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Victory Farm Stormwater Management and Stream Stabilization January 21St, 2021 Public Meeting

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Victory Farm Stormwater Management and Stream Stabilization January 21St, 2021 Public Meeting Victory Farm Stormwater Management and Stream Stabilization January 21st, 2021 Public Meeting City of Gaithersburg Department of Public Works, Environmental Services Division Agenda o Introductions o Background o Watershed Basics 1 2 3 4 o MS4 Basics (Municipal Separate Storm Sewer System) o City Funding o Drainage Area Characteristics o Project Goals o Water Quality vs. Water Quantity o Stable watercourse o Environmental Stewardship o Safety o Community Involvement o Project Design o Three part design o Type of facility o Features and purpose o Priority Considerations o Associated Improvements o Next Steps o Exit Survey Introductions City of Gaithersburg Pearce Wroe, P.E. Stormwater Project Manager Mike Weyand Watershed Specialist Engineering Consultant, CPJ & Associates, Inc. Robyn Barnhart, P.E. Section Head, Public Sector Division Introductions: Who are you? Chesapeake Bay Watershed Maryland’s Watershed Basins Maryland’s 8 Digit Watershed Maryland’s 8 Digit Watershed & City of Gaithersburg Maryland’s 12 Digit Watersheds Next Slide Watershed Basics Victory Farm Pond Drainage Area ~450 Acres Watershed Basics o Water flows downhill o A Watershed is; o “a region or area bounded peripherally by a divide and draining ultimately to a particular watercourse or water body” – Merriam-Webster o An area where all of water drains to the same place o Scales of Watersheds o Chesapeake Bay o Local watersheds – Seneca Creek, Muddy Branch, Rock Creek o Sub Local watersheds – Middle Seneca Creek - Whetstone Run o Project Level Watersheds – We’ll call these “Drainage Areas” Watershed Basics o Runoff o When water falls from the sky and drains overland o Hard surfaces or impervious area creates more runoff o Rainfall intensity and volume affect runoff amounts o Runoff can wash off pollutants and carry them to our waterways MS4 Permit Basics o The City must meet regulatory requirements based on the Federal Clean Water Act o The City is issued an MS4 Permit from the State of Maryland o Mandates new treatment of 20% of the City’s previously untreated impervious area o New treatment includes new Stormwater Facilities, Stream Restoration practices, or other alternative measures like street sweeping practices o We refer to new treatment as “Credits” which are equal to 1 impervious acre of new treatment o Several other requirements of the Permit, but impervious area treatment tends to be the most expensive. MS4 Permit Basics Dedicated Capital Outreach Staff Improvement Funding & Projects Education Stormwater Management Fee Illicit City Discharge Dedicated Rainscapes Stormwater Detection Rewards Program Stormwater Management Fund “Pay as you go” Operations, Development Maintenance, Structure Review Inspection, Reporting Victory Farm Stormwater Project Introductory Poll Questions Project Goals o Improve Water Quality in the Chesapeake Bay Watershed o Improve Water Quality in the Middle Seneca Creek Watershed o Meet the MS4 Permit Requirements o Meet goals safely o Meet goals in a fiscally responsible way o Meet goals in a socially equitable way o Provide improvements to the community including natural resources, amenities, education, connectivity, aesthetics… Project Goals Also Quality Quantity Important! Project Goal Pollution Volume Removal Toxicity Rate Prevent Health Flooding Prevent Nutrients Erosion Victory Farm Drainage Area Basics o Middle Seneca Creek, Whetstone Run Tributary o ~ 450 Acres Drainage Area o ~ 140 Acres Impervious Area in Gaithersburg o ~ 125 Acres of untreated impervious in Gaithersburg o ~ 3,800 Residential Addresses Victory Farm Drainage Area Basics Pond Retrofit Quick Facts o Wet Extended Detention Pond o ~ 1.5 acres of permanent pool area o 4,000,000 gallons of treatment o ~50% permanent pond, 50% temporary o 1” of rainfall in 24 hours o 90% average annual rainfall Alternatives Aka: Parts of the Solution Rain Garden Micro-Bioretention https://eoimages.gsfc.nasa.gov/images/imagerecords/88000/88742/ https://mygreenmontgomery.org micro_bioretentionfilter_hunt%20valley_web.jpg Alternatives Aka: Parts of the Solution Green Roof Pervious Pavement https://www.montgomerycountymd.gov/DGS- https://www.montgomerycountymd.gov/water/wqpc/rates.html OES/Resources/Images/GreenRoofs/GreenRoofEMTOC.jpg Alternatives Aka: Parts of the Solution Rain Barrels http://sustainmc.org/2017-rain-barrel-promotion/ Alternatives by # Facilities Pond Micro-Bioretention Rain Gardens Green Roofs 70,000 Rain Barrels Pervious Pavement Rain Barrels 6,000 Rain Gardens 150 Ac. Green Roof 45 Ac. Pervious Pav. 600 Micro- Bioretention 1 Pond & 2 Stream Restoration Alternatives by Construction Cost Pond $25M Micro-Bioretention