Post on 26-Mar-2015
transcript
Channel Migration Hazard Maps for eastern Jefferson County Rivers
2004
Susan PerkinsPerkins Geosciences
---Watershed Professionals Network, LLC
Intent of Hazard Maps
FEMA Flood Insurance maps have limited application in planning areas safe for development
CMZ delineations help reduce risks to human communities by guiding development away from areas at risk of channel erosion
Channel migration is not limited to areas below 100-year flood elevation
Channel migration may not be limited to HMZ CMZ delineation allows planners and managers to
weigh the relative risks of human development with habitat preservation over time
Slide courtesy of Wash. Dept. of Ecology
Channel Migration Hazard Maps
CMZs were reclassified into:
Disconnected Migration Areas (DMA)
High hazard Moderate hazard Low hazard
Disconnected Migration Areas
substantial, continuous bank protection structure OR
demonstrated commitment and ability of government agencies to prevent future channel migration
consulted with Jefferson County Public Works Dept. and Natural Resources Division
DMAs may still be subject to flooding
Deltas
from Rosgen (1996)
Dosewallips River
Dosewallips River Brinnon levee – Reach B
Duckabush River
Duckabush River
failed riprap Reach B
Big Quilcene River
Big Quilcene right bank levee
Little Quilcene River
Hazard Delineation
Hazard Level
HIGH near river, or
low former channel
MODERATE
LOW farther from river,
surface
unchannelized
Approx. Time Frame
< 50 years
50-100 years
> 100 years
Hazard Delineation Geomorphic channel type Type of channel migration
lateralavulsions
Sediment regime transportdeposition
Constraints on channel migration
Geomorphic Channel Types
Delta/alluvial fan Wandering river Straight and entrenched
currently, but formerly meandering
Straight and narrow (always)
Delta/alluvial fan reaches Distributary channels Avulsions Depositional Major constraints on
channel migration Cross-floodplain roads Extensive levees or
revetments
Reaches Little Quilcene A Big Quilcene A Duckabush A Duckabush B Dosewallips A Dosewallips B
Delta/alluvial fan hazard assignment
High Hazard Zone
HCMZ (+AHZ +EHA where not blocked by constraints)
Moderate Hazard Zone
Remainder of AHZ and EHA
Wandering river reaches
Meandering with bend cutoffs and avulsions
Straight at times following avulsions
Depositional, or mixed transport and deposition
Low, active floodplain No or few constraints on
channel migration
Reaches Little Quilcene B Little Quilcene D Big Quilcene B Big Quilcene E Duckabush C Dosewallips D Dosewallips G
Wandering river avulsion example -- Duckabush
old channel
new channel
side channel and island -- Duckabush
Wandering River hazard assignment
High Hazard Zone
HCMZ +AHZ +1/2 of EHA width
Moderate Hazard Zone
Remainder of EHA
Straight and Entrenched reaches:CMZ now narrower than historic
Minor to no bend growth, or bends occur within a narrower zone than HMZ
Sediment transport, or sediment supply where bed has degraded
Was channelized in past Entrenched Discontinuous revetments
constrain channel migration
Reaches Big Quilcene C Big Quilcene D
Straight and Entrenched example – Big Quilcene D
Straight and Entrenched example – Big Quilcene C
Straight and Entrenched hazard assignment
High Hazard Zone
½ to 2 EHA widths from the year 2000 active channel edge
Moderate Hazard Zone
Remainder of EHA
C
D
B
E
Big Quilcene
Straight and Narrow reaches Minor to no bank erosion, no
bend growth Few bends, held in place by
erosion-resistant geology Sediment is transported
through reach No man-made constraints on
channel migration Bedrock or glacial bluffs
constrain channel
Reaches Little Quilcene C Big Quilcene F Dosewallips C Dosewallips E Dosewallips F
Straight and Narrow hazard assignment
High Hazard Zone
HCMZ
Moderate Hazard Zone
AHZ + EHA
Dosewallips
E
D
Limitations of CMZ Maps Maps show relative risk, not a precise
prediction of time or location Not all High and Moderate hazard
areas will be occupied in next 100 yrs Though unlikely, some Low hazard
areas could be occupied Maps do not depict flood risk Maps do not depict landslide risk If erosion-resistant bank materials are
present, risk would be lower Transitional zones at downstream
ends of straight reaches should be reassessed every 10 years.
Confidence Level
Generally high 63+ years of historical data Excellent LIDAR maps Field work Migration distance >> map error
Lower in upstream ends No LIDAR migration distance < map error
Lower Hoh River historic channels
Lower Hoh River CMZ hazard map
Hoh River 2004