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

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

1. Water quantity & flow mis-timing

2. Morphological modifications channel / riparian zone (3. Excessive erosion/ sedimentation)

4. Sediment quality deterioration

5. Water quality deterioration

•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

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?

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

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

Upton

Pulham Withiel Florey

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

Brandt 2000

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…

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

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

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

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

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

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

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

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: