1. Predicting THE EFFICIENCY OF BUFFER STRIPS IN REMOVING sediments and nutrients from overland flow
Hossein Ghadiri
Sina Akram
Bofu Yu
Griffith School of Environment
Environmental Futures Centre
Griffith University, Brisbane, Australia

2. What are Vegetative Buffer Strips?
Zones/strips of vegetation used to slow down runoff and reduce sediment and pollutant delivery to water bodies.
Sometimes called filter strips, riparian zones, grass barriers, grass hedges, buffer strips.
Vegetation includes crops, grasses, pastures, trees, shrubs, native/non native species and combinations of these
Can vary in width, plant density or cover, plant height

3. What are Vegetative Buffer Strips?
Or simply un-cut or un-burnt strips of crop or pasture through which surface runoff passes on its way to surface water bodies

4. How do Vegetative Buffer Strips (VFS) Work?
They reduce sediments, nutrients & pollutant transport through a combination of:
settling (deposition) from overland (surface) flow
infiltration (subsurface/root zone)
Adhesion to soil and plants

5. Deposition From Overland Flow
Rainfall
Stiff vegetation buffer e.g. vetiver grass
Runoff carrying sediments/
pollutants
Ponded area in front of buffer. Velocity slowed so sediment deposits out
Less sediment/ pollutants in runoff due to buffer

6. Trapping Efficiency of Buffers
Literature Source
Trapping efficiency %
Total sediment
Total P
Soluble
Prosser et al.,1999
17 to 99
6 to 97
-225 to 90
Dorioz et al., 2006
53-98
-64 to 93
-83 to 93
McKergow et al. 2004a,b
(Extreme conditions in Far North Queensland)
TSS Bedload>80
-33 to 64
Efficient at trapping sediment
Fairly efficient at trapping particulate-sorbed nutrients
Less efficient for soluble nutrients and fine sediment (clay, silt)-enrichment
As you can see, this highly popular system is highly unreliable, highly variable, and very unpredictable

7. Available Buffer Strip Models
There are quite a few models but none can reliably predict the efficiency of buffer strips or provide help with their designs:
WEPP model (Water Erosion Prediction Project),
Kentucky model,
TRAVA model of Deletic (2001, 2005)
Model by Newham et at. (2005)
These are a few models with Buffer Strip components but all have one or more of the following constraints:
They do not simulate deposition in the ponded area upslope of buffer.
They can not cope with the time dependency of trapping efficiency of buffer strips.
They can not deal with the changes in particle size and concentration of sediment.

8. Griffith University Modeling Work
We began developing an understanding of processes occurring in and around the buffer strips as an initial step to develop a predictive model.
We used an artificially made strip (nails) for our early work to provide us with exact characteristics such as density, diameter, width, height, gaps etc. (Hussein, Yu, Ghadiri & Rose (2007) Journal of Hydrology; Hussein, Ghadiri, Yu & Rose (2007) Soil Science Society of America Journal).

9. Flow Through Nail Strips
Back water
Hydraulic jump

10. Flow Hydrology and Sediment Trapping Efficiency Studies
Deposition upstream of nail bed

11. Characterizing backwater and hydraulic jump

12. Model Development
Our first attempt was to develop theoretical models describing flow characteristics and the magnitude & extent of deposition in the backwater and then tested them using experimental data.

13. VBS (or GUSED-VBS)
VBS is an implementation of our theoretical approach.
VBS simulates the evolution of the deposited layer by dynamically adjusting bed elevation, water profiles and velocity as a result of sediment accumulation.

14. VBS
VBS attempts to solve the following set of coupled ordinary differential equations:
where y is the water depth (m); x is the distance in down slope direction (m); Sf is the frictional slope (m/m); So is the bed slope (m); Fr is the Froude number, q is the unit discharge (m2/s); ci is the sediment concentration in size class i (kg/m3); vi is the fall velocity for size class i (m/s); F is the fraction of stream power available for entraining sediments; ρ is the water density (kg/m3); σ is the wet density of sediments (kg/m3); V is the velocity of flow (m/s); zo is the bed surface elevation (m), t is the time (s); c is the total sediment concentration (kg/m3); and λ is the porosity (m3/m3).

