Category Archives: dissertation research

Hurricane Harvey update

The Greater Houston Metro just got pounded by one of the largest storms (in terms of rainfall) on record: Hurricane Harvey. There was widespread flooding across all parts of the metro, but my home and Rice university were largely spared. The skies are clear today, after ~5 days of nearly continuous rainfall. However, the cleanup effort for this disaster will last years and cost billions of dollars. Nonetheless, I am certain that Houston will come back stronger and better than ever. This ordeal has made me even more proud to be a Houstonian.

At my house, we had a small leak in an interior bathroom, but because we received so much rain over the duration of the event, the roof became saturated and the sheetrock partially collapsed. No one was hurt, thankfully, and repairs are already underway.

I am currently working on an NSF RAPID grant, which I compiled the following figure for. This figure shows total rainfall during the Harvey event with the City of Houston (CoH) labeled. The precipitation data were collected from the National Weather Service and then summed to produce the total rainfall numbers for the below plot. I am proposing some work on the Brazos River (BR) so this feature is also labeled.

Outreach module — Flooding risk in low-lying landscapes

I have put together an outreach module that describes some of the risks of flooding in low-lying landscapes. The module runs in Matlab, either within a licensed environment, or with the Matlab Runtime Environment (which is available to anyone).

Accompanying the GUI is a worksheet that steps through all the aspects of the GUI and attempts to demonstrate the principles of flooding in deltas without detailing the math or physics behind the model. My hope is that it will be helpful to High School educators in their course work.

So far, I have only written a worksheet targeted at 9-12th graders, but plan to write two more (one for younger students, and one for more advanced undergraduate/graduate students) worksheets in the near future.

Below is a demonstration of the GUI. The full information for the module (including the source code) is on my GitHub here. The project website is here.

 

 


This material is based upon work supported by the National Science Foundation (NSF) Graduate Research Fellowship under Grant No.145068 and NSF EAR-1427177. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

China field work 2017

I’m heading off tomorrow for my third summer field campaign in China. That also means that I’ve completed three years of my PhD.

I always seem to act a bit introspective around this time of the year, reevaluating decisions, remembering achievements, and reliving failures. This year hasn’t been especially easy for me; I’ve lost two important people in my life, and I’ve struggled to get my research moving at a pace that I feel is fast enough. Nonetheless, I’ve done a lot of good things this year too; I will be an author on (at least) two papers coming out this year, I won a service award for my department, and I am starting a new series of symposia at Rice that I’m especially excited about.

Our field campaign marks another journey halfway around the world to collect data on one of the most exceptional rivers in the world. My research goals for this year are to collect water column data during a flood. My analysis of the last two years’ data suggests that at high discharges, the concentrations of sediment become sufficiently high to dampen turbulence in the flow and introduce “density stratification”. I’m hoping to constrain the development of the density stratification in the lower Yellow River during this field campaign.

I’ll be working with some of the best people on our field campaign. Our campaign will be led by myself, Brandee Carlson, my advisor Jeff Nittrouer, and Hongbo Ma, and we will be helped (immensely) by Tian Dong, Chenliang Wu, Eric Barefoot, Dan Parsons, and Austin Chadwick.

Cheers for another year! Wish us luck!

Brandee Carlson and me towards the end of our 2016 field campaign

The exceptional sediment load of fine-grained dispersal systems: Example of the Yellow River, China — Ma et al., 2017

We’ve just published an exciting new paper in Science Advances which assess the transport of sediment in fine-grain river systems. The research is driven by Postdoctoral Researcher Hongbo Ma, who is the first author on the publication. I led the field survey and processed the Multibeam data of the Yellow River channel bed that you see in a few of the figures in the paper.

Hongbo has identified a physical explanation for why fine-grain rivers are able to move so much sediment. In short, it has to do with the organization of the channel bed, whereby dunes are wiped out at high Froude number flows with a small grain size on the bed. This reduces the form drag in the river and allows for more skin friction on sediment to bring into suspension. Hongbo continues to make strides in identifying a “phase transition” in sediment transporting systems that helps to explain the observations made in the Yellow River.

