Presentation to Con-Com on Crooked Spring −9/2013


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Photos used in this presentation are by Chelmsford Open Space Stewardship.
The work parties were all COSS work parties and funded by COSS.

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Presentation to Con-Com on Crooked Spring −9/2013

  1. 1. Crooked Spring Reservation
  2. 2. 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
  3. 3. 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
  4. 4. 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
  5. 5. Crooked Spring Reservation • Draft Master Plan by Zuelke, Larson & Freitus Landscape Architects
  6. 6. 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
  7. 7. 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
  8. 8. Crooked Spring Dam - Problem CURRENT CONDITION • Untreated stormwater runoff from Graniteville Road Outlet (not the only source) depositing significant silt – sand behind the dam
  9. 9. 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
  10. 10. Crooked Spring Dam - Problem CONDITION PRIOR TO LOWER LEVEL OUTLET FAILURE • Noxious or invasive pond weed dominating open water area when ponded
  11. 11. 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
  12. 12. 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
  13. 13. 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 •
  14. 14. 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