Environment Prepared for: Prepared by: Town of Sanbornton AECOM Manchester, NH 60163921 September 2012 Black Brook Watershed Management Plan Environment Prepared for: Prepared by: Town of Sanbornton AECOM Manchester, NH 60163921 September 2012 Black Brook Watershed Management Plan _________________________________ Prepared By: Don Kretchmer and Al Pratt _________________________________ Reviewed By: Jake San Antonio AECOM Environment i Contents 1.0 Introduction and Water Quality Summary ................................................................... 1-1 2.0 Phosphorus Target ........................................................................................................ 2-1 2.1 Numeric Water Quality Target ............................................................................................. 2-1 3.0 LLRM Model of Current Conditions ............................................................................. 3-1 3.1 Hydrologic Inputs and Water Loading ................................................................................. 3-4 3.2 Nutrient Inputs ...................................................................................................................... 3-5 3.3 Phosphorus Loading Assessment Summary...................................................................... 3-8 3.4 Phosphorus Loading Assessment Limitations .................................................................... 3-9 3.5 Lake Response to Current Phosphorus Loads................................................................. 3-10 3.6 Critical Conditions .............................................................................................................. 3-12 3.7 Seasonal Variation ............................................................................................................. 3-13 3.8 Loading Model Development Summary ............................................................................ 3-13 4.0 Evaluation of Alternative Loading Scenarios ............................................................. 4-1 4.1 Natural Environmental Background Phosphorus Loading ................................................. 4-3 4.2 Build Out Analysis ................................................................................................................ 4-4 4.3 Reduction of Loads to Meet In-lake Short-Term TP Goal of 6.3 g/L Mean In-Lake Concentration ....................................................................................................................... 4-5 4.4 Distribution of Load in Black Brook Watershed .................................................................. 4-5 5.0 Options for Managing Phosphorus Loading to Lake Winnisquam from the Black Brook Watershed. ............................................................................................... 5-1 5.1 Land Development ............................................................................................................... 5-1 5.1.1 Existing Land Development Protection ................................................................ 5-1 5.1.2 Considerations for Management of Land Development ...................................... 5-3 5.2 Septic Systems .................................................................................................................... 5-4 5.3 Roads and Stormwater Management ................................................................................. 5-4 5.3.1 Road Maintenance ................................................................................................ 5-5 5.3.2 Culvert Cleaning/Maintenance ............................................................................. 5-6 5.3.3 Stormwater Management Practices ..................................................................... 5-6 5.4 Timber Harvesting................................................................................................................ 5-9 5.5 Agriculture / Field Management .......................................................................................... 5-9 5.6 Black Brook - Site-Specific, Non-Point Source Management Measures ........................... 5-9 AECOM Environment ii 5.6.1 Road Drainage BMP’s ........................................................................................ 5-11 5.6.2 Driveway Runoff .................................................................................................. 5-25 5.6.3 Field Management (Agriculture /Golf Course/Cemetery) .................................. 5-27 6.0 Implementation Plan ..................................................................................................... 6-1 6.1 Phosphorus Management Summary .................................................................................. 6-2 6.1.1 Road Maintenance and Storm Water Drainage Improvements .......................... 6-2 6.2 Implementation Schedule .................................................................................................... 6-7 7.0 Public Outreach and Education ................................................................................... 7-1 8.0 Monitoring Plan ............................................................................................................. 8-1 9.0 Potential Sources of Funding ....................................................................................... 9-1 10.0 References ................................................................................................................... 10-1 AECOM Environment iii List of Appendices Appendix A Black Brook/Lake Winnisquam Water Quality Target Memorandum August 25, 2011 Appendix B Model Parameter Tables Appendix C Example Ordinances Appendix D Selected BMP Designs Appendix E NHDES Watershed Management Fact Sheets Appendix F NHDES Septic System Fact Sheets List of Tables Table 1-1: Characteristics of Lake Winnisquam, NH ................................................................................ 1-1 Table 1-2: Lake Winnisquam - Pot Island Basin ....................................................................................... 1-4 Table 3-1: Pot Island Basin and Black Brook Water Budget .................................................................... 3-5 Table 3-2: Land Use Categories by Subwatershed (Existing Conditions) .............................................. 3-7 Table 3-3: Pot Island Basin of Lake Winnisquam Phosphorus Loading Summary ................................ 3-9 Table 3-4: Predicted In-Lake Total Phosphorous Concentration for Pot Island Basin under Current Conditions using Empirical Models .................................................................................. 3-11 Table 3-5: Predicted In-Lake Chlorophyll a and Secchi Disk Transparency Predictions based on an Annual Average In-Lake Phosphorous Concentration of 6.6 μg/L ............................ 3-12 Table 4-1: Comparison of Phosphorous Loading Scenarios for the Pot Island Basin, Lake Winnisquam ........................................................................................................................ 4-2 Table 4-2: Lake Water Quality Response to Different Loading Scenarios for the Pot Island Basin, Lake Winnisquam ............................................................................................................... 4-3 Table 4-3: Phosphorus Loading to Black Brook by Land Use (Current Conditions) ............................... 4-6 Table 4-4: Phosphorus Loading to Black Brook by Land Use (Buildout Conditions) .............................. 4-7 Table 4-5: Land Use Categories by Subwatershed (Buildout Conditions) ............................................... 4-1 Table 5-1: Best Management Practices Selection Matrix ......................................................................... 5-1 Table 6-1: Recommended Measures to Manage Phosphorous Loading to Lake Winnisquam – Load Reduction Estimates for Road Drainage Improvements ......................................... 6-3 Table 6-2: Recommended Measures to Manage Phosphorus Loading to Lake Winnisquam – Cost Estimates for Road Drainage Improvements .................................................................... 6-5 Table 6-3: Implementation Schedule ......................................................................................................... 6-7 AECOM Environment iv List of Figures Figure 1-1: Lake Winnisquam Location and Bathymetry ................................................................... 1-3 Figure 2-1: Lake Winnisquam Water Quality Target ................................................................................. 2-1 Figure 3-1: Conceptual model of Lake Winnisquam for LLRM model assessment of Black Brook ........ 3-2 Figure 3-2: Black Brook Watershed Land Use .......................................................................................... 3-3 Figure 5-1: Sites for Best Management Practice Implementation .......................................................... 5-10 Figure 5-2: Sites for Best Management Practice Implementation - Western Headwaters .................... 5-11 Figure 5-3: Sites for Best Management Practice Implementation .......................................................... 5-12 AECOM Environment v List of Photos Photo 1: Black Brook Road, Shoulder Grading, Looking West (BMP 3)................................................ 5-12 Photo 2: Black Brook Road, Proposed Sediment Trap (BMP 1) ............................................................ 5-12 Photo 3: Huse Road, Southern Portion