ESS 424 Exam 2 || Already Passed.
What is the definition of a "watershed"? correct answers Area where any surface or subsurface
flow drains to the same outlet.
How do the runoff rate per unit area and runoff volume per unit area generally change as the
watershed area increases? correct answers volume increases-density/intensity decreases
What are some cases in which the watershed area may be very hard to define? correct answers
water flowing underground, flowing between political boundaries/counties/states
Know the component of a storm hydrograph inducing the axes. Indicate 3 things (pieces of
information) about the storm hydrograph we are generally most interested in. correct answers 3
things are peak discharge, lag time and peak rainfall. The graph stars linear and then becomes a
parabola shaped like a hill to depict river discharge, also includes a bar chart to depict rainfall. x-
axis: time, y-axis: rainfall (mm), River Discharge (cumecs).
Why does runoff not begin immediately with the beginning of rainfall? correct answers
Infiltration, vegetation, waterways basins, ditches not filled yet, barriers slowing flow, initial
losses ,wetting front
State the two different definitions of the time of concentration correct answers Time at which
entire watershed is contributing, When the most hydrologically remote areas are contributing
In most relationships to estimate time of concentration, the travel time for overland flow and for
channel flow are treated separately. Why? correct answers They flow at different rates
What factors are important in the relationship we saw to estimate overland flow time? correct
answers Steepness, length of steepness, manning's factor (roughness)
What factors are important in the relationship we saw to estimate channel flow time? correct
answers Length and steepness of slope
Explain why a design storm duration equal to the time of concentration gives the maximum
design runoff rate? correct answers When whole watershed is contributing, then max amount can
be achieved because there is more water
Know the factors which control runoff. correct answers 1)Local infiltration controls: Soils and
geology - remember hydrologic groups; Vegetation and residues; Antecedent moisture
conditions
2) Topography
3) Watershed orientation and shape
How does the watershed size affect how the watershed responds to storms? correct answers
Bigger watersheds can carry more, takes more to affect them, smaller can be affected by for
example, convective storms which are strong and fast, not be able to move water fast enough?
, Describe why a storm that is first light then heavy produces more runoff than the opposite pattern
correct answers Because it gives time for watershed to fill up before getting to the major part
Explain why a 50-year rainfall event may not produce a 50-year runoff event correct answers
Different factors that go into the creation of runoff such as soil characteristics, topography,
antecedent moisture conditions, storm shape, storm duration and intensity.
What are the two common uses of runoff estimation? correct answers -Design for storage, need
to know proper size, Pick longer duration than the time of concentration
-Design for control, duration = time of concentration, limit how fast water comes out
A "design storm" is a hypothetical rainfall depth on which we will base our runoff estimation.
What two factors are used to determine the design storm depth? correct answers - Cost failure,
return period/frequency and based on how much you want to spend
- Duration of storm event, depends on what you're designing
On what basis is the design storm return period chosen? correct answers Design return period is
chosen based off the cost of failure of the design, Higher cost failure, longer return period
The method we discussed for estimating runoff volume is called what? correct answers SCS
direct runoff (curve number) method
The runoff volume estimation method we discussed uses what 2 parameters as inputs? correct
answers - P: precipitation depth based on desired return period and duration
- S: Controlled by all things that control infiltration, states how entire system will respond to
rainfall, Uses CN, Gives us insight on infiltration based on particular land uses
Let's say that you have 4 rainfall maps to work with: A) 2-yr 1-hr, B) 2-yr 24-hr, C) 50-yr 1 hr,
D) 50-yr 24-hr. Which would you most likely pull information from if, for your small watershed,
you were designing
:a) a culvert under a small country road ____________
b) a culvert under a major highway ____________
c) a livestock pond ____________
d) a reservoir above a subdivision ____________ correct answers A
C
B
D
The initial abstraction in most cases is Ia = 0.2 * S. What does the S in this relationship represent
(not just the name, but what it means)? correct answers S is sometimes referred to as the storage
variable and in this relationship it represents what fractions of storage in the surface that must be
filled up before runoff begins
The initial abstraction in most cases is Ia = 0.2 * S. What does the 0.2 in this relationship
represent (not just the name, but what it means)? correct answers - "Initial abstraction"
What is the definition of a "watershed"? correct answers Area where any surface or subsurface
flow drains to the same outlet.
