Site-Specific and Regional Probable Maximum Precipitation (PMP) Studies Ed Tomlinson, PhD 2009 Western Regional Dam Safety Forum San Francisco, California January 15, 2009 PO Box 680 Monument, Co 80132 (719) 488-9117 e-mail: awaadmin@comcast.net www.appliedweatherassociates.com Site-Specific and Regional PMP Studies – Current HMRs – Completed Site-Specific and Regional Studies – Key Tasks – Procedures – Examples
Probable Maximum Precipitation • Definition: The theoretically theoretically greatest depth of precipitation for a given duration that is physically physically possible possible over a given storm area at a particular geographic location at a certain time of year geographic location (HMR 59, 1999) • T ypes of PMP studies: – G eneralized (Hydrometeorological Reports, HMRs) – S ite-Specific • Individual drainage basin • Regional Current NWS HMRs
Regional PMP Studies EPRI Wisconsin/Michigan PMP Study 1993 Nebraska Statewide PMP Study 2008 Arizona Statewide PMP Study Begin Feb 2009 Recent FERC Approved Site- Specific PMP Studies Upper and Middle Dams FPLE Maine Hydro 2003 Great Lake Sacandaga/Stewarts Bridge Reliant Energy 2005 Blenheim Gilboa Pump Storage 2008 New York Power Authority
Current FERC PMP Studies Lewis River, Washington PacifiCorp Initiated Nov 2008 State Site-Specific PMP Studies Approved Colorado Ohio New Jersey Nebraska New York Potential new studies Oregon California Virginia Pennsylvania
Method for Computing PMP Values • N o physically based theoretical method (yet) • O bserved extreme rainfall events are used • T hree primary steps – Maximization – Maximization procedures are applied – Transpositioning – Transpositioning procedures are applied – Enveloping – Enveloping procedures are applied • Storm separation method used in HMRs 57 & 59 Do Site-Specific PMP Studies Provide Improved PMP Values? • Attributes: – More storms considered – New technologies used – Problems in the HMRs corrected – Topographic features addressed
Key Tasks for Site-Specific PMP Studies • I dentify extreme storm types – E valuate the use of HMR procedures for each storm type • I dentify unique topography – M oisture depletion by upwind mountain barriers – P recipitation enhancement/decrease – E ffects on storm center location • R eview HMR procedures used for the basin location – I dentify inconsistent assumptions Examples of Site-Specific PMP Study Findings • H omogeneous Climate Regions – S elect storms for each region – T ransposition only in the same region
Examples of Site-Specific PMP Study Findings • S torm Maximization, Dewpoint Analyses – 12-hour vs 6-hour persisting dewpoints – 12-hour persisting vs 6-hour average • Observed dewpoint values • Hour 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 • Td 68 67 69 69 72 75 75 74 75 73 70 69 68 65 66 65 65 65 67 66 • ! Rainfall Event ! • 12-hour persisting: 65 • 6-hour persisting: 72 • 6-hour average: 74
Updated Procedures • Maximum Dewpoint Climatology – HMR 59 produced new dewpoint climatologies • 3- and 12- persisting maximum dewpoint values – Nebraska statewide PMP study produced new dewpoint climatologies • 6-, 12-, and 24-hour average dewpoint values • 20-, 50- and 100-year return frequencies • Sea surface temperatures used for some storms – Subjective techniques used in HMRs 57 & 59 • Not documented – Objectives techniques used in other site-specific PMP studies Recalculation of the Distance from the Coast curve in HMR 51
Examples of Site- • FPL Maine Hydro Upper and Middles Dams Specific PMP Study – Upwind moisture barrier Results defined – Sea surface temperatures used Hysplit Upwind Trajectories
Barrier Moisture Depletion Analysis Southeastern Inflow Barrier - South Arm to Sugarloaf Mountain Average Barrier Height - 830 Meters / 2725 Feet 1400 1300 1200 1100 1000 Barrier Elevation (Meters) 900 800 700 600 500 400 300 200 100 0 1 51 101 151 201 251 301 351 401 451 501 551 601 651 701 751 801 851 901 951 1001 1051 Cell Number
