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Prepared by: The Center for Watershed Protection, Aquafor Beech Ltd., & Step by Step

September, 1999

Section 1: Introduction

This Section provides the rationale for the study and an outline of study objectives. In brief this study addresses the impact on channel form associated with gravel extraction practices and associated instream works for flood hazard management. This was done through a literature review of gravel extraction case studies, the development of a conceptual model for the explanation and prediction of channel response to gravel extraction, completion of a case study in a Vermont stream and validation of the model through application to the Vermont case study. Finally these findings were used to formulate recommendations for the management of Vermont streams as a basis for further discussion.

Section 2: Literature Review

In this Section a comprehensive review of case studies pertaining to the morphological impacts of gravel extraction from numerous States as well as Europe, Africa, New Zealand and Canada is described. In total observations from 70 different river systems in 11 countries were reviewed and summarized. Morphological impacts were found to be consistent for rivers of similar form and size regardless of geographical location, climate and topography. Consequently, generalities can be made from collective assessment of case studies. In general the morphological impacts varied depending upon the location of the reach relative to the mined reach, the size of the watershed, the amount of gravel extracted relative to the supply, stream type (braided, meandering, sinuous or straight) and whether extraction practices were active or inactive. In the majority of instances flood hazard benefits were short lived and the gravel mining resulted in the de-stabilization of the channel with a commensurate increase in property loss and aesthetic and habitat degradation in both the mined reach and reaches upstream and downstream of the zone of mining. The type of mining also had a bearing on the degree of morphological impact. The stripping of gravel bars had less impact then pit mining within the river. Pit mining within the floodplain was only an issue when lateral migration of the channel resulted in capture by the pit and avulsion of the channel system.

Section 3: Conceptual Morphological Response Model

The above studies were used to formulate a model for the prediction of morphological impacts using a decision tree approach. The model represents a comprehensive and unique approach to the prediction of the response of gravel bed rivers to a disturbance affiliated with gravel extraction and associated flood hazard reduction measures. The model provides a suitable format for the development of a smart systems computer model. Such a model would provide practioners and decision makers with a systematic methodology for the prediction of the morphological impacts associated with gravel extraction and associated instream works for the reduction of flood hazard. Further, the model would be suitable for use in costing proposed mitigation works and therefore an instrumental step in the development of a prioritization algorithm for the allocation of limited resources.

Section 4: Granville Case Study

This Section deals with an analysis of historic aerial photographs for the White River through the Town of Granville (the "subject" channel). Photographs were available for the years of 1939, 1962, 1974 and 1995 for this region. The 1939 and 1974 photo series were subsequent to major flood flow and "maintenance" events (gravel mining and flood hazard mitigation works). The 1962 and 1995 photo series were taken 5 and 22 years after such events respectively.

The "subject" channel was subdivided into three distinct reaches: Reach 1 (upstream of the zone of mining); Reach 2 (the zone of mining from the Bowl Mill Bridge to a point downstream of the confluence of the White River and Alder Meadow Brook) and Reach 3 (downstream of the zone of mining to the first crossing of Route 100).

The White River through the "subject" reach has experienced channel "maintenance" on four confirmed occasions since 1938 and possibly on a fifth occasion in the late 1920's.

The photographs were digitized and corrected for scale based on ground proofing. Morphometric parameters including the length of the thalweg, the width of the active channel, channel surface area, maximum and average normal shift were then determined for each photo year or Epoch. The same analysis was conducted for a "reference" channel. The West Branch of the Tweed River near Pittsfield was selected for this purpose because instream modifications were believed to be minimal and land use, topography, climate and watershed size were similar to that of the "subject" reach.

Pairwise comparison on the observations by Epoch indicates that the White River through the zone of mining has narrowed and straightened. Maximum and average normal shift were not determined for this reach because of the influence of maintenance activities. Downstream of the mined reach the channel has straightened and widened. Maximum normal shift has increased indicating increased lateral instability while average normal shift has declined. The later observation is consistent with channel straightening. The reach upstream of the zone of mining was not impacted because geologic controls prevent the headcutting of nickpoints and other grade discontinuities. In contrast, the Tweed River was found to relatively constant over the study period with a slight increase in width and normal shift. The morphological response of the White River is significant in comparison to the "reference" stream. The observed responses are also consistent with the observations reported from the literature review.

Section 5: Validation of The Conceptual Morphological Response Model

This Section describes the application of the conceptual model to the Granville case study. The model was applied to the three Reaches as defined in Section 4. Reach 1 showed no impact because of the step-pool form and bedrock control. Reach 2 has been subject to extensive gravel extraction and instream "maintenance" practices since the 1920's resulting in widening of the active channel and channel incisement. Large Woody Debris, large scale roughness elements (large boulders) and riparian vegetation have also been modified through the years. The model was also applied to the most downstream reach. Although some bank stabilization works are evident in this reach it is largely unmodified directly through gravel extraction practices.

Reach 2 is dominated by erosional forms resulting in Valley Formation. This process results in the formation of a new active-floodplain channel system inset within the existing system but at a lower elevation. This scenario was adequately predicted by the proposed model. The new channel has an increased flow conveyance capacity and consequently provides the intended flood hazard reduction but does so at the expense of considerable loss of property within the mined reach and the de-stabilization of the channel downstream of the zone of mining. The downstream reach is dominated by sedimentation forms leading to aggradation and the formation of chutes and cutoff channels. The formation of the bifurcation in 1998 was satisfactorily predicted by the model. Flood hazard in this lower reach initially increased as a consequence of the maintenance works. The development of the bifurcation resolved the imbalance between the elevated sediment load an the lack of stream competence by decreasing channel length and thereby increasing longitudinal slope and stream power. This interim quasi-stable form occurred with the loss of tillable farmland.

The proposed model indicates that eventual stabilization of Reach 2 and the commensurate decline in total sediment yield together with the fining of the sediment load may once again de-stabilize Reach 3. The lower reach will attempt to increase its flow length and thereby decrease its longitudinal slope to reduce its stream power to match its sediment load characteristics. It may accomplish this through increased meander development and propagation rates.

It was concluded that the proposed model provides a useful tool for the prediction of channel response to a disturbance for channel systems similar to the White River through the Granville reach. Further testing and development of the model is recommended for general application to Vermont streams.

Section 6: Flood & Erosion Hazard Management

The results of the literature review, Granville case study and the conceptual model were used to outline a general flood and erosion hazard management approach regarding instream works and gravel extraction practices in gravel bed streams in the State of Vermont. The recommendations are organized around watershed size and stream type (braided, meandering, straight) and they are intended for discussion purposes only.

In terms of watershed size channel systems of less than 38.6 mi2 (100 km2) were found to be very sensitive to instream works. In contrast channel systems in watersheds exceeding 386 mi2 (1000 km2) were found to be the least sensitive. Regarding stream type, braided channel systems were found to be the least sensitive while meander and straight channel systems were progressively more sensitive respectively. Consequently, gravel extraction in small channel systems is not recommended. Gravel extraction in moderately size watersheds may be permitted in braided systems and selected instances if a well defined management plan is followed. This may be defined using a sediment budget approach based on selected particle size fractions such that stream sediment load requirements downstream of the mined reach are satisfied. The instream programs must address issues of channel form and particle roughness, bed material gradation and structure, and the preservation of riparian vegetation and floodplain connectivity. Similarly, gravel extraction in large watersheds, particularly in braided channel systems, may be allowed in a controlled manor following completion of a well defined management plan.


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