Subject dramatic video footage of urban flash flooding
Subject: dramatic video footage of urban flash flooding from 10 Jan 2011 Date: Wed, 12 Jan 2011 10: 17: 57 +1100 Two days ago there was a dramatic flash flood in Toowoomba (pop 130, 000) in the mountains west of Brisbane, Australia. The water came up in about an hour and was in the middle of the day so there is amazing amateur video footage. One of the best is of the rising limb downtown http: //www. youtube. com/watch? v=k. YUpk. PTcq. PY&feature=player_embedded A good reason not to have car parking next to storm water drains. Not an uncommon problem in urban areas as we had similar problems in Newcastle two years ago with storm drains, bridges and culverts clogged with cars making the flooding much worse. Prof Garry Willgoose, Director, Centre for Climate Impact Management (C 2 IM), The University of Newcastle, Australia.
Weather and Precipitation Learning Objectives • Be able to describe weather systems responsible for precipitation, explain the basic processes and concepts involved and calculate atmospheric properties involving water in the atmosphere – Humidity, vapor pressure, dew point, latent heat, stability • Be able to interpret point precipitation measurements by plotting cumulative and intensity hyetographs • Be able to construct and interpret intensity-durationfrequency graphs • Be able to calculate area average precipitation CEE 3430 – Spring 2011 David Tarboton 2
An Example (problems 1. 13 and 1. 14 actually) Time P min in 0 15 30 45 60 75 90 105 120 135 0 0. 1 0. 4 1 1. 5 1. 8 2 2. 3 2. 4 Time Q hr cfs 0 0 1 100 2 200 3 400 4 800 5 700 6 550 7 350 8 250 9 150 10 100 11 50 12 0 CEE 3430 – Spring 2011 Calculate • Cumulative Mass Curve • Rainfall Hyetograph • Infiltration Volume • Runoff Ratio David Tarboton 3
Why Study Weather/Climate? Hydrology as we know it is driven by the climate, primarily precipitation, but also temperature and radiation. To understand the variability in hydrology we need to understand something about the weather and climate.
From Dingman, 1994
General Circulation of the Atmosphere, Bedient Fig 1 -3 a
Three-Cell Model From Simon Wang
Three-Cell Model: Scientific evolution Halley Thermally direct circulation forcing air towards equator Hadley Earth’s rotation and the conservation of linear momentum cause the Trade Winds Ferrel Coriolis force deflects winds toward the east and pulls air from south + Conservation of angular momentum 170 years! Slide from Simon Wang
Coriolis Effect http: //www. youtube. com/watch? v=_36 Mi. CUS 1 ro&feature=related CEE 3430 – Spring 2011 David Tarboton 9
Formation of Precipitation 1. Source of moisture 2. Lifting mechanism (orographic or heating) 3. Phase change from vapor to water - Energy 4. Small nuclei or dust for droplet formation 5. Droplets must grow as they fall to earth From Bedient
Lifting Mechanisms From Bedient
ATMOSPHERIC STABILITY Dry Adiabatic Lapse Rate (10 0 C/km) Altitude Environmental Lapse Rate Moist Adiabatic Lapse Rate (6 0 C/km). Lower due to release of latent heat of condensation from moist air Temp Modified from Bedient
Atmospheric Stability Stable Ambient lapse rate < adiabatic lapse rate of lifted parcel From Brutsaert, 2005 Unstable Ambient lapse rate > adiabatic lapse rate of lifted parcel
How much water can the air hold? Saturation vapor pressure es(t). The maximum vapor pressure that is thermodynamically stable. mb, for T in o. C See Goff-Gratch (1946) for more precise equation or Lowe (1977) for polynomials for efficient evaluation es(Ta) ea Td Ta Relative humidity. Vapor pressure relative to saturation vapor pressure. (usually expressed as %) Dew point. Td. The temperature to which a parcel of air has to be cooled at constant (vapor) pressure to reach saturation
Ralph, F. M. , P. J. Neiman, G. A. Wick, S. I. Gutman, M. D. Dettinger, D. R. Cayan, and A. B. White (2006), Flooding on California's Russian River: Role of atmospheric rivers, Geophys. Res. Lett. , 33, L 13801, doi: 10. 1029/2006 GL 026689.
Major Thunderstorm From Bedient
Fronts and Low Pressure • Cold/Warm Front • Lifting/Condensation • High and Low Pres • Rainfall Zone • Circulation Issues • Main weather makers From Bedient
From Bedient Hurricane Katrina in the Gulf - 8/29/2005 Katrina 08 -28 -2005 at 17: 00 UTC
Measuring Rainfall - Tipping Bucket • Recording gage • Collector and Funnel • Bucket and Recorder • Accurate to. 01 ft • Telemetry- computer From Bedient
The Hyetograph § Graph of Rainfall Rate (in/hr) vs Time (hr) at a single gage location § Usually plotted as a bar chart of gross RF § Net Rainfall is found by subtracting infiltration § Integration of Net Rainfall over time = Direct RO Vol (DRO) in inches over a Watershed From Bedient
Mass Curves & Rainfall Hyetographs From Bedient
Intensity-Duration-Frequency • IDF design curves • All major cities • Based on NWS data • Various return periods & durations • Used for drainage design of pipes & roads • Used for floodplain designs - watersheds From Bedient
Design Rainfalls § Design Storm from HCFCD and NWS § Based on Statistical Analysis of Data § 5, 10, 25, 50, 100 Year Events § Various Durations of 6 to 24 hours Six Hour Rainfall From Bedient
From Bedient Rainfall Averaging Methods
Thiessen Polygons - Areal Average Rainfall from Gages • Connect gages with lines • Form triangles as shown • Create perpendicular bisectors of the triangles • Each polygon is formed by lines and WS boundary P = S (Ai*Pi) / AT From Bedient
RADAR Rainfall Estimates § NEXRAD provides real-time data on a ~16 km 2 (6 mi 2) grid § Each estimate represents an average rainfall amount over the entire 4 x 4 km 2 area § NEXRAD rainfall estimates compare well with point rain gage measurements (r 2 ~ 0. 9) From Bedient
Summary • The atmosphere is the major link between oceans and continents • The hydrologic cycle is shaped by conditions in the atmosphere with precipitation as the major input • Precipitation due to atmospheric moisture, lifting, cooling, condensation, release of latent heat, instability • Precipitation variability is summarized statistically in intensity-duration-frequency curves used for design • Spatial averaging methods are used to calculate precipitation over a watershed CEE 3430 – Spring 2011 David Tarboton 28
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