sign in sign up
<<
Home , Stormwater

Victory Stormwater Management and 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 Funding o Area Characteristics o Project Goals o 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 ’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 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, practices, or other alternative measures like 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  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 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 , amenities, education, connectivity, aesthetics… Project Goals Also Quality Quantity Important!

Project Goal Volume Removal

Toxicity Rate

Prevent Health Flooding

Prevent Nutrients Victory Farm Drainage Area Basics o Middle Seneca Creek, Whetstone Run 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-

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 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 Gardens Rain

150 Ac.

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 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 areas o provide small amount of water quality benefits o Dry pond provides some water quantity control but not to standards

Outflow Wetland

Wetland Wetland Wetland Pond Existing Conditions

Weir and Control Structure - Upstream and Control Structure - Downstream

Pond Interior – Facing Upstream Top of 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 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 and much wider . o Process washes tons of 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 1A & 2 o Reduce in-stream erosion and minimize of sediment in Victory Farm Pond and 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/ Access

Proposed Forebay Modify Proposed Riser Extended Detention Raise Proposed Sewer Overtopping Proposed Proposed Protection Permanent Aquatic Pool Bench Proposed Riser Modification

Proposed 11” Diam. Extended Opening Detention

Permanent Pool

Block Existing Overtopping Protection o Dam Safety requirements for High and Significant Hazard require the facility to safely convey the Probable Maximum (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 Impacts from Pond Retrofit o Aquatic within and along the perimeter of the permanent pool will help absorb nutrients and provide a balanced aquatic ecosystem. o Will attract amphibian (frogs, salamanders, etc.) and (smaller) fish species o The resulting aquatic ecosystem will have a balance of prey and predator species o Trees within the permanent pool will be removed o Providing 50% of the water quality treatment volume through extended detention minimizes the loss of trees o Areas impacted by proposed grading will be replanted o Except no woody vegetation is allowed on the dam or within 15’ of the toe of dam Vegetated Pond Fringe Examples

Aquatic vegetation provides buffer between the water’s edge Native species foster aquatic ecosystem development and adjacent land within pond Example Native Vegetation for Pond Fringe Examples of Pond Retrofits

In Construction 1 Year After Construction

5 Years After Construction 5 Years After Construction Proposed Stream Stabilization o Utilize natural channel design approach to promote aquatic and mimic natural stable conditions including and pool sequences required for a healthy stream o Restore stable heights o Provide floodplain connectivity allowing stream forces to spread across floodplain reducing erosion and downstream sedimentation o Mimic natural stable channel conditions with stable bank heights and floodplain connectivity to promote aquatic life o Natural channel design requires additional area and tree removal than simply hard armoring of eroded banks o Hard armoring does not address erosive forces but deflects them downstream causing additional erosion problems in the future o Can be very expensive o MS4 permit credits for stopping erosion and transport of sediment down stream Proposed Stream Stabilization Tributary 1A

Riffle

Pool

Step Pools Proposed Stream Stabilization Tributary 2

J-Hook Riffle Step Pools

Pool Wetland Creation Typical Stream Structures o Bankfull Bench

o Provides area during large storm events, for stream forces to spread across floodplain to reduce erosion Typical Stream Structures o Riffle

o Controls slope and elevation

o Water moves faster, removing fine and oxygenating the stream Typical Stream Structures o Cross Vane

o Dissipates , deflects stream flow to center of channel and creates pools

o Provides grade control to prevent changes in bed elevation Typical Stream Structures o J-Hook

o Dissipates energy, deflects stream flow to center of channel, reduce bank erosion and creates pools

o Can also provide grade control to prevent changes in bed elevation Typical Stream Structures o Step Pool

o Dissipates energy, deflects stream flow to center of channel and creates pools

o Provides grade control to prevent changes in bed elevation

o Used to address greater elevation changes Environmental Impacts from Stream Stabilization o Regenerates and protects the health of watersheds and aquatic ecosystems o Stream grading and access designed to minimize tree loss by utilizing asphalt path with mulch overlay to minimize tree root impact o Trees located within grading limits for new stable natural channel will be removed o Many of these trees are undercut or otherwise subject to future damage and death due to active erosion o Temporary impacts during construction restored in-kind or better o Trees are planted to provide riparian buffer for the stream Common Riparian Trees and Plants Examples Trees

Common Hackberry

Live Stakes

Black Gum Kentucky Coffee Tree Black Willow

Shrubs Buttonbush Buttonbush

Silky Dogwood Examples of Stream Stabilization What to Expect During Construction o Duration o Approximately 8-10 months (weather dependent) o Construction Hours o Monday through Friday, 7AM – 4PM o Safety o Work limits will be fenced with high visibility orange construction safety fence o Traffic o Access off Saybrooke Oaks Blvd o Noise o Contractor is required to comply with City of Gaithersburg’s Noise Ordinance o Sediment o Contractor will be required to comply with City of Gaithersburg’s Permit and not track dirt onto Associated Improvements

o We want to hear from you. What fits best in your community o Connectivity? o Additional Environmental benefits? o Educational amenities? o Recreational amenities? o Public Art? o What matters to you? Next Steps

o Absorb the wonderful feedback we’re receiving o Respond to your feedback on the project page o Make adjustments as necessary and develop 60% plans o Determine if another public meeting is needed o Develop 90% plans and have another public o Alert the community about pending construction o Construct o Enjoy Victory Farm Stormwater Project Exit Poll o Please use the following link to answer a brief survey about how we did and what you think about this project. https://www.surveymonkey.com/r/X3KH3YW