Design of a Sediment Mitigation System for Conowingo Dam Presenters: Sheri Gravette Kevin Cazenas Said Masoud Rayhan Ain Sediment Plume Sponsor: Faculty Advisor: Lower George Donohue Susquehanna Riverkeeper Agenda • Context • Stakeholders • Problem/Need Statement • Mission Requirements • Design Alternatives • Technical Approach • Preliminary Results • Project Management 2 Chesapeake Bay and The Susquehanna River • Chesapeake Bay is the largest estuary in the United States • 3 largest tributaries of the Bay are the Susquehanna, Potomac and James rivers – Provide more than 80% of the Bay’s freshwater • Susquehanna River is the Bay’s largest tributary – Provides nearly 50% of freshwater to the Bay – Flows from NY to PA to MD Map of the Chesapeake Bay Watershed Source: The PA Dept. of Environmental Protection 3 Lower Susquehanna River and Water Quality • Flows through Pennsylvania and Maryland • Quality of water is vital to the bay’s health – Improvement in water quality thus far can be attributed to US Army Corp. of Engineers • Provides power for turbines in hydroelectric plants and clean water to people • Contains 4 Dams: York Haven, Safe Harbor, Holtwood, Conowingo – York Haven, Safe Harbor and Holtwood have reached steady state - dam has completely silted up and is no longer able to retain sediment; dams are at maximum capacity Map of Conowingo Reservoir Source: US Army Corps of Engineers, (2013) 4 Conowingo Dam • Constructed in 1928 • Southernmost Dam of the Lower Susquehanna sediment • Location of Conowingo Hydroelectric Station – Mainly provides power to Philadelphia, PA – A black start power source – Provides 1.6 billion kWh annually • Traps sediment and nutrients from reaching the Chesapeake Bay – Water quality is closely related to sediment Conowingo Dam deposition Source: J. Schroath – Traps ~1.5 million tons annually 5 Flow and Sediment in the Conowingo Reservoir • Rouse Number: 휔 푃 = 푠 푢∗ 휔푠=Sediment fall velocity 푢∗=shear velocity Holtwood Dam • Rouse number defines a concentration profile of sediment – Determines how sediment will be transported in flowing water – Rate of particle fall velocity versus strength of turbulence acting to suspend the sediment • Most of suspended sediment is located directly behind the dam (areas away from Conowingo Dam turbines) Rouse Number for Medium Silt Particle at 30,000 cfs 6 Source: S. Scott (2012) Probability of Flow Rate at Conowingo Dam (2010-2012) 0.002 0.0018 0.0016 0.0014 0.0012 0.001 Steady State Probability 0.0008 Transient 0.0006 0.0004 0.0002 0 Flow (cfs) 7 Data Source: USGS, 96 rates/day Lower Susquehanna River: Steady State vs. Transient State Current Steady State: river flow rate Transient state: river flow rate higher less than 30,000 cfs than 300,000 cfs – Sediment/nutrients enters Chesapeake – Major Scouring event: enhanced erosion Bay at low-moderate rate of sediment due to significantly increased flow rates and constant interaction of water with the Dam Chesapeake Bay: Before and After Tropical Storm Lee Source: MODIS Rapid Response Team at NASA GSFC 8 Impact of Major Scouring Events on the Chesapeake Bay Natural Yearly Ecosystem Cycle vs. Effects of Previous Storms Source: Dennison, W.C., T. Saxby, B.M. Walsh, Eds. (2012). 9 Impact of Major Scouring Events on the Chesapeake Bay Natural Yearly Ecosystem Cycle vs. Effects of Previous Storms Source: Dennison, W.C., T. Saxby, B.M. Walsh, Eds. (2012). 10 Chesapeake Bay Total Maximum Daily Load (TMDL) • Established by US Environmental Protection Agency in conjunction with Obama’s Clean Water Act • Actively planned since 2000 • Covers 64,000 square miles in NY, PA, DE, MD, WV, VA, and DC • Sets limits for farmers, plants, dams, and other organizations that dump sediment/nutrients into dam • Designed to fully restore Bay by 2025 – 2017: 60% of sediment/nutrients reduction must be met 11 Susquehanna Contribution to TMDL Watershed limits to be attained by 2025 are as follows: • 39,222 tons of nitrogen per year (46% of Chesapeake TMDL reduction) • 1,719 tons of phosphorus per year (30% of Chesapeake TMDL reduction) • 893,577tons of sediment per year (30% of Chesapeake TMDL reduction) 12 Project Scope Within Scope Out of Scope • Main concern is mitigation of • Prevention of increased sediment/nutrients currently sediment/nutrients arriving from deposited directly behind dam upriver (steady-state problem) • Storm surge/scouring events, • Entirety of the Chesapeake Bay which is a transient problem TMDL (steady-state problem) (river flow rate > 300,000 cfs) 13 Sediment Deposition at Conowingo Dam 100% 200 Sediment Deposition • If sediment deposition Expected 90% ) Threshold reaches maximum 80% capacity: 150 70% • Scouring events would (million tons (million 60% further devastate the Chesapeake Bay 50% 100 ecosystem 