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Hydrology and Precipitation (a review and application). CEE 6/5460 David Rosenberg. Learning Objectives. Identify hydrologic cycle components and equations important to manage storm water Infer the appropriate rainfall intensity and hytograph from a depth-duration-frequency chart
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Hydrology and Precipitation(a review and application) CEE 6/5460 David Rosenberg
Learning Objectives • Identify hydrologic cycle components and equations important to manage storm water • Inferthe appropriate rainfall intensity and hytograph from a depth-duration-frequency chart • Recall basic probability principles • Calculate the risk of a detention basin overtopping during it’s design life CEE 6/5460 – Water Resources Engineering
Which components are important to engineers designing storm water systems?
1. Hydrologic cycle (cont.) How do we quantify flow through cycle components? CEE 6/5460 – Water Resources Engineering
Time-series of daily precipitation at USU http://climate.usurf.usu.edu/products/data.php
Precipitation intensity The rate at which rain or snow occurs [L/T] What rainfall intensity should we use? • Depends on the storm duration and precipitation • Likelihood of the event • Often specified in design criteria • Read depth from a rainfall depth-duration-frequency curve • Intensity (i) = depth / duration CEE 6/5460 – Water Resources Engineering
Precipitation Depth-Duration-Frequency Curves for Layton, UT Source: NOAA, 2008, http://www.nws.noaa.gov/oh/hdsc/index.html
Rainfall intensity (cont.) Example 1. What is the expected rainfall intensity of a 3-hour storm with a 10-year recurrence interval? Example 2. Draw the hourly storm hytograph (intensity versus time). CEE 6/5460 – Water Resources Engineering
4. Probabilities Purpose To quantify and represent uncertainty in an uncertain world Basic properties • 0 ≤ pi ≤ 1, for all possible outcomes i • Probability of jointly occurring independent events P(A∩B) = P(A) P(B) (product rule; intersection) What do probabilities represent? • Relative frequency of outcomes (repeatable events) C A B CEE 6/5460 – Water Resources Engineering
4. Probabilities (cont.) Return Period (T) Expected time T to wait for the next event size Q or larger [years] Probability that event Q will occur in any year = P(0) = 1/T Reliability Probability that a design/structure will safely pass Probability that structure will not fail (no catastrophic event Qs) Probability that Q will NOT occur over an n-year period Probability that Q will NOT occur in any year = 1 – P(0) = 1 – 1/T Probability that Q will not occur in n-years = (1 – P(0))n Risk Probability that at least one Q will occur = 1- Reliability = 1 – (1 – 1/T) n CEE 6/5460 – Water Resources Engineering
System risk for different magnitude events over various observation periods Risk = 1 – (1 – 1/T) n CEE 6/5460 – Water Resources Engineering
Risk (cont.) Example 3. A 3-hour storm generates 2 inches of precipitation and will overtop a Layton, UT storm water detention basin. What is the risk a 3-hour storm will overtop the basin during the 25-year life of the basin? CEE 6/5460 – Water Resources Engineering
Wrap up • Today’s key points and learning objectives • Thursday: rainfall-runoff CEE 6/5460 – Water Resources Engineering