NRCS Mohawk Watershed Symposium

Natural Channel Restoration after Irene and Lee

a Schoharie County Opportunity

Will VanDeValk, Area Engineer USDA/NRCS

2013 Mohawk Watershed Symposium 1

NRCS Emergency Watershed Protection Program

Several towns approached NRCS in late 2011 seeking technical and financial assistance

State Office staff evaluated sites for EWP eligibility 10 sites (reaches) on 6 Streams were determined eligible Significant Threat to Public Health and Safety 75% NRCS Funding for construction (25% Local Match req’d) NRCS determined that a holistic approach to stream

restoration is critical to long-term success 2

Platter Kill/Flat Creek Road (5,500 LF reach)

Approx. 40-ft high raw bank near upstream end of 5,500 LF reach

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Loose, saturated, highly-erodible bank Bed & bank armoring washed away by flood

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Approx. 50-ft high near vertical bank Bed & lower bank armoring washed away by flood

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Home site destroyed by channel relocation

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15-ft wide x 7-ft high box culvert beneath roadway

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Little Schoharie Reach A (9,600 LF reach)

Overwidened channel w/ loose berms

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Little Schoharie Reach B (8,200 LF reach)

40-ft high raw bank below laboratory facility

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Little Schoharie Reach B (8,200 LF reach)

Downcutting of exposed clay/silt material

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Little Schoharie Reach C (6,000 LF reach)

Original meandering channel, abandoned by flood

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Little Schoharie Reach C (6,000 LF reach) Approx. 20-ft high eroding bank near Brooky Hollow Evidence of considerable downcutting

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Little Schoharie Reach D (6,700 LF reach)

Sharp bend @ Huntersland Rd., 10’ – 15’ downcutting

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Conditions on 1/24/12 (~ 5 mos. after Irene)

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Conditions on 3/06/13 (~ 18 mos. after Irene)


Sloughing of bank near residence as stream downcutsg

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Line Creek/Mill Valley Rd. (10,400 LF reach)

Home site destroyed by channel relocation

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Line Creek/Mill Valley Rd. (10,400 LF reach)

Stream adjacent to road & residences

Some hard-armoring or structural solutions may be required

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Damage Survey Report (DSR) Process Field Procedures (began in Jan. 2012)

GPS-locate features & estimate lengths & heights Office Procedures (Conceptual – Level)

Enter field data into GIS system Preliminary Hydrologic & Hydraulic analyses Conceptual solution (reconnect stream w/ floodplain,

remove berms, restore meander pattern, size channel for 1 to 2 year flows, provide grade-control, revegetate banks, hard-armoring only where necessary)

Quantity estimates Cost Estimating (completed 3/27/12)

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Loss of stream meander Raw Bank Height & Area

estimates ~ 885,000 sq. ft. of raw

bank along 5.6 miles (Reaches A – D)

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Project Finances (NRCS estimates)

10 Project sites on 6 streams covering approximately 47,300 LF (9 miles) of stream

Construction Cost estimated at $21.2M

NRCS share (75%) = $15.9M NYS has agreed to fund Local share (25%) = $5.3M

NRCS will fund up to $1.59M for allowable non-

construction costs (administration, engineering, etc.)

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Project Plan – Schoharie County EWP

County under contract with AECOM to “Provide Assessment, Design and Construction Administration for Support of Natural Stream Restoration Projects”

Assessment & Design in 1st half of 2013 Construction to begin this summer, with completion by

mid-January, 2014 (EWP condition)

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Fluvial Geomorphology

The study of how landforms change with time under the influence of rivers and streams

Frequently takes a “watershed scale approach” Attempts to work with a river’s natural processes

vs. Conventional Channelization

Tends to make the river conform to a preconceived regular shape

Often used in short reaches, to meet very specific criteria (e.g. 100-year flood)

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Conventional Channelization

Wide, trapezoidal channel

Often straight

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• Contain Floods



Often straight

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• Contain Floods • Levees sometimes

used



Often straight

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• Contain Floods

• Levees sometimes used

• Low flows wide & shallow

Natural Channel Design

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• Irregular shape w/ low-flow channel

• Meandering

• Channel contains ~ 1.5 year flood (Bankfull Flow)


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• Meandering


• Higher flows use floodplain


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• Meandering


• Higher flows use floodplain • Low flows confined

Side-by-side Comparison

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• Natural Channel Design •Conventional Channelization

Low Flows Low Flows


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Bankfull (~ 1.5 yr Flood)

Design Flood (>>1.5 yr flood)


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Large Floods

Large Floods (if berms are used)

Energy Balance of Streams (Lane, 1955)

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Regional Curves – Flow

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Energy Dissipation in Stable Streams

Bed Roughness Form Resistance - Step Pools (steeper streams) - Riffle/Pool sequences (moderate slope streams) - Meander Pattern (flatter streams) Floodplain acts like an “Energy Relief Valve”

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Rock cross-vanes for energy redirection, grade control & sediment transport

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Man-made approaches

Bed Material is Important!

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Summary – Natural Channel Design

puts the ~ 1.5 year flood in the channel and larger floods out onto a floodplain

Low-Flow conditions, Water Quality, Habitat, Ecosystem Function, and Resilience are all important

In areas w/out an adequate floodplain, other methods (or a combination of methods) may be necessary

Requires extensive data collection & analysis should provide Schoharie County with many stream

corridor benefits

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Slide 40

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Technology

NRCS Mohawk Watershed Symposium