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Hydrologically-Connected Roads:
An Indicator of the Influence of Roads
on Chronic Sedimentation, Surface Water
Hydrology, and Exposure to Toxic Chemicals
by Michael J. Furniss, Sam Flanagan, and Bryan McFadin
To assess the potential for roads to affect water quality
and aquatic habitats, a simple indicator—the amount of road hydrologically
connected to the stream network—is useful to indicate the potential
for several important adverse effects:
· Delivery of road-derived sediments to streams
· Hydrologic changes associated with subsurface flow interception,
concentration, and diversion; increased drainage density; and extension
of the stream network
· The potential for road-associated spills and chemicals
applied to roads to enter streams.
This indicator can help to distinguish between roads
that have the potential for these effects--those that are connected
to streams--and roads that do not have these effects or potential--unconnected
roads.
Background
A study conducted in the western Cascades
of Oregon (Wemple et al., 1996), detailed the mechanisms whereby
inboard ditches on roads extend the stream network, change runoff
timing, and increase peak flows. They found that 57% of roads segments
studied were linked to surface flow paths. Primary linkages were
ditches draining to road-stream crossings and ditches draining to
gullies below cross-drains. A study of cross-drains in Idaho (Haupt,
1959), used a regression equation to relate road and slope characteristics
to down slope sediment movement. Haupt found sediment obstructions
below cross-drains and cross-drain spacing intervals were the most
important factors in predicting sediment travel distances below
cross-drains. The study also noted that where road drainage points
were a sufficient distance from streams, the drainage discharge
re-infiltrated and did not contribute sediment to streams. Piehl
et al. (1988) found that 38% of 515 cross-drains sampled in the
coast range of Oregon had gullies below their outlet. Outlet erosion
increased with increased ditch length. Bilby et al (1989) reported
that 34% of ditch drainage points examined drained directly into
streams. They noted that the most effective method of preventing
road sediment from reaching streams is to drain ditches onto the
forest floor where it can infiltrate before reaching a stream.
What is the hydrologic connectivity of roads?
Roads frequently generate overland flow from relatively
impermeable running surfaces and cutslopes. In addition, the interception
of interflow at cutslopes can generate substantial amounts of runoff,
converting subsurface flows to surface flows. Where these surface
flows are continuous between roads and streams, such as where inboard
ditches convey road runoff to stream channels, the road generating
or receiving the runoff is considered “hydrologically connected”
to the stream network. In other words, a hydrologically-connected
road becomes part of the stream network. Wherever a hydrologic connection
exists, accelerated runoff, sediments and road-associated chemicals,
such as spills or oils, generated on the road surface and cutslope
have a direct route to the natural channel network and surface waters.
We propose that this indicator be referred to as: Hydrologically-Connected
Road. Equivalent terms include: “hydrologic integration of
roads and streams,” “stream-connected road,” or “stream network
extension by roads.”
A working definition of Hydrologically-Connected Road,
is:
“Any road segment that, during a ‘design’
runoff event, has a continuous surface flow path between any part
of the road prism and a natural stream channel.”
Hydrologic connectivity will tend to increase
with increasing intensity and duration of precipitation or snowmelt,
and with increasing antecedent soil moisture content. A suitable
“design” runoff event for many purposes might be the 1-year, 6-hour
storm, with antecedent moisture conditions corresponding to the
wettest month of the year, or similar expression of precipitation
depth, statistical frequency, duration, and antecedent soil moisture
status.
The indicator may be expressed as the length
or proportion of road connected, or length or proportion of road
network in an analysis area or reporting unit (such as a watershed)
that is connected. In addition, estimates can be made of the amount
or proportion of road than can be ‘disconnected’ through road improvements
such as increased cross drainage.
