Independent Evaluation of Recent Flooding in New Hampshire July 2008 Federal Emergency Management Agency Department of Homeland Security 500 C Street, SW Washington, DC 20472 This document was prepared by URS Group, Inc. 200 Orchard Ridge Drive, Suite 101 Gaithersburg, MD 20878 Contract No. HSFEHQ-06-D-0162 Task Order HSFEHQ-08-J-0307 15707307.001007 Acknowledgements The project team would like to thank the New Hampshire Department of Environmental Services Dam Bureau for hosting the initial kickoff public meeting at their offices in Concord on December 12, 2007, and for making arrangements and hosting the public progress meeting held at the Wayfarer Inn in Bedford on March 18, 2008. We would also like to thank the City of Rochester for hosting the final public meeting held at Spaulding High School on June 25, 2008. TABLE OF CONTENTS ACRONYMS AND ABBREVIATIONS .......................................................................................................... v EXECUTIVE SUMMARY............................................................................................................................. vii SECTION ONE FLOODING IN SOUTH CENTRAL AND SOUTHEASTERN NEW HAMPSHIRE: ITS CAUSE AND RECOMMENDATIONS FOR FUTURE MITIGATION..............................................................................................................1-1 1.1 Introduction – The Purpose and Scope of This Study ............................. 1-1 1.2 Organization of This Report .................................................................... 1-1 SECTION TWO THE MAY 2006 AND APRIL 2007 EVENTS IN PERSPECTIVE...............................2-4 2.1 Comparing Conditions Leading up to the Events.................................... 2-5 2.1.1 Precipitation in the Months and Weeks Leading up to the Events........................................................................................... 2-6 2.1.2 Streamflow Before the Events ..................................................... 2-7 2.1.3 Comparing Rainfall and Snow during the Events........................ 2-9 2.2 Comparing Runoff and Flooding caused by the Two Events................ 2-14 2.3 Comparing these Events with Past Flood Events .................................. 2-19 2.4 Just How Severe were these Events?..................................................... 2-22 2.5 Can They Happen Again?...................................................................... 2-23 SECTION THREE DAM OPERATIONS DURING THE APRIL 2006 AND MAY 2007 EVENTS.............3-1 3.1 Overview.................................................................................................. 3-1 3.2 Types of Dams in Southern New Hampshire .......................................... 3-1 3.2.1 Flood Control Dams..................................................................... 3-1 3.2.2 Dams that Provide Significant Local Flood Control Benefits ........................................................................................ 3-1 3.2.3 Dams that Provide Limited Local Flood Control Benefits .......... 3-2 3.2.4 Run-of-River Dams...................................................................... 3-2 3.3 Evaluation of Selected Dams................................................................... 3-2 3.3.1 Dams Evaluated in Detail ............................................................ 3-2 3.3.2 Actual Operations........................................................................ 3-4 3.3.3 Simulations of What Actually Happened During the Events ...... 3-7 3.3.4 Alternative Operations Evaluated in What-If Scenario Simulations ................................................................................ 3-10 3.4 Key Findings.......................................................................................... 3-13 SECTION FOUR FLOODPLAIN MANAGEMENT.................................................................................4-1 4.1 Overview.................................................................................................. 4-1 4.2 Did Land Use Development Make the Flooding Worse?........................ 4-1 P:\GAITHERSBURG\FEMA\HMTAP PROJECTS - 2006 CONTRACT\15707307 NH FLOODING\NEW HAMPSHIRE FLOODING ANALYSIS 7-28 FOR FINAL REVIEW BM.DOC\30-JUL-08\\ i TABLE OF CONTENTS 4.3 Erosion, Sediment, and Woody Material: Do They Aggravate Flooding? ................................................................................................. 4-4 4.3.1 Erosion and Sediment .................................................................. 4-4 4.3.2 Woody Material........................................................................... 4-5 4.4 Is the information developed by the National Flood Insurance Program accurate?.................................................................................... 