15. VBS where n is the Manning’s roughness coefficient.
Equation (1) is the governing equation for steady gradually varied flow, namely the backwater equation (Chow, 1959). The first term on the right hand side of equation (2) is the sink term due to deposition; the second term due to re-entrainment of deposited sediments (Rose et al., 2003). The frictional slope in Equation (1) is evaluated using the Manning’s equation:
where n is the Manning’s roughness coefficient.
Bed slope and total concentration are given by:

16. GUSED_VBS
The coupled ordinary differential equations are solved for each time step. (Runge–Kutta method with adaptive step-size control)
A routine implemented using Cash–Karp parameters for the embedded Runge–Kutta method (Cash and Karp, 1990; Press et al., 1992).
The bed profile was updated at the end of each time step.
This in essence assumes a series of steady state solutions of flow and sediment transport and deposition.

17. Model Input
These can be generated from the GUSED model if rainfall/slope/soil type/ cover etc is known
Data needed to run the model are:
Incoming flow rate             
Incoming sediment concentration.
Incoming sediment sizes  
Slope
Wet density of sediment
Porosity
Water density
Manning's n
Backwater depth at start of buffer
Time intervals & total time
Distance increments upstream of buffer and total distance
The inputs for the model are readily obtainable from many field and laboratory studies

18. Model Output: Water and sediment profiles behind buffer
 Mass of deposited sediment in the backwater
 Amount and size distribution of sediment passing through the buffer

19. Experimental Program to Test the Model

20. Experimental work in GUTSR
Buffer system set up inside flume (vetiver grass)
Buffer strip was 30 cm width (similar to a row of vetiver in arable field after 1 year growth)
Applied flow & sediment
Subcritical flow ( Froude <1)
3 types of sediment
3 slopes
Measured resultant flow depths, runoff, sediment deposition, particle size, nutrients

21. Sediment deposition

22. Experimental Results: Sediment deposition in the backwater
Coarse texture soil
Fine texture soil

23. Comparison of simulated versus measured water profiles

24. Comparison simulated versus measured data sediment profiles

25. Simulated versus measured particulate P retention
Simulated lower than measured data but followed similar trends –
P retention decreased with increasing fineness of soil and with increasing slope

26. Overall Performance of the Model
The model successfully simulated:
Water profiles, with and without sediment.
Sediment profiles.
Mass of sediment deposited upslope of the buffer.
Particle sizes of deposited sediment
Overall the model appears to provide a very good start for estimating sediment retention by grass strips at subcritical flow.

27. Potential of VBS Model as a Design Tool Modelled trapping efficiency of buffer strips

28. Further Development of our Buffer Strip Model
Further development to include inside and downstream of buffer
Include coupled flow – surface & root-zone
Expand into trapping of P and other sorbed nutrients
Fine sediment and reduced soluble P
Ponded area in front of buffer. Velocity slowed so coarser sediments (with particulate-P) are deposited
Grass buffer
Infiltration followed by P transformations in the soil matrix
Rainfall
Runoff carrying sediments, particulate- P and soluble P

29. Griffith University Previous Researches
Expansion of the VBS Model

30. Modelling Water Profile Inside the Buffer
Theory
Upstream prior to BW BW D1
Dmax
Dmax +/- f(d).
The value of d depends on uniformity of plants in buffer
LBF = length of buffer
Decay curve
Dmax +/-d D1 D2 D3
(D3>D1) L

31. Water profile simulation
Variable n ≈ a Fr -1

32. Conclusion
A new theoretically-based buffer strip model is being developed (VBS)
The first version of the model satisfactory predicts water profiles, sediment deposition & deposition of particulate nutrients in the backwater.
Theories for the inside buffer and downstream side of the buffer strip has also been developed.
Field and flume testing the new expanded version of VBS is currently being conducted.
The next step will be to account for the infiltration of water, sediment and nutrients inside the buffer strip (coupled surface/rootzone flow).
The addition of soluble nutrient trapping component will complete the process and produces a field applicable model.

33. Thank you

36. Froude Number (F) and Roughness Coefficient (n)
Froude Number is a dimensionless number, for shallow surface water is:
Where V is flow velocity and C is a characteristic water wave propagation velocity, g is gravitational acceleration, A is cross-sectional area and B is free surface width.