What hit us Notes on the Physical Geography of Hurricanes Irma and Maria

Transcription

1 What hit us Notes on the Physical Geography of Hurricanes Irma and Maria Carlos E Ramos-Scharron Department of Geography & the Environment and LLILAS UT-Austin

2 Objectives Central theme is MAGNITUDE How is the concept of magnitude described by Physical Geography, Hydrology, & Geomorphology? Apply those concepts to specific aspects of Hurricanes Irma and Maria

3 Magnitude How to adequately describe it?

4 About magnitude A definition: A quantitative measure expressed usually as a multiple of a standard unit. What are appropriate metrics to describe Hurricanes? Size? Barometric pressure? Wind velocity?

5 1) Mag = f (Wind velocity) Kinetic Energy = f (Velocity 2 ) Accumulated Cyclonic Energy Index H. Irma s Cat. 5 winds in the British Virgin Islands

6 2) Mag = f (Scale of your measurement) Zoomed too far in or too far out? What is an appropriate scale? Differences due to scale but also varying methods Preliminary maps (do not distribute) Rain Gauges Max = 963 mm Doppler Radar Max = 1200 mm Satellites Max = 370 mm

7 3) Mag = f (drivers - resistance) Magnitude dependent on the meteorological phenomena, but also the landscape Sensitivity (sociological concept assoc. to vulnerability; risk): How prone & what are the consequences? Geomorphic thresholds: Likelihood of change in landform shape by an event is spatially variable From:

8 3) Mag = f (drivers - resistance) Over 40,000 landslides mapped, but concentrated on a certain range of slope steepness, lithology (Utuado batholith), and land cover (forest & coffee farms) Landslide densities (#/km 2 ) exceed those for Guatemala during H. Mitch Preliminary map (do not distribute) Landslide density map: # slides per unit area

9 Portion of Rio Yauco Watershed 3) Mag = f (drivers - resistance) Coffee farms steep, on weak geologic substrate, highly manipulated terrain Preliminary map (do not distribute) 1,000 Meters

10 4) Mag = f (effects / recovery) How long does it take for the affected area to return to pre-event conditions? IF they return Resilience: recuperating capacities Sandy beaches example Playa Córcega, Rincón

11 4) Mag = f (effects / recovery) Coral reefs example: growth rates few mm yr -1 up to 2 cm yr -1 Hurricane Hole, St. John, US Virgin Islands [Before (left) and After (right) H. Irma & Maria Videos by Caroline Rogers-USGS]

12 5) Mag = f (event / previous events) Steady state equilibrium: Landscape does change as a response to small & large events but vacillates around an average condition the landscape is used to this! Previous well-documented event in PR in terms of streamflow was Hurricane Hortense in 1996

13 HORTENCIA 5) Mag = f (event / previous events) More than ½ stations had higher peak flows for Maria than Hortense (larger watersheds La Plata, Manatí, Añasco) Five stations reported peak flows exceeding or near world records 4.0 Q P MARIA / Q P Preliminary data (do not distribute) Maria > Hortense Maria < Hortense WATERSHED AREA (KM 2 ) Rio La Plata at Comerio

14 5) Mag = f (event / previous events) 10% & 13% of streams had recurrence intervals > 100 & 500 years, respectively Flooding covered ~2% of watersheds (Harvey ~9% of Houston area) Some streams underwent avulsion (major geomorphic transformation) Río Grande de Añasco

15 Closing remarks The entire US Caribbean was exposed to major hurricanes in 2017 Magnitude signatures for both events are locally remarkable Physical geography s views on hurricanes could be valuable in contextualizing socioeconomic impacts