Labs Sediment Transport Mechanics
Sediment Transport Mechanics by Greg Tucker & Bob Anderson, Spring 2013
Modeling assignment # 1. Experiments with water volume balance in buckets and basins
Goals: You will gain experience with: setting up a code in IRF format; setting up simple arrays; stepping incrementally in time; and in plotting your results.
a) Write a MATLAB code that answers the following question: given a fixed input of water to the bucket, and a hole in the base of the bucket, how will the water level in the bucket evolve? Assume that the bucket starts with no water in it. Produce a plot of water level as a function of time.
b) Now allow the inputs to vary through time, in particular allow the input to oscillate sinusoidally. Specify a period and amplitude of the oscillation about the mean. Again, answer how the water level in the tank will vary through time. Produce a two-part plot in which you display both input and output of water on a top plot, and the water level on the lower plot.
Answers to Leaky Bucket assignment # 1a &1b.
1.Bob Anderson's demonstration code
Now let’s take this to a little bit more real setting, a closed basin lake in which there is input solely from runoff from the surrounding landscape, and outputs solely from evaporation from the lake surface. One of the most famous examples in the US is Mono Lake in eastern California at the foot of the Sierras.
We start with a generic lake basin, and move on to a more real basin after developing a simple model.
Consider a simplistic closed lake basin, called Snowcone Basin, shown in the figure below.
The lake basin: The drainage basin has an area Ab. There are no outlets. The geometry of the basin is a cone, with the tip of the cone at an elevation of 1000 m, and a spread angle for the cone of a.
The climate: On the average, it precipitates in the basin, uniformly (by which I mean evenly over the entire area), at a rate P (in meters of water equivalent per year). The climate is such that it also evaporates off any standing water in the basin at a mean annual rate E (also in meters per year).
The runoff: The precipitation is imperfectly converted to runoff when the precipitation falls on any surface but the lake itself, some of it being lost to transpiration and evaporation from the surface of the basin and its vegetation before the water can make it to any lake in the basin. The conversion factor, called the runoff coefficient, is . Of course, any precipitation that falls on the lake is not affected by this factor. Given this information, let us now ask several questions of the lake in this basin. Note that in all cases, E>P. (You should ponder how you could possibly get a lake under such circumstances!)
a) Assume that the lake in the basin has achieved a steady state condition, meaning that it has found an equilibrium in which the lake level and the lake area are constant in time. Write out the equation for the water balance that this implies (rate of volume into the lake = rate of volume out of the lake), and solve it for the expected altitude of the lake surface, z.
b) Now let’s ponder a specific basin. Let the area of the basin be 1000 km2 and the spread angle of the basin be 4°, an angle typical of the slopes of alluvial fans. Let the precipitation, P, be 0.1 m/yr, the runoff coefficient be 0.5, and the evaporation, E, be 2.5 m/yr. Assuming that these conditions have prevailed for some long time, at what altitude, zl, do you expect to see the modern shoreline? Now plot the expected lake level as a function of a range of possible mean annual precipitation, from P=0 to P=0.8m/yr.
c) We now travel to the edges of the modern lake in Snowcone Basin, and discover that there are lake shores well above the level of the modern lake. We measure the altitude of the paleo-shoreline to be 1250 m. Quantitatively, what can you tell us about the climatic conditions that might have prevailed at the time of the formation of this shoreline? I would like to know by what percentage of the modern precipitation the past precipitation must have been enhanced to raise the lake to this level, if the evaporation is held constant, and by how what percentage the evaporation in the past must have been reduced relative to the present, if the precipitation is held constant. (Likely the answer is of course some combination of these effects...and please assume the runoff coefficient is also unchanging.)
That gives you some sense of what controls the steady state lake level. Another type of question we can ask of this simple system is one of time scales. Think about how fast the system should respond to changes in the climatic forcing. While the basin geometry remains constant over these time scales, as it is determined by long term tectonic and landscape evolution processes, the climate can change very rapidly, causing changes in the lake surface elevation, the hence lake area and the lake volume. Clearly, this is important to think about if the lake water is being put to some human use, or if we have to deal in some way with expanded lake surfaces. For example, I-80 runs right along the shoreline of the Great Salt Lake, at an elevation not more than a few meters above it!
To answer this last question, construct a lake model in which you track water balance in the lake, and predict a history of lake level. We ask what is the characteristic time scale for responding to a change in climate. Does it take a decade or a century to respond? Is there hysteresis, meaning does it respond with one timescale when rising, another when lowering?
d) To accomplish this, construct a generic lake model, and spin it up from zero water in the basin until it reaches steady state (or very close to it). You will have to explore to find the appropriate time step and the appropriate model duration. Please show us histories of lake level, area and volume. Now assess this characteristic time of the lake. In radioactive systems we often use the “half-life” of a system, but more formally, it is better to define the characteristic time by determining the time it takes to reach (1-(1/e)) of its steady state value.
Answers to Closed Lake assignment # 1c &1d.
