head cuts and knickpoints identify, prevent & stop georganne bowman september 30, 2011
TRANSCRIPT
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Head cuts and Head cuts and KnickpointsKnickpoints
Identify, Prevent & StopIdentify, Prevent & Stop
Georganne BowmanGeorganne BowmanSeptember 30, 2011September 30, 2011
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outlineoutline
• Stream Response to flow changes
• Stream restoration in the US– Challenges and expenses
• Channel stabilization– Headcuts and knickpoints
• How to Identify, prevent and stop
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Stream ResponseStream Response
• Streams form over time in response to flow regime and geology (conductance and resistance)
• Urbanization changes the frequency, magnitude and duration of flood events
• These in turn increase the amount of work that the river/stream can do.
• This causes channel incision and destabilization
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Work and PowerWork and Power
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Competence vs CapacityCompetence vs Capacity
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Restoration In the USRestoration In the US
• Type of projects:– Stream Habitat improvement– Bank Stabilization– Water Quality– Dam removal
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RestorationRestoration
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Channel StabilizationChannel Stabilization
• Streams are dynamic systems reacting to:– stream flow, – slope or – velocity changes
• The new channel – Cut off from previous
flood plain
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What is a Headcut?What is a Headcut?
• Headcut or knickpoint is an abrupt change of gradient of a stream. It is often attributed to a fall in base level: this initiates a knick point which then travels upstream. It may alternatively be due to a change in rock type or load size, or to tributary entry.
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Stream erosionStream erosion
• The stream expends kinetic energy in "trying" to eliminate the nickpoint.
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Partch PropertyPartch Property
• 7 acres with 25 acres draining to it.
• Pasture land, grazing for horses/goats
• Dashed blue line, channelized
• Objectives:– Livestock safe access to pasture– Stop erosion– Ability to drive across stream– Budget $3000
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Issues with siteIssues with site
• 3 channels converging• Swampy area
– Horses and vehicles could not cross
– Headcut migrating approximately 2 feet per year.
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Step poolsStep pools
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Demonstration siteDemonstration site
• John Sullivan did the design & ordered the supplies
• Homeowner did the backhoe work and stone placement
• Grasspave donated and used for driving lane
•
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Lessons LearnedLessons Learned
• It’s not enough to slow the velocity at the outlet structure.
• We also have to consider the change in elevation from the bottom of the discharge to the stream bed
• Stream restoration is expensive and rarely works.