Examples of Site- Specific PMP Study Results Southeast M oisture Inflow Barrier - The G reen, Berkshire, and Taconic M ountains Average Barrier H eight M inus 100 Feet: 2040 Feet 4 00 0 3 50 0 3 00 0 Barrier Height in Feet 2 50 0 2 00 0 1 50 0 1 00 0 50 0 0 0 20 4 0 6 0 8 0 10 0 12 0 1 40 16 0 B arrier W idth in M iles Average Barrier H eight Average Barrier H eight M inus 100 Feet Barrier Terrain H eight
Northeast Moisture Inflow Barriers: Tuxedo Lake Drainage Basin Effective Barrier Height: 1400 feet 3000 Northeast Inflow Barrier 1 Northeast Inflow Barrier 2 Average Elevation: 1,461 feet Average Elevation: 1,230 feet Length: 66 miles Length: 85 miles 2500 2000 1500 1000 500 0 Barrier 1 Average Elevation Barrier 2 Average Elevation Northeast Inflow Barrier 2 Northeast Inflow Barrier 1 Effective Barrier Height Within Basin Orographics
Within Basin Orographics Within Basin Orographics
StormName: Hallett, OK Storm Adjustment for Wanahoo, NE StormDate: 02-Sep-1940 AWA Analysis Date: 5/10/2007 17-Aug Temporal Transposition Date Lat Long Moisture InflowDirection: SSE@ 75 miles Stormcenter location 36.23 N 96.57 W Basin E levation 1,300 feet 930 StormRepdewpoint location 35.23 N 96.06 W Storm E levation feet Transposition dewpointlocation 40.20 N 95.88 W E ffective Barrier Heigh t 1,300 feet 41.25 N 96.66 W Basin location Notes:DAD values taken fromUSACEStorm Studies SW 2-18 The stormrepresentative dew point is 76.0 F with total precipitable water above sea level of 2.99 inches. The in-place maximumdew point is 79.0 F with total precipitable water above sea level of 3.44 inches. The transpositioned maximumdew point is 80.0 F with total precipitable water above sea level of 3.60 inches. The in-place stormelevation is 930 which subtracts 0.24 inches of precipitable water at 76.0 F The in-place stormelevation is which subtracts inches of precipitable water at 930 0.27 79.0 F The transposition basin elevation at 1,300 which subtracts 0.38 inches of precipitable water at 80.0 F The inflow barrier/basin elevation height is 1,300 which subtracts 0.38 inches of precipitable water at 80.0 F The in-place stormmaximization factor is 1.15 The transposition/elevation to basin factor is 1.02 The barrieradjustment factor is 1.00 The total adjustment factor is 1.17 Observ edStormDepth-Area-Duration 6Hours 12Hours 18 Hours 24Hours 30 Hours 36Hours 48 Hours 60Hours 72 Hours 10 sq miles 18.4 23.4 23.6 23.6 23.6 23.6 0.0 0.0 0.0 100 sq miles 14.7 19.2 19.4 19.6 19.7 19.8 0.0 0.0 0.0 200 sq miles 12.5 17.6 17.8 18.0 18.1 18.3 0.0 0.0 0.0 500 sq miles 9.7 15.4 15.6 15.7 15.8 16.2 0.0 0.0 0.0 1000 sq miles 7.9 13.3 13.4 13.6 13.7 14.1 0.0 0.0 0.0 5000 sq miles 4.3 7.3 7.4 7.5 7.7 7.8 0.0 0.0 0.0 10000 sq miles 3.0 5.3 5.4 5.5 5.6 5.7 0.0 0.0 0.0 20000 sq miles 2.0 3.9 4.1 4.2 4.3 4.4 0.0 0.0 0.0 AdjustedStorm Depth-Area-Duration 6Hours 12Hours 18 Hours 24Hours 30 Hours 36Hours 48 Hours 60Hours 72 Hours 10 sq miles 21.5 27.4 27.6 27.6 27.6 27.6 0.0 0.0 0.0 100 sq miles 17.2 22.5 22.7 22.9 23.1 23.2 0.0 0.0 0.0 200 sq miles 14.6 20.6 20.8 21.1 21.2 21.4 0.0 0.0 0.0 500 sq miles 11.4 18.0 18.3 18.4 18.5 19.0 0.0 0.0 0.0 1000 sq miles 9.3 15.6 15.7 15.9 16.0 16.5 0.0 0.0 0.0 5000 sq miles 5.0 8.5 8.7 8.8 9.0 9.1 0.0 0.0 0.0 10000 sq miles 3.5 6.2 6.3 6.4 6.6 6.7 0.0 0.0 0.0 20000 sq miles 2.3 4.6 4.8 4.9 5.0 5.2 0.0 0.0 0.0 Stormor StormCenter Name Hallett, OK StormDate(s) 2-Sep-1940 StormType MCC StormLocation 36.23 N 96.57 W StormCenter Elevation 930 Precipitation Total & Duration 24.00 Inches 12-hours USACE StormStudies SW 2-18 StormRepresentative Dewpoint 76.0F 12hr average StormRepresentative Dewpoint Location 35.23 N 96.06 W MaximumDewpoint 79.0F Moisture Inflow Vector SSE@ 75Miles In-place Maximization Factor 1.15 Temporal Transposition (Date) 17-Aug Transposition Dewpoint Location 40.20 N 95.88 W Transposition Max imumDewpoint 80.0F Basin Elevation 1,300 Transposition to Basin Adjustment Factor 1.02 Higherof Basin Elevation - Inflow Barrier Height 1,300 Elevation Adjustment Factor 1.00 Total Adjustment Factor 1.17
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