Yellow River at Hukou Waterfall. The river here is a bedrock-alluvial river, but this image provides a good demonstration of the comparatively massive volume of sediment transported by the Yellow River

You can get the paper here, or uploaded to my site as a pdf here.
There is a full article about the research with loads more information, including a video interview, from the Rice press department here. And an article from the National Science Foundation (funding organization) here. A Scientific American video explaining some of the research is available here.

 

Vibracore extraction tripod engineering drawings — Vibracore system

For our research in China, I was charged with building a Vibracore system. The Vibracore works by utilizing a concrete vibrator to rapidly vibrate an upright thin-walled aluminium pipe into the sand/dirt/mud below. A tripod is then set up over the in-ground pipe to pull it up from the ground. The pipe (now called a core I suppose…) is then cut open with a saw and analyzed/sampled.

dimetric view of assembled tripod

This system is nothing we invented, although I’m not sure of its origin. I based the design for our tripod on an apparatus that our colleague John Anderson has in his collection of field equipment. Our only substantial modification to the design was to make the legs of our system separable so that instead of a solid 10′ pipes of aluminium, we have two 5′ pipes, joined by a coupler. This is quite useful for us, since we send our system to China each year, and it makes it much easier to handle for shipping.

I recently made some engineering drawings of our system for a colleague and figured I would share them here in case they may be helpful to others. You can find the plans as a .pdf file here, or explore the system in three dimensions in the software they were designed in (OnShape CAD) at this link.

example drawing: head assembly top plate

Rice crew Vibracoring the Yellow River delta

In the future, I hope that my colleague Brandee Carlson (who leads the research using the Vibracore) and I can write a bit of an updated guide to the system based on our experiences using the system in the field, but for now I’ll just leave you with a few references for the system design below.

Land-based Vibracoring and Vibracore analysis: Tips, Tricks, and Traps. Occasional Paper 58. Thompson, T. A., Miller, C. S., Doss, P. K., Thompson, L. D. P., and Baedke. 1991.

Collection and analysis techniques for paleoecological studies in coastal-deltaic settings — Robert A. Gastaldo

Building a simple delta numerical model: Part VI

This will be the final piece of the model that we need to get to have a working code for delta growth: the time routine. We will define a few more terms in order to set up the model to be able to loop through all the time steps and update the evolving delta.

T = 500; % yrs
timestep = 0.1; % timestep, fraction of years
t = T/timestep; % number of timesteps
dtsec = 31557600 * timestep; % seconds in a timestep

T is the total time the model will be run for (in years), timestep is the fraction of year that will be simulated with each timestep, expressed in seconds as dtsec, and t is the number of timesteps to loop through. Now we simply take our block of code that we’ve built up to calculate all the propertyies of the delta (slope, sediment transport, deposition, etc.) and surround it with a for statement:

for i = 1:t
[S] = get_slope(eta, nx, dx); % bed slope at each node
[H] = get_backwater_fixed(eta, S, H0, Cf, qw, nx, dx); % flow depth
U = Qw ./ (H .* B0); % velocity
[qs] = get_transport(U, Cf, d50, Beta);
qsu = qs(1); % fixed equilibrium at upstream
[dqsdx] = get_dqsdx(qs, qsu, nx, dx, au);
[eta] = update_eta(eta, dqsdx, phi, If, dtsec);
end

To explain in words the above block, now, we are going to go through every timestep i and calculate the slope of the bed (eta) everywhere in the model domain, then we use this slope (and other defined parameters) to determine the flow depth and velocity through the entire delta. The velocity is used to calculate sediment transport, and the change in sediment transport over space produces a change in the channel bed elevation over time (i.e., the Exner equation). Finally, we return to the top of the loop to calculate a new slope based on our new bed.

