Quantcast HYDROLOGY/MATERIAL TRANSPORT (Cont.)

 
  
 
from the surface in shallow streams and at 0.2 units from the surface and bottom in deeper streams.
Total discharge is then summarized over the entire cross section by adding partial discharges calculated
for each subsection.
Material transport refers to materials transported as either dissolved substances, suspended
solids, or as the bed load. Dissolved substances will be discussed later. Suspended solids transported
in a stream are a function of the weight of the suspended material (often represented by grain size due
to analytical techniques) and velocity (Table 1.3.2). Materials in movement along a stream bottom
constitute the stream's bed load. The amount of suspended material in a stream impacts physical
characteristics of the stream water such as heat adsorption and penetration/scattering of light.
Measurement of material transport is often conducted by direct measurement of total, suspended, and
dissolved solids and indirectly using measurements of turbidity (light scattering by particulates).
Material transport is related to the type of substrate which is a function of the area geology and
hydrologic conditions. Substrates vary from eroded bedrock to fine clays and channels can exhibit
combinations of materials sized between boulders and clays. The hydraulics, morphology, and
substrate availability usually result in a sorting of substrate by size throughout the channel. For instance,
heavier material will settle in areas of decreased flow such as deep areas, banks, and bends. The
permanency of this sorting is subject to the magnitude of flow (Table 1.3.2).
The hydrology and material transport associated with reservoir operations greatly influences the
water quality in reservoir tailwaters. For instance, a flood control structure will retain high flow while
maintaining a controlled discharge. This type of operation greatly influences retention time (Figure
1.3.7) and provides for particulate settling in the reservoir, decreasing transport to the tailwater, and
relatively "steady-state" conditions in the tailwater during the high flow release. This is most often true
when high flow events have appreciably impacted reservoir water quality resulting in near-
homogeneous conditions. After flood control releases have been curtailed, routine releases are often
reflective of the inflow hydrograph and a more "natural" hydrograph is established with biological and
chemical conditions reflective of reservoir release water quality and riverine processes. During low flow
periods, only a minimum flow, usually very similar to the inflow, is maintained in the release. Minimal
flow often occurs when project operations, such as recreation in the upstream reservoir, require a
stable or established surface elevation or during periods of low flow.
Hydropower production results in a more structured hydrograph (Figure 1.3.8a) than flood
control hydrographs (Figures 1.3.8b and 1.3.8c). A hydropower project also routinely adds a more
detailed temporal component based on demand for electricity (Figure 1.3.9) and often
impacts daily (diurnal) temperature and dissolved oxygen cycles (Jabour et al. 1996; Ashby et al.
1995). Daily heating and photosynthesis are often disrupted as a result of increased discharge during
"peak" demands for electricity and new conditions are established that reflect discharge water quality
(Figure 1.3.10). For example, pre-generation temperatures and dissolved oxygen
1.3-8

 


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