21 October 2014 Science Briefs

SNAMP Pub #28: Seasonal Accumulation and Depletion of Local Sediment Stores of Four Headwater Catchments

Article Title: Seasonal Accumulation and Depletion of Local Sediment Stores of Four Headwater Catchments

Authors: Sarah E. Martin, Martha H. Conklin and Roger C. Bales

Research Highlights:


  • Using turbidity versus discharge relationships, we infer that localized, in-channel sources dominate sediment supply in these catchments.

  • In all four forested watersheds, fall flow events were most likely to produce a turbidity response than events during other seasons, despite the fact that the largest storms typically do not occur during fall.

  • Fall events occur after a summer dry period, when loose sediment can accumulate in the channel, so there is more material available to transport with the first rains of the season.


Research Questions:

The goal of this study was to use long term discharge, turbidity (sediment), and precipitation data from forested mountain catchments to answer: (1) What are the seasonal trends in turbidity patterns and what do they imply about sediment production and transport throughout the water year in these catchments? (2) What are the turbidity patterns associated with individual storm events and what do they imply about sediment sources and transport? (3) How does the source of water (i.e., rain, snow-melt, and rain on snow) to the stream affect the discharge and turbidity response of the stream? What do all these imply about water flow pathways and sediment movement in these catchments?

Background:

An understanding of sediment movement in small headwater catchments in mountain areas is important as these can be significant sediment source areas. This is especially true in California where 60 percent of the water comes from the Sierra Nevada and most of the major river systems contain dams where storage area can be greatly reduced by accumulating sediments. Understanding the patterns of turbidity events and source areas of sediment within watersheds will allow managers to mitigate sediment sources and to better plan for turbidity-related impacts to downstream water quality.

This research analyzes the seasonal turbidity patterns as well as patterns of turbidity versus discharge in four 1 km2 headwater catchments on the western slope of the Sierra Nevada, California for 3 years (2010-2012). Catchments are divided into two field sites, each with two headwater catchments containing perennial streams. The paired catchments were chosen because they are comparable in size, gradient, discharge, aspect, and vegetation cover. These catchments are located at the snow-rain transition zone, with snow making up roughly 40 to 60 percent of average annual precipitation.

Fine sediments that remain in suspension and cause turbidity signals in streams can come from hillslope or in-channel sources. Turbidity events in this region are reported to be infrequent and of short duration. Turbidity event patterns can vary on multiple time scales as the controlling factors on erosion and sediment transport vary. The timing of turbidity versus discharge [termed hysteresis] can help provide insight into the proximity of the sediment sources and into whether or not sediment depletion is occurring.

Turbidity was measured at 15 minute intervals using back-scattered light from a near infrared LED that illuminated a sample of the water column. Stream stage was measured at two locations using pressure sensors located 100 to 300 meters apart. In addition, there were two meteorological stations per site, as well as snow depth instrumentation. Rating curves and stream discharge profiles, were created for each catchment and used to calculate discharge. Sediment hysteresis loops were created by plotting turbidity versus discharge for each flow event and classified by shape of those plots into clockwise, counterclockwise, linear, figure eight, or complex hysteresis shape categories.

Results:

Water years 2010 and 2011 were above average for rain and 2012 was below average. Turbidity events varied greatly in magnitude for 2010 to 2012 and not all storm events produced a turbidity signal.

In all four watersheds, fall flow events were most likely to produce turbidity signals, despite the fact that the largest storms typically do not occur during that season. A possible explanation is that during summer base flow, channel banks typically dry out and crumble producing stores of loose sediment in the channel. Fall rain events occur after this summer dry period, when there is more material available to transport with the first rains of the season. The first storm moves a large portion of sediment out of the local area and as the season and year progresses, less and less loose, easy to move sediment is available. Bank surveys were conducted at the end-of-summer low-flow periods each year. In many of these surveys, a pile of accumulated sediment was observed at the toe of banks. This accumulated sediment provides supporting evidence to our conceptual model.

Another possible explanation for fall having more turbidity events is that fall rain events show the most abrupt discharge increases. Discharge data indicates that fall events had the highest average flow increases above seasonal background levels. .

A clockwise hysteresis loop shape for individual storm events was dominant for all catchments and most seasons. The mainly clockwise shaped events imply that localized in-channel sources dominate sediment supply in these catchments. Extended discharge events with multiple consecutive discharge peaks showed a shift toward more linear and counter clockwise hysteresis patterns. This shift indicates depletion of localized, loose, easy to transport sediment sources and a shift toward more distant or more consolidated sources.

Conclusions:


  1. Rain events in fall are the most likely events to produce a turbidity signal despite not being the largest flow events.

  2. Seasonal and event-scale accumulation and depletion cycles govern sediment transport in these streams.

  3. Localized, in-channel sources of sediment dominate sediment production in the study watersheds.

  4. Because in-channel sources dominate sediment production, the type of precipitation (rain vs. snow) does not have much effect on turbidity.

Full Reference:

Martin, S.E.; Conklin, M.H.; Bales, R.C. Seasonal Accumulation and Depletion of Local Sediment Stores of Four Headwater Catchments. Water 2014, 6, 2144-2163

This paper is available at: http://www.mdpi.com/journal/water.

For more information about the SNAMP project and the water team, please see: http://snamp.cnr.berkeley.com/water

No Comments Yet

Post a New Comment