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| Titel |
Patterns and contributions of floodplain and legacy sediments remobilized from Piedmont streams of the mid-Atlantic U.S. |
| VerfasserIn |
Mitchell Donovan, Andrew Miller, Matthew Baker, Allen Gellis |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250101486
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| Publikation (Nr.) |
 EGU/EGU2015-641.pdf |
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| Zusammenfassung |
| The perceived role of streambank erosion as a contributor to watershed sediment yield is an
important driver of policy decisions for managing downstream impacts in the United States.
In the Piedmont physiographic province of the eastern U.S. and in other regions of the south
and midwest, the issue of “legacy” sediment stored in stream valleys has long been
recognized as a consequence of rapid deforestation and erosive agricultural practices
following European settlement. Remobilization of stored floodplain sediment by bank erosion
is frequently cited as a dominant component of watershed sediment budgets, with legacy
sediment comprising the largest portion of this source. However there are few published
studies documenting spatially extensive measurements of channel change throughout the
drainage network on time scales of more than a few years. In this study we document 1)
rates of sediment remobilization from Baltimore County floodplains by channel
migration and bank erosion, 2) proportions of streambank sediment derived from
legacy deposits, and 3) potential contribution of net streambank erosion and legacy
sediments to downstream sediment yield within the Mid-Atlantic Piedmont. We
measured gross erosion and channel deposition rates over 45 years within the fluvial
corridor along 40 valley segments from 18 watersheds with drainage areas between
0.18 and 155 km2 by comparing stream channel and floodplain morphology from
LiDAR-based digital elevation data collected in 2005 with channel positions recorded on
1:2400-scale topographic maps from 1959-1961. Results were extrapolated to estimate
contributions to watershed sediment yield from 1005 km2 of northern Baltimore
County.
Results indicate that legacy sediment is a dominant component (62%) of the sediment derived
from bank erosion and that its relative importance is greater in larger valleys with broader
valley floors and lower gradients. Although mass of sediment remobilized per unit channel
length is greater in these downstream valleys, a majority of remobilized sediment (62%) is
coming from first- and second-order tributaries because they represent the largest
fraction of cumulative channel length in the drainage network. Floodplain segments
are discontinuous along low-order tributaries but sediment contributions reported
here are adjusted to account for the percent valley length bordered by floodplain
sediments. Average annual lateral migration rates ranged from 0.04-0.19 m/y with
higher rates along larger streams; however, when scaled by channel width, we find
that on average streams are migrating 2.5% of channel width across all drainage
areas.
Direct measurements reported here account for in-channel deposition, but not floodplain
deposition. Other studies in the region have demonstrated that redeposition on floodplains is
an important component of the sediment budget and are necessary to avoid overestimating
streambank erosion contributions to watershed sediment yield. We therefore adjust our
measured sediment contributions by estimating the mass of sediment redeposited on
floodplains within our study area. With this adjustment, extrapolated net stream bank
sediment yields (72 Mg/km2/yr) are equivalent to 70% of the estimated average Piedmont
watershed yield (104 Mg/km2/yr) cited by previous authors. Furthermore, our results
demonstrate that measurements over adequate spatial and temporal scales- rather than
short-term, localized observations- are required to accurately capture and measure patterns of
streambank erosion across the drainage network. It is important to note that upland erosion
rates- not measured here- have been reported with equivalent and greater magnitude for
forested and cropland areas within the Maryland Piedmont and therefore should not be
assumed to contribute only 30% of the total. |
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