>Research Questions
- How have rates of channel migration changed over the available historic aerial photographic record?
- Can BeachTools be effectively used to estimate rates of channel planform change?
- How have human modifications altered the planform characteristics of the Pomperaug River?
Study Area
The Pomperaug watershed covers a 90 square mile area (56,958 acres) in western Connecticut and is underlain by a
regionally significant stratified drift aquifer. The watershed has undergone a series of human modifications
including extensive de-forestation during colonial settlement, and a more recent shift from agriculture to
urban/suburban development and reforestation in the last ~50 years. Several channel reaches have been stabilized
with rip-rap as near-channel development has increased. However several "natural" reaches show signs of active
channel migration.
Preparation of Photographs
Aerial photographs for a total of five years (1944, 1970, 1985, 1990, and 1995) were collected, scanned, and
georectified to 1994 orthorectified aerial photographs. The root mean squared (RMS) error for each rectified photograph
was used to create a buffer around the digitized centerline of each river reach in each photograph.
To reduce the error caused by distortion near the boundaries of the photographs, a circular region was buffered
and clipped around the midpoint of each photograph. As a result, each new individual clipped layer contained only
the section of digitized channel centerlines from the most accurate areas of the photograph.
Each individual layer was then snapped to join all individual clipped river segments to obtain a single
feature representing the channel location for each year.
Quantification of Channel Change
BeachTools, an extension in ArcView 3.1, was used to quantify shifts in channel position. After digitizing centerlines
for each year of photography, the centerlines were buffered and BeachTools was used to produce an estimate of
planform change over time.
Despite geo-rectification, in some cases edges of aerial photographs did not line up well. In these cases,
line segments were artificially linked. None of these segments were incorporated into the final traces because
the clipping procedure eliminated the edges of the photograph. In other cases, the clipped segments did not align well
and were manually snapped.
Conclusions and Future Research
Results show that, over the available historic aerial photographic record, rates of channel changes have increased
while sinuosity has decreased. This is likely the result of human modifications of the channel and floodplain
environment which have resulted in simplification of channel planform through artificial reduction in sinuosity.
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| Photos above illustrate the changes in
land use with time along one section of the Pomperaug River |
Further work is needed to investigate the accuracy of the BeachTools method of planform change analysis. We plan
to compare the BeachTools results with a more labor and time intensive polygon-based assessment of planform change.
1Channel Planform
Channel Planforms, or patterns, are the forms that a river channel can take (e.g. Straight, Meandering or
Braided.) Planforms develop for a number of reasons, most of the planforms listed above are in continuum, that is
that they move between straight, meandering and braided channels. The planforms depend upon the width: depth ratio,
bank stability, sediment transport rate and bed load transport. Sediment Transport rate itself is dependant upon
stream power. Increased bedload is associated with braiding, and increased suspended sediment is associated
with meandering channel forms.