That’s it! Our delta model is now complete and we can outfit it with all sorts of bells and whistles to test hypotheses about delta evolution in the natural (or experimental) world. A powerful tool indeed!

Below is a simple movie output from this model that shows the results of our hard work! The complete code for the delta model can be found here.

Note that there is a small instability that grows at the front of the sediment wedge, this isn’t a huge problem depending on what you want to do with your model, but you can tweak dx and dt to make things run “smoother” (see the CFL number for more information).

This material is based upon work supported by the National Science Foundation (NSF) Graduate Research Fellowship under Grant No.145068 and NSF EAR-1427177. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Building a simple delta numerical model: Part V

Now we need to add a routine to update the channel bed, based upon the calculated change in sediment transport over space from the previous step. We’ll use the calculation from the last Part of the tutorial (get_dqsdx) in order to update the channel bed (η) at the end of each timestep. Define the following parameters

phi = 0.6; % bed porosity
If = 0.2; % intermittency factor

where If is an intermittency factor representing the fraction of the year that the river is experiencing significant morphodynamic activity. We are basically assuming that the only major change to the river occurs when the river is in flood. One year is the temporal resolution for the model, which we’ll define in the next Part.

function [eta] = update_eta(eta, dqsdx, phi, If, dtsec)
eta0 = eta;
eta = eta0 - ((1/(1-phi)) .* dqsdx .* (If * dtsec));
end

This module works simply by multiplying our vector for change in sediment transport capacity over space to the Exner equation (reproduced below) to evaluate the change in bed elevation over time (i.e., at the next timestep).

There isn’t really anything exciting to show at this stage, as we’ve only calculated the change in the bed for a given dqsdx vector, which represents only a single timestep. In the next Part, we’ll add a time routine to the model, completing the setup required for the simple delta model.

This material is based upon work supported by the National Science Foundation (NSF) Graduate Research Fellowship under Grant No.145068 and NSF EAR-1427177. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Building a simple delta numerical model: Part IV

In this part of “Building a simple delta numerical model”, we’ll write the part of our model that will solve the “Exner” equation, which determines changes in bed elevation through time. We’ll start with the module in this post, and then implement the time-updating routine in the next post of this series.

We’re going to make an assumption about the adaptation length of sediment in the flow in order to simplify the Exner equation. The full Exner equation contains two terms for sediment transport, a bed-material load and a suspended load term. The bed material load is probably transported predominantly as suspended load in an actual river, but because it is in frequent contact and exchange with the bed (i.e., suspension transport is unsteady), the flow and bed can be considered to be in equilibrium over short spatial scales, or rather, there is a short adaptation length.

It has been suggested that in fine-grain rivers the adaptation length of the flow to the bed conditions, and vice versa, is so long that sediment stays in suspension over a sufficiently long distance that the fraction of sediment mass held in suspension must be conserved separately from the bed fraction of sediment. We’re going to neglect this idea, because 1) we’re considering medium sand which likely has a short suspension period, and 2) the process may actually not be that important for fine-grain rivers either (An et al., in prep).

So, the Exner equation is then given by (Equation 1):

where λp = 0.6 is the channel-bed porosity, η is the bed elevation, t is time, qs is sediment transport capacity per-unit channel width, and x is the downstream coordinate. The form of Equation 1 shows that a reduction in the sediment transport over space (negative ∂qs/∂x) causes a positive change in the bed elevation over time (∂η/∂t).

