Area Precipitation And Design Storms Flashcards
Method - Arithmetic Average
RA = 1/n * sum of rainfall at each rain gauge
Method - Thiessen Method
Method
1. Triangulation (area)
2. Draw perpendicular bisectors to create polygons
3. Intersect catchment boundary
RA = (sum of rainfall at each rain gauge*the area of the polygon) / the total area
Method - Isohyetal
- Obtain contours
- Measure the area between every two neighbouring isohyets
- Get average rainfall for regions between two isohyets. Then get the weighted average using formula
RA = sum of (Avi/A)* Rvi
Method - Manual calculation
Count squares
Method - spatial interpolation
IDW method
1. Choose a radius around the target location R
2. Within that boundary draw lines from the rainfall measurements to target location.
3. Calculate weight using inverse relation between that and distance
wi= 1/di^p where p =2
4. Calculate sum of individual weights W
5. Finally
R = 1/W * sum of Ri *wi
Design storm steps
- Point value: obtain synthetic rainfall value with desired frequency and duration at the centroid of the catchment
- Catchment/areal value: apply an ARF to represent the areal value using the point value obtained
- Temporal distribution: Disaggregate the catchment storm depth over the time domain using a predefined profile which represent the most likely event process
Procedure for rainfall frequency analysis
- decided the duration by considering the concentration time of the catchment
-produce a series of annual maxima of the rainfall that has a desired duration
-use the frequency series to fit a theoretical PDF curve - by varying the duration, repeat the procedure above, a set of Depth-duration-frequency DDF curves can be obtained
Depth-Duration-Frequency DDF
-can tell the rarity of an event
Areal Reduction Factor
Used to reflect the difference when deriving the return period of the rainfall values from observations
ARF = 1-bD^-a
where..
D: duration in hours
a and b: related to area
A: taken from the catchment using a table
