Chapter 4 Flood Frequency Analysis for the Suncook River
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Mechanisms governing avulsions in transient landscapes: Analysis of the May 2006 Suncook River avulsion in Epsom, New Hampshire by Mariela C. Perignon Submitted to the Department of Earth, Atmospheric and Planetary Sciences in Partial Fulfillment of the Requirements for the Degree of Bachelor of Science in Earth, Atmospheric and Planetary Sciences at the Massachusetts Institute of Technology June 1, 2007 Copyright 2007 Mariela C. Perignon. All rights reserved. The author hereby grants M.I.T. permission to reproduce and distribute publicly paper and electronic copies of this thesis and to grant others the right to do so. Author Signature redacted Department of Earth, Atmospheric, an Planetary Sciences June 1, 2007 Certified by Signature redacted John B. Southard Signature redacted Thesis Supervisor Accepted by ARCHIVES Samuel A. Bowring MASSACHUSETS INSTITUTE OF TECHNOLOGY Chair, Committee on Undergraduate Program 2 2017 The author hereby grants to MIT permission to OCT 4 reproduce and to distribute publicly paper and electronic copies of this thesis document In LIBRARIES whole or in part in any medium now known or hereafter created. Mechanisms governing avulsions in transient landscapes: Analysis of the May 2006 Suncook River avulsion in Epsom, New Hampshire By Mariela C. Perignon Submitted to the Department of Earth, Atmospheric, and Planetary Sciences June 1, 2007 In Partial Fulfillment of the Requirements for the Degree of Bachelor of Science in Earth, Atmospheric, and Planetary Sciences ABSTRACT Avulsions, or rapid changes in the location of a river, usually occur in environments such as deltas, floodplains, and alluvial fans where net-deposition can raise the bed of the river above its floodplain. Avulsions are less frequent in transient landscapes, such as New England, where topography and hydrography are still responding to recent glaciation. One of these rare avulsions occurred during a 100-year flood on the Suncook River, Epsom, NH, between May 14 and 15,2006. We studied the Suncook River event to develop a model for the drivers of avulsions in transient landscapes. We suggest that a strong substrate in the parent channel, such as bedrock or immobile boulders, can facilitate an avulsion by preventing incision and driving water overbank. Easily erodible substrates in the path of the new channel can also contribute to avulsions by allowing a knickpoint to migrate quickly upstream and create a channel with a more favorable slope during a single flood. Based on Slingerland and Smith's (2004) model, we also propose that a low water-surface slope in the parent channel could be a direct driver for avulsions. In the Suncook River, this low water-surface slope was created in the backwater of a small mill dam in the parent channel. A 200-year flood that occurred in the Suncook River in 1936 did not create an avulsion. We suggest that ice floats could have damaged the dam and increased the water-surface slope of the parent channel, making an avulsion less favorable and reducing the depth of water flowing overbank. The topography in the path of the 2006 avulsion, which was lowered by activity in a sand pit starting in the 1960s, probably prevented water from finding a new path. We believe that these anthropogenic modifications directly contributed to the occurrence of the May 2006 avulsion in the Suncook River. These conditions are common throughout New England, and could increase the risk of avulsions in the region. Thesis Supervisor: John B. Southard Title: Professor Emeritus, Department of Earth, Atmospheric, and Planetary Sciences Research Supervisor: Scott R. Miller Title: Visiting Lecturer, Department of Earth, Atmospheric, and Planetary Sciences 2 Acknowledgements I would like to thank Professor John Southard for ignoring the fact that he is retired and continuing to teach and support undergraduate research. I would also like to thank Dr. Scott Miller for his support and helpful reviews. Numerous discussions with Chad Wittkop, formerly with the New Hampshire Geological Survey and now at the University of Wisconsin - Eau Claire, were the base for most of my work. Chris Sherratt, from Lindgren Library, has been an invaluable resource in my search for obscure literature. I wish more people knew how wonderful the library staff is. I would especially like to thank Professor Sam Bowring for believing in the potential of undergraduates. The interest he has expressed in this project is the one thing that made me finish my thesis. Vicki McKenna has also been a priceless resource throughout my undergraduate career. Somehow she makes everything work. I would also like to thank Pat Walsh. There is no specific reason related to my thesis why I would like to thank her, but she is awesome and should be included in my acknowledgements. Roxana Safipour was my field assistant. I would really like to thank her for wading through a half-frozen river with me in the name of science. She has also been a great friend. I would also like to thank Andy Wickert for numerous long nights of work and conversation. Finally I would like to thank my family for allowing me to pursue my education abroad, and for letting me play with dirt when I was little. It was so much more fun than dolls. 