California method Flashcards
what is the california method of annual flood flow calculation?
(W1 + W2 / 2) x depth. multiply resultant area by 3.
gives area for a 100 year flood
Where is stream width measured from?
the high water mark which is indicated by a definite change in vegetation.
How is round pipe area calculated?
(Pipe diameter / 2)squared x pai
what are some characteristics of the high water mark?
edge of scour (alluvium)
start of terrestrial veg
rafted debris
California good for small streams, what about big streams?
Big streams require increased caution due to much higher volumes of water. Rational formula is used for big streams.
What is the rational formula?
Q (m3/sec) =CiA/360 -i=rainfall (mm/hr) -A=drainage area in ha -C=runoff coefficient Gives flow for 10 year flood multiply by 1.6for 100 year flood.
What is runoff coefficient based on?
Surface cover and physiography.
What are the limitations of the california method?
- does not account for log culverts
- roughness of channel and slope of pipe not accounted for.
What is important to remember about the relative lengths of precipitation events?
Short rainfall events are more intense than long ones.
What is time of concentration?
Time taken for water to flow from most remote part of watershed to culvert. Controlled by topography, geology and land use in the watershed.
What is important for culvert design with respect to Tc?
The culvert should be designed for a storm duration that will generate maximum flow.
How is Tc calculated?
square root of drainage area in km2 will line up with the appropriate curve that represents to topography of the watershed.
ex. if 65ha watershed, convert to .65km
square root of .65 will line up with topo curves.
in this case Tc=1.5, giving an i value of 16mm/h
Why are culvert sizes built to be 80% full at max flow?
So that debris won’t block the culvert
What defines pipe size?
Hw/D should be 0.8
