Rain on snow (ROS) events can dramatically impact the
pattern of water delivery to streams. When rain falls
on snow, water does not infiltrate the soils, as it normally
does. Instead, water runs over the surface of the ground
into the receiving stream network. This can result is
high water levels in streams (high peaks on the hydrograph).
The OWEB manual describes watersheds as having potential
impact from ROS events if two conditions are met in 20%
or more of the watershed area: (1) less than 30% crown
closure and (2) elevations suitable for ROS events (undefined
in OWEB manual)..
As more water enters the stream network, water velocities
increase, so does the capacity of the water to erode banks
and downcut streambeds. Lack of forest cover (zones designated
"open" in the
CLAMS95
data) increases the likelihood of such stream channel modifications
from ROS events.
ROS zones are areas where conditions exist that can
POTENTIALLY
lead to ROS events. This is to say, that ROS events have
a greater probability of occurring in ROS zones. It does
not mean that ROS events always occur in these zones,
especially in our study area. ROS events can have return
intervals of years to tens of years.
Results from this analysis can be used to locate 6th
Field Watersheds that may experience ROS disturbances
to stream networks.
We have also included the ROS polygons so that they can
be located below the 6th Field Watershed scale & the
elevation zones for future planning.
Considering elevation only, approximately
66.8% of the study area fell within the 0-1000 ft. elevation
zone, 27.3% within the 1001-2000 ft. zone, 5.4% within the
2001-3000 ft. zone, and only 0.6% of the study area was
greater than 3001 ft. ROS areas accounted for only 273
ha (ca. 0.01%) of the study area. None of the 6th field
watersheds are at risk for increased peak flow due to ROS
events because none had more than 20% of its area both open
and above 2000 ft.
Click To Enlarge
Avg: 0.00063 ha of ROS/ ha watershed
Max: 0.01660 ha of ROS/ ha watershed
Min: 0.00000 ha of ROS/ ha watershed
Please address questions or comments
to:
Dr. Ralph Garono
Principal Investigator
Earth Design Consultants, Inc.
rgarono@earthdesign.com
Laura Brophy
Principal Investigator
Green Point Consulting
brophyl@peak.org |
homepage 
