Exeter Great Dam RemovalManaging Aging Infrastructure to

Improve Resiliency and Aquatic Habitat

Presented by

Jake San Antonio, PE

NETWC Lightning Session | September 12, 2018

Great Dam Impoundment

NH 108/Court

Street Bridge

Railroad Bridge

Gilman Park

Great Dam Structural Features

Exeter Library

Great Dam Site

Level 1 bullet

– Level 2 bullet 3. Great Dam

4. Fish Ladder

Kimball

Island

4. Fish Weir

1. Headworks

Exeter Library

2. Low Level Outlet

Mother’s Day Flood, May 2006

Decision Making – A Long Story

1981 • Town Takes Ownership of the Dam

2000 • NHDES Dam Bureau Issues Letter of Deficiency

2007 • Phase 1 (Dam Modification) Final Report for the Town of Exeter (Wright-Pierce)

2008 • Riverbank Scour/Design Impacts to Water Quality (Wright-Pierce)

2009 • Geomorphic Assessment (Bear Creek Environmental/Fitzgerald)

2010 • Water Supply Alternatives Study – Final Report (Weston & Sampson)

2013 • Great Dam Removal Feasibility and Impact Study (VHB)

2014 • Town Warrant Article No. 8 Passes – Authorizes Funding for Removal

2015 • Design and Permitting

2016 • Dam Removal and River Restoration

Feasibility Study - Benefits of Removal

Flood Resiliency – lower flood elevations upstream

Avoid flood damage costs ($1.7M-$3.4M over 10 years)

Public Safety – eliminate unsafe dam structure

– Class A Dam (Low hazard)

– Great Dam does not pass the 50-year flow with 1 ft freeboard

– Great Dam does not meet stability criteria

Avoid dam maintenance costs ($180-620k over 30 years)

Fish Passage – barrier removed, anadromous fish restoration

Water quality (thermal stratification, dissolved oxygen)

Restore 21 miles of river to free-flowing condition

Design Considerations

Stabilize channel and banks for the 100-year design event

Accommodate fish passage

Maintain water intake functionality

Accommodate sediment transport

Prevent channel head cut (High Street bridge scour)

Mitigate historic impacts

2017

GOLD

AWARD

Design Consideration – Flood Resiliency

Design Consideration – Flood Resiliency

Design Consideration - Fish Passage

Target Species:

– Alewife (Alosa pseudoharengus), anadromous

– Blueback herring (A. aestivalis), anadromous

– American eel (Anguilla rostrata), catadromous

Channel Geometry – slope, drop heights, boulder placement,

turbulence

Iterative design, ongoing research (NOAA/NHF&G)

Fish Weir

Existing GroundGreat Dam

Existing Ground

Pool Bottom

(El. 11.25)

Design Consideration - Fish Passage

Construction (2016)

Post-Construction Monitoring

Funding/Permit requirement

Annual Report (for 5 years)

Photo Documentation

Cross-Section Surveys - Channel Depth & Slope

Vegetation recovery – 75% in 3 years (met!)

Fish Counts - Alewife Presence (NH Fish & Game)

Spring Migration Survey – Channel micro-revisions

Jake San Antonio | jsanantonio@vhb.com | 617.607.2710

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Offices located throughout the east coast