Schoharie Creek at Prattsville, NY James G. MacBroom, P.E. January 30, 2012 On Behalf of NYDEP
Watershed Characteristics 237 sq. miles steep forest lands narrow valleys slightly sinuous gravel bed semi-confined with alternating floodplains slope near Prattsville is 0.002 ft/ft
River System Elements • Thalweg • Channel • Channel Banks • Riparian Wetlands • Floodplains • Terrace Scarps • Terraces
Watershed Management
FEMA Floodplain Map Rte 23 Bridge Downtown Prattsville Floodplain Schoharie Creek Batavia Kill Flow Direction
Schoharie Creek Hydrology
Huntersfield Creek Bridge Opening 6 Ft High by 52 Ft Wide Waterway Area is 312 Sq Ft Single Span, no piers Base Flood is 2100 CFS Gross Velocity is 7 FPS Measured Channel Width is 36 Ft Regional Bankfull Width is 42 Ft Conclusion; Bridge Size is Reasonable
Flowage Barriers and Vectors
Existing Flood Profiles Schoharie Creek Prattsville Plan: Ex Short MMI Bankfull 12/2/2011 Geom: Ex Short FEMA MMI Channel Geometry Flow: FEMA Bankfull (short) Discharges Milone & MacBroom Inc for NYDEP R1 R1 Legend Main Street Profile WS 500 yr 1170 Similar to 50 yr flood WS 100 yr WS 50 yr WS 10 yr Rte 23 Bridge impact Ground 1160 1150 Elevation (ft) 1140 Fish Weir Potential River Bed Elevation 1130 15331 Fish Block Weir 13050 Prattsville Cen 15090 FEMA E 9504 FEMA C 1120 10116 10328 10643 11000 11309 11611 11915 12218 12519 12747 13350 13655 13963 14267 14573 14877 15495 15801 16101 16344 16650 16955 17261 17563 17866 3315 3620 3925 4226 4532 4841 5148 5362 5670 5980 6283 6587 6846 7152 7460 7769 8076 8382 8683 8984 9284 9804 4000 6000 8000 10000 12000 14000 16000 18000 Main Channel Distance (ft)
Existing Section D Center of Prattsville- Existing Conditions Schoharie Creek Prattsville Plan: Ex Short MMI Bankfull 12/2/2011 Geom: Ex Short FEMA MMI Channel Geometry Flow: FEMA Bankfull (short) Discharges River = R1 Reach = R1 RS = 12594 FEMA D 12594.15 2.40141 Milone & MacBroom Inc for NYDEP .2 .102 . .036 . .1 .2 1210 0 0 Legend 5 6 5 WS 500 yr 1200 WS 100 yr WS 50 yr WS 10 yr 1190 Ground Ineff 1180 FEMA model assumed Bank Sta Main Street Elevation (ft) Floodplain “turned off” 1170 500 yr flood 100 yr flood 50 yr flood 1160 Terrace at 50 year flood level 1150 10 yr flood stays in banks Lower floodplain 1140 Bankfull flow elevation, below top of berm Correct ~230ft bankfull width 1130 0 200 400 600 800 1000 1200 1400 1600 Station (ft)
Total Flow Cross Section Area Schoharie Creek Prattsville Plan: Ex Short MMI 11/30/2011 Geom: Ex Short FEMA MMI Channel Geometry Flow: FEMA (Short Geo) Discharges Milone & MacBroom Inc for NYDEP R1 R1 Legend Area 100 yr 16000 FLOW 14000 12000 Area (sq ft) 10000 8000 6000 PRATTVILLE 4000 5000 10000 15000 20000 25000 30000 Main Channel Distance (ft)
Flood Hazard Reduction Hydrologic Techniques: – Reduce or contain peak flow rates at watershed scale – Floodwater storage dams, wetland preservation, restoration of floodplain connectivity – Example: NRCS floodwater storage dams in the Batavia Kill
Flood Hazard Reduction Hydraulic Techniques: – Decrease flood water elevations – Remove or reduce flow contraction points at bridges or narrow channel sections – Increase flow capacity of channels and floodplains – Use broad low velocity floodways – Divert floodwaters around sensitive areas
Flood Hazard Reduction Barrier Techniques: – Physically separate floodwaters from developed areas – Install levees, floodwalls, or fill material – Could require interior drainage pump stations, removable panels at road crossings, and considerable maintenance
