Sediment Delivery to the Lower Neches River and the Effects of Impoundments (Town Bluff Dam-Sam Rayburn Dam) - PowerPoint PPT Presentation

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Sediment Delivery to the Lower Neches River and the Effects of Impoundments (Town Bluff Dam-Sam Rayburn Dam)

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deprive downstream river reaches of their sediment supply. ... side of newly constructed reservoirs aggrade in response to the new hydrologic gradient. ... – PowerPoint PPT presentation

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Title: Sediment Delivery to the Lower Neches River and the Effects of Impoundments (Town Bluff Dam-Sam Rayburn Dam)


1
Sediment Delivery to the Lower Neches River and
the Effects of Impoundments (Town Bluff Dam-Sam
Rayburn Dam)
2
It is well known that reservoirs serve as
sediment traps, which deprive downstream river
reaches of their sediment supply. Under most
circumstances, stream reaches and floodplains on
the upstream side of newly constructed reservoirs
aggrade in response to the new hydrologic
gradient.  Stream reaches below dam outfalls
are scoured and their channels deepened. Water is
sediment-free, except for scour.
3
Dams designed for flood control, such as Town
Bluff, tend to lessen peak flows.  River
geomorphology tends to be a function of peak
flows, so the Neches River might be expected to
adjust over time to dam-induced,
lower-than-historic peak flows. These lower
peak flows are possibly accompanied by lower
current velocities and lower sediment transport
potentials.
4
In some cases, a deficiency of floods leads to
aggradation downstream. This effect has been
documented. See(http//www.usace.army.mil/inet/u
sace-docs/eng-manuals/em1110-2-1418/c-2.pdf)and(
http//www.usbr.gov/lc/region/g2000/assess/appc.ht
m) Aggradation seems to follow a two-step
process, 1) initial degradation of the stream
bed, followed by 2) activation of local sediment
sources as tributaries respond to the increased
localized hydrologic gradient.
5
One requirement seems to be a ready source of
easily erodable material from either the
floodplain or surrounding uplands.  A second
requirement is that this local source be
available in equal or greater quantities than
the material captured by thereservoir.
6
These conditions have not been adequately
documented for the Neches River, but evidence
from the adjoining Trinity River Watershed
suggests that conditions conducive to river
aggradation might occur within the Gulf coastal
plains. See http//64.233.187.104/search?qcac
heHYw0RgV337wJwww.twdb.state.tx.us/RWPG/rpgm_rpt
s/2002483442.pdfnechessandbardevelopmenthlen
7
Preliminary results suggest that since the
construction of the Lake Livingston Dam there
has been no detectable reduction in sediment
delivery to the lowermost reaches of the Trinity
River, and to Trinity Bay. Flow regimes
downstream of Livingston Dam have not been
greatly modified, and there is no flow-related
decline in sediment transport capacity. There is
also no shortage of available sediment in the
lower Trinity. Sand is abundant, and there is no
evidence of depletion of sandy bars since the dam
was constructed. Floodplain accretion is
occurring, also indicating that the river is not
sediment supply-limited.
8
There is evidence of channel incision associated
with reduced sediment loads immediately
downstream of the lake, and as far downstream as
Romayor, and evidence of reduced post-dam
sediment yields at Romayor. However, there is no
evidence of a post-dam reduction in sediment
yield further downstream at Liberty. Sediment
yield data also show that sediment storage in the
lower Trinity valley--which is independent of,
and occurred both before and after impoundment of
Lake Livingston--is a more important control over
sediment transport to the estuary than sediment
trapping in Lake Livingston.
9
(No Transcript)
10
This provides evidence that the area of the river
between Romayor and Liberty serves as a
depocenter for the Trinity
11
The source of sediment is apparently reworked
material from the adjoining floodplain. Geomorph
ic effects downstream from dams are a complex
interaction of river size, hydrologic regime,
environmental setting, the history and morphology
of the stream reach, as well as operation of the
impoundment.  Depositional loci are largely a
function of sea level in such regimes.
12
There are several means of documenting floodplain
aggradation 1) Physical structures within the
channel2) Historic photography3)
Dendrogeomorphology (examination of root crowns,
flaring, and growth rings)4) In-stream sediment
analysis
13
Ramifications of aggradation include 1)
Reduced volumetric capacity a the river
channel2) Flooding at lower flow volumes
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