3 Table of Contents Title page Abstract 1 Acknowledgements 2 Table of Contents 3 List of Figures 4 List of Tables 6 8 Chapter 1: Introduction 9 Chapter 2: Events That Led to the Suncook River Avulsion - Geographic and Geologic Setting 13 2.1: Events Leading to the May 2006 Flood 2.2: Geometry of the Avulsion Site 13 2.3: The May 2006 Avulsion 14 2.4: Geologic Setting 17 20 Chapter 3: Reconstructing the Flooding History of the Suncook River 23 3.1: Finding a Proxy for the Discharge of the Suncook River 3.1.1: Comparing Mean Daily Discharges 24 3.1.2: Comparing Daily Discharges for the Shared Recording 26 Period 27 3.1.3: Normalized Daily Discharges for the Shared Recording Period 28 3.2: Reconstructing a Continuous Hydrograph for the Suncook River 3.3: Estimated Discharge in the Suncook River During the May 2006 32 Flood 33 Chapter 4: Flood Frequency Analysis for the Suncook River 4.1: Methods 35 4.2: Results and Analysis 35 37 Chapter 5: Drivers for the May 2006 Suncook River Avulsion 39 Chapter 6: Geologic Conditions Affecting the May 2006 Suncook River Avulsion 43 6.1: Prevention of Incision in the Parent Channel by Armoring 6.2: Role of Substrate Strength in the Migration of Knickpoints 43 6.3: Estimates of Knickpoint Migration Rate 45 49 Chapter 7: Discussion 52 4 Chapter 8: Conclusions 55 References 60 Appendix A 67 5 List of Figures Fig. 1. Deltaic lobes of the Mississippi River since 5000 years ago 10 Fig. 2. Accumulated precipitation for New England between May 11 and 14 May 18, 2006 Fig. 3. Road map of the area surrounding Epsom, NH 15 Fig. 4. Important features of the Suncook River avulsion site 16 Fig. 5. Photograph of Cutter's Pit on May 16, 2006 showing breached 18 ridge and terraces Fig. 6. Aerial photograph of avulsion site on May 17, 2006 19 Fig. 7. FEMA flood insurance rate map showing location of newly 20 carved channel, outside of the 100-year and 500-year floodplains Fig. 8. Locations and descriptions of the USGS discharge gages in the 25 Soucook, Piscataquog, and Suncook Rivers Fig. 9. Mean daily discharge hydrographs for the Suncook, Soucook, and 27 Piscataquog Rivers Fig. 10. Examples of daily discharges for the Suncook, Soucook, and 29 Piscataquog Rivers during the time of shared record Fig. 11. Examples of the daily discharge for the Suncook, and the 30 normalized discharges for the Soucook and Piscataquog Rivers during the time of shared record Fig. 12. Scatter plot comparing the daily normalized Soucook River 31 discharges and the normalized Piscataquog River discharges to the measured Suncook River discharges Fig. 13. Map of flood recurrence intervals exceeded during the May 2006 33 flood in New Hampshire Fig. 14. Flood recurrence interval analysis for the Suncook River 38 Fig. 15. Slingerland and Smith (2004) plot for avulsion types based on 40 water surface slope ratios 6 Fig. 16. Parent and avulsion channel map, with measurements for bed 41 surface slope Fig. 17. Bed surface slope ratio for the Suncook River in Slingerland and 42 Smith's model Fig. 18. Photograph of primary channel showing boulder reaches 44 Fig. 19. Diagrams of the hypothetical outcome of obstacles to the 47 migration of the knickpoint Fig. 20. Longitudinal profiles for the parent and new channel of the 48 Suncook River obtained through field measurements 7 List of Tables Table 1. Major floods in the history of New Hampshire that have affected 23 the Merrimack Basin Table 2. Discharge and exceedance probabilities expected for floods of 38 increasing recurrence interval 8 Chapter 1 Introduction The positions of channels in deltas, alluvial fans, and floodplains change frequently on geological time scales. Abrupt changes, referred to as avulsions, create new channels that capture some or all of the flow of a previously established river. The relocation of rivers during avulsions contributes to the distribution of sediment on floodplains, and thus directly affects the patterns of drainages and the architecture of fluvial sedimentary deposits on the surface. Avulsions are also common in modem drainage systems, and often cause havoc in our attempts to control the paths of rivers. The Mississippi River delta, for example, has changed position at least seven times in the past 5000 years (Kolb and Van Lopik, 1966) (Figure 1). Fisk (1952) also recognized the Atchafalaya-Mississippi bifurcation as an incipient avulsion. If this switch were allowed to occur, the Mississippi would run a new course farther west than it is now, leaving the port of New Orleans dry and severely disrupting commercial cargo traffic in the United States.