Cross-Section w/ Deleted Encroachments Schoharie Creek Prattsville Plan: D. MMI smooth no encroachments 12/2/2011 Geom: Alt Short smooth del encroahments Flow: FEMA (Short Geo) Discharges River = R1 Reach = R1 RS = 12594 FEMA D 12594.15 2.40141 Milone & MacBroom Inc for NYDEP .2 .102 . .036 . .06 .2 1210 0 0 Legend 5 6 5 WS 500 yr WS 100 yr 1200 WS 50 yr WS 10 yr 1190 Ground Bank Sta 1180 Elevation (ft) 1170 500 yr flood 100 yr flood 1160 50 yr flood 100 yr flood inundates most of valley bottom 1150 10 yr flood just below top of banks 1140 1130 0 200 400 600 800 1000 1200 1400 1600 Station (ft)
Alternate Comparison Center of Prattsville Schoharie Creek Prattsville Plan: 1) Plan 01 11/30/2011 2) A. no bridge 12/2/2011 3) B. No Encroach 12/2/2011 4) C. no brid, encr 12/2/2011 5) D. Smooth, no En 12/2/2011 Geom: Ex Short FEMA MMI Channel Geometry Flow: FEMA (Short Geo) Discharges River = R1 Reach = R1 RS = 12594 FEMA D 12594.15 2.40141 Milone & MacBroom Inc for NYDEP .2 . . .036 . .1 .2 1210 1 0 0 Legend 0 5 6 2 5 WS 100 yr - Plan 01 1200 WS 100 yr - A. no bridge Impact of initial alternatives, 100 yr flood WS 100 yr - C. no brid, encr 1190 WS 100 yr - B. No Encroach WS 100 yr - D. Smooth, no En Ground 1180 Elevation (ft) Bank Sta Main Street 1170 1160 1150 1140 1130 0 200 400 600 800 1000 1200 1400 1600 Station (ft)
Alternate Comparison 500 feet downstream of Center of Prattsville Schoharie Creek Prattsville Plan: 1) Plan 01 11/30/2011 2) A. no bridge 12/2/2011 3) B. No Encroach 12/2/2011 4) C. no brid, encr 12/2/2011 5) D. Smooth, no En 12/2/2011 Geom: Ex Short FEMA MMI Channel Geometry Flow: FEMA (Short Geo) Discharges River = R1 Reach = R1 RS = 13050 Prattsville Cen 13049.80 2.48771 Milone & MacBroom Inc for NYDEP .149 .095 . .036 . .095 .156 1200 0 0 Legend 6 5 5 WS 100 yr - Plan 01 WS 100 yr - A. no bridge 1190 Impact of initial alternatives, 100 yr flood WS 100 yr - C. no brid, encr WS 100 yr - B. No Encroach 1180 WS 100 yr - D. Smooth, no En Ground Elevation (ft) Bank Sta 1170 1160 1150 1140 1130 0 200 400 600 800 1000 1200 1400 1600 1800 Station (ft)
Profile Comparison Schoharie Creek Prattsville Plan: 1) Plan 01 11/30/2011 2) A. no bridge 12/2/2011 3) B. No Encroach 12/2/2011 4) C. no brid, encr 12/2/2011 5) D. Smooth, no En 12/2/2011 Geom: Ex Short FEMA MMI Channel Geometry Flow: FEMA (Short Geo) Discharges Milone & MacBroom Inc for NYDEP R1 R1 Legend 1170 Impact of initial alternatives, 100 yr flood WS 100 yr - Plan 01 WS 100 yr - A. no bridge WS 100 yr - B. No Encroach WS 100 yr - C. no brid, encr 1160 WS 100 yr - D. Smooth, no En Ground 1150 Elevation (ft) Fish Weir 1140 1130 15331 Fish Block Weir 13050 Prattsville Cen 18923 Batavia Kill 1120 15090 FEMA E 6695 FEMA B 10116 10328 10643 11000 11309 11611 11915 12218 12519 12747 13350 13655 13963 14267 14573 14877 15650 15953 16253 16496 16802 17107 17412 17717 18019 18324 18618 4377 4686 4994 5208 5516 5825 6131 6436 7001 7306 7614 7923 8231 8532 8833 9134 9438 9654 1110 6000 8000 10000 12000 14000 16000 18000 Main Channel Distance (ft)
Effect of Slope Increasing Slope Reduces Water Elevation Almost 4 Feet
Effect of Smoothening and Cut
Effect of Dredging and Slope
Preliminary Conclusions • Increasing Peak Flow Rates • Channel Slightly Incised • Rare Events Overtop Developed Terrace • Significant Energy Loss At Bridge • Low Gradient Channel at Hunterfield Creek Bar • Dredging & Widening Floodway Help • Filling Terrace will Raise Flood Levels
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