In the last post we calculated a vector qs for the sediment transport capacity everywhere, and now we simply need to calculate the change in this sediment transport capacity over space (x). We’ll do this with a winding scheme and vectorized version of the calculation for speed. This guide is not meant to be comprehensive for solving PDEs or even winding calculations, and we already did a similar calculation for slope, so I’ll just present the code below:

au = 1; % winding coefficient
qsu = qs(1); % fixed equilibrium at upstream

function [dqsdx] = get_dqsdx(qs, qsu, nx, dx, au)
dqsdx = NaN(1, nx+1);% preallocate
dqsdx(nx+1) = (qs(nx+1) - qs(nx)) / dx; % gradient at downstream boundary, downwind always
dqsdx(1) = au*(qs(1)-qsu)/dx + (1-au)*(qs(2)-qs(1))/dx; % gradient at upstream boundary (qt at the ghost node is qt_u)
dqsdx(2:nx) = au*(qs(2:nx)-qs(1:nx-1))/dx + (1-au)*(qs(3:nx+1)-qs(2:nx))/dx; % use winding coefficient in central portion
end

We’ve introduced two new parameters in this step, a winding coefficient (au) and an upstream sediment feed rate (qsu). The sediment feed rate constitutes a necessary boundary condition for solving the equation. Here, we set this boundary to be in perfect equilibrium by defining the feed rate as the transport capacity of the first model cell; however, this condition has interesting implications for the model application (more discussion on this in Part VI!).

The calculation then, for a single t (which we implicitly set to be 1 second with out sediment transport calculation), the change in sediment transport capacity over space is maximized in the backwater region.

This vector is converted to the commensurate change in bed elevation over this single t by multiplying by (1/(1-λp)). Of course, we can consider many ts at once, and so the vector would again be multiplied by the timestep (Δt). We’ll consider this in the next post, where we add a time loop to the model, such that on ever timestep, the backwater, slope, sediment transport and change in bed elevation are each recalculated and updated so that we have a delta model which evolves in time!

The code for this part of the model is available here.

 

This material is based upon work supported by the National Science Foundation (NSF) Graduate Research Fellowship under Grant No.145068 and NSF EAR-1427177. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Building a simple delta numerical model: Part III

In this part of “Building a simple delta numerical model”, we’ll simply develop a module for calculating sediment transport at all locations within our model domain. This is naturally a critical piece of the model to get right, because the magnitude of sediment transport that occurs within the delta system will have a direct correlation with the magnitude of geomorphic change to the delta system. Just as before, we’ll be working our module for the Mississippi River, and so we will use the Engelund and Hansen, 1967 formulation for bed-material load sediment transport.

It is worth noting here that there are countless sediment transport equations published within the literature, with applicability for different scenarios, covering the entire range of alluvial rivers, the shelf environment, to turbidity currents. A nice review of some relations is given by Garcia, 2008, who produces the plot below with six selected relations, where τ* is shear stress (Shields number) and q* is a dimensionless sediment transport.

garcia_formulas

The only points I hope to make with this figure are that stress and transport are nonlinearly related, and that many of the relations are quite similar. In fact, the majority of sediment transport equations are based on an excess-shear stress formulation, where, as shown above, the sediment transport prediction scales with the shear stress. The Engelund and Hansen, 1967 equation is given in its dimensional form by (Equation 1)

eh_formula

where qs is the sediment transport per unit width (m2/s), β is an adjustment coefficient discussed below, R = 1.65 is submerged specific gravity of the bed sediment, = 9.81 m/s2 is gravitational acceleration, is the grain size in question, Cf is the coefficient of friction, ρ = 1000 kg/m3 is the fluid density, and τ = ρ Cf U2 is the fluid shear stress, where is depth-averaged fluid velocity.

The solution to this equation is obtained by simple algebra, and so I will display the code calculations below without any explanation.


U = Qw ./ (H .* B0); % velocity

function [qs] = get_transport(U, Cf, d50, Beta)
% return sediment transport at capacity per unit width
% 1000 = rho = fluid density
% 1.65 = R = submerged specific gravity
% 9.81 = g = gravitation acceleration
u_a = sqrt(Cf .* (U.^2));
tau = 1000 .* (u_a.^2);
qs = Beta .* sqrt(1.65 * 9.81 * d50) * d50 * (0.05 / Cf) .* ...