Water, sediment, and chemical runoff generated on the
road prism can enter the natural stream channel network in a variety
of ways (Figure 1):
· Inboard ditches delivering runoff to a stream at a road-stream
crossing
· Inboard ditches delivering runoff to a cross drain (culvert,
dip, waterbar, etc.) where sufficient discharge is available to
create a gully or sediment plume that extends to a stream channel
· Other cross-drainage features, such as waterbars or dips,
that discharge sufficient water to create a gully and/or sediment
plume that extends to a stream channel
Roads sufficiently close to streams so that the fillslope
encroaches on the stream, such as at road-stream crossings
· Landslide scars on the road-fill that expose bedrock
and create a surface flow path to an adjacent channel.
Figure 1. Sketch showing ways that roads can be connected to streams.
Inboard ditches, gullied cross-drain discharges, sediment plumes
below cross-drain discharges, and road-stream crossings create connected
surface flowpaths between roads and natural stream networks. Disconnecting
roads from streams involves limiting the concentration of surface
discharge and using permeable soils on the forest floor and road
fill slopes to infiltrate runoff and convert it to subsurface flow
before it can reach a stream.
A specific road segment is either hydrologically
connected or not. Partial connection can be conceived of, but is
not needed for most applications.
What can be done about it?
Road treatments to “disconnect” roads
from streams—to reduce the amount of Hydrologically-Connected-Road—are
usually simple, inexpensive, and effective in reducing road effects
and risks to water quality and aquatic habitats. Increasing cross
drain spacing or outsloping is the most common technique for reducing
or eliminating hydrologic connectivity. Not all road segments
can be disconnected, and specific solutions require site-specific
engineering. Accomplishments can be measured both as the length
and proportion of road disconnected, and the associated decrease
in drainage density. Erosional yields can be estimated for the
disconnected road segment and reductions in delivered sediment volumes
quantified.
Additional discussion
of this topic and methods of assessment can be found in Flanagan
et al. (1999), and in the recently released Forest Service Roads
Analysis Guide (USDA 1999). Readers can link to the Water-Road Interaction Technology Series Documents from the
STREAM Web page (www.stream.fs.fed.us) or go directly to: (www.fs.fed.us/news/roads/DOCSroad-analysis.shtml)
to download documents.
References
Bilby, R.E.; Sullivan, K.; Duncan, S.H. 1989.
The generation and fate of road-surface sediment in forested watersheds
in southwestern Washington. Forest Science 35: 453-468.
Flanagan, S.A.; Furniss, M.J.; Ledwith, T.;
Ory, J.; Thiesen, S.; Love, M.; Moore, K. 1998. Methods for inventory
and environmental risk assessment of road drainage crossings. Water/Road
Interaction Technology Series. No. 9877 1809-SDTDC. San Dimas CA:USDA,
Forest Service, Technology and Development Program. 56 p.
Haupt, H.F. 1959. Road and slope characteristics
affecting sediment movement from logging roads, J. Forestry, 57(5):
329-339.
Piehl, B.T.; Beschta, R.L.; Pyles, M.R. 1988.
Ditch-relief culverts and low-volume forest roads in the Oregon
Coast Range. Northwest Science 62(3): 91-98.
U.S. Department of Agriculture, Forest Service.
1999. Roads Analysis: informing decisions about managing the national
forest transportation system. Miscellaneous Report FS-643. Washington,
DC: U.S. Dept. of Agriculture Forest Service. 222 p.
Wemple, B.C; Jones, J.; Grant, G. 1996. Channel network
extension by logging roads in two basins, western Cascades, Oregon.
Water Resources Bulletin 32(6): 1195-1207.
Mike Furniss is a hydrologist on assignment with the Stream
Systems Technology Center, Rocky Mountain Research Station, Fort
Collins, CO. Mike is stationed on the Six Rivers National Forest,
Eureka, CA; (707) 441-3516; mfurniss@fs.fed.us.
Sam A. Flanagan was a geologist on the Six Rivers National
Forest, he is presently with the National Marine Fisheries Service,
Arcata, CA.
Bryan McFadin was a hydrologist on the Six Rivers National
Forest, he is presently with the California North Coast Regional
Water Quality Control Board, Santa Rosa, CA.
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