17 TH RRC ANNUAL NETWORK CONFERENCE. Planning, delivery and evaluation of our rivers: challenges and choices. Tweet us #RRCBlackpool

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1 17 TH RRC ANNUAL NETWORK CONFERENCE Tweet us #RRCBlackpool Planning, delivery and evaluation of our rivers: challenges and choices We would like to thank the sponsors of the conference who support discounted places

2 Hydraulic modelling for river restoration: methods for minimizing (not just flood) risk. Eric Gillies 1,2 17th River Restoration Conference cbec eco engineering UK; 2. University of Glasgow School of Engineering (Hon. Res. Fellow).

3 Minimum modelling requirements to minimize risk in river restoration 1D xc Eric Gillies 1,2 95 th Scottish Freshwater Group Meeting 1. cbec eco engineering 2D mesh UK; 2. University of Glasgow School of Engineering. Craigburn restoration River Nith 2D model

4 Minimum modelling requirements to minimize risk in river restoration Eric Gillies 1,2 95Risk th Scottish that restoration Freshwater will Group cause Meeting damage Risk that restoration will not achieve its objective 1. cbec eco engineering UK; 2. University of Glasgow School of Engineering.

5 Minimum modelling requirements to minimize risk in river restoration Eric Gillies 1,2 95 th Scottish Freshwater Group Meeting 1. cbec eco engineering UK; 2. University of Glasgow School of Engineering. Flooding River Severn Flooding River Dulnain

6 Minimum modelling requirements to minimize risk in river restoration Eric Gillies 1,2 95 th Scottish Freshwater Group Meeting 1. cbec eco engineering UK; 2. University of Glasgow School of Engineering. Haugh Head ford restoration project Allt Lorgy restoration project

7 Modelling reduces uncertainty When restoration projects are put out to tender, modelling is often mandated: The specification is typically for Flood Risk modelling, as flood may pose the greatest risk (N.B. Flood Risk model spec. often leads one to a particular choice model software). But why take any risk if no improvement to habitat or natural process can be demonstrated? These eco-hydraulic issues can be modelled alongside flood risk (often requiring a different model spec. to above). For river restoration, it is best to use a model that can assess both flood risk and improvements to natural river process.

8 Existing conditions may suggest the wrong model choice It is important to consider future river state, or restoration design option, when selecting the appropriate hydraulic model: Straight reach: 1D model suggested Bo Burn restoration (contains OS data, Crown Copyright 2014) Slight natural recovery as a result of sediment supply- Restoration to allow lateral process to continue.

9 River restorations generally increase hydraulic complexity River restoration often involves some intervention in the water environment: Re-alignment, bed re-profiling Structure removal Sediment injection Bar creation, large wood placement, riparian vegetation Embankment removal, reconnection to floodplain Wetland creation Restorations generally increase LATERAL FLOW, or allow natural lateral process to recommence Allt Lorgy restoration

10 Real in-channel fluid/sediment behaviour is complex, 3D Large scale eddies Surface waves, super-elevation Shear flow (from bed viscosity) Helical (secondary) flow Water surface level varies mainly in streamwise direction (with slope), but also in cross stream direction A subset of these flow phenomena important for flood risk A different subset important for eco-hydraulics & for natural river process. Turbulent eddies Vertical flow Lateral flow Viscous interstitial flow (hyporheic)

11 What can a 1D in-stream model capture? Chosen model: 1D Surface waves (can usually capture jumps) Shear flow (from bed viscosity) Manning friction for each xc panel Water surface level captured accurately in streamwise direction (at xc s only) this is main driver for flood risk n lwd n c n fp wse x Sediment transport- vertical bed aggradation/erosion

12 Desire to use 1D models Many 1D or 1D/2D flood models already exist (pressure to re-use these in restoration design). The greatest model unknown is usually bed friction (typically Manning friction)- there is a large literature and toolset for estimating 1D model friction (e.g. UK CES roughness advisor). CES output (Wallingford Software Ltd.)

13 Chosen model: 1D/2D Weir equation link Crude momentum transfer to floodplain Floodplain levels and velocities relatively accurate n fp In-stream modelling is the same as 1D modelno lateral flow n lwd n c? Better than 1D for flood risk, but inadequate to assess eco-benefits of restoration (other than NFM)

14 Reminder: Much of complex real environment important for eco-hydraulic assessment & flood risk Large scale eddies Surface waves, super-elevation Shear flow (from bed viscosity) Helical (secondary) flow Water surface level varies mainly in streamwise direction (with slope), but also in cross stream direction Turbulent eddies Vertical flow Lateral flow Viscous interstitial flow (hyporheic)

15 What more can a 2D in-stream model capture? Able to assess flood risk (including increased risk from lateral flow) AND assess ecohydraulics/restoration efficacy (excellent stability in low flow) Large scale eddies Chosen model: 2D Surface waves, trans-critical flow, superelevation Shear flow (from distributed Manning n) Helical flow (some codes) Turbulent eddies (good turb. models) Lateral flow Sediment transport- Good bed shear stresses; Vertical bed aggradation and erosion

16 2D setup- survey requirements Expert, Feature based approach capturing breaks of slope and bedforms that result in hydraulic roughness (>3pts/m 2 in complex areas and 0.5pts/m 2 in uniform areas), in addition to traditional cross sections. Total station and RTK GPS. At some sites, on some days, RPAS (drone) surveying (digital photogrammetry) possible of floodplain, bed and substrate. Site still needs some ground survey.

17 Chosen model: 2D Fully 2D (in-stream and floodplain) model is often the minimum requirement to assess both flood risk, bed evolution and restoration efficacy (eco-hydraulics)

18 Conclusions: Any intervention in nature involves risk River hydraulics, sediment transport, discharge (climate) are complex and contain many uncertainties or processes we cannot yet model. Restoration options, do nothing or intervention involve uncertain changes to flood risk, and in-stream process & habitat. It is necessary to minimize flood risk. Are we missing a trick by not using the best technology to also assess the efficacy of the restoration design? Bed change, Haugh Head ford removal There is no point taking the risk without a quantified benefit. 2D in-stream models represent the best tools to assess river restoration. 2D model need not be more expensive than a 1D or 1D/2D model.

19 17 TH RRC ANNUAL NETWORK CONFERENCE Tweet us #RRCBlackpool Planning, delivery and evaluation of our rivers: challenges and choices We would like to thank the sponsors of the conference who support discounted places