Regulate Instream Sand Mining Process

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Regulate Instream Sand Mining Process

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Regulate Instream Sand Mining Process

Regulate Instream Sand Mining Process

Regulate Instream Sand Mining Process

Strategies used to manage instream mining range widely, and in many jurisdictions there is no effective management. One strategy is to define a redline, a minimum elevation for the thalweg (the deepest point in a channel cross section) along the river, and to permit mining so long as the bed does not incise below this line (as determined by annual surveys of river topography). The redline approach addresses a problem common to many past permits, which have specified that extraction is permitted “x feet below the channel bed” or only down to the thalweg, without stating these limits in terms of actual elevations above a permanent datum. Thus the extraction limits have migrated vertically downward as the channel incises.

Current approaches to managing instream mining are based on empirical studies. While a theoretical approach to predicting the effects of different levels of gravel mining on rivers might be desirable, the inherent complexity of sediment transport and channel change and the lack of adequate data on channel form, sediment transport, and gravel extraction overtime, make firm, specific predictions impossible at present. Sediment transport models can provide an indication of potential channel incision and aggradation, but all such models are simplifications of a complex reality, and the utility of existing models is limited by unreliable formulation of sediment rating curves, variations in hydraulic roughness, and inadequate understanding of the mechanics of bed coarsening and bank erosion (NRC 1983).

In 1995, the US Department of Transportation issued a notice to state transportation agencies indicating that federal funds will no longer be available to repair bridges damaged by gravel mining, a move that may motivate more careful regulation of gravel mining in rivers by states.

The “Replenishment Rate” Concept

Another approach to managing gravel mining is to estimate the annual bedload sediment supply from upstream, the “replenishment rate”, and to limit annual extraction to that value or some fraction thereof, considered the “safe yield”. The replenishment rate approach has the virtue of scaling extraction to the river load in a general way, but bedload transport can be notoriously variable from year to year. Thus, this approach is probably better if permitted extraction rates are based on new deposition that year rather than on long-term average bedload yields. More fundamentally, however, the popular notion that one can extract at the replenishment rate without affecting the channel ignores the continuity of sediment transport through the river system. The mined reach is the “upstream” sediment source for downstream reaches, so mining at the replenishment rate could be expected to produce hungry water conditions downstream.

Dunne et al. (1981) stressed because actual bedload transport is variable from year to year, estimated average annual bedload inflow rates may not be applicable in most years. Replenishment can be estimated year-to-year, either riverwide (based on sediment rating curves), or based on site-specific deposition. The latter approach is used on the Mad River in California, where a committee of scientists visits extraction sites annually, reviews semi-annual cross section survey data, estimates the amount of deposition over the flow season, and recommends an extraction amount, location, and method (Klein et al. 1999).

In estimating annual (or annual average) replenishment rates, it is important to recognize that using sediment transport equations yields an estimate of theoretical bedload sediment transport capacity, which is commonly less than actual load, as the latter is limited by actual sediment supply from the basin.

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