41 D4. Geomorphology (Posters)

Monday, 18 March 2019
1:30 PM - 5:30 PM
Casco Bay Hall

MULTIPLE FLOOD INDICATORS IN A BEDROCK-CONTROLLED REACH OF NESHAMINY CREEK, PENNSYLVANIA

Paper number: 41-4
Program: Technical Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

Intense flood events cause substantial reorganization in small bedrock-bound and engineered streams, often leaving a variety of indicators of flow stage height, direction, and magnitude. At Tyler State Park (Bucks County Pennsylvania) a 50-m-long reach along the east bank of Neshaminy Creek revealed several flow indicators downstream of a dam: distribution of sandstone and mudstone clasts, imbrication angle and azimuth, orientation of transverse gravel bars, obstacle shadow aprons, and direction of vegetation indicators (leaning trees and trunk-wrapped macrophytes). Following a rainstorm in October 2018, twenty-four sites were sampled along the gravel bar proximal to the stream, with maximum gravel sizes ranging from 10-50 cm (overall maximum: 152 m). Clast size increased upslope, largely due to rockfall talus from nearby cliffs (Stockton Fm, Triassic). Additionally, this trend is consistent with disc-shaped cobbles and boulders remaining higher on the profile due to large surface areas and limited ability to roll downslope. Multiple indicators demonstrate a range of flow impact directions from N44E to N40W, which is consistent with the southerly direction of the stream. Scour niches in mudstones beneath resistant paleo-channel conglomerate overhangs acted to diver the flood flow in an easterly direction from the thalweg. The previous flow height of at least 1.3 m was indicated by wrapped vegetation and is consistent with flow conditions recorded on preceding hydrographs. Our findings demonstrate the applicability of an integrated field database in providing a means of rapid assessment of the interaction between landforms, bedload, vegetation, and fluvial processes.

Geological Society of America Abstracts with Programs. Vol. 51, No. 1

doi: 10.1130/abs/2019NE-327852

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