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Rainfall_Lab_11

# Rainfall_Lab_11 - Rainfall Lab Rain Measurement Rainfall...

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Rainfall Lab January 24, 2011 Rain Measurement, Rainfall Time Series, Rainfall Return Period Overview This lab has three objectives: 1) Detailed examination of rainfall measurement instrumentation 2) Time series analysis of rainfall 3) Design storm return period, peak flow prediction and culvert sizing The assignment is broken into 3 parts (rainfall histogram, storm return time, culvert sizing) – you will be provided with data; guidelines for the analyses are provided in this document. Introduction Measuring the rain is the most fundamental part of the water budget. This lab will first take a hands-on look at the different methods that we use to measure rainfall, and then look at some actual rainfall data to examine patterns and distribution. Finally, we will use information for the rainfall time series to design a culvert. Rainfall arrives in patterns that vary from time-to-time and place-to-place. Consider the following two graphs of annual rainfall and frequency distributions between Gainesville (Florida) and Seattle (Washington) for the same period of record (1948-2006).

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Our first objective will be to describe the rainfall pattern from a new data set (Gainesville Florida between 1900 and 2006). Usually, what we want to know the total rainfall for a year, the degree to which that annual rainfall is reliable (how variable is it), when the rainfall arrives. In particular, what we’ll infer from actual data is: 1) How complete is the data record? How many days/months/years do we have actual data for? 2) What are the annual totals for the period of record; what is the mean annual rainfall; what is the standard deviation? 3) What are the average monthly rainfall totals for the period of record; what are the standard deviations? 4) What is the frequency distribution of rainfall events (in % of total days) for the period of record? Our second objective is to determine the rainfall associated with engineering design requirements for flow control/conveyance structures. Engineering projects for dealing with stormwater are designed based on rainfall intensity. While it is actually the flow from the rain that the structures must withstand, flow data sufficient for statistical calculations are rarely available. Instead, we use rainfall data which are available for many areas, and frequently have long records. The obvious problems are that not all rainfall becomes runoff, and it takes time for that rainfall which does become runoff to get to the point in the landscape where we are concerned about dealing with flow. A variety of rainfall-runoff relationships (e.g., the “Rational Method”, the NRCS Curve Number, finite element process-modeling) are used with “design” storms to statistically compute expected flows for structures like bridges and culverts. For example, the design storm for a temporary logging culvert might be a 10 year/1 hour storm (Q 10 ); a design storm for a permanent logging road might be a 25year/1 hour (Q 25 ) storm; and a design storm for a permanent public access road might be a 100 year/1 hour (Q 100 ) storm. That is, as our risk tolerance gets smaller, we design for increasingly rare events (this is a general theme of any kind of risk analysis).
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Rainfall_Lab_11 - Rainfall Lab Rain Measurement Rainfall...

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