        (tau ./ (1000 * 1.65 * 9.81 * d50)) .^ 2.5;
end

Let’s first simply examine the behavior of Equation 1, in its dimensionalized form, for the Mississippi River. We need to define the grain size and adjustment coefficient to use in our model; we’ll follow from the literature and use 300 μm for the grain diameter, assuming this makes up the entirety of the bed, and β = 0.64 an empirical adjustment (Nittrouer et al., 2012; Nittrouer and Viparelli, 2014).

Evaluating the equation over a range of velocities from 0 to 3 m/s, we find that the nonlinear behavior shown in the above figure is also true for the Engelund and Hansen formulation.

 

Now, let us implement this into our delta model. Following from earlier, the flow depth and velocity are related by the cross sectional area and discharge, where for a rectangular cross section, the velocity = Qw / H B. We therefore can take our calculation of flow depth from the previous lesson, and solve for velocity at each location. We then simply plug this into the relation for τ given above and solve Equation 1.

 

The bottom plot reveals the behavior of the flow velocity calculated through the backwater region (shown in red), and the resulting sediment transport calculated (shown in green). The nonlinearity between stress (i.e., velocity) and transport is again revealed by the change in slope of the lines through the backwater region. Following from left to right along the green curve, it becomes obvious that there is a decrease in transport downstream. If there is more sediment being transported upstream than downstream, then there must be sediment that is deposited to the bed somewhere along the channel bed between ‘upstream’ and ‘downstream’.

This thought experiment represents one half of the famous Exner equation, which we will explore in the next part, and will provide the last module to complete our delta model. My complete code for the sediment transport calculation in our backwater delta model can be found here.

 

References

1. Engelund, F. & Hansen, E. A monograph on sediment transport in alluvial streams. (Technisk Vorlag, Copenhagen, Denmark, 1967).
2. García, M. H. Sediment Transport and Morphodynamics in Sedimentation engineering processes, measurements, modeling, and practice 21–163 (American Society of Civil Engineers, 2008).
3. Nittrouer et al., Spatial and temporal trends for water-flow velocity and bed-material sediment transport in the lower Mississippi River. 2012. Geological Society of America Bulletin, 5 (Table 1).
4. Nittrouer, J. A. & Viparelli, E. Sand as a stable and sustainable resource for nourishing the Mississippi River delta. Nature Geoscience, 7, 350–354 (2014).

This material is based upon work supported by the National Science Foundation (NSF) Graduate Research Fellowship under Grant No.145068 and NSF EAR-1427177. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Building a simple delta numerical model: Part II

Taking the simpler form of the backwater equation, that is, assuming that width does not vary over the channel reach, we find an expression for changing flow depth along the channel streamwise coordinate x: (Equation 1)backwater_fixed_eqnwhere S is the channel bed slope, Cf  is the dimensionless coefficient of friction, and Fr is the Froude number: (Equation 2)

A couple quick notes about the overall delta model we are developing herein:

  1. The code for this entire model will be developed in Matlab syntax, however, the math is all just math, and the code could easily be translated to another programming language.
  2. The model will be set up for the conditions of the Mississippi River, to what I feel are the best constraints published in the literature. Surely not everyone will agree with some of my choices.

We set up our model domain by defining some parameters:

L = 1200e3; % length of domain (m)
nx = 400; % number of nodes (+1)
dx = L/nx; % length of each cell (m)
x = 0:dx:L; % define x-coordinates
start = 63; % pin-point to start eta from (m)
S0 = 7e-5; % bed slope
eta = linspace(start, start - S0*(nx*dx), nx+1); % channel bed values
H0 = 0; % fixed base level (m)
Cf = 0.0047; % friction coeff

bw_schem_label

Each of these terms are necessary for defining the boundary conditions the backwater calculation requires. We will additionally need to define the water discharge (Qw), and it’s width averaged derivative qw = Qw / B, where B is the channel width. We’ll use a fixed width of 1100 m (Nittrouer et al., 2012) and a discharge of 10,000 m3/s; since width is fixed in our model, qw = 31.8182 m2/s.

