Abstract The largest UNESCO World Heritage Site in the UK is found in Cornwall and west Devon, and its designation is based specifically on its heritage for metalliferous mining, especially tin, copper and arsenic. With a history of over 2000 years of mining, SW England is exceptional in the nature and extent of its mining landscape. The mining for metallic ores, and more recently for kaolin, is a function of the distinctive geology of the region. The mining hazards that are encountered in areas of metallic mines are a function of: the Paleozoic rocks; the predominant steeply dipping nature of mineral veins and consequent shaft mining; the great depth and complexity of some of the mines; the waste derived from processing metallic ores; the long history of exploitation; and the contamination associated with various by-products of primary ore-processing, refining and smelting, notably arsenic. The hazards associated with kaolin mining are mainly related to the volume of the inert waste products and the need to maintain stable spoil tips, and the depth of the various tailings’ ponds and pits. The extent of mining in Cornwall and Devon has resulted in the counties being leaders in mining heritage preservation and the treatment and remediation of mining-related hazards.

Abstract We investigated the viability of ground-penetrating radar (GPR) as a method to estimate the quantity of sediment stored behind the Merrimack Village Dam on the Souhegan River in southern New Hampshire. If the predam riverbed can be imaged, the thickness and volume of the reservoir deposit can be calculated without sampling. Such estimates are necessary to plan sediment management after dam deconstruction. In May 2008, we surveyed six cross sections with a Mala Geosciences ProEx 100 MHz GPR. In a related study, topographic surveys were conducted in 2008–2009 to monitor the sediment flux associated with the removal of the Merrimack Village Dam in August 2008. Within a month of the removal, these surveys mapped the predam riverbed in the uppermost cross sections in the former impoundment. We compared these surveys to our interpreted GPR images for one cross section to determine a calibrated velocity for the impounded sand of 0.043 ± 0.020 m/ns. We also estimated the radar velocity of the deposit by analyzing hyperbolic reflections in the GPR images, and found a similar result (0.039 m/ns). Using the calibrated velocity, we estimated a total volume of sediment stored behind the Merrimack Village Dam of 66,900 ± 9900 m 3 , which compares well to a previous estimate (62,000 m3) based on a depth-to-refusal survey. Our findings indicate that GPR is a useful technique for quantifying impounded sediment prior to dam removal in reservoirs containing 1–10 m of sand overlying a coarser predam riverbed, but it may be less effective in settings with finer and/or thicker impounded sediment.

Abstract The removal of obsolete and unsafe dams for safety, environmental, or economic purposes frequently involves the exploration of sediments trapped within the impoundment and the subsequent assessment of sediment management needs and techniques. Sediment management planning requires a thorough understanding of the watershed’s surficial geology, topography, land cover, land use, and hydrology. The behavior of sediments is influenced by their age, consolidation, and stratigraphy. All watersheds have a history that helps forecast sediment loads, quality, gradation, and stratigraphy. Impounded sediment deposits may include coarse deltas and foreset slopes, fine or coarse bottom deposits, cohesive or organic matter, and wedge deposits immediately behind the dam. Some watersheds have anthropogenic pollutants from agricultural activities, mining, industries, or urban runoff. The volume and rate of sediment release during and after small dam removal can be limited by active management plans to reduce potential downstream impacts. Management strategies include natural erosion, phased breaches and drawdowns, natural revegetation of sediment surfaces, pre-excavation of an upstream channel, hazardous waste removal or containment, flow bypass plans, and sediment dredging.

Abstract The 2005 removal of the 3.6-m-high Munroe Falls Dam from the middle Cuyahoga River, Ohio, provided an opportunity to assess dam removal channel-evolution models and to anticipate impacts from additional dam removals on the Cuyahoga River. Preremoval geomorphic and sedimentologic conditions were characterized. Monitoring of the river response to dam removal has continued for 5 yr. The dam removal lowered base level and increased flow velocity upstream of the former dam site. Postremoval, the initial channel response was rapid incision to the predam substrate, followed by rapid lateral erosion of the exposed impoundment fill. Four to nine months after removal, dewatering and vegetation of the exposed impoundment fill greatly reduced the rate of lateral erosion. For 2.5 yr post -removal, sandy bar forms were present upstream of the former dam, and sand was transported under all flow conditions of the year. Subsequently, the bed has become armored with gravel. Downstream of the former dam site, the channel aggraded with sand, causing flow to occupy meander bend chutes that had formerly only been active during high flow. A sandy deltaic feature has accumulated 3.3 km downstream in the impoundment created by the Le Fever Dam. The impacts of the Munroe Falls Dam removal are generally well described by published channel-evolution models with minor exceptions due to local geology and hydrology. The similarities between the Munroe Falls and Le Fever Dam impoundments suggest that this study can aid in understanding the impacts of the possible future removal of the Le Fever Dam.

Abstract For safety and environmental reasons, removal of aging dams is an increasingly common practice, but it also can lead to channel incision, bank erosion, and increased sediment loads downstream. The morphological and sedimentological effects of dam removal are not well understood, and few studies have tracked a reservoir for more than a year or two after dam breaching. Breaching and removal of obsolete milldams over the last century have caused widespread channel entrenchment and stream bank erosion in the Mid-Atlantic region, even along un-urbanized, forested stream reaches. We document here that rates of stream bank erosion in breached millponds remain relatively high for at least several decades after dam breaching. Cohesive, fine-grained banks remain near vertical and retreat laterally across the coarse-grained pre- reservoir substrate, leading to an increased channel width-to-depth ratio for high-stage flow in the stream corridor with time. Bank erosion rates in breached reservoirs decelerate with time, similar to recent observations of sediment flushing after the Marmot Dam removal in Oregon. Whereas mass movement plays an important role in bank failure, particularly immediately after dam breaching, we find that freeze-thaw processes play a major role in bank retreat during winter months for decades after dam removal. The implication of these findings is that this newly recognized source of sediment stored behind breached historic dams is sufficient to account for much of the high loads of fine-grained sediment carried in suspension in Mid-Atlantic Piedmont streams and contributed to the Chesapeake Bay.