FINAL EXAM – ANSWER KEY
NR 285/Geog 295: Environmental Hydrology
Below are 5 questions that require quantitative as well as qualitative responses. Choose 4 of the 5 questions
and answer them as completely as possible (i.e., you may chose to eliminate one question). You should spend no
more than six hours total on this exam. Each question is worth 25 total points, distributed among the
component sub-questions. Formatting and submission guidelines are provided at the end of this document.
1. One mid-winter day, a 0.75 meter snowpack with a density of 0.25 g/cm3 covers the roughly 15 mi2 area of
Burlington, Vermont. For this snowpack (a) estimate the total volume of water stored in the snow, (b) briefly
outline the conditions that would be required before snowmelt will begin, (c) use the average daytime
atmospheric conditions given below to determine the rate of snowmelt once the conditions above are met, and
(d) indicate how the presence of a forest (rather than the open urban setting) might affect the rate of snowmelt if
you considered the same snowpack and atmospheric conditions, but for a rural forested setting.
Average daytime atmospheric conditions:
Incoming shortwave radiation: 40 cal / cm2 hr
Incoming longwave radiation: 8 cal / cm2 hr
Outgoing longwave radiation: 12 cal / cm2 hr
Surface albedo: 40%
Sensible heat flux: 5 cal/ cm2 hr
Latent heat flux: 0 cal / cm2 hr
Air temperature: 5o C
Soil temperature -1o C
2. A proposal has been submitted to harvest the hardwood forest on a 500 acre watershed on the hilly outskirts
of Chittenden County to provide wood fuel to heat the town school. The proposed plan includes the removal of
approximately 30% of the forest on the watershed on recurring 20-year rotations. Assess the likely hydrologic
effects of the proposal by providing (a) two independent quantitative estimates of the change in total annual
runoff immediately following the harvest and (b) a single estimate of the change in the peak discharge for the
one-year recurrence interval 24-hour storm event, drawing upon concepts and/or methods we have covered in
class. In your answer, also include (c) a narrative describing how you would expect changes in annual runoff to
evolve over the rotation cycle of the proposed plan drawing upon readings we have covered in class, and (d) an
appraisal of the ways in which runoff processes in this setting would be changed if the landscape is eventually
converted to suburban housing covering 30% of the watershed area.
3. You have been asked to estimate the likely flow conditions associated with a storm event on a rural,
ungauged lowland river in Vermont. High water marks along the channel margin following the storm indicate
that the flow reached a depth of at least 3 meters at some point. For this channel and event: (a) estimate the peak
discharge associated with the storm event, (b) indicate whether the channel topped its banks during the event, (c)
indicate the fraction of the channel bed that would likely have been mobile during the peak of the event, and (d)
estimate the likely recurrence interval associated with this storm event, explaining the thinking behind your
answer.
100
Channel conditions and grain size
distribution (at right):
80
Percent finer than
channel width: 12 m
Mannings roughness: 0.033
channel bed slope: 0.8%
bank full discharge: 6000 cfs
90
70
60
50
40
30
20
10
0
0.01
0.1
1
10
Particle size (mm)
100
1000
4. You’ve been asked to design a stormwater detention system for a 10 square mile watershed in Vermont that
has a maximum point intensity-duration-frequency storm event pattern shown in the graph below. This 10
square mile area is a residential area generally characterized by ½ acre lots on D type soils with a combination
of oaks and pines among lawns that are in good condition. The soils are 30 feet thick over a bedrock confining
layer. The initial design criteria are that you have to detain the storm water volume contained in the 2 year/12
hour storm.
a) What is the average intensity of rainfall that you
would expect to occur over the 10 square mile area?
b) What is the total rainfall volume that will fall on
this watershed during the event? c) How much of this
rainfall volume do you expect to have to detain? Note:
You do not need to run a model here, though a semiquantitative estimate is possible. I’m more interested
in your narrative rationale for the volume of rainfall
that will end up as runoff and why. Decide on a value
for runoff volume, justify it, and the use in the next
section. d) Local stormwater design criteria state that
