
depth to bedrock, very coarse sediments, reduced discharges,
greatly reduced sediment supply
Brandt 2000

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. 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. 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. 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. 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. 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. 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. 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.  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: