Dubinski Ian M, Wohl Ellen
Department of Geosciences, Colorado State University, Ft. Collins, CO 80523-1482, USA.
Environ Manage. 2007 Jul;40(1):147-60. doi: 10.1007/s00267-005-0176-4. Epub 2007 May 24.
The Gunnison River in the Black Canyon of the Gunnison National Park (BCNP) near Montrose, Colorado is a mixed gravel and bedrock river with ephemeral side tributaries. Flow rates are controlled immediately upstream by a diversion tunnel and three reservoirs. The management of the hydraulic control structures has decreased low-frequency, high-stage flows, which are the dominant geomorphic force in bedrock channel systems. We developed a simple model to estimate the extent of sediment mobilization at a given flow in the BCNP and to evaluate changes in the extent and frequency of sediment mobilization for flow regimes before and after flow regulation in 1966. Our methodology provides a screening process for identifying and prioritizing areas in terms of sediment mobility criteria when more precise systematic field data are unavailable. The model uses the ratio between reach-averaged bed shear stress and critical shear stress to estimate when a particular grain size is mobilized for a given reach. We used aerial photography from 1992, digital elevation models, and field surveys to identify individual reaches and estimate reach-averaged hydraulic geometry. Pebble counts of talus and debris fan deposits were used to estimate regional colluvial grain-size distributions. Our results show that the frequency of flows mobilizing river bank sediment along a majority of the Gunnison River in the BCNP has significantly declined since 1966. The model results correspond well to those obtained from more detailed, site-specific field studies carried out by other investigators. Decreases in the frequency of significant sediment-mobilizing flows were more pronounced for regions within the BCNP where the channel gradient is lower. Implications of these results for management include increased risk of encroachment of vegetation on the active channel and long-term channel narrowing by colluvial deposits. It must be recognized that our methodology represents a screening of regional differences in sediment mobility. More precise estimates of hydraulic and sediment parameters would likely be required for dictating quantitative management objectives within the context of sediment mobility and sensitivity to changes in the flow regime.
位于科罗拉多州蒙特罗斯附近的甘尼森国家公园黑峡谷（BCNP）内的甘尼森河，是一条由砾石和基岩混合而成的河流，有季节性的支流。流量在上游立即由一条导流隧道和三座水库控制。水力控制结构的管理减少了低频、高水位流量，而这种流量是基岩河道系统中主要的地貌作用力。我们开发了一个简单模型，用于估算BCNP中给定流量下的泥沙运移范围，并评估1966年流量调节前后不同水流状态下泥沙运移范围和频率的变化。当无法获得更精确的系统实地数据时，我们的方法提供了一个筛选过程，用于根据泥沙运移标准识别区域并确定其优先级。该模型使用河段平均床面剪应力与临界剪应力的比值来估算给定河段特定粒径的泥沙何时会被运移。我们利用1992年的航空摄影、数字高程模型和实地调查来识别各个河段，并估算河段平均水力几何形态。通过对岩屑堆和泥石流扇沉积物的卵石计数来估算区域冲积物粒度分布。我们的结果表明，自1966年以来，BCNP中甘尼森河大部分河段河岸泥沙被水流运移的频率显著下降。模型结果与其他研究人员进行的更详细、针对特定地点的实地研究结果非常吻合。在BCNP中河道坡度较低的区域，显著的泥沙运移水流频率下降更为明显。这些结果对管理的影响包括植被侵入活动河道的风险增加，以及冲积物长期导致河道变窄。必须认识到，我们的方法只是对泥沙运移区域差异的一种筛选。在泥沙运移和对水流状态变化的敏感性背景下，可能需要更精确的水力和泥沙参数估计来确定定量管理目标。
