Channel Evolution of a Restored Low Gradient, Sand Bed Stream

Abstract

Stream restoration design and construction relies on an accurate prediction of stable dimension, pattern, and profile within the bounds of expected dynamic equilibrium. This study charts the morphologic evolution of a small, restored headwater stream in response to hydrologic, hydraulic, and geologic conditions in a low gradient, coastal plain system. The goal of the project was to define an expected dynamic equilibrium for constructed streams in the geomorphic regime and equate changes in stream form to fluvial and geotechnical parameters for predictive analysis.

The restored stream reach was a previously straightened stream near Cove City, NC in the Lower Coastal Plain. The channelized stream was degraded due to direct cattle access and lack of features for aquatic habitat. Restoration involved removing the cattle, constructing a new floodplain, and re-meandering the channel. Log sill and rootwad structures were used to protect the stream bed and banks. The floodplain and banks were planted with a mixture of hydrophytic, herbaceous, and woody vegetation. The constructed stream was a low gradient, sand bed channel with an equilibrium bed slope of 0.001 m⁄m, cross sectional area of 1.9 m2, top width of 4.8 m, maximum depth of 0.7 m, and W⁄D of 12. The stream and riparian zone were instrumented for monitoring hydrology and surveying permanent cross sections.

Rapid development of channel morphology occurred in the first year of monitoring. No significant changes in channel dimension or profile were recorded after the first year post construction. Significant deviations within the stream features were related to structures. Cross sections with root wads had a 12% steeper median side slope than non-structured stream sections. The channel downstream of log sills was 20% deeper than the median cross sectional depth.

The dimensional response to fluvial hydraulics and geotechnical parameters was a complex interaction between stream power, shear stress, inundation, water table gradient, and discharge. The survey information was treated as a longitudinal data set, and multivariate statistical analysis was used to relate changes in stream dimension to stream power, shear stress, inundation, shallow groundwater seepage, and discharge processes. Inundation was directly related to increases in stream bank side slopes. Shallow groundwater gradient measured within the stream banks was a significant explanatory variable of direct changes in channel width, while the hydraulic gradient measured in the floodplain and discharge were inversely proportional. Maximum depth was directly related to discharge, but shear stress and stream power were not significant.

In order to study in detail the response in bank side slope to shallow groundwater seepage, a soil lysimeter experiment was conducted using soil from the stream site. Banks were constructed in the lab and a high water table gradient was used to induce seepage at the bank face. Bank side slope was varied between experiments, and each test was run until failure. The observed failure mechanism was due to small, pop-out failures and liquefaction of the underlying sandy soil. Positive pore water pressure in the upper loam horizon reduced apparent cohesion and promoted bank collapse. Bank failures occurred along linear failure planes that were similar to the initial bank slope. An increase in bank slope was observed to increase slope stability.

Scour pools that developed downstream of the log sills at the restored stream were modeled using River2D. The survey data and information on roughness and hydrology was used to model the two-dimensional, depth-averaged velocity profile upstream of the logs and within the pools. The maximum depth of scour was modeled using empirical relationships based on the morphologic jump, headloss over the sill, meander radius of curvature, median bed particle size, and flow turbulence. The predicted and measured scour depths were significantly correlated in a mixed linear⁄interactive multivariate model. The model parameters included a morphologic jump term, turbulence, and median bed material size. The scour pool depth was dependent on downstream conditions and tailwater depth. An empirical relationship was derived relating headloss over the weir for prediction of downstream energy condition.

Stream restoration design and construction techniques are specific to geomorphologic regime. This study provides scientific knowledge related to stream evolution under fluvial and geotechnical processes in the Lower Coastal Plain of North Carolina. The results can be used to evaluate channel equilibrium conditions, improve construction practices, and predict the implications of designed channel form and structures.