Submitted to: Geomorphology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/20/2001
Publication Date: N/A
Interpretive Summary: To reduce erosion from unstable channels, rock and concrete structures are often constructed within the channels. In theory, this will abate erosion upstream of the structure by reducing the slope of the channel bed and the erosive power of the flowing water. Downstream from such structures, as with flood-control structures such as dams, stream water carries significantly less sediment because much of the sediment is trapped upstream of the structure. Because the water emanating from the structure carries very little sediment, downstream flows often erode the channel downstream of such structures. An evaluation of channelized Hotophia Creek, located in northern Mississippi, was carried out to evaluate the effect of grade-control structures installed along the creek by comparing actual amounts of erosion (with structures) to a hypothetical scenario of erosion without the structures. Results indicate that more sediment was eroded from the channel bed and banks with the grade-control structures than would have occurred without the structures. The structures could have been more effective if they had been constructed during the period of channel adjustment dominated by bed erosion.
Technical Abstract: Geomorphological evaluation was used to estimate spatial and temporal trends of bed and bank erosion along an 18-km length of Hotophia Creek, Mississippi. The evaluation was undertaken for two scenarios of channel adjustment in response to the 1961-1963 channelization of Hotophia Creek. While the Case 2 scenario represents the 'actual' response of the channel and includes the effects of installing a series of grade control structure (GCS), Case 1 (without structures) represents a hypothetical scenario in which the channel is left to adjust naturally. The geomorphological evaluation relies on the availability of reliable channel-survey data to develop empirical bed and bank response models for each adjustment scenario, together with simple bank stability modelling to predict future rates of bank erosion. Results indicate that channel-erosion rates decline non-linearly with respect to time since 1961, for both adjustment scenarios. However, by the year 2050 the Case 2 (with structures) adjustment scenario results in the cumulative removal of some 663,000 (9%) extra tons of sediment relative to the Case 1 (without structures) scenario. Most (63%) of this excess is derived from enhanced bed erosion during 1976-1985 and 1985-1992, with the remainder derived from increased bank erosion during 1985-1992. These results show that, for optimal effectiveness, GCS must be installed in incising channels early in the adjustment cycle. If this is not possible then great care must be exercised in choosing the location and spacing of structures to optimize the potential for structures to prevent upstream migrating erosion, while minimizing the trapping of coarse material that may promote bed recovery downstream.