Regulated Rivers - Hydro-Environmental Analysis: Freshwater Environments - page 56

inlet control, the low in the culvert is controlled by the water-surface elevation at the inlet (head-
water depth) and water can low through and out of the culvert faster than it can enter it. Under
inlet control, the head of the culvert acts like a weir or a sluice gate where the inlet controls the
low, not the length, roughness of the barrel, or outlet. Under inlet control, the low passes through
a critical depth at or just downstream of this location. Therefore, under inlet control the low in the
barrel of the culvert will always be in a state of shallow-velocity and high-velocity lows, known as
supercritical lows where the velocity is greater than the celerity of a gravity wave (gY) 0.5 , where
g is the gravitational acceleration and Y is the depth and where downstream disturbances cannot
travel upstream. A culvert lowing full is often considered the most “hydraulically eficient” since it
is conveying the maximum possible low.
Culverts are most commonly designed with the goal of maximizing hydraulic eficiency in order
to maximize conveyance while minimizing pipe sizes and costs. Commonly not included in the
development of the design is the impact of the culvert on the aquatic life and the geomorphic condi-
tions of stream channels.
Of course, culverts do have many beneits, such as reducing looding. However, culverts can
also have many detrimental effects. One prevailing cause of those detrimental effects is basing the
design of culverts on maximizing hydraulic eficiency, thereby minimizing pipe sizes and costs,
without considering the environmental impacts. One of those environmental impacts is the isola-
tion of communities by acting as barriers to ish movement. Five common conditions at culverts that
create migration barriers are (Washington 2003):

• Excess drop at the culvert outlet

• High velocity within the culvert barrel

• Inadequate depth within the culvert barrel

• Turbulence within the culvert

• Debris and sediment accumulation at the culvert

The irst four of these conditions could result from the design for inlet control. Other impacts of
culverts include direct habitat loss, water quality degradation, upstream and downstream channel
impacts, and impacts during channel maintenance and construction (Washington 2003).
So, how many culverts are there? This is presently not known and it has stimulated the question
in the U.S. Department of Agriculture (USDA Forest Service Roads Analysis, [1999, p. 67, AQ(10)]:
“How and where does the road system restrict the migration and movement of aquatic organisms;
what aquatic species are affected and to what extent?” One result is the development of a procedure
to identify barriers to ish passage, the “National inventory and assessment procedure for identify-
ing barriers to aquatic organism passage at road-stream crossings,” by Clarkin et al. (2005). One
example of the methods developed to screen culverts for ish passage is the system developed by
Taylor and Love (2002) for adult and juvenile anadromous salmonids. In this system, culverts are
categorized as “red,” “gray,” or “green” as deined by (Clarkin et al. 2005) the following:

• Green: Conditions are assumed adequate for the passage of all salmonids. Even the

weakest-swimming life stage (juveniles) can pass the crossing during the entire period of
migration.

• Gray: Conditions may not be adequate for all salmonid species or life stages presumed

present. Additional analyses are required to determine the extent of the barrier for each
species and life stage.

• Red: Conditions do not meet passage criteria over the entire range of migration lows for

even the strongest-swimming species and life stage (adults) presumed present. Assume
“passage condition inadequate.”

Examples of red, gray, and green culverts are illustrated in Figure 3.14.