CEE 577 |
28 March 2006 |
Closed book, 1 sheet of notes allowed.
Answer 2 of the following 3 questions. Please state any additional assumptions you
made, and show all work.
I.
(50%) Whole Hog
creek flows through and area of the North Carolina Piedmont that is heavily
populated with pig farms. The creek
discharges into a lake used for recreational purposes. Concern over phosphorus loading of the lake
has led to a short-term study of nitrogen imputs during storm events. Data from this brief, 8-day survey period is
shown below.
a.
Determine the
total phosphorus concentration for each day using the standard log-log model
b.
Estimate the mean
mass loading of phosphorus (in kg/d) into the reservoir based on these values.
Day |
Flow (m3/s) |
Total Phosphorus
Concentration (µg/L) |
1 |
0.2 |
|
2 |
0.5 |
12 |
3 |
4.2 |
|
4 |
13.1 |
55 |
5 |
2.2 |
|
6 |
8.3 |
|
7 |
2.3 |
|
8 |
0.9 |
|
II.
(50%)
Year |
Output (batteries/yr) |
2005 |
5,800,000 |
2010 |
5,100,000 |
2015 |
4,400,000 |
2020 |
3,700,000 |
2025 |
3,000,000 |
This is a problem with: (1) a pre-existing concentration that
decays; (2) a step loading in 1/1/05, and a linear loading with a negative
slope.
III. (50%) On a
separate sheet of paper, answer any five (5) of the following questions.
A.
Describe the difference between mechanistic and
empirical modeling
·
Mechanistic is based on a some mechanism grounded in
first principles or theory; a deductive approach
·
Empirical is based on a statistical analysis of real
environmental data. The model need not
resemble actual fundamental relationships; an inductive approach
B.
Is a first order reaction always faster than a zero
order reaction? Explain.
·
No. It depends on the reaction rate constant. First order loss processes do show a higher
rate of reaction at the beginning than at the end. Zero order rates are constant throughout.
C.
Sketch out a qualitative concentration vs distance (downstream) profile for a river with a point
discharge at x=0 and a distributed source that runs from x=3 to x=6 miles. Show qualitatively how increases in the 1st
order decay rate for a pollutant changes this profile.
Note that these are arbitrary concentration levels
D.
Explain the relationship between Secchi-disk
depth and lake trophic state. Why is there such a relationship?
·
As trophic state increases,
there is more algal growth, more light scattering due to algal cells and Secchi depth is smaller, because of the inability to see
the Secchi disk at greater depths
E.
Describe 3 different methods for determining stream
velocity.
·
Dye: inject it and follow concentration versus time at
a point downstream
·
Current Meter: hand-held or suspended meter to measure
velocity at various depths and cross-sections
·
Surface floating device: (oranges), only get surface
velocity that way, look at different locations across stream width
·
Drogue: follow movement of device, sail can be set to
different depths
·
Theoretical relationship based on roughness, slope,
etc: e.g., Manning’s equation
F.
Describe 2 different ways of measuring drainage basin
area
·
Cut and weigh: trace drainage basin on topo map, cut it out and weigh paper
·
Counting squares: same as above, but use graph paper
and determine area by counting squares
·
Planimeter method:
trace out basin in topo map
·
GIS delineation: requires GIS software and proper
elevation data
G.
Describe the factors that determine re-aeration in
rivers, and contrast this with the factors that determine re-aeration in
lakes. In your description, relate
micro-scale processes (molecules) to macro-scale (bulk water or air)
·
Mixing is drives it.
2-film model interprets this as reducing the stagnant film thickness
·
Water side mixing is most important, and it is determined
by turbulence and water depth
·
Water side turbulence is high with high velocity flow
and roughness of stream channel. High
flow and turbulence is cause by elevation drop (slope, free fall over a weir), and
by wind turbulence on the surface
·
Air side mixing is less important but affected by wind
speed also