How do the runoff rate per unit area and runoff volume per unit area generally change as the
watershed area increases? correct answers volume increases-density/intensity decreases
What are some cases in which the watershed area may be very hard to define? correct answers
water flowing underground, flowing between political boundaries/counties/states
Know the component of a storm hydrograph inducing the axes. Indicate 3 things (pieces of
information) about the storm hydrograph we are generally most interested in. correct answers 3
things are peak discharge, lag time and peak rainfall. The graph stars linear and then becomes a
parabola shaped like a hill to depict river discharge, also includes a bar chart to depict rainfall. x-
axis: time, y-axis: rainfall (mm), River Discharge (cumecs).
Why does runoff not begin immediately with the beginning of rainfall? correct answers
Infiltration, vegetation, waterways basins, ditches not filled yet, barriers slowing flow, initial
losses ,wetting front
State the two different definitions of the time of concentration correct answers Time at which
entire watershed is contributing, When the most hydrologically remote areas are contributing
In most relationships to estimate time of concentration, the travel time for overland flow and for
channel flow are treated separately. Why? correct answers They flow at different rates
What factors are important in the relationship we saw to estimate overland flow time? correct
answers Steepness, length of steepness, manning's factor (roughness)
What factors are important in the relationship we saw to estimate channel flow time? correct
answers Length and steepness of slope
Explain why a design storm duration equal to the time of concentration gives the maximum
design runoff rate? correct answers When whole watershed is contributing, then max amount can
be achieved because there is more water
Know the factors which control runoff. correct answers 1)Local infiltration controls: Soils and
geology - remember hydrologic groups; Vegetation and residues; Antecedent moisture
conditions
2) Topography
3) Watershed orientation and shape
How does the watershed size affect how the watershed responds to storms? correct answers
Bigger watersheds can carry more, takes more to affect them, smaller can be affected by for
example, convective storms which are strong and fast, not be able to move water fast enough?
, Describe why a storm that is first light then heavy produces more runoff than the opposite pattern
correct answers Because it gives time for watershed to fill up before getting to the major part
Explain why a 50-year rainfall event may not produce a 50-year runoff event correct answers
Different factors that go into the creation of runoff such as soil characteristics, topography,
antecedent moisture conditions, storm shape, storm duration and intensity.
What are the two common uses of runoff estimation? correct answers -Design for storage, need
to know proper size, Pick longer duration than the time of concentration
-Design for control, duration = time of concentration, limit how fast water comes out
A "design storm" is a hypothetical rainfall depth on which we will base our runoff estimation.
What two factors are used to determine the design storm depth? correct answers - Cost failure,
return period/frequency and based on how much you want to spend
- Duration of storm event, depends on what you're designing
On what basis is the design storm return period chosen? correct answers Design return period is
chosen based off the cost of failure of the design, Higher cost failure, longer return period
The method we discussed for estimating runoff volume is called what? correct answers SCS
direct runoff (curve number) method
The runoff volume estimation method we discussed uses what 2 parameters as inputs? correct
answers - P: precipitation depth based on desired return period and duration
- S: Controlled by all things that control infiltration, states how entire system will respond to
rainfall, Uses CN, Gives us insight on infiltration based on particular land uses
Let's say that you have 4 rainfall maps to work with: A) 2-yr 1-hr, B) 2-yr 24-hr, C) 50-yr 1 hr,
D) 50-yr 24-hr. Which would you most likely pull information from if, for your small watershed,
you were designing
:a) a culvert under a small country road ____________
b) a culvert under a major highway ____________
c) a livestock pond ____________
d) a reservoir above a subdivision ____________ correct answers A
C
B
D
The initial abstraction in most cases is Ia = 0.2 * S. What does the S in this relationship represent
(not just the name, but what it means)? correct answers S is sometimes referred to as the storage
variable and in this relationship it represents what fractions of storage in the surface that must be
filled up before runoff begins
The initial abstraction in most cases is Ia = 0.2 * S. What does the 0.2 in this relationship
represent (not just the name, but what it means)? correct answers - "Initial abstraction"
