Flood Mitigation Needs Assessment Six Mile Creek Tompkins County, New York October 2003 MMI #2343-01 Prepared for: Tompkins County Planning Department 121 East Court Street Ithaca, NY 14850 Edward Marx, Commissioner of Planning Kate Hackett, Water Resources Planner Katie Borgella, Land Programs Manager Prepared by: Milone & MacBroom, Inc. 716 South Main Street Cheshire, CT 06410 TABLE OF CONTENTS Executive Summary 1.0 Introduction 1.1 Background and Purpose ................................................................................................. 1-1 1.2 Six Mile Creek Pilot Watershed ...................................................................................... 1-5 1.3 Project Stakeholders......................................................................................................... 1-8 1.4 Principles of Watershed Management ........................................................................... 1-10 1.5 Watershed Delineation and Nomenclature .................................................................... 1-12 2.0 Inventory of Existing Conditions 2.1 Geologic and Geomorphic Background........................................................................... 2-1 2.2 Terrain.............................................................................................................................. 2-6 2.3 Existing Land Uses within the Six Mile Creek Watershed.............................................. 2-6 2.4 Water Quality................................................................................................................... 2-9 2.5 A Review of Past Studies on Six Mile Creek ................................................................ 2-10 2.6 Hydrology and Flooding History of Six Mile Creek ..................................................... 2-13 3.0 Watershed and Stream Needs Assessment 3.1 Overview of Field Investigations..................................................................................... 3-1 3.2 Stream Profile and Control Points ................................................................................... 3-1 3.3 Needs Assessment by Stream Segment ........................................................................... 3-2 3.3.1 Segment #1 – Cayuga Inlet to Van Natta's Dam ............................................................. 3-2 3.3.2 Segment #2 – Van Natta's Dam to Burns Road............................................................... 3-3 3.3.3 Segment #3 – Burns Road to Banks Road....................................................................... 3-4 3.3.4 Segment #4 – Banks Road to Middaugh Road ................................................................ 3-7 3.3.5 Segment #5 – Middaugh Road to Valley Road near Route 330 in Brooktondale ........... 3-7 3.3.6 Segment #6 – Valley Road near Route 330 in Brooktondale to Boiceville Road ........... 3-8 3.3.7 Segment #7 – Boiceville Road to Creamery Road .......................................................... 3-8 3.3.8 Segment #8 – Creamery Road to Six Hundred Road (Slaterville Springs) ..................... 3-9 3.3.9 Segment #9 – Six Hundred Road to Headwaters in Dryden.......................................... 3-11 3.4 Surveyed Cross Section Geometry ................................................................................ 3-11 3.5 Channel Incision ............................................................................................................ 3-19 3.6 Complex Response......................................................................................................... 3-21 3.7 Slope and Sinuosity........................................................................................................ 3-22 Flood Mitigation Needs Assessment Tompkins County October 2003 i 4.0 Priority Issues and Recommendations 4.1 Priority Issue # 1 – Streambank Erosion.......................................................................... 4-1 4.1.1 Stream Dynamics............................................................................................................. 4-1 4.1.2 Sediment Budget and Transport Mechanisms ................................................................. 4-3 4.1.3 Types of Erosion.............................................................................................................. 4-4 4.1.4 Bank Stabilization............................................................................................................ 4-7 4.1.5 Streambank Erosion Recommendations .......................................................................... 4-8 4.2 Priority Issue #2 – Water Quality .................................................................................. 4-11 4.3 Priority Issue #3 – Need for Coordinated Planning Efforts........................................... 4-12 4.4 Management Practices ................................................................................................... 4-16 5.0 Implementation Strategy 5.1 Management Practices, Program Operation and Coordination........................................ 5-1 5.2 Funding Considerations ................................................................................................... 5-2 5.3 Implementation and Future Needs................................................................................... 5-3 LIST OF TABLES Table 1-1 Partial List of Project Stakeholders...................................................................... 1-8 Table 1-2 Summary of Subwatershed Areas...................................................................... 1-15 Table 1-3 Summary of Stream Segment Designations ...................................................... 1-16 Table 1-4 Correlations of Subwatersheds to Stream Segments ......................................... 1-16 Table 2-1 Summary of USGS Stream Gauge Data at Bethel Grove.................................. 2-14 Table 3-1 Hydraulic Geometry of Selected Stream Segments........................................... 3-12 Table 3-2 Adverse Impacts Due to Channel Incision......................................................... 3-21 Table 3-3 Segment Data..................................................................................................... 3-22 Table 4-1 Types of Mass Soil Failures................................................................................. 4-7 Table 4-2 Segment Restoration Priority............................................................................. 4-10 Table 4-3 Relationship of Imperviousness to Water Quality............................................. 4-14 Table 4-4 Primary Watershed Management Functional Groups........................................ 4-16 Flood Mitigation Needs Assessment Tompkins County October 2003 ii LIST OF FIGURES Figure 1-1 Location Plan – Tompkins County ...................................................................... 1-2 Figure 1-2 Watersheds within Tompkins County.................................................................. 1-4 Figure 1-3 Six Mile Creek Location Map.............................................................................. 1-6 Figure 1-4 Watershed Delineation....................................................................................... 1-14 Figure 1-5 Sub-Watershed Structure ................................................................................... 1-17 Figure 2-1 Summary of Regional Channel Evolution ........................................................... 2-2 Figure 2-2 Land Use within the Six Mile Creek Watershed ................................................. 2-8 Figure 3-1 Six Mile Creek at German Cross Road – Depth of Channel Below Bridge Deck ......................................................................................................... 3-5 Figure 3-2 Cross Section @ Karig Property u/s German Cross Road................................. 3-13 Figure 3-3 Bend Cross Section – Karig Property ................................................................ 3-14 Figure 3-4 Gully Cross Sections – Karig Property.............................................................. 3-15 Figure 3-5 Cross Section 165 Feet d/s Banks Road ............................................................ 3-16 Figure 3-6 Cross Section @ Barille Site.............................................................................. 3-17 Figure 3-7 Sinuosity vs Slope in Six Mile Creek ................................................................ 3-23 Figure 4-1 Relationship of Imperviousness to Water Quality............................................. 4-15 LIST OF APPENDED FIGURES Figure I Watershed Delineation and Nomenclature Figure II Stream Profile Flood Mitigation Needs Assessment Tompkins County October 2003 iii Executive Summary Study Purpose and Need The Tompkins County Planning Department has retained Milone & MacBroom, Inc. (MMI) to conduct a Flood Mitigation Needs Assessment for numerous watersheds in the County as part of an ongoing planning and mitigation effort. The intent is to restructure the Flood Hazard Mitigation Program to address a more holistic watershed approach. This will enable individual projects to be assessed and evaluated based on their merit and function within the framework of the overall watershed needs. The following objectives have been identified for the subject planning initiative: ¾ to evaluate effective flood mitigation in Tompkins County; ¾ to re-emphasize watershed approaches through the development of a strategy to address watershed needs; ¾ to consider cumulative