Crooked Spring Reservation

Selected History - Crooked Spring Dam •Constructed 1930’s Napachi Gun Club •Reservation purchased 1965 – 37.44 acres •Site Assessment in 1971-1972 •Reconstructed in 1973 •Last engineering inspection February 2007

•Mass Hazard Code – “Significant -high hazard” •Fair condition •Failed riser replaced March 2010 •Declassified with MA DCR •Scheduled for re-inspection in 2012

Crooked Spring – Dam Design •120-lf earthen dam (NGVD 119.5’) • 30-lf long & 12-ft high vertical steel sheet spillway weir (NGVD 117’) •Stone riprap lined stilling basin below weir •Flood prevention riser 12” half-round CMP and 24” CMP conveyance to tailrace •Stop-log controlled

Crooked Spring Dam - Hydrology • Pond capacity 9.3

acre-feet • 0.88 m2 (560 acres)

watershed • Pond surface 1.5 acre

at OHW • Maximum pond 3.6

acre area • Winter flow 80 GPM

at 1 FPS • Dam can pass 96% of

inflow (305 cfs) from a 100-year storm event (320 cfs) estimated at 6.5 inches over 24 hours

• Crooked Spring Road culvert capacity 62 CFS

Crooked Spring Reservation • Draft Master Plan by Zuelke, Larson & Freitus Landscape Architects

Crooked Spring Dam - Problem

1974 USDA SCS Overview Plan

2010 Riser Replacement

• Failure of flood control conveyance pipe below the NE portion of the earthen dam • Pond draining via leak below dam (NGVD 110’) AND board leakage through riser (≤ NGVD 117’)

12’ half riser replaced in 2010

Crooked Spring Dam - Result CURRENT CONDITION

• Dewatered

• Run-of-river operation w/o storage (inflow=outflow) @ < NGVD 110’ (LLO pipe invert elev.)

• Functions as a stream during low flows

• Debris plugs will function as low-head impoundment immediately after moderate or heavy rainfall events (short –term only)

• Erratic water levels

Crooked Spring Dam - Problem CURRENT CONDITION

• Untreated stormwater runoff from Graniteville Road Outlet (not the only source) depositing significant silt – sand behind the dam

Crooked Spring Dam - Result • Pond water volume storage

capacity mostly eliminated

• Poor water quality from warm water temperature & low oxygen levels in summer

• Does not freeze solid in winter

Crooked Spring Dam - Problem CONDITION PRIOR TO LOWER LEVEL OUTLET FAILURE

• Noxious or invasive pond weed dominating open water area when ponded

Crooked Spring Dam - Result • Annual dieback and decomposition of noxious weed contributing to additional

silt deposits & shallow environment with poor water quality

• Eliminating plant diversity (type of plants) – degrading the habitat

Crooked Spring Dam - Alternatives SHORT-TERM • Replace failed CMP – rebuild earthen dam ($$S)

Benefit – maintains pond condition in short-term for aesthetics Detriment – Costly for a band-aid fix – restores noxious vegetation

• Decommission duplicative flood-prevention low level outlet & maintain stream run-of-river at current spillway elevation (0 $) Benefit – Allows complete structural assessment of dam to fill in

gaps of the 2007 survey Benefit – $ that would be spent on short-term repair of unnecessary

feature could be used to assess condition and/or used toward a permanent solution

Benefit – Temporary control of noxious vegetation (not permanent solution)

Detriment – Alters view-shed Detriment - May allow other invasive plants to colonize dewatered

area

Crooked Spring Dam - Considerations

INTERIM STEPS

• Engineer re-evaluate condition of dam & identify cost to maintain and/or to remove

• Eliminate unnecessary discharges contributing to silt and sand deposits in pond

• Identify sustainable management regime for ecosystem and passive recreational use – Reservation Baseline Assessment and revised Reservation Master Plan

• Investigate options and funding sources (grants) to remove accumulated silt and restore depth of lower pond to open water

•

Crooked Spring Dam - Alternatives LONG-TERM • Dredge Lower Pond then Remove Dam • Dredge Lower Pond then Repair Dam @ existing elevation

(NGVD 117’) • Dredge Lower Pond, Notch Dam @ lower elevation and

restablish low-flow stream channel • Repair Dam without removal of sediment in impoundment

– Ecological succession to eliminate pond

Dredging requires assessment of source of funding, quality of

sediment and probable disposal options and jurisdiction for permits from both state and federal regulatory agencies