Understanding

Question 1

What is the difference between the velocity profiles of laminar and turbulent flow? Can you draw them?

Answer 1

Laminar is parabolic and turbulent is logarithmic

Question 2

Explain turbulent smooth flow. Can you draw the profile?

Answer 2

1) Roughness elements along the boundary are contained within the laminar sub layer. 2) Dependant on the Reynold’s Number due to viscosity.

Question 3

Explain turbulent rough flow. Can you draw the profile?

Answer 3

1) Roughness elements along the boundary protrude outside of the laminar sub layer. 2) Dependant on roughness height only as it produces form drag. 3) No skin friction.

Question 4

What is important to consider when a channel is not wide?

Answer 4

Sidewall shear stress as well as bed friction

Question 5

What happens in the corners of a channel that is not wide (ratio of 4)?

Answer 5

Combination of sidewall and bottom shear stresses increases the shear in the corners causing the velocity to drop. Fluid is carried along the bed into the corners, causing secondary currents to occur reducing peak velocity.

Question 6

What is local acceleration?

Answer 6

The acceleration of a fluid particle at a point with respect to time.

Question 7

What is advective acceleration?

Answer 7

Acceleration of a fluid particle along a pathline or streamline with respect to distance.

Question 8

What boundary conditions in an estuary would lead to local acceleration?

Answer 8

Tide is the main contributor to local acceleration in an estuary.

Question 9

What boundary conditions in an estuary would lead to advective acceleration?

Answer 9

Change in area with distance along an estuary would give a change in velocity with distance which would therefore result in advective acceleration.

Question 10

What does the term ‘So’ represent?

Answer 10

The bed slope of a channel

Question 11

Do you know the term that represents advective acceleration?

Answer 11

Beta(U/g)(dU/dx)

Question 12

Do you know the term that represents local acceleration?

Answer 12

(1/g)*(dU/dt)

Question 13

What does (dh/dx) represent?

Answer 13

Longitudinal variation in depth

Question 14

What term represents the head loss due to friction per unit length?

Answer 14

(TAUo)/pgR

Note - p is rho (density)

Question 15

If the groundwater level is at river level and there has been no rain for a significant amount of time, what can be assumed?

Answer 15

Flow is steady and therefore no local acceleration

Question 16

What equation can be used to calculate normal depth?

Answer 16

u = q/hn

where hn - normal depth

Question 17

If an effluent is is put into a river, what would the impact of a weir being built in between a barrage and the discharge site on the concentration of the effluent?

Answer 17

• Weir will slow down flow and increase turbulence at weir.
• This increases retention time allowing the decay to have more time to be effective.
• Therefore the concentration would be lower at barrage

Question 18

How would constructing a weir affect the flood risk upstream and downstream?

Answer 18

• unlikely to affect downstream.

- upstream at greater risk due to higher water levels

Question 19

What design consideration can be taken when designing a weir to minimise flood risk upstream?

Answer 19

• make weir as long as possible.
• oval shaper, or
• incline weir to main flow direction, e.g. Llandaff

Question 20

Name 7 key economic and EIA issues that need to be addressed before the construction of a barrage?

Answer 20

1) Justify project & cost in terms of needs & benefits.
2) Identify main potential hydro-environmental issues (flooding, water quality etc).
3) Assess scope of work during construction.
4) Collect & analyse existing baseline data.
5) Perform environmental impact assessment analysis.
6) Identify need for any mitigating circumstances.
7) Plan environmental monitory programme (pre & post construction).

Question 21

What are four disadvantages of using a physical model? (SAFE)

Answer 21

1) Scaling - cannot scale sediment, mixing etc.
2) Adaptability - hard to refine once made, e.g. make it bigger etc.
3) Flexibility - cannot include certain bits, e.g. wind stress.
4) Expense - generally more expensive than numerical or computer models.

Question 22

Why would a computer model be better than a physical one?

Answer 22

Would be cheaper, easier to scale, more flexible and adaptable.

Question 23

What are the main physical processes that could introduce uncertainties into numerical hydrodynamic models? (MOTTaWF)

Answer 23

i) Momentum correction factor, ii) Open boundary date, iii) Turbulence coefficient, iv) bed Topography accuracy, v) Wind stress, and vi) bed Friction

Question 24

What are the 9 main computer model simulations that should be undertaken for a barrage project?