4-5 4.4.1 The Status of FEMA Floodplain Mapping in Southern New Hampshire.................................................................................... 4-5 4.4.2 Is the Floodplain Information Accurate? ..................................... 4-8 4.5 Are the State’s Dam Safety Regulations Adequate?.............................. 4-15 4.6 Are flood forecasts accurate and are they used effectively to anticipate and respond to flooding events?............................................ 4-17 4.6.1 The Role of NHDES in Forecasting Floods .............................. 4-17 4.6.2 The National Weather Service’s Role in Forecasting Floods.... 4-19 4.6.3 How Well Did the NWS Predict the Flood Events in Southern New Hampshire? ........................................................ 4-21 4.6.4 Value of Forecasts...................................................................... 4-33 4.7 is the response at all levels of government during flood emergencies adequate and effective?..................................................... 4-35 SECTION FIVE WHAT CAN BE DONE TO MITIGATE THE IMPACT OF FUTURE FLOODS?........5-1 5.1 Reducing flooding.................................................................................... 5-1 5.1.1 Operations and Maintenance........................................................ 5-2 5.1.2 Security ........................................................................................ 5-4 5.1.3 Structural Improvements.............................................................. 5-4 5.1.4 Dam Removal .............................................................................. 5-5 5.1.5 Erosion, Sediment, and Woody Material..................................... 5-6 5.2 Improving Floodplain Management ........................................................ 5-9 5.2.1 FEMA’s National Flood Insurance Program ............................... 5-9 5.2.2 FEMA’s Mitigation Planning and Grants Programs.................. 5-11 5.2.3 Emergency Operations and Communications Improvements.... 5-11 5.3 Improving Forecasting and Response.................................................... 5-12 5.3.1 Availability of Forecasts............................................................ 5-12 5.3.2 Accuracy of Forecasts................................................................ 5-13 5.3.3 Riverine Risk Management Tool............................................... 5-14 SECTION SIX RECOMMENDATIONS TO IMPROVE FLOODPLAIN MANAGEMENT ...................6-1 6.1 Achieving Accurate Floodplain Mapping................................................ 6-1 6.2 Improved Floodplain Management.......................................................... 6-2 6.3 Emergency Operations and Communications Improvements.................. 6-3 6.4 Recommendations to Control Erosion, Sediment, and Woody Material.................................................................................................... 6-4 6.4.1 Sediment and Woody Material Removal..................................... 6-4 6.4.2 Stormwater Permitting Issues and Best Management Practices ....................................................................................... 6-4 P:\GAITHERSBURG\FEMA\HMTAP PROJECTS - 2006 CONTRACT\15707307 NH FLOODING\NEW HAMPSHIRE FLOODING ANALYSIS 7-28 FOR FINAL REVIEW BM.DOC\30-JUL-08\\ ii TABLE OF CONTENTS 6.5 Apply Vermont’s “Fluvial Erosion Hazard Methodology” to New Hampshire Waterways............................................................................. 6-5 6.6 Recommendations Made Previously........................................................ 6-5 6.6.1 Recommendation No. 1 – Reminder letters to dam owners ........ 6-5 6.6.2 Recommendation No. 2 – Coordination meeting in anticipation of runoff season........................................................ 6-6 6.6.3 Recommendation No. 3 – Cleaning of debris and woody material from the railroad trestle upstream of the Kelley’s Falls Dam on the Piscataquog River and from the Bucks Street Dams on the Suncook River. ............................................. 6-6 SECTION SEVEN RECOMMENDATIONS TO IMPROVE FLOOD FORECASTING..............................7-1 7.1 Improved Flood Forecasting Summary ................................................... 7-1 7.2 Access to Currently Available NWS Forecasts ....................................... 7-1 7.3 Improve and Expand NWS Forecasts...................................................... 7-2 7.3.1 Additional Forecast Points........................................................... 7-2 7.3.2 Smaller Modeling Time Step....................................................... 7-3 7.3.3 Longer Forecast Period................................................................ 7-3 7.4 Revitalize and