Environmental Water Allocation Hydraulics Dr L Beevers Heriot Watt University, it UK l.beevers@hw.ac.uk
Overview Why hydraulics in EWA? Different types of flows Theory Case studies Review
Why do we need hydraulics? Water quality needs: Flow, velocity Fish biologists need: Critical depth or velocity Hydrology gives flows m 3 /s or day? Vegetation specialists need: Flood inundation and levels Geomorphologists need: Hydraulic shear stress and velocity
Why do we need hydraulics? Important to characterise a variety of flows: Low flows and flood flows Water quality needs: Flow, velocity Fish biologists need: Critical depth or velocity Hydrology gives flows m 3 /s or day d=water depth v=water velocity P= wetted perimeter w=water width Hydraulics Links discharge to flow components d, v, P, w Geomorphologists need: Vegetation specialists need: Flood inundation and levels Hydraulic shear stress and velocity
So Hydraulician links the two Takes discharge or water balance and translates it to: El levation (m AD) 44 43.5 43 42.5 Water Levels XSEC 4 cross-section Qmean Q95 Qmed Q5 Q-5yr Q-10yr Q-25yr Q-50yr Q-100yr Q-200yr Qbf Water level/depth 42 Flow width Velocity (varying) 51 50.5 50 49.5 49 48.5 Wetted perimeter evation (m AD) el 48 47.5 47 0 5 10 15 Width (m) Depth-averaged velocities: CS1 (ch.65) for Qmed cross-section max R 0 0 2 4 6 8 10 12 14 16 18 20 width (m) default R min R 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 velocity (m/s)
Hydraulic Theory A river channel is complex, natural system. How do we establish an understanding of how flows move through the system? We use approximations to the physics of the system and we learn from data. First the some flows then some theory!
Diverse habitats need flow variety Low flow Ephemeral (intermittent flow)
Diverse habitats need flow variety Flood flows: turbulent (broken wave/chaotic) Uniform flow: (smooth, rippled) Pools/riffles; chute; waterfalls
So: diversity is best!
Short exercise: flow types Have a look at the photos given out in pairs or threes Identify different types of flow evident in the pictures Based on your previous experience what kind of species would be suited to this habitat in your country?
Scales, scales, scales Spatial and temporal scales are crucial. Do we want to predict flows for 10 minutes, 5days or a year? Do we need to know the velocity every 5cm across a 50m channel? Spatial scales: We need to consider a basin-scale in order to predict the movement of discharge through the system. Reach-scale variety can be important for nonmigratory species We need to consider the local flow features by representative cross-sections.
Spatial p scales scales: BasinBasin-scale Reach Reach--scale Cross Cross--section - local
Hydraulic prediction - xsec We need to consider the local flow features by representative cross-sections sections Models are built from representative cross-sectionssections
A good datum
But: Consider river corridor, including floodplains
River corridor width main channel width
How about this one?
main channel width corridor width
Short exercise: cross-sectionssections Have a look at the photos given out in pairs or threes Identify the cross-section you would measure if you were surveying Indicate it on the photo
Key physical principles Mass is conserved. Newton s Second Law: Force is equal to rate of change of momentum
How? We consider the reach between two cross sections. Mass conservation says that if more goes into this reach than leaves, then the water level will rise by a depth corresponding to the increase in volume. For Newton s Second Law things are more difficult.
Forces on water Gravity Resistance from the bed and banks due to material and vegetation. Narrowing of river channel. Pressure due to depth of water. All these are approximated and then used in Newton s Second Law.
For completeness: The de St. Venant Equations:
Simplification Assume that the flow does not change with time so it is steady. Then the above equation can be reduced to what is called the Backwater Equation. http://www.lmnoeng.com/channels/gvf.htm Given a downstream depth and knowing the slope we can calculate depths by working our way up the channel.
Further simplification The channel shape does not vary rapidly it is uniform. So S 0 =S f In words: Bed friction is balanced by the gravity slope
How do we measure friction? Historically: Experimental and fullscale measurement Analysis produces empirical formula Input parameters Cross-section, streamwise slope, bed material.
Mannings and Chezy formula n Manning s coefficient R hydraulic radius=area/wetted Perimeter S slope Wikipedia Manning s Formula
How do we get n or C? Look-up tables on the web and in books: http://www.engineeringtoolbox.com/manning s-roughness-d_799.html ht http://www.lmnoeng.com/manningn.htm http://wwwrcamnl.wr.usgs.gov/sws/fieldmeth / /fi ld h ods/indirects/nvalues/index.htm Open Channel Hydraulics, Ven Te Chow.
More sophisticated tools In more complex projects where funding and time allow it is possible to use more sophisticated modelling tools in 2 and 3 dimensions. Delft3D, MIKE21, etc. But it doesn t always have to be complicated
River Idle Restoration Project
River Idle Restoration Project
Kingcausie Burn: Methodology Et Extensive baseline assessment (hydrology, water quality, geomorphology gy and ecology) No existing geomorphological information Detailed input into watercourse realignment design Sediment sampling Channel capacity checks Channel stability checks CES software used to support decision making
Kingcausie Burn: Fluvial Audit Morphologically diverse Dominated by fine gravel and coarse sand transport Coarser bed particles appear to be stable Some localised incision Woody debris and leaf litter widespread causing steps in the channel
Kingcausie Burn: Design Conveyance Estimator Software (CES) Simplified hydraulics Engineering proposals iteratively modified: Design channel alignment, gradient and form (2 stage) Inform channel morphology and substrate composition (based on Hjulström) Channel stability and bank sediment elevation (m AD) e Depth-averaged velocities: CS1 (ch.65) for Qmed 51 1.6 50.5 50 49.5 49 48.5 0.6 cross-section 48 default R 0.4 47.5 min R 0.2 47 max R 0 0 2 4 6 8 10 12 14 16 18 20 width (m) 1.4 1.2 1 0.8 velocity (m/s)
Kingcausie Burn: Recommendations Two stage channel designed Overall gradient, sinuosity/alignment decided Cascade designed (1:11 gradient) Gravel-bed Cobble cascade (large cobbles) Plunge pool Gravel-bed (large cobbles or boulders) overlain with small cobbles Bank sediment dictated by bed rock formations but vegetation consideration included. d Bed morphology agreed Water Levels XSEC 4 Capable of transporting up to pebble sized particles Elevation (m AD) 44 43.5 43 42.5 cross-section Qmean Q95 Qmed Q-5yr Q-10yr Q-25yr Q-50yr Q-100yr Q-200yr Qbf 42 0 5 10 15 Width (m)
Summary Why hydraulics in EWA? It is the link between flow rate and detailed requirements Different types of flows Different flows in the environment Theory Some of the mathematical theory Measurement Cross-sections, Case studies