Introduction to Probable Maximum Precipitation (PMP) Estimates

Background

This tutorial and guide develops the Probable Maximum Precipitation (PMP) estimates using guidance and information in Hydrometeorological Report Number 51 (HMR51), Hydrometeorological Report Number 52 (HMR52) and the built-in HMR52 functionality in the meteorologic model of HEC-HMS.  The workshop is intended to demonstrate how to develop a periodic assessment / semi-quantitative risk assessment (PA/SQRA) level PMP estimate for watersheds within the Eastern US.  Watersheds in the West are covered by HMR documents 55a, 49, 57, and 58/59.  The procedure for developing the PMP in the West is different than what is found in HMR 51 and 52. The biggest differences include only one PMP index map for Western US HMR documents and the lack of a pre-defined spatial PMP storm pattern (due to the complex terrain). 

Goals

The general objectives of this workshop are summarized below: 
1)    Apply the HMR52 Storm meteorologic model option in HEC-HMS and manually maximize the PMP precipitation upstream of Sayers Dam.
2)    Use the optimization trial to automatically adjust the HMR52 Storm meteorologic model parameters to maximize flow and stage.
3)    Alter HMR52 ellipsoids in ArcMap and import them into HEC-HMS to illustrate the optimized PMP location and orientation.

Data

Foster Joseph Sayers Dam is located on Bald Eagle Creek in Centre County, PA.  Sayers dam is about 14 miles upstream of the confluence of Bald Eagle Creek and the West Branch Susquehanna River at Lock Haven, PA.  The drainage area upstream of Sayers dam is approximately 339 square miles, while the drainage area at the confluence with the West Branch Susquehanna is approximately 781 square miles.  The primary purpose of Sayers Dam was to provide flood control for the downstream reach of Bald Eagle Creek and West Branch Susquehanna River.  Sayers Dam was completed in 1969 and consists of a rolled earth-fill embankment about 6,835 feet long with a top width of 25 feet.  The maximum height of the dam is 100 feet above the stream bed at elevation 683 feet.