Runoff Flashcards
Factors affecting runoff?
-Climatic factors
Precipitation
Evaporation
-Human built structures
Dams, reservoirs, bridges
-Geographic factors
Watershed size, shape, land use, soil
Channel storage capacity
Limitations of Rational method
-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
Advantages of Rational method?
-ease of use
-runoff coefficients readily available for lots of land types
-intensities available for most regions
SCS (Soil conservation service) curve number method?
-widely used to estimate runoff based on land use and treatment
-developed from studies on small agricultural watersheds
-soil conservation act passed in 1935
What does the curve number depend on?
-select curve number based on conditions and type of cover
SCS hypothesis?
-based on the fact that ratio of infiltration to storage is equal to ratio between direct runoff to precipitation minus abstractions
Soil group A?
-high infiltration rates
-high rate of water transmission
-very well drained
-sands, silts, gravel
Soil group B?
-Moderate infiltration rates
-moderate rate of water transmission
-moderately well to well drained
Soil group C?
-low infiltration rate
-poorly drained
-low water transmission
-soil layer that impedes downward movement such as clay or sandy loams
Soil group D?
-very slow infiltration rate
-high water table
-low water transmission
-heavy plastic clays, saline soil
Moisture condition AMC 1
Dry but not to wilting point
Moisture condition AMC 2
Average case for annual floods
Moisture condition AMC 3
Soil is mainly saturated
Estimation of CN values
- Identify soil
- Identify land use
- Identify hydrologic conditions
- Determine CN for normal conditions (AMC 2)
- Modify CN to AMC 1 and AMC 3 if needed
Limitations to SCS method
-sensitivity to the curve number
-lack of clear guidance on how to vary atmospheric moisture conditions (AMC)
-fixing the initial abstraction ratio
Benefits of SCS Curve number method
-simplicity
-only need one parameter
-widespread acceptance
Time of concentration
Time required for drop of water to travel from the most remote point of the watershed to the outlet
Dickenson time concentration
Developed from several rural watersheds with slope between 3 and 10%
Federal aviation administration time concentration
-based on theory of overland flow and using kinematic wave approach
-intended for use in airfield drainage
-frequently used for urban areas
