Probable Maximum Precipitation Study

Virginia Floodplain Management AssociationWorkshop

Thursday, October 29, 2015

Robert T. Bennett, PE, RA, CFMDCR Dam Safety & Floodplain Management

Definition(4VAC50-20-50.H) Probable maximum

precipitation means the theoreticallygreatest depth of precipitation for a givenduration that is meteorologically possibleover a given size storm area at aparticular geographical location at aparticular time of year with no allowancemade for future long term climatic trends.

Why PMP? Emergency Spillway Capacity70% Of Dam Failures Due to OvertoppingTo Prevent Failures, Dams Must Pass:100% PMP for New High Hazard Dams90% PMP for Existing High Hazard Dams50% PMP for Significant Hazard DamsAccurate 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 andRecreation, on behalf of the Virginia Soil and WaterConservation Board, shall utilize a storm-basedapproach in order to derive the Probable MaximumPrecipitation (PMP) for locations within or affecting theCommonwealth. The PMP revisions shall be based onaccepted storm evaluation techniques and take intoaccount such factors as basin characteristics that affectthe occurrence and location of storms and precipitation,regional and basin terrain influences, availableatmospheric moisture, and seasonality of storm types.

• .

The results shall be considered by the Virginia Soil andWater Conservation Board in its decision to authorizethe use of the updated PMP values in ProbableMaximum Flood calculations, thus replacing the currentPMP values. Such PMP revisions shall be adopted bythe Board if it finds that the analysis is valid and reliableand will result in cost savings to owners for impoundingstructure spillway construction or rehabilitation efforts.

§ 2. The development of the methodology shall becompleted by December 1, 2015.

§ 3. Owners of impounding structures with spillway designinadequacies who maintain coverage under aConditional Operation and Maintenance Certificate inaccordance with the Board's Impounding StructureRegulations (4VAC50-20) shall not be required torehabilitate the spillway of their impounding structureuntil the analysis required under § 1 has beencompleted and reviewed by the Virginia Soil and WaterConservation Board. Such owners shall remain subjectto all other requirements of the Dam Safety Act (§ 10.1-604 et seq.) and regulations.

2. That in addition to other sums made available, theDepartment of Conservation and Recreation isauthorized to utilize up to $500,000 in unobligatedbalances in the Dam Safety, Flood Prevention andProtection Assistance Fund established pursuant to §10.1-603.17 of the Code of Virginia or the Dam SafetyAdministrative Fund established pursuant to § 10.1-613.5 of the Code of Virginia to contract out for theanalysis required under § 1.

3. That an emergency exists and this act is in force fromits 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 EngineeringArthur Miller – AECOMJeff Orrock – Meteorologist NOAAStephen 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. 51Probable Maximum Precipitation Estimates, United States

East of the l05th MeridianU.S. DEPARTMENT OF COMMERCE

NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION

U.S. DEPARTMENT OF THE ARMYCORPS OF ENGINEERS

Washington, DCJune 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 Types1. 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 1942Virginia 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 Factor2. Moisture Transposition Factor3. Orographic Transposition Factor

Probable Maximum Precipitation Study for Virginia

5. PMP Values DevelopedArea: 2.5 square mile gridDurations: 1 – 72 hoursPMP Evaluation Tool:• PMP values in each 2.5 square mile grid• Watershed overlay will give average PMP

Probable Maximum Precipitation Study for Virginia

Task 5Develop 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, CFMDCR Dam Safety & Floodplain

Managementrobert.bennett@dcr.virginia.gov