Development of a fish habitat mapping tool to assess flow changes on fish habitat utilisation Paul Gratton
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 m 3 /s • Maximum abstraction of 20 m 3 /s (10 m 3 /s per turbine) • Q99 at Stoke is 24.19 m 3 /s – HEP scheme would therefore reach maximum abstraction at <Q99 • Weir crests raised to increase head drop Proposed HEP Residual Reduction in River flow Percentile abstraction weir flow over-weir (m 3 /s) (m 3 /s) (m 3 /s) 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: Life stage Species 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 D 50 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 D 50 particle size at each sediment sampling point. • Calculated critical shear stress compared with modelled boundary shear stress to assess whether the D 50 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
Thank you Paul Gratton Principal Fisheries Scientist p.gratton@apemltd.co.uk 0161 442 8938
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