August 2006 El Paso. Policy Factors

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1 August 2006 El Paso Flooding: Technical and Policy Factors John Walton University of Texas at El Paso El Paso, Texas UTEP

2 History Deficiencies in El Paso Arroyo drainage systems were multitudinous and obvious Many years of poor designs and lax enforcement Rare but below design basis storm caused extensive damage UTEP

3 Overview Examples of Failures Technical and Educational Issues Policy and Enforcement UTEP

4 Upstream Development Downstream hydraulic structures not upgraded to reflect upstream development Large arroyo diverted into small w/o proper update of FEMA map & flows No sedimentation basins Warning ignored by city and FEMA UTEP

31 Arroyo Upstream Sediment Generation w/o Basin UTEP

32 Silver Springs UTEP

33 A Well Designed Channel! UTEP

35 Shadow Mountain Box Culvert Clogs with Sediment UTEP

36 Once water jumps from arroyos, it s hard to know where it will go. UTEP

38 Behind Blockbuster UTEP

39 Behind Blockbuster UTEP

40 Blockbuster from Behind note orographic clouds UTEP

43 Braided Stream Arroyos carry water and sediment Natural arroyos dynamically adjust to balance sediment transport and water discharge Lined drainage canal located at temporal location of braided stream channel No berm or sedimentation basin Channel shifted during storm onto street UTEP

46 Upper Part of Development UTEP

54 Shear Stress Failure UTEP

55 Shear Stress in Bend UTEP

56 ^2 l bf ê f t ^ r ess, l e ar St r She Shear Stress Actual Shear Stress Allowable Shear Stress UTEP Width of Channel, ft

57 Failure Between Houses UTEP

58 Failure at Left Turn UTEP

60 Rock Wall Washed Out UTEP

61 Textbook Works Textbook calculation predicts failure of the unlined, nocohesive berm at the location of maximum shear stress Berm failed at point of maximum shear stress Flooding damage occurred far from failure location UTEP

62 Other Examples Detention ponds not designed to rapidly drain before next storm arrives Detention ponds with no outlet t Flooding of house from water originating on the roof UTEP

63 Technical/ Educational Subtle probability concepts Meteorological concepts Practicing i engineers are generalists Data inadequate to properly define design basis storm in complex terrain Precipitation data obtained in a rain shadow (El Paso Airport) applied to mountain front locations UTEP

64 The probability of a 100 yr or greater storm in any one year is? 1. 1/100, by definition iti 2. < 1/ > 1/ none of the above UTEP

65 Answer: There are many 100 yr storms: 1 minute, 10 minute, 30 minute, 1 hour, 6 hour, 1 day, 2 day, 1 week The probability of each type = 1/100; the probability of any one of the set > 1/100 UTEP

66 What is a Design Basis Storm? 100 year (or other chosen return period) storm at storm duration equal to the time of concentration At location of concern, not at location of historical data collection UTEP

67 Time of Concentration Rivers days Arroyos an hour or two Subdivisions minutes Parking lot, streets seconds to minutes UTEP

68 It Rains More Near the Franklins Than at the Airport (Orographic Mammary Clouds) ~1.7x 17x UTEP

69 Measured Precipitation versus AIRPORT IDF Curve Note that a proper Arroyo IDF curve would be ~1.7 x higher The longer the time interval, the more extreme the event becomes using any dataset. from: Dave Novlin, Tom Gill Official Data (Airport) Unofficial Reports Educated Guess for Dr. Green UTEP School

70 El Paso Storm Exceeded 100 yr storm at duration of 2 days No legitimate data indicate properly defined design basis storm was exceeded at location of failure Most of the observed engineering failures should not have occurred UTEP

71 Current 100 year storm & orographic effects confusion provide plausible deniability it s God s fault..... couldn t beus Will future engineering designs and regulatory reviews be more rigorous? UTEP

72 Recommended Changes IDF curve revised to reflect mountainside location Sedimentation basins where unlined Arroyos are first lined Proper enforcement by City of El Paso and FEMA Higher standards by all involved UTEP

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