Finally, to solve the above backwater equation, we’ll need to find the local slope at all nodes — our bed holds a constant slope so we could just skip this step and send S0 directly into the calculation, but we’ll be wise beyond our present experience, and prepare our calculation now for the future when our bed slope is constantly changing in time and space. A simple slope calculation is completed by a central difference calculation, applying forward difference and backward difference calculations at the domain upstream and downstream boundaries, respectively.

function [S] = get_slope(eta, nx, dx)
% return slope of input bed (eta)
S = zeros(1,nx+1);
S(1) = (eta(1) - eta(2)) / dx;
S(2:nx) = (eta(1:nx-1) - eta(3:nx+1)) ./ (2*dx);
S(nx+1) = (eta(nx) - eta(nx+1)) / dx;
end

As described in the previous section, we will use a predictor-corrector scheme to find a solution to Equation 1. This is not meant to be a comprehensive guide on solving ODEs, and so I will only summarize the method here.

  1. We first can evaluate the flow depth H at the downstream boundary, where elevation is fixed (H = H0 – η = 21 m).
  2. Then we predict the value at the next upstream node by solving Equation 1 for dH/dx at our known downstream-most node, using the inputs described above and H and S at the known node. This yields dH/dxp = 6.5784e-05.
  3. Multiplying dH/dx by dx = 3000 m and subtracting this change from the known H at the downstream-most node, we find our predicted value Hp for the next upstream node.
  4. We now repeat the evaluation of Equation 1 with the same inputs except using our new Hp value in Equation 2. This yields our corrected change in flow depth over x:
    d
    H/dxc = 6.5663e-05.
  5. We then combine the results of these two predictions in flow depth change to give our best solution to the ODE Equation 1 at the downstream node. We combine the values using the trapezoidal rule for Hi = H(i+1) – ((0.5) * (dH/dxp + dH/dxc) * dx), giving a predicted change in flow depth of 0.1972 m, for a depth at Hi of 20.8028 m (it was 21 m at the downstream node).
  6. repeat steps 2-5 solving for each node moving up the model domain until the final node is determined.

This method is fairly stable for most discharges, but may fail when discharge is very low. In thsi case, the backwater region becomes condensed, and the rate of change in H becomes very large over a very small distance. My code for the predictor corrector implementation can be found as the get_backwater_fixed() function within the complete backwater_fixed model. Below is a gif showing the results of a range of input discharges to our model.

 

and a still multi-surface example to demonstrate the variability.

 

The low and moderate discharge water surface curves in the above figure (solid and dashed lines) demonstrate the classical convex-up (aka M1, e.g., Chow, 1959) condition that characterizes lowland delta systems during most of the time. The third, high discharge, water surface profile exemplifies a convex-down water surface profile. This condition, called the M2 condition has been recently demonstrated to have importance to the cyclic processes of avulsion and delta-lobe growth (e.g., Chatanantavet et al, 2012; Ganti et al., 2016).

 

Building a simple delta numerical model: Part III

 

References:
1. Nittrouer et al., Spatial and temporal trends for water-flow velocity and bed-material sediment transport in the lower Mississippi River. 2012. Geological Society of America Bulletin (Table 1).
2. Chow, Ven Te. Open Channel Hydaulics. N.p., 1959. Print. McGraw-Hill Civil Engineering Series.
3. Chatanantavet, Phairot, Michael P. Lamb, and Jeffrey A. Nittrouer. Backwater Controls of Avulsion Location on Deltas. 2012. Geophysical Research Letters 39.1
4. Ganti, V., A. J. Chadwick, H. J. Hassenruck-Gudipati, B. M. Fuller, and M. P. Lamb. Experimental River Delta Size Set by Multiple Floods and Backwater Hydrodynamics. 2016. Science Advances 2 (5).

This material is based upon work supported by the National Science Foundation (NSF) Graduate Research Fellowship under Grant No.145068 and NSF EAR-1427177. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.