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