Micro- Rain Gardens $130M $90M Bioretention Green Roofs Pervious Pavement Green Roof Rain Gardens $16M Rain Barrels Pervious Pavement $7M Rain Barrels $3.2M Pond and Stream Restoration Alternatives by Land Area Pond Micro-Bioretention 45 Ac. Rain Gardens Pervious Green Roofs 65 Ac. Pavement Pervious Pavement 150 Ac. Rain Barrels Green Roof Rain Gardens 5 Ac. Rain 40 Ac. Barrels Micro- Bioretention 2.5 Ac. Pond and Stream Restoration Existing Site Conditions and Proposed Project Design Site Vicinity Map Lake Whetstone Victory Farm Pond Access Route Aerial Image of Site Stream Tributary 2 Pond Stream Tributary 1A Aerial Image – Looking Northeast Pond Existing Conditions o Existing dry pond with 4 man-made wetland areas o Wetlands provide small amount of water quality benefits o Dry pond provides some water quantity control but not to current standards Outflow Wetland Wetland Wetland Wetland Pond Existing Conditions Weir and Control Structure - Upstream Weir and Control Structure - Downstream Pond Interior – Facing Upstream Top of Dam Existing Pond Hazard Classification o Ponds are classified as either low, significant or high hazard o Classification based on dam breach study results that determine if there is a change in hazard should the pond embankment should fail for various storms events o Existing pond is currently classified as significant hazard o Due to change in hazard at Goshen Road o Retrofit must meet current State dam safety requirements o Emergency Action Plan (EAP) must be prepared for facility o EAP outlines actions to be taken by Emergency Management Officials should a situation arise at the dam Stream Existing Conditions o Stream degraded with accelerated erosion from increased runoff volumes o Stream is attempting to widen or incise to provide additional area to handle greater peak runoff due to upstream development. o Process will reach an equilibrium point over time, decades or longer, but result in a larger stream channel and much wider floodplain. o Process washes tons of sediment downstream and results in uncontrolled loss of trees which may cause additional damage when they fall o Many existing trees are undercut or subject to future damage and death due to active erosion Tributary 1A Existing Conditions Upstream End Middle Section Near Downstream End Discharge into Pond Wetland Area Tributary 2 Existing Conditions Upstream End Middle Section Middle Section Near Downstream End Proposed Project Design o Stabilize Tributaries 1A & 2 o Reduce in-stream erosion and minimize deposition of sediment in Victory Farm Pond and Lake Whetstone o Mimic natural stable channel conditions o Modify riser structure to create a permanent pool with extended detention to capture nutrients and provide water quality treatment o Extended detention dewaters over 24 hours after storm event o Provides improved water quantity control o Meet Dam Safety requirements for existing High Hazard dam o Provide overtopping protection o Minimize impacts to existing sewer line o Enhance site aesthetics with extensive plantings Proposed Pond Retrofit Access from Saybrooke Oaks Blvd Proposed Proposed Safety Bench/ Grading Access Proposed Forebay Modify Proposed Riser Extended Detention Raise Proposed Sewer Overtopping Proposed Proposed Manhole Protection Permanent Aquatic Pool Bench Proposed Riser Modification Proposed 11” Diam. Extended Opening Detention Permanent Pool Block Existing Pipe Overtopping Protection o Dam Safety requirements for High and Significant Hazard dams require the facility to safely convey the Probable Maximum Flood (PMF) storm event (27.5” of rain in 6 hours) and the Half PMF storm with 1’ of freeboard o Storm flows that overtop an earth embankment can erode the downstream face of the dam leading to a complete failure of the dam o Existing dam is overtopping by ~3’ for PMF storm and ~1.5’ for Half PMF storm o To prevent overtopping, the existing concrete weir wall would have to be removed and rebuilt almost twice as long; Extremely Costly o A more cost-effective option is to provide overtopping protection which armors the top and downstream face of the dam with a material that is more resistant to the erosive forces Overtopping Protection o Overtopping Protection options include concrete, rock riprap, articulated concrete block and HydroTurf o HydroTurf is less costly than other options and blends better with natural surroundings o “Impermeable geomembrane protected by engineered turf which is infilled with HydroBinder, resulting in a fiber reinforced high strength concrete matrix.” Pond Safety o 10’ wide safety bench provided at the extended detention water surface elevation o 5’ wide aquatic bench provided 6”-12” below the permanent pool elevation prior to deeper water o Aquatic bench fully planted to provide a vegetative buffer around pond Extended Safety Detention Bench Permanent Pool Aquatic Bench Environmental
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