Delta sediment measurements to support numerical modeling of sediment-turbidity
Scott Wright, David Schoellhamer, Tara Morgan, Dan Whealdon-Haught, Matt Marineau
USGS Sacramento
CWEMF annual meeting 17 April 2012, Sacramento CA
U.S. Department of the Interior U.S. Geological Survey Sediment-turbidity modeling in the Delta
Delta sediment modeling is useful for a variety of reasons: - fish habitat (e.g. Delta smelt) - tidal wetlands, restoration, sea-level rise - navigation - levee integrity
Robust models require data for set- up, calibration, and testing
The Delta is large and very complex. Several data gaps exist with respect to development and testing of numerical sediment models
We began a project to fill these data gaps in December 2010, funded by the Federal Task Force on the Delta Sediment model data requirements
Hydrodynamics:
- bathymetry and levee topography - flows and velocities at upstream model boundaries - water levels at downstream boundary and interior - bed roughness (e.g. Manning’s n, drag coefficients)
Sediment:
- Sediment loads at upstream model boundaries - Sediment concentrations at interior points - Size distributions of sediment on the bed - Sediment erosion/entrainment rates - Size distributions and settling velocities of suspended sediment Sediment-turbidity monitoring
17 turbidity and sediment flux stations, co-located with flow gages
Network is designed to monitor incoming sediment loads and track movement of turbidity and sediment throughout the Delta Sediment-turbidity monitoring Sac River @ Freeport
Miner Slough
Georgiana Slough
Cache Slough
Mokelumne River
Sac River @ Rio Vista
Old River Sediment-turbidity monitoring
Turbidity is converted to sediment concentration using periodic SSC measurements and site/instrument- specific calibrations
Combined with flow data, this yields sediment flux at the stations and allows for computation of sediment budgets for various Delta regions
Tara Morgan will be talking in more detail about these data during IEP on Thursday at 3:30 pm Size distributions of sediment on the bed
Seasonal sampling, before and after the winter wet season, at 30 sites (flow gage locations)
3 samples collected at each site, channel center and both channel margins
Document regions of deposition and erosion (by changes in bed sediment texture) Size distributions of sediment on the bed
Delta channels are primarily sand bedded
Channel margins are finer in general than channel center
Central/south Delta and Cache Slough are generally finer than northern Delta
Small distributary channels are generally very coarse, but not always. Ongoing work to explain this phenomenon Bed sediment erodibility/entrainment
Erosion Microcosm System
Gomex corer Subsample
Core is collected and subjected to known rates of shear stress, erosion is measured
Flow path Bed sediment erodibility/entrainment
Recent example from Mokelumne River 1 System has been deployed on Mokelumne River, Middle 0.5 River, Little Potato Slough, Franks Tract, Grizzly Bay, and south SF Bay Shear stress, Pa stress, Shear 0 0 2000 4000 6000 8000 10000 12000 Additional sampling will 200 occur at Delta sediment sites and San Pablo Bay
100
Turbidity, NTU Turbidity, 0 0 2000 4000 6000 8000 10000 12000 Elapsed time, sec Suspended sediment size distributions and settling velocity – floc camera
Samples are collected and video is recorded in situ (on the boat) of individual flocs settling
Edge detection used to measure size, sequential images used to measure settling velocity Suspended size and settling velocity – floc camera
System has been deployed at all of the Delta sediment stations at several locations in SF Bay
5 overlaid images Suspended size and settling velocity – floc camera Near-bed hydro- and sediment dynamics
Instrument package rotated between Delta sediment flux sites to measure in situ near-bed processes, such as velocity, turbulence, turbidity, and suspended- sediment concentration and size distribution
Data in this talk are from the San Joaquin River at Stockton
LISST-100X: volume Delta instrument package concentration and particle size
ADV: 3D velocity and turbulence
Multi-parameter sonde: depth, turbidity, temperature, conductivity Bed roughness-flow resistance
ADV measures near-bed velocity and turbulence Bed roughness-flow resistance
Flow resistance computed from velocity and shear stress
San Joaquin River at Stockton Size distributions of suspended sediment
Measured in situ with LISST-100X
Primarily flocculated particles – samples are ~75% silt/clay by mass and ~40% silt/clay by volume (in situ)
Floc density ~ 1.5 g/cm3 Size distributions of suspended sediment
Median particle size is inversely related to shear stress
Coarsest sizes are also inversely related to shear stress, while medium sizes are directly proportional
Indicates cycles of floc formation and break up Size distributions of suspended sediment
High shear breaks up flocs, decreasing concentration of >100 um particles and increasing concentration of <100 um particles
Large flocs reform around slack tide
10-100 um range also includes coarse silt and sand primary particles Summary
- We are compiling an extensive dataset on sediment transport processes in the Delta; the project is ongoing
- Several observations will be of interest to ongoing modeling efforts, such as the manner in which sediment disperses through the Delta during Sacramento River floods, and suspended- sediment particle and floc dynamics
- Several research questions remain, such as the link between bed sediment size, cohesiveness, and erodibility; and the distribution of suspended sediment at channel junctions
- This dataset will allow robust testing of numerical models and improved confidence in their predictive capabilities. Data are available (provisionally) by contacting us; several articles are in preparation Questions?
Scott Wright David Schoellhamer [email protected] [email protected]
916-278-3024 916-278-3126