Flows and Transport in Rivers - Hydro-Environmental Analysis: Freshwater Environments - page 83

4 Flows and Transport in Rivers

Measurement and Analysis

4.1 INTRODUCTION
Rivers are lotic systems (lotus, from lavo , to wash) and are generally dominated by lows, as opposed
to lakes and reservoirs, which are lentic systems ( lenis , to make calm). The lows in rivers have pro-
found impacts on their physical, chemical, and biological characteristics. Therefore, the measure-
ment and analysis of lows are a critical component of river management.
Flow simply refers to the lux of water passing though some deined area, such as a cross section
of river, as illustrated in Figure 4.1, during some inite interval of time, in units of cubic length per
time (e.g., cubic meter per second and cubic foot per second). The low (Q), or lux, is the product of
the water velocity (U, length/time) and the area (A, length 2 ); (Q = UA). To determine the low, the
velocity and area are typically measured and then used to compute it.
While theoretically simple, in practice the “devil is in the details.” For example, velocities are not
constant over the channel width or depth (cross section), so multiple area and velocity measurements
are required to compute lows. One question then is: how many measurements are required and at
what accuracy? Also, the variations in velocities over the cross section and length of the river have
large impacts on the transport of materials carried by the low.
Another part of the analysis of lows is determining which lows or which low components are
important. In many cases, continuous measurements are required and/or measurements correspond-
ing to a water quality sampling event. Flows are required for water quantity/quality management
in order to determine a low budget or low balance, to determine, for example, how much water
is entering and leaving a river reach (section of river). Flows are required in water quality studies
to determine both the lux of materials into a system and the transport of materials through it. The
load or lux of materials (mass/time) into a river reach is computed from Q*C, where Q is the low
(L 3 T -1 ) and C is the concentration of some material (mass L -3 ) (suspended solids, dissolved oxygen,
nutrients, etc.). So, if the lows are not known or not determined as part of a ield study, the loads
(luxes) cannot be determined. For example, if the determination of monthly or yearly average loads
into a river reach were of importance for addressing some management question, then both lows
and concentration data would be required at a suficient frequency to compute an average. Similar
to a water budget, it is often necessary to construct a budget of materials of water quality interest,
such as sediments, which requires both sediment and low data at all points of inlow or outlow.
Flows also impact the transport of materials and that transport is a fundamental process having
large impacts of water quality variations (Martin and McCutcheon 1999).
The lows that are typically used for hydraulic design or the determination of regulated lows
are some relatively rare lows as characterized by their low magnitude and return interval. For
example, in studies of loods, the 100-year low is often used, so low data are required to estimate
the low magnitude that would occur once every 100 years. For water quality studies, it is often the
low low that is important, such as the 7Q10 low. The 7Q10 low is the 7-day average low that is
expected to occur once every 10 years. For biota or for regulated lows, often some minimum low
is used. That minimum low could be protective of biota or a low that satisies some downstream
demand. More commonly today, for the protection of biota or to determine whether some hydro-
logic alteration has taken place, low components include not only the magnitude and return inter-
val, but also the duration, timing, and rate of change of lows (Poff et al. 1997; Richter et al. 1996,