Strategy for Observing NFM-Related Reductions in Flood Peaks Nick Chappell and Dave Kennedy

Strategy for observing NFM-related reductions in flood peaks Nick Chappell and Dave Kennedy

Q-NFM

Image: www.uniquehomestays.com Strength of field-observed evidence for delivery Gold of flood reduction benefits of individual NFM standard features: the evidence scale

Field- Field-observed Field-observed Estimate of max Field-observed observed overland-flow or overland-flow or storage potential field parameter storage change storage change or infiltration / flood (eg permeability, synchronous with without feature- evaporating area hydrograph roughness) local streamflow pertinent local of each NFM change reductions change streamflow data feature

Simple accounting Capable of informing procedure describing national or international extent of NFM research base implementation Strength of field-observed evidence for delivery Gold of flood reduction benefits of individual NFM standard features: the evidence scale

Keswick Flood Action Group meet EA

Keswick Flood Action Group meet EA Simple accounting Capable of informing procedure describing national or international extent of NFM research base implementation Observe (measure/monitor) flood hydrograph?

Velocity-Area methods “measure discharge directly at one point in time (person present)”

Dilution Gauging methods Observe (measure/monitor) flood hydrograph?

Combine with continuous (15-min) stream level measurements (‘stage’) Observe (measure/monitor) flood hydrograph?

Calibrate the stage values to spot discharge values (even during floods)

Complex (require /s) 3 many large flood

events studied) Spot discharge (m discharge Spot

Noisy e.g. due flow regime change (large uncertainty – hard to see change) Stage (m)

Have to dash to field for every large flood in particular to measure spot discharges Observe (measure/monitor) flood hydrograph?

Calibrate the stage values to spot discharge values (even during floods)

Captain WN McClean calibrating his weir from a bowsun’s chair on River Garry nr Invergarry House (around 1913-15) Photo: SEPA

Have to dash to field for every large flood in particular to measure spot discharges Observe (measure/monitor) flood hydrograph?

Feasible solution…?

Install a gauging structure that forces stream through ‘critical flow’ and that performs according to a theoretical calibration (‘rating’)

Weir

Critical flow by a fall (‘hydraulic drop’)

But

Required stilling pool accumulates sediment

Impedes passage of fish & macroinvertebrates Observe (measure/monitor) flood hydrograph?

Feasible solution…?

Install a gauging structure that forces stream through ‘critical flow’ and that performs according to a theoretical calibration (‘rating’)

Flume

Critical flow by a constriction (‘hydraulic drop’)

Do not obstruct passage of fish & macroinvertebrates

If installed in correct location…

Self cleaning (not require manual digging out of sediment after every rain event) Observe (measure/monitor) flood hydrograph?

Feasible solution…?

Install a gauging structure that forces stream through ‘critical flow’ and that performs according to a theoretical calibration (‘rating’)

Only requires a few spot checks on stage-discharge calibration Cost limits size of structure and hence size of basin gauged (< ca 1km2, 250 acres)

FRPB flume CEH mountain flume ca 1 km2 Lower Hafren ca. 4 km2 £500 flume >£50,000 flume Costs increase exponentially Key issue, not basin size, but magnitude of intervention-effect relative to peak-flow (linked to basin size)

Reproduced under Lancaster University Digimap licence

South Peak Slow the Flow Flume

Flume Key issue, not basin size, but magnitude of intervention-effect relative to peak-flow (linked to basin size)

Reproduced under Lancaster University Digimap licence

South Peak Slow the Flow Flume

Flume ‘leaky-dam’ under-construction Accurate quantification of reductions in flood peaks – most likely to be observable

For example…

1/ Where wetted-canopy evaporation (‘interception loss’) can be increased several fold (eg 2-5) during large storms across a large proportion of the basin

2/ A basin with very low infiltration (cm/hr), where interventions have increased infiltration by a factor of 5-10 or more across much of the basin

3/ Where large surface storage change (m3 /hr) in flood event is created relative to peak channel flow (m3/hr)

While having…  Highly accurate gauging station  A robust ‘reference’ to measure change against

Advisable to not attempt to monitor unless likely to see NFM-related change Robust ‘reference’?

For example…

Type 1: Gauging station immediately upstream and downstream (with no major channel flows entering) eg bracketing a series of in-channel Intervention woody dams After Type 2: An adjacent basin (also gauged) lacking the NFM-intervention = extensive NFM features (eg reference moorland conifer planting basin next to forested basin – emulating optimal state after tree planting)

Type 3: A single reference gauging station eg where change in storage during storm (m3 /hr) is a Before significant proportion of peak channel flow (m3/hr)

Type 4: A single gauging station monitored before Control and after an intervention added (if not surface storage - requires exceptional Time Series Analysis to capture changing rain-flow dynamics with minimal uncertainty) Combining 2 & 4 = BACI design (Before-After Control-Intervention) Map: Cumbria County Council

Locations of flume-observed NFM effects across Cumbria

in development

Standardised FRPB Flumes Map: Cumbria County Council

Locations of flume-observed NFM effects across Cumbria

in development

Bessy Gill Flumes Type 4 design (channel realignment)