40% • All Susquehanna River Percent Capacity Percent 30% sediment would flow 50 20% through to the Chesapeake Sediment Deposition Deposition Sediment Bay 10% • Deposition potential – 0 0% 1929 1936 1943 1950 1957 1964 1971 1978 1985 1992 1999 2006 2013 2020 2027 expected sediment Year Sediment Deposition in Conowingo Reservoir; Construction to 2008 with Gap Prediction deposited over a given Source of Data: Hirsch, R.M., (2012) time 14 Conowingo Reservoir: Relationship Between Scoured Sediment Load and Flow Rate 14 Sediment Load • Scoured sediment y = 221373e4E-06x follows an exponential ) 12 Threshold Expon. (Sediment Load) curve with relation to 10 water flow (million tons (million 8 • Current threshold set at a 75% decrease 6 from the trend line 4 • Scour Potential – Scoured Sediment Load Load Sediment Scoured expected sediment 2 scoured with a given 0 flow rate 0 200,000 400,000 600,000 800,000 1,000,000 Flow Rate (cfs) Sediment Scoured from Conowingo Reservoir Based on Flow Rate Source: LSRWA (2013) 15 Agenda • Context • Stakeholders • Problem/Need Statement • Mission Requirements • Design Alternatives • Technical Approach • Preliminary Results • Project Management 16 Primary Stakeholders Objective(s) Issue Lower Susquehanna - Find alternative uses for the sediment - Cost to remove sediment is high from Riverkeeper and Stewards of stored behind Conowingo Dam Reservoir is high the Lower Susquehanna, Inc. - Highlight vulnerabilities in (SOLs) environmental law - Minimize effects of major scouring events to the Chesapeake Bay Chesapeake Waterkeepers - Protect and improve the health of the - Cost to remove sediment is high from Chesapeake Bay and waterways in the Reservoir is high region Maryland and Pennsylvania - Maintain healthy waters for fishing and - Cost to remove sediment is high from Residents (Lower recreation Reservoir is high Susquehanna Watershed) - Improve water quality of the watershed - Receive allocated power from Hydroelectric Dam Exelon Generation – owner of - Obtain relicensing of Conowingo Dam - Sediment build up has no impact on Conowingo Dam prior to its expiration in September energy production 2014 - Maintain profit Federal Energy Regulatory - Aid consumers in obtaining reliable, - Pressure to update dam regulations Commission (FERC) efficient and sustainable energy services - Define regulations for energy providers 17 Stakeholder Tensions and Interactions ------- Aids in sediment removal ------- Does not aid or potentially aids in sediment removal 18 Agenda • Context • Stakeholders • Problem/Need Statement • Mission Requirements • Design Alternatives • Technical Approach • Preliminary Results • Project Management 19 Problem Statement - Conowingo Reservoir has been retaining a majority of the sediment flowing down the Susquehanna River - Major scouring events in the Lower Susquehanna River perpetuate significant ecological damage to the Chesapeake Bay - This ecological damage is caused by increased deposition of sediment and nutrients in the Bay 20 Need Statement • Need to create a system to reduce the environmental impact of scouring events • Need is met by reducing the sediment and nutrients currently trapped behind Conowingo Dam • Reduction is to be done while maintaining energy production in order to help satisfy FERC standards, and eventual TMDL regulations. 21 Agenda • Context • Stakeholders • Problem/Need Statement • Mission Requirements • Design Alternatives • Technical Approach • Preliminary Results • Project Management 22 Mission Requirements MR.1 The system shall remove sediment from the reservoir at a load rate greater than or equal to 1.5 million tons annually. MR.2 The system shall reduce sediment scouring potential by 75%. MR.3 The system shall allow for 1.6 billion kWh power production annually at Conowingo Hydroelectric Station. MR.4 The system shall facilitate Susquehanna watershed limits of 39,222 tons of nitrogen, 1,719 tons of phosphorus, and 893,577 tons of sediment per year by 2025. MR.5 The system shall facilitate submerged aquatic vegetation (SAV) growth in the Chesapeake Bay. 23 Agenda • Context • Stakeholders • Problem/Need Statement • Mission Requirements • Design Alternatives • Technical Approach • Preliminary Results • Project Management 24 Sediment Mitigation Alternatives 1. No Mitigation Techniques – Sediment remains in reservoir 2. Hydraulic Dredging – Sediment removed from waters – Product made from sediment 3. Dredging & Artificial Island – Initially: Sediment is dredged to make an artificial island – Over time: Sediment is slowly forced through the dam into bay Conowingo Dam Source: D. DeKok (2008) 25 1. No Mitigation 2. Hydraulic 3. Dredging & Techniques Dredging Artificial Island WHAT HOW • Sediment will reach capacity • Normal Flow: < 30,000 cfs