Probable Maximum Precipitation Study Virginia Floodplain Management Association Workshop Thursday, October 29, 2015 Robert T. Bennett, PE, RA, CFM DCR Dam Safety & Floodplain Management
Definition (4VAC50-20-50.H) Probable maximum precipitation means the theoretically greatest depth of precipitation for a given duration that is meteorologically possible over a given size storm area at a particular geographical location at a particular time of year with no allowance made for future long term climatic trends.
Why PMP? Emergency Spillway Capacity 70% Of Dam Failures Due to Overtopping To Prevent Failures, Dams Must Pass: 100% PMP for New High Hazard Dams 90% PMP for Existing High Hazard Dams 50% PMP for Significant Hazard Dams Accurate PMPs Essential for Public Safety
Legislation - 2014 HB 1006 Delegate Kathy J. Byron SB 582 Senator Thomas A. Garrett, Jr. 1. 1. That the Department of Conservation and Recreation, on behalf of the Virginia Soil and Water Conservation Board, shall utilize a storm-based approach in order to derive the Probable Maximum Precipitation (PMP) for locations within or affecting the Commonwealth. The PMP revisions shall be based on accepted storm evaluation techniques and take into account such factors as basin characteristics that affect the occurrence and location of storms and precipitation, regional and basin terrain influences, available atmospheric moisture, and seasonality of storm types..
The results shall be considered by the Virginia Soil and Water Conservation Board in its decision to authorize the use of the updated PMP values in Probable Maximum Flood calculations, thus replacing the current PMP values. Such PMP revisions shall be adopted by the Board if it finds that the analysis is valid and reliable and will result in cost savings to owners for impounding structure spillway construction or rehabilitation efforts.
2. The development of the methodology shall be completed by December 1, 2015. 3. Owners of impounding structures with spillway design inadequacies who maintain coverage under a Conditional Operation and Maintenance Certificate in accordance with the Board's Impounding Structure Regulations (4VAC50-20) shall not be required to rehabilitate the spillway of their impounding structure until the analysis required under 1 has been completed and reviewed by the Virginia Soil and Water Conservation Board. Such owners shall remain subject to all other requirements of the Dam Safety Act ( 10.1-604 et seq.) and regulations.
2. That in addition to other sums made available, the Department of Conservation and Recreation is authorized to utilize up to $500,000 in unobligated balances in the Dam Safety, Flood Prevention and Protection Assistance Fund established pursuant to 10.1-603.17 of the Code of Virginia or the Dam Safety Administrative Fund established pursuant to 10.1-613.5 of the Code of Virginia to contract out for the analysis required under 1. 3. That an emergency exists and this act is in force from its passage
Applied Weather Associates Completed PMP Studies across country Arizona Ohio Wyoming Texas Tennessee Valley Authority Federal Energy Regulatory Commission Nuclear Regulatory Commission
Technical Review Board Mathew Lyons State Conservation Engineer USDA NRCS Kenneth Fearon Federal Energy Regulatory Commission John Harrison Schnabel Dam Engineering Arthur Miller AECOM Jeff Orrock Meteorologist NOAA Stephen Rich Southeast Weather Consulting
Technical Review Board Meetings July 8, 2014 November 18, 2014 April 7,8, 2015 October 6, 7, 2015
National Weather Service
HYDROMETEOROLOGICAL REPORT NO. 51 Probable Maximum Precipitation Estimates, United States East of the l05th Meridian U.S. DEPARTMENT OF COMMERCE NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION U.S. DEPARTMENT OF THE ARMY CORPS OF ENGINEERS Washington, DC June 1978
Advantages of New PMP Study More Storms Considered New Technologies Used Problems/Unknowns Corrected Topographic Features Addressed Updated Climatologies Used Improved Resolution
1. Review Previous Studies AWA PMP Studies Hydrometeorological Reports USACE/USGS Storm and Flood Analyses
2. Storm Search & List Development Storm Search to Identify Significant and Transpositionable Storms in Region Identify Storms in Hydrometeorological Reports and Other PMP Studies Identify Most Significant Flood Events Identify Extreme Storm Types 1. Local Storms (Thunderstorms) 2. General Storms (Frontal System) 3. Hurricanes and Tropical Storms
78 Storms Analyzed for Virginia PMP Study Max Rainfall 34.91 Smethport PA 1942 Virginia Storms Max Rainfall: 28.39 Rapidan VA 1995 27.23 Tyro VA 1969 23.44 Portsmouth VA 2010 22.56 Montebello VA 1985 20.22 Upper Sherando VA 2003
Virginia Storms Max Rainfall (continued) 19.77 Big Meadows VA 1942 19.22 Yorktown VA 1999 15.66 Coeburn VA 1977 15.13 Little River VA 1949 14.38 Richmond VA 2004
3. Storm Analysis Analyze Extreme Storms with Storm Precipitation Analysis System Produces Gridded Rainfall Analysis Produces Required Data Sets Older Storms Re-Analyzed
4. Storm Maximization, Transpositioning, Orographic Analysis Adjustment Factors Calculated For Each Storm: 1. Maximization Factor 2. Moisture Transposition Factor 3. Orographic Transposition Factor
Probable Maximum Precipitation Study for Virginia
5. PMP Values Developed Area: 2.5 square mile grid Durations: 1 72 hours PMP Evaluation Tool: PMP values in each 2.5 square mile grid Watershed overlay will give average PMP
Probable Maximum Precipitation Study for Virginia Task 5 Develop PMP Values will be provided on a gridded basis or other format Appropriate durations, 1-hr, 6-hr.as needed Not confined to 72-hrs ~2.5mi2 Analyze the orographic effects of elevated terrain Transposition limits for each storm will determined - Use the procedures developed in previous PMP studies - Precip frequency data to calculate the Orographic Transposition Factor - Corrects stippled region in HMR 51/52
Questions? Robert T. Bennett, PE, RA, CFM DCR Dam Safety & Floodplain Management robert.bennett@dcr.virginia.gov