Development of a fish habitat mapping tool to assess flow changes on - - PowerPoint PPT Presentation

Paul Gratton Development of a fish habitat mapping tool to assess flow changes on fish habitat utilisation

Introduction

• Details of sites and overview of

development proposals

• Assessment methodology
• Scope
• Field data collection
• Data processing
• Results
• Conclusions and applications of

methodology to other assessment areas

Site locations

Nottingham

Importance of weir pools

• High energy environments
• Depositional features provide important spawning habitat

Stoke Weir – proposed HEP scheme

Stoke – abstraction regime

• Scheme to be operated based on a hands-off level, equivalent to a hands off

flow of ~3 m3/s

• Maximum abstraction of 20 m3/s (10 m3/s per turbine)
• Q99 at Stoke is 24.19 m3/s – HEP scheme would therefore reach maximum

abstraction at <Q99

• Weir crests raised to increase head drop

Percentile River flow (m3/s) Proposed HEP abstraction (m3/s) Residual weir flow (m3/s) Reduction in

• ver-weir

flow

Q1 374.24 20 354.24 5% Q5 247.61 20 227.61 8% Qmean 86.35 20 66.35 23% Q50 59.44 20 39.44 34% Q75 39.01 20 19.01 51% Q95 28.14 20 8.14 71%

Assessment scope

• Environment Agency and Natural England raised concerns regarding impacts on

fish habitat

• Understanding of potential geomorphological adjustments required
• Hydraulic and habitat modelling assessment approach agreed, focusing on the

following species/life stages:

Species

Life stage

Adult Juvenile Spawning

Barbel (rheophilic) ✓ ✓ ✓ Roach (eurytopic) ✓ ✓ ✓ Salmon ✓ Lamprey ✓ ✓

• Habitat assessed at flows of Q95, Q75, Q50, Qmean and Q5

Data needed for habitat assessment

• Three key sources of data used to assess habitat quality:
• Substrate

Collection of data on-site

• Depth

Hydraulic modelling

• Velocity

Sediment sampling

• Sediment samples collected across each site
• Samples dried and sieved to calculate particle size distributions
• Results summarised as a D50 value for each point in millimetres

Hydraulic data and modelling

• ADCP survey completed of all weir pools to capture channel bathymetry
• 2D hydraulic models produced, calibrated against long term stage data
• Modelling used to produce the following outputs:
• Depth
• Velocity
• Depth averaged shear stress
• Modelling results generated for baseline (existing scenario) and future HEP

scenario

Stoke – 2D hydraulic modelling results - velocity

Q95 before Q95 after

Stoke – 2D hydraulic modelling results - velocity

Q75 before Q75 after

Stoke – 2D hydraulic modelling results - velocity

Q50 before Q50 after

Stoke – 2D hydraulic modelling results - velocity

Qmean before Qmean after

Stoke – 2D hydraulic modelling results - velocity

Q5 before Q5 after

Stoke – 2D hydraulic modelling results - depth

Q95 before Q95 after

Stoke – 2D hydraulic modelling results - depth

Q75 before Q75 after

Stoke – 2D hydraulic modelling results - depth

Q50 before Q50 after

Stoke – 2D hydraulic modelling results - depth

Qmean before Qmean after

Stoke – 2D hydraulic modelling results - depth

Q5 before Q5 after

Calculating habitat suitability

• Habitat suitability indices used to convert depth, velocity and substrate

into a combined habitat suitability value

• Habitat suitability value =

Depth suitability x velocity suitability x substrate suitability

• E.g. cell with velocity of 0.3 m/s, depth of 0.6 m and D50 of 60 mm =

0.9 x 0.8 x 1 = 0.72 Rheophilic spawning habitat

Habitat modelling results – rheophilic spawning Q95 - existing Q95 - future

Habitat modelling results – rheophilic spawning Q75 - existing Q75 - future

Habitat modelling results – rheophilic spawning Q50 - existing Q50 - future

Habitat modelling results – rheophilic spawning Qmean - existing Qmean - future

Habitat modelling results – rheophilic spawning Q5 - existing Q5 - future

Refinement of abstraction regimes

• Changes made to abstraction regimes following initial modelling, focusing

primarily on impacts at low flows (Q95/Q75)

• Increase in the HOF passing over the weir
• Weir crest raising delayed until moderate flows
• Hydraulic modelling and fish habitat modelling completed for a second

time to consider potential improvements from changes

Refinement of abstraction regime Q95 – original proposal Q95 – revised proposal

Refinement of abstraction regime Q75 – original proposal Q75 - revised proposal

Refinement of abstraction regime Qmean – original proposal Qmean – revised proposal

Geomorphology

• Modelled boundary shear stress used to assess changes in mobility of bed

material.

• Boundary shear stress is the force per unit area (N m-2) exerted by the flow on

the channel bed.

• Bedload transport is a threshold phenomenon: occurs when boundary shear

stress exceeds critical shear stress (i.e. the boundary shear stress required to entrain a grain of a given diameter).

• Critical shear stress (𝜐"#) can be calculated using Shields equation:

𝜐"# = 𝜄𝑕 𝜍( − 𝜍 𝐸

• 𝜄 = Shields parameter (assumed 0.06)
• 𝑕 = gravitational acceleration (9.81 m s-2); 𝜍(= density of sediment (2650 kg

m-3); 𝜍 = density of water (1000 kg m-3)

• 𝐸 = grain size (mm)

Geomorphology

• Critical shear stress calculated for the D50 particle size at each sediment

sampling point.

• Calculated critical shear stress compared with modelled boundary shear

stress to assess whether the D50 particle size would be above or below the threshold of motion for baseline and proposed conditions.

• Results indicated minimal changes in weir pool geomorphology following HEP

installation due to:

• 1. Low modelled boundary shear stress throughout most of the weir pool

at all flows under both existing and proposed condition.

• 2. Coarse bed material (and therefore high critical shear stress).
• Therefore, bed material movement is limited under both existing and

proposed conditions.

Stoke – geomorphology

Q95 before Q95 after

Stoke – geomorphology

Q75 before Q75 after

Stoke – geomorphology

Q50 before Q50 after

Stoke – geomorphology

Qmean before Qmean after

Stoke – geomorphology

Q5 before Q5 after

Other applications for modelling method

• Assessments of water company abstractions as part of AMP7

WINEP investigations

• Review of time-limited abstraction licences due for renewal
• Use as a predictive tool in assessing risk of deterioration in WFD

status under changes in abstraction

• Drought permit/order applications as part of EAR process,

particularly at high profile or designated sites

Paul Gratton Principal Fisheries Scientist p.gratton@apemltd.co.uk 0161 442 8938

Thank you