detention facilities can not be more than 5 feet deep
and it is recommended that no single facility in a
multi-facility system (if that becomes necessary)
should be greater than 40,000 ft2 in area. Can you
recommend a single facility or will you have to go
with a multi-facility system? e) No matter what you
do, the structural engineering consultants tell you that
for safety reasons it is a bad idea to expose the system
even one time to a 50 year/1 h storm while the system
is under construction. The client you are working with
(a city council) is willing to tolerate a 5% chance that
this type of storm might occur after the construction starts. If this is the case, then once you have broken ground
on the system, what is the maximum time you can take (in whole years) to complete the project without
exceeding the client’s risk tolerance? f) The structural engineering consultants have also told you that if there
are more than five 10 year/1 h storm events over the design life of the system (30 years), then there is some
maintenance that should be done to ensure that the system operates at optimal performance. The client wants to
know whether this is likely or not, so that they can decide whether to budget for this maintenance cost or not. If
there is less than a 1-in-5 chance of this occurring, the client is willing to not budget for the maintenance. What
do you recommend?
5. Consider the reach width and depth data in the table at the right.
You’ve been asked to do the preliminary planning for a slug addition
experiment in which you will compare the flow at two different
reaches in the watershed. The first reach is at a point that drains 50
square miles and the other drains 25 square miles. Other
characteristics of the two reaches are provided in the table below.
Parameter
Velocity
Incision ratio
Slope
Substrate
Location
Land use
Conductivity
25 Sq. Mile
50 Sq. Mile
watershed
watershed
0.5
0.6
1.16
1.05
0.03
0.01
gravel/cobble smooth sand
mountains
plain
forest
agriculture
73
234
Units
m/s
dimensionless
m/m
uS/cm
Watershed
Reach ID
size
(#)
(Sq. miles)
1
71.3
3
69.3
5
67.5
6
56.3
7
54.1
8
42.1
9
41.5
11
26.2
17
17.8
18
17.3
19
16.9
27
9.9
33
13.5
Width
(feet)
116.0
108.0
95.0
84.0
87.0
73.0
87.0
75.0
64.0
62.0
58.5
33.5
43.0
Depth
(feet)
2.53
3.00
3.07
2.62
1.90
2.88
2.20
2.29
2.10
2.22
1.83
1.96
1.68
5 (continued)
a) What is the width and depth that you would expect for the 25 and 50 sq. mile reaches? Quantify you answers.
b) What is the optimal mixing length you should expect for these two reaches?
c) How much Rhodamine WT dye should you use for the experiments at these two sites? Assume that the dye
comes in the standard stock solution of 20% mass/volume (200 g/L). Note: It is unnecessary to worry about the
specific gravity of the stock material. To address this problem you will need to make some assumptions and you
will need to fill in at least one important blank with a preliminary estimate.
d) Sketch a graph of what you would expect to see if you were collecting data with a properly calibrated (and
operational) field fluorometer at the optimal mixing length in the two streams. Provide a detailed narrative
explanation of the patterns you have sketched.
General guidelines for preparing and submitting your responses:

Use any of the concepts, readings, exercises or labs we have covered this semester in crafting your answers.

For answers requiring calculations, show your work in full to receive full credit.

If you need to make an assumption to answer a question, do so and clearly state your assumption(s).

Start each numbered question on a new page with your name included on every page, so that we may
separate questions and grade individually.

We strongly encourage you to craft your narrative response in MS/Word in standard formatting (Times
Roman, 11pt, single spaced, doubled sided, 1” margins) with supplemental hand written notes and
calculations as needed. Embedded Excel spreadsheets or other graphics are fine. If you choose to hand
write your entire response, ensure that they are legible. Responses with illegible writing or undecipherable
logic can not be graded. (Partial credit will be awarded wherever we can.)

Your responses are due no later than 6:00pm on Friday, December 7 to Beverley in Old Mill 202. We
prefer a hard copy submission to avoid the possibility that what we receive is not exactly what you intended
to submit or that we would fail to receive your response before we leave town for the weekend. If you need
to make other arrangements, please speak to Beverley directly.