Landscape lab

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Name:____________________
ESS – 211 Lab
Modeling Landscape Evolution
This lab is adapted from materials found at http://www.niu.edu/landform/home.html
In this lab you will investigate how mathematically based geomorphic models are
constructed and use a simple numerical (computer) model to see how an artificial
landscape responds to changes in several geomorphic variables. Although some parts of
the lab are to be done in groups of 3, you will turn in your own individual lab packet
Learning goals: Students will
- Recognize basic features of drainage networks and drainage basins
- Understand factors that influence rates and patterns of landscape evolution
- Predict the landscape patterns that result from changes in geomorphic variables
- Test predictions, and explore the effects of different factors that influence
landscape evolution over geologic timescales
Tasks:
- Read introductory material in lab and online and answer associated questions
- Complete WILSIM tutorial and associated questions
Part 1 – Introduction
Landforms are the result of Earth-surface processes that operate over geological
timescales (typically thousands to millions of years), and they can provide important
clues to past processes and events related to global change and even human impacts.
Landform evolution involves complicated interactions among physical processes and
environmental factors, including underlying rock structures, tectonics, rock types, climate
and climate changes, and human activities – all occurring over a wide range of spatial and
temporal scales. Unfortunately, human lifetimes are too short to observe long-term
landform evolution directly. Moreover, because the factors that influence Earth-surface
processes and their interactions are complex, using observations of modern processes
alone it can be difficult to infer which underlying factors give rise to the landscapes we
see today and to understand how Earth-surface processes work over geologic time.
Computer simulation is a great tool to help us understand the complex effects of a variety
of physical and geological processes that interact to influence landform evolution over
geologic time. There are many models out there, but few are easily accessible and simple
enough to use without specialized software. The Web-based Interactive Landform
Simulation Model (WILSIM) we will use today is designed to help you better understand
landform evolution from the comfort of any computer with an internet connection and a
Java-enabled web browser. Using WILSIM, you will be able to explore and observe how
landforms evolve as you change different parameters (such as rock erodibility, rainfall
intensity, and/or tectonic rock uplift rate) interactively. But first, we will review some
general concepts and principles of landform evolution.
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Drainage networks and drainage basins
We can represent most continental landscapes by dividing the world into hillslopes and
rivers. We can observe their characteristic forms today, but what controls their evolution?
Ultimately, the rivers form the “skeleton” of the landscape and set the lower boundary
condition for hillslopes, so we must consider river networks and the factors that influence
their shapes to understand the landscape as a whole.
One of the most commonly observed patterns of river
systems, or drainage networks, is the branching
dendritic pattern see in the image on the left (from the
Greek dendrite, which means tree). This self-similar
pattern can be observed from the small scale of a rill on
a newly eroded surface to the continental scale
drainages that evolved over long geological times (e.g.,
the Mississippi, Amazon, Congo, and Yellow River
systems). A question we will address today is: How do
branching drainage networks get started, and what
controls their evolution?
Left: Dendritic drainage pattern in Yemen (space shuttle
photograph from http://www.solarviews.com/cap/
earth/yemen.htm)
A drainage basin (also called catchment or watershed)
includes both the river network and the hillslopes in
between. Every drop of water within the area covered by
the drainage basin travels down hill and down slope, and
eventually gets funneled to a single point. This point is
usually at the exit of the basin (like in the diagram shown
on the right), although there are such things as internally
drained basins, which have no outlet. This point can be a
body of water, such as a river, lake, or ocean, or even a dry
lake or a point where surface water is lost underground.
Each drainage basin is separated topographically from
adjacent basins by a drainage divide, which is an area of
elevated topography such as a ridge or mountain. Drainage
basins are self-similar and nested (see sub-basins in the
diagram); they drain into other drainage basins in a hierarchical pattern, and they have
scale-independent characteristics. As a result, it can be tough to tell the scale of a
drainage basin on a map or sketch without a scale bar!
(1) On the image on the upper left of this page, draw the outlines (drainage divide)
of three nested drainage basins.
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What factors influence landform evolution?
Drainage basin evolution involves complicated interactions among exogenous factors like
climate, climate changes, and human activities, and endogenous factors like tectonics
(which drives long-term rates of rock uplift) and underlying rock types and structures. A
useful concept for understanding how rivers respond to such factors is Stream Power, the
“carrying capacity” of a river.
Stream Power = K Am Sn
where K is a constant related to the erodibility of the substrate, A is the area drained by
the river (related to discharge, which is the volume of water transported/time), and S is
the local slope of the channel. The values of the exponents m and n describe the relative
importance of slope and discharge in causing erosion (m and n are debated, but are
generally accepted to be in the range of 0.5 to 2).
Every reach of a drainage network has a certain sediment carrying capacity (stream
power). If sediment influx from hillslopes exceeds this capacity, deposition occurs.
(2) If the carrying capacity (power) exceeds the sediment load, what occurs?
Consider the following four important factors:
2. Erodibility. The durability of the surface rock or soil and how easily it can be eroded
certainly influence landscape evolution.
(3) On a natural landscape, what are the primary causes or processes responsible
for variation in erodibility of the land surface?
(4) A detachment limited (bedrock) river has much lower erodibility than a
transport limited (alluvial) river. Which do you think will respond more quickly to
perturbations (e.g., increased rainfall).
1. Climate. Climatic variables play a key role in drainage basin form, in river and
hillslope form and process, and in the evolution of a drainage basin through time. Annual
variations in temperature, precipitation, and seasonality of precipitation work together to
influence the degree of chemical and physical weathering of hillslope materials, the depth
of weathered materials or soils that develop, and, importantly, to determine the vegetation
type and percentage of cover across a landscape. Vegetation cover in turn helps control
hillslope form and mass movement process, and therefore many attributes of the drainage
basin.
