Runoff

Question 1

Factors affecting runoff?

Answer 1

-Climatic factors
Precipitation
Evaporation

-Human built structures
Dams, reservoirs, bridges

-Geographic factors
Watershed size, shape, land use, soil
Channel storage capacity

Question 2

Limitations of Rational method

Answer 2

-only provides peak flow rate
-assumes C is constant
-assumes tc is constant
-Assumes rainfall and intensity is constant over the entire watershed
-assumes 10 year event produces 10 year runoff event

Question 3

Advantages of Rational method?

Answer 3

-ease of use
-runoff coefficients readily available for lots of land types
-intensities available for most regions

Question 4

SCS (Soil conservation service) curve number method?

Answer 4

-widely used to estimate runoff based on land use and treatment
-developed from studies on small agricultural watersheds
-soil conservation act passed in 1935

Question 5

What does the curve number depend on?

Answer 5

-select curve number based on conditions and type of cover

Question 6

SCS hypothesis?

Answer 6

-based on the fact that ratio of infiltration to storage is equal to ratio between direct runoff to precipitation minus abstractions

Question 7

Soil group A?

Answer 7

-high infiltration rates
-high rate of water transmission
-very well drained
-sands, silts, gravel

Question 8

Soil group B?

Answer 8

-Moderate infiltration rates
-moderate rate of water transmission
-moderately well to well drained

Question 9

Soil group C?

Answer 9

-low infiltration rate
-poorly drained
-low water transmission
-soil layer that impedes downward movement such as clay or sandy loams

Question 10

Soil group D?

Answer 10

-very slow infiltration rate
-high water table
-low water transmission
-heavy plastic clays, saline soil

Question 11

Moisture condition AMC 1

Answer 11

Dry but not to wilting point

Question 12

Moisture condition AMC 2

Answer 12

Average case for annual floods

Question 13

Moisture condition AMC 3

Answer 13

Soil is mainly saturated

Question 14

Estimation of CN values

Answer 14

1. Identify soil
2. Identify land use
3. Identify hydrologic conditions
4. Determine CN for normal conditions (AMC 2)
5. Modify CN to AMC 1 and AMC 3 if needed

Question 15

Limitations to SCS method

Answer 15

-sensitivity to the curve number
-lack of clear guidance on how to vary atmospheric moisture conditions (AMC)
-fixing the initial abstraction ratio

Question 16

Benefits of SCS Curve number method

Answer 16

-simplicity
-only need one parameter
-widespread acceptance

Question 17

Time of concentration

Answer 17

Time required for drop of water to travel from the most remote point of the watershed to the outlet

Question 18

Dickenson time concentration

Answer 18

Developed from several rural watersheds with slope between 3 and 10%

Question 19

Federal aviation administration time concentration

Answer 19

-based on theory of overland flow and using kinematic wave approach
-intended for use in airfield drainage
-frequently used for urban areas