Answer 24

1) simulate and analyse modifications to tidal currents, e.g. eddies, turbine planes etc.
2) simulate and analyse conditions for pre and post construction and short and long term effects on water quality.
3) simulate wave transformation features.
4) simulate peak water elevations for flood inundation calculations and flood extent.
5) simulate suspended sediment concentration distributions, both pre and post barrage.
6) simulate morphological changes as a result of barrage and changes to coastline.
7) simulate water quality, turbidity, light intensity and salinity level changes.
8) use simulations to optimise design and operation of barrage.
9) assess changes in dispersion and diffusion processes.

Question 25

What are the 6 key stages for setting up a computer model for an estuary (both with or without a barrage)?

Answer 25

1) specify open boundary condition, both at the barrage site and domain limits, including rivers and WwTW inputs.
2) specify various empirical parameters, such as bed friction, turbulence, dispersion and diffusion coefficients.
3) specify sediment properties, e.g. size, type.
4) choose extent of domain and relevant grid size.
5) calibrate and verify model with fluid data.
6) digitise bed topography and set up the data of the bed elevations in the model.

Question 26

What does each term in the general advective-diffusion equation for pollutant transport represent?

dC/dt + dUC/dx = D*(d^2C/dx^2) - kC

Answer 26

1) change in concentration with time at a point
2) transport of a pollutant with the mean flow
3) spreading of a pollutant due to longitudinal dispersion and turbulent diffusion.
4) kinetic decay of a pollutant

Question 27

What fluid parameters could affect each term in the general advective-diffusion equation for pollutant transport?

dC/dt + dUC/dx = D*(d^2C/dx^2) - kC

Answer 27

1) Would be affected by flood or tide in an estuary and time varying discharge from outfall.
2) Convergence or divergence of estuary or change in depth due to dredging.
3) Change in turbulence properties of flow, e.g. depth or velocity or roughness characteristics, change in velocity distribution.
4) higher decay due tom temperature or light

Question 28

What is the significance of the hydraulic radius?

Answer 28

It measures a balance between the weight component of flow down a slope relative to the resisting force due to boundary shear.

Question 29

What 6 steps need to be taken when measuring the flow across a wide river?

Answer 29

1) Measure the velocity in straight uniform reach. Length should not be less than 3x the river width, measuring section mid-way along.
2) Require fairly uniform stage discharge (h vs Q) curve, i.e. h unaffected by tide, downstream tributaries etc.
3) Require good sensitivity, i.e. appreciable changes in h for measured changes in Q.
4) Require a stable cross-section, i.e. from seasonal weed growth.
5) For low flows, velocity should be in measuring range.
6) Site needs to be accessible.

Question 30

In a river when is it maximum velocity?

Answer 30

When du/dz = 0

Question 31

What is the general difference between T90 and T50?

Answer 31

One is the time taken for 90% of the concentration to decay and the other is the same but for 50% of the concentration to decay

Question 32

State the continuity equation in words.

Answer 32

The net mass of fluid entering and leaving a control volume in time dt is equal to the mass change within the control volume in the same time.

Question 33

On the 19.5mark turbine question, what is the random equation he somehow comes up with for the mass of outflow?

Answer 33

rhoAoCdsqrt(2gh)dt

Question 34

On the 19.5mark turbine question, what is the random equation he somehow comes up with for the change of mass in control volume?

Answer 34

rhoAdh

Question 35

Some disadvantages of a numerical model?

Answer 35

• need high level of technical expertise
• need a lot of empirical coefficients often not known, e.g. roughness.
• many processes often not well understood, e.g. sediment transport decay.

Question 36

What is each term in the one-dimensional momentum equation?

Sf = So - dh/dx - beta(U/g)(dU/dx) - (1/g)(dU/dt)

Answer 36

1) Friction Slope
2) Bed Slope
3) Longitudinal variation in depth.
4) Advective acceleration.
5) Local acceleration

Question 37

What are the steps for the direct step method?

Answer 37

1) Assume a value for h and work out A, U and U^2/2g.
2) Work out E = h + U^2/2g
3) Work out the hydraulic radius and input it into manning’s to get the friction slope.
4) Step 2 onwards - calculate dE = E2 - E1 and Sf = 1/2(Sf1 + Sf2).
5) Work out So - Sf and calculate dx = dE/(So - Sf)