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(5) give an example of how a change in one climate variable might change the
capacity of rivers to erode and transport sediment.
Variable:
How will a change in this variable increase/decrease stream power?
1. Steepness. We learned in class that the flux of soil due to soil creep processes on a soilmantled hillslope is proportional to the local surface slope (or steepness). We can see
from the stream power equation that the steepness of the slope over which the streams
flow and erode also factors into the power of rivers to transport and erode sediment.
(6) Where in the landscape do you think the deposition (accumulation of eroded and
transported sediments) would be most unlikely to occur?
A. In places where the slope is very steep.
B. In places where the slope is very gentle.
C. In places where there is a change in slope from steep to gentle.
D. In places where the stream enters an open water body such as lake or ocean.
2. Tectonics. Tectonics refers to deformation of the Earth’s crust and surface, which
serves to create changes in land surface slope (such as the formation of mountains and
plateaus). A slope change from steep to gentle in the land surface could lead to the
reduction of stream carrying capacity and to deposition at the base of the slope, usually in
the form of alluvial fans or coalesced alluvial fans (see picture below of alluvial fans at
the base of the mountain range).
(7) The picture below shows an alluvial fan. How could tectonics influence the high
amount of deposition occurring in this setting?
Aerial view of Lost River Mountains, alluvial fan, and floodplain of Big Lost River,
Butte and Custer counties, Idaho, U.S.A. Source: http://www.gly.uga.edu/railsback
/FieldImages.html
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Part 2 – The model
Go to: http://www.niu.edu/landform/home.html. Read the How It Works section and
respond to the following.
(8) Briefly describe how the Cellular Automata (CA) algorithm works.
(9) What are some of the geomorphic variables that the CA algorithm doesn’t
incorporate that might be important?
Part 3 – Run the model
Read through the tutorial and then run the following animation scenarios IN GROUPS
OF 3. For all of the scenarios read the questions before you start the runs, watch the
changes through the runs, and then look at the snapshots, profiles, and hypsometries after
the runs are over. Record your own observations on your own lab packet to turn in.
Model A.
Initial Conditions – given (100,000 iterations)
Erodibility – given, uniform at 0.05
Climate – constant at 0.1
Tectonics – no uplift
(10) What process seems to be controlling landscape evolution in scenario A?
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(11) How does the hypsometry change through time? (Remember what hypsometry
means? See last page of this lab handout to jog your memory).
(12) After the 100,000 iterations are complete, click on the snapshots tab and look at
the 4 images. When does most of the dissection take place?
(13) How far up the block does the dissection go?
Model B.
Initial Conditions – CHANGE SLOPE TO 0.3
Erodibility – given, uniform at 0.05
Climate – INCREASING SET LOW AT 0.1 AND HIGH AT 0.15
Tectonics – no uplift
(14) Describe how this scenario is different from Model A.
(15) Is the landscape response in Model B what you expected given that you
increased the slope and rainfall?
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Model C.
Initial Conditions – CHANGE SLOPE TO 0.2
Erodibility – given, uniform at 0.05
Climate – SET TO 0.1
Tectonics – BREAK AT Y=50, AND SET TOP TO 0.0003
(16) What type of geologic structure is this scenario imitating?
(17) How does this scenario change the sediment profile in the Profiles tab?
(18) Is the uplift rate faster than the erosion rate for the uplifted block? How can
you tell?
(19) What features are forming from streams flowing off the uplifted block? Does
this make sense?
Model D.
Initial Conditions – CHANGE SLOPE TO 0.1 AND INCREASE ITERATIONS
TO 500,000
Erodibility – BREAK AT Y=50, SET TOP TO 0.05 AND BOTTOM TO 0.01
Climate – SET TO CONSTANT = 0.1
Tectonics – BREAK AT Y=50, AND SET TOP TO 0.0001
(20) Wow -- this is a strange situation! Why are there two vastly different elevation
platforms?
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(21) How do the sizes of the “alluvial fans” forming at the uplift break (Y=50) relate
to their upstream drainages? Does this make sense?
(22) Even with the slower uplift rate is the uplift still much faster than the CA-based
erosion rate of the uplifted block?
(23) Do the extra 400,000 iterations change how the final landscape comes out
compared to the 100,000 iterations you ran in A,B,C? If so what changes?
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Part 4 – Test your own hypothesis
Come up with your own different simulation that is designed to test a hypothesis or the
sensitivity of the model to a variable.
(24) What is the hypothesis you would like to test or sensitivity you would like to
determine in Model E?
Choose variables below, and describe why you chose each.
Variable
Initial Conditions –
Justification
Erodibility –
Climate –
Tectonics –
(25) Did your scenario have the effect that you thought it would? If Yes, what does
that confirm? If No, why not?
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Hypsometry: Recall from Google Earth lab…
The shape of the Earth at all scales, from individual landforms up to mountain ranges
and ocean basins, is defined by the distribution of elevation. For convenience, we
measure elevation relative to mean sea level. Obviously this datum is well defined
around the coasts, but it has to be established by careful surveying and calculation at
locations far from the ocean. You can think of it as the height to which water would rise
in a canal cut across a continental interior. More technically, sea level defines the geoid,
a surface of equal gravitational potential over the Earth. The shape of the geoid is
influenced both by the Earth's rotation and the distribution of mass below the surface.
The global frequency distribution of elevation is shown in the plot below, called a
hypsometric curve. It shows the cumulative percentage of the Earth's surface area below
any elevation on the y-axis. Notice how the most common elevations in the histogram in
Fig. (a) correspond to the flatter portions of the curve in Fig. (b).
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