Influences of High-flow Events on a Stream Channel Altered by Construction of a Highway Bridge: A Case Study

Northeastern Naturalist, 2009 by Hedrick, Lara B, Welsh, Stuart A, Anderson, James T

Abstract

Impacts of highway construction on streams in the central Appalachians are a growing concern as new roads are created to promote tourism and economic development in the area. Alterations to the streambed of a first-order stream, Sauerkraut Run, Hardy County, WV, during construction of a highway overpass included placement and removal of a temporary culvert, straightening and regrading of a section of stream channel, and armourment of a bank with a reinforced gravel berm. We surveyed longitudinal profiles and cross sections in a reference reach and the altered reach of Sauerkraut Run from 2003 through 2007 to measure physical changes in the streambed. During the four-year period, three high-flow events changed the streambed downstream of construction including channel widening and aggradation and then degradation of the streambed. Upstream of construction, at a reinforced gravel berm, bank erosion was documented. The reference section remained relatively unchanged. Knowledge gained by documenting channel changes in response to natural and anthropogenic variables can be useful for managers and engineers involved in highway construction projects.

Introduction

Natural stream channels are achieved by allowing streams to develop a stable dimension, pattern, and profile. In a stable system, the streambed neither aggrades nor degrades, and its sediment load is consistently transported (Allen 1995, Schumm 1977). Alteration of the natural stream channel may lead to channel instability, which occurs when a streambed is degraded by scouring processes, or excessive sediment deposition leads to aggradation (Rosgen 1996).

Wolman (1967) initially categorized stages of stream channel change in response to urbanization. The first stage is equilibrium and stream channel stability. As development and construction begin in the second stage, sediment delivery rates increase leading to channel aggradation. The third stage is an urban landscape with increased areas of impervious surfaces leading to decreased sediment inputs and channel degradation due to flash discharges with low sediment yield (Wolman 1967). Subsequent studies on effects of urbanization indicate that stream channels respond to early stages of construction with an increase in sediment influx resulting from erosion of exposed, unvegetated channel banks (Urbana and Rhoads 2003) and the land surface due to recontouring and leveling (Wohl 2000). Enlargement of the floodplain can occur as sediment material the stream cannot carry is deposited as floodplain alluvium (Graf 1975). Additional responses to increased urbanization include channel widening (Colosimo and Wilcock 2007, Grabel and Harden 2006, Hammer 1972), channel incision (Booth 1990, Doyle et al. 2000), erosion of unarmoured banks, and aggradation of the streambed (Colosimo and Wilcock 2007, Grabel and Harden 2006, Hess and Johnson 200 1 ).

Road construction along stream corridors, including road crossing and stream channel alteration, changes the structure, function, and stability of stream channels (Albanese and Matlack 1998, King and Ball 1965). Road crossings such as bridges and culverts can influence stream hydraulics and sediment transport (Duck 1985; Johnson 2002, 2006). Bridges and culverts often restrict flow across the floodplain due to high embankments or approaches to the bridge or culvert. Bridges can either be single span, with no pillars in the stream, or multiple span, with one or more pillars in the stream. Pillars in the stream alter the natural flow regime and cause scouring upstream, and deposition downstream. A stream channel that was straightened and constricted with steep banks may not allow flow to intercept the floodplain. The importance of the floodplain is to dissipate the energy of flows exceeding the effective discharge (Ward et al. 2002). If a culvert is present, water can back-up upstream creating localized channel widening. If the flow is forced to remain in the channel instead of intercepting the floodplain, it will increase the sheer stress and velocity, resulting in bank erosion and failure, and streambed degradation (Graf 1975, Johnson 2002, Richardson and Davis 2001).

Roads that cross a stream at mid-slope and bridge spans built on cut-andfill material can be sources for debris flows. Debris flows are rapid movements of soil, sediment, and organic matter down steep stream channels. Heavy rains can trigger landslides of the fill material and, if near a stream, can result in a debris flow. Debris flows can move downstream, encounter a road or culvert, and either continue movement of fill downstream or deposit it. The major impact of debris flows is movement and rearrangement of sediment. Debris flows mainly occur during floods and are most severe on small, steep stream segments (Jones et al. 2000). If the stream cannot carry the sediment load, it may be deposited on the floodplain, creating new sediment bars, and enlarging current ones by vertical accretion (Graf 1975).

The stability of a stream is associated with a balance between variables such as width, depth, velocity, slope, sediment volumes, and sediment sizes. Changes in a stream's dimension, pattern, and profile due to changes in these variables can result in deteriorated water quality (Trimble 1997, US EPA 1994), reduction in quality and diversity of habitat, negative impacts on aquatic communities (Jones et al. 1999, Rabeni and Smale 1995), and land loss through erosion (Hammer 1972, Rosgen 1996). Monitoring a stream over time can be used to determine if the stream is aggrading, degrading, or laterally eroding, and can provide information on stream response to alteration. This article presents a case study of a first-order stream that was altered by channelization, placement and then removal of a culvert, and creation of a reinforced gravel berm in association with construction of a highway overpass. During construction of a highway overpass across the stream, four periods of high flows were documented. We predicted that changes to the streambed associated with road construction would cause a decrease in stream stability that we would be able to document through measurements of longitudinal profile and stream cross-sectional area. Our objectives were to collect data from a reference reach upstream of the construction zone and an altered reach within and downstream of the construction zone, and compare these to determine if construction activity and channel alteration affected streambed response to high-flow events.