Precipitation - Hydrology - Lecture Slides, Slides of Water and Wastewater Engineering

Download Precipitation - Hydrology - Lecture Slides and more Slides Water and Wastewater Engineering in PDF only on Docsity! Precipitation and IDF Curves 1 Docsity.com Objectives • Know different forms of precipitation • Know what a return frequency is • Know what an IDF curve is • Know how to create an IDF curve 2 Docsity.com Snow • Ice crystals 5 Docsity.com Drizzle • Mist; slow settling rates (.04 in/hr) 6 Docsity.com Rime • White, opaque deposits of ice granules which are separated by trapped air. Formed by rapid freezing of supercooled water drops impinging on exposed objects. 7 Docsity.com Factors Responsible • Cyclonic-Lifting of air converging into a low- pressure area – Frontal (warm or cold) – Nonfrontal • Convective-rise of warm, lighter air in colder denser surrounding • Orographic-lifting over mountains 10 Docsity.com Precip Data • National Weather Service (NWS) collects and publishes data • Point rainfall collected in vertical cylindrical rain gauges (dia. = 8”) 11 Docsity.com ; So S fs “| lege | & \ \ ® \ \ \ | AH \s _ — rf a «| \ * \ Legend: : cr anor ATA Sy, | CREATE 1 HOURLY ‘omen tase rtmon tearmn me \ - . + 12 Figure 1,--Station index map. Docsity.com  | [ONE INCH is 25.4mm FIG. 2-[ Typical monthly distribution of precipitation in. vorious climatic regions. From: Hydrology For Engineers by Ray K. Linstey, dr., Max A. Kohler ond Joseph & L. 4. Pouthus, Copyright 1958 by the Mc Graw-Hill Book Co, Used with permission of McGraw-Hill Book Company. Docsity.com Effect of Mountains See next slide • Discontinuities in the precipitation maps caused by the Rocky Mountains, the Cascades, and to a lesser extent, the Appalachian Mountains 16 Docsity.com ONE INCH is 25.4mm ‘ es 2-2 Average annual precipitation in the United States. . e (After National Weather Service). Docsity.com • Data is usually displayed in the form of isohyetal lines on geographical maps (total amount of rainfall in inches for a specific storm duration and for a specific recurrence interval) 20 Docsity.com Determining Intensity • Previous map--- shows 1” of water in 15 minutes. Convert to intensity. • 4” of water in 60 minutes • Intensity is 4” per hour 21 Docsity.com Intensity • 6” of precipitation falls in 2 hours • Rainfall intensity is __ per hour? 22 Docsity.com Hydro-35 • Contains precipitation info for storms with a very short duration (<= 1 hour) • Other documents TP-40 (eastern US), Atlas 2, etc. cover longer durations and other geographical areas 25 Docsity.com Updated Data • NOAA’s NWS Precipitation Frequency Data Server • http://hdsc.nws.noaa.gov/hdsc/pfds/ 26 Docsity.com Hydro-35 • 200 weather stations • 60 years of record 27 Docsity.com UM enh hist: Intensity - Duration - Frequency Curves — Fe5 years — F=10 years — F=20 years — F=30 years — F=60 years [7 — F=120 years |] ve = E = = 7) c a = Time (rnin) http:/Awww.mathworks.com/matlabcentral/fileexcha nge/9740-intensity-duration-frequency-curves Docsity.com  se INTENSITY (MILLIMETRES ¥¢ HOUR) RAINFALL INTENSITY-DURATION- FREQUENCY CURVES FOR SINGAPORE ISLAND RAINFALL % XIONGddY 304 6 87 a4D a B50 6H 70 8H 90 108 ET) 388 480 508 688 70888080 1908 19992 TIME «MINUTES? http:/Awww.pub.gov.sg/general/code/Pages/SurfaceDrainagePart & 2-7.aspx Docsity.com Design Frequencies • It is not economically feasible to provide protection against the largest flood that could occur. • For large projects (bridges/dams) a 100-yr frequency is often used • For smaller projects design year depends on agency, type of facility, etc. 32 Docsity.com Example • The probability of precipitation amounts equaling or exceeding a 2-yr storm is • P=1/2=0.5=50% • In other words, each year there is a 50% chance that precipitation will exceed that of a 2-yr storm 35 Docsity.com Relationship to design • If culverts are designed to pass flows of annual probability P, then it implies that on average P*N culverts will be overtopped each year, where N is the total number of culverts. • If a county has 100 culverts and they’re designed for a 2-yr frequency then on average 50 culverts will be topped each year 36 Docsity.com Probabilities over Time • What if you want to know the probability of a flood occurring over a specified interval of time (ex. during the construction of a dam) • J=1-(1-P)N 37 Docsity.com