flooding of comparable magnitudes but shorter durations and thus plugs did not form during these periods. In Figure 4.5 the normal conditions represent periods in which the El Niño index does not exceed 0.5 or fall below -0.5 for more than two months during the year. Therefore, the influence of the El Niño and La Niña periods can be compared individually rather than simply comparing these periods against one another. It appears from Figure 4.5 that the La Niña periods are characterized by a duration of overbank flows that is generally much shorter than the normal conditions and the El Niño periods are characterized by having a longer period of discharge greater than two thousand cubic feet per second than the normal conditions but of comparable periods for discharges greater than three thousand, four thousand, or five thousand cubic feet per second. A possible explanation for the El Niño events having a similar or shorter duration of discharges that are greater than three thousand, four thousand, and five thousand cubic feet per second is due to the presence of dams upstream that prevent large peaks by releasing a greater quantity of water over a longer period of time. Therefore, although the El Niño events typically experience a greater overall discharge during the year, this period is not subjected to longer durations of high discharges. Overall it appears that it is less likely that a relatively long duration of overbank flow will occur during La Niña periods and it is more likely that a relatively long duration of overbank flow will occur during El Niño periods in regions with an overbank discharge of less than approximately three thousand cubic feet per second. Figure 4.4: Average Days per Year with an Average Discharge beyond a Threshold Magnitude between 1986 and 2012 for La Niña, El Niño, and Normal Conditions [Rainwater 2013] : Description of Plug Formation The flow chart as shown in Figure 5.1 below illustrates the process that is believed to produce sediment plugs on the Middle Rio Grande. This process has been categorized into four stages to illustrate this process. The first stage is entitled the causing factors since these factors represent the initial circumstances that enable the resulting effects in stage two. The spring runoff duration and sequence however has been separated from the spring runoff magnitude because it is not expected that this factor contributed to the resulting effects. The spring runoff duration and sequence and the resulting effects are expected to be capable of forming a sediment plug, perching, and avulsion but it would require multiple seasons. However, when combined with
from the dam. Similarly to the reduction in peak flows, this aggradation would serve to reduce the channel slope as shown in Figure 5.3 below. Figure 5.2: Lane’s Balance Influence of Slope [Rainwater 2013] Figure 5.3: Zones of Erosion, Transport, and Deposition [Rainwater 2013] (Equation 5.1) 5.1.3 Floods and Droughts A process resulting from a cycle of droughts and floods may contribute to channel aggradation because of a low sediment transport capacity during drought periods and then due to overbank flow during flood periods. During drought periods the channel filling with sediment leads to reduced bank height. Then when the next peak flow occurs, more flow goes overbank than otherwise would if there was not a deposition of bed sediments during drought. Evidence of plug formation following periods of drought is shown in Figure 4.1a-d. A process resulting from a cycle of droughts and floods that contributes to low bank height, variations in the channel width, and channel perching is shown in Figure 5.4 below. Figure 5.4: Flood and Drought Influence on Channel Geometry [Rainwater 2013] 5.1.4 High Sediment Supply and Low Transport Capacity A high sediment concentration may be a consequence of overbank flow that contains a relatively small amount of sediment and therefore a disproportionately large amount of sediment remains in the channel. Also, a low transport capacity may be a result of a low flow velocity, geometric factors such as a relatively flat slope or a large width-depth ratio, or an increase in the channel roughness. Overbank flow may also contribute to a large width-depth ratio as illustrated in Figure 5.4 as well as a low flow velocity since the flow with the highest velocity is located near the surface of the channel and therefore overbank flow selectively removes the flow with the greatest transport capacity. 5.1.5 Spring Runoff Magnitude, Duration, and Sequence The spring runoff discharge must exceed the overbank discharge in order for overbank flows to occur during this period. In addition to the spring runoff magnitude, a short-term
Reclamation (2012). “Bosque del Apache Sediment Plug Baseline Studies.” Annual Report 2011, Technical Service Center, Denver, CO. Shrimpton, C. and Julien, P.Y. (2012). Middle Rio Grande, Assessment of Sediment Plug Hypotheses. Report prepared for Reclamation. Colorado State University, Fort Collins, CO, 48p. Shrimpton, C. (2012). “Hypotheses of sediment plug formation” M.S. Technical Report, Colorado University, Fort Collins, CO, 62p. U.S. Geological Survey. (n.d.). USGS Data for the Nation. USGS Water Data for the Nation. Retrieved August 27, 2013, from J. M. U.S. Department of the Interior, Bureau of Reclamation. (2010). Middle Rio Grande Peak Discharge Frequency Study