Environmental Engineering Reference
In-Depth Information
(a)
(b)
Figure 2.1.
Typical (a) pool and (b) riffle.
Source
: Organization for the Assabet river Stream Watch (2005a,b). Photo by
Suzanne Flint.
canals, have low currents, high temperatures and low
dissolved oxygen in the summer, and are typically turbid.
The
pool/riffle ratio
or
bend/run ratio
is calculated
by dividing the distance between riffles or bends by the
width of the stream, respectively. The pool/riffle ratio is
used to classify streams with higher slope (mountain,
piedmont, and valley), and the bend/run ratio is used to
classify slow-moving lowland streams. An optimum
value of these ratios is in the range of 5-7 (Novotny,
2003), with ratios greater than 20 corresponding to
channels that are essentially straight and are poor
habitat for many aquatic species. Disruption of the run-
riffle-pool sequence has detrimental consequences on
macroinvertibrate and fish populations, while habitat
diversity is related directly to the degree of meandering
in natural and channelized streams (Karr and Schlosser,
1977; Zimmer and Bachman, 1976, 1978).
TABLE 2.1. Flow Velocity versus Type of Substrate
Velocity
(cm/s)
(ft/s)
Type of Substrate
<20
<0.7
Silt and bottom muck
20-40
0.7-1.3
Silt and sand
40-50
1.3-1.6
Sand
>50
>1.6
Gravel and rocks
Source
: uSEPA (1983b).
general, clean and shifting sand and silt is the poorest
habitat. Bedrock, gravel, and rubble on the one side and
clay and mud on the other side, especially when mixed
with sand, support increasing biomass. Substrate with
more than 50% cobble gravel is regarded as excellent
habitat conditions; substrate with less than 10% cobble
gravel is regarded as poor habitat. Watercourses with
swift velocities (>50 cm/s [1.6 ft/s]) that have cobble and
gravel beds have the greatest invertebrate diversity
(DeBarry, 2004).
Embeddedness
is a measure of how much of the
surface area of the larger substrate particles is sur-
rounded by finer sediment. This provides a measure of
the degree to which the primary substrate (e.g., cobble)
is buried in finer sediments. The embeddedness measure
allows evaluation of the substrate as a habitat for
benthic macroinvertibrates, spawning of fish, and egg
incubation. Gravel, cobble, and boulder particles with
0-25% fraction surrounded by fine sediments are excel-
lent habitat conditions; gravel, cobble, and boulder par-
ticles with greater than 75% fraction surrounded by fine
sediments are poor habitat conditions.
2.2.2 Substrate
Substrate
is the material that makes up the streambed.
Sand and gravel are common substrate materials. The
type of substrate is influenced significantly by the veloc-
ity of flow in the stream, and a typical relationship
between the type of substrate and the velocity in a
stream is given in Table 2.1. It is also useful to note that
sand settles in streams where velocities are less than
25-120 cm/s (0.8-3.9 ft/s), gravel settles in streams
where velocities are less than 120-170 cm/s (3.9-5.6 ft/s),
and erosion of sand and gravel riverbeds occurs at
velocities greater than 170 cm/s (5.6 ft/s) (DeBarry,
2004). Stream velocities below 10 cm/s (0.3 ft/s) are
typically categorized as slow, 25-50 cm/s (0.8-1.6 ft/s) as
moderate, and greater than 50 cm/s (1.6 ft/s) as swift. In
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