Basic Hydrology Water Quality: Sediment production and transport

Sediment in streams u The significance of sediment in streams has two aspects –the water quality aspect is related to suspended sediment unwanted suspended sediment can cause domestic water quality problems, and can cause physical damage to fish – supply and movement of coarse sediment in streams is a channel morphology issue excessive coarse sediment in streams can adversely affect fish habitat and fill in water control structures

Sediment movement in streams u Sediment movement in stream channels can be categorized by grain size or by the process that transports it –suspended sediment consists of fine particles of silt and clay that are carried by the water column suspended sediment movement is independent of flow rate –wash load is a term that refers to sediment that is carried by the water column, that has grain size distribution smaller than the channel bed

–saltating bed load consists of fine to medium sand movement is dependent on the velocity of flow: at low flow it may not move, but at high flow it may bounce high up into the water column –behaviour may be similar to suspended sediment for fine sand at high flow –bed load consists of coarse sand, gravel and larger sized particles that are transported by rolling along the channel bed. bed load transport is also dependent on flow rate there is a threshold flow velocity needed to get it moving

Grain size classes

Sediment production vs. transport u Sediment production refers to the addition of new sediment to the channel system –landslides –surface erosion –channel bank erosion u Sediment transport refers to the movement of sediment that is already in the channel system

Types of sediment transport systems u Supply limited –most rivers are supply limited in terms of sediment production –the stream has the ability to move all the sediment that is supplied to it, hence sediment transport is limited by the supply –sediment transport tends to exhibit pulse behaviour, sediment movement occurs mostly on the hydrograph rising limb –hysteresis occurs where sediment concentration for a given flow is greater for rising than falling streamflow

Suspended sediment pulse, Russell Creek

SS concentration vs. discharge November 1991 Russell Creek Supply limited Rising Falling

Spray River near Banff, 1973 Supply limited

Sediment transport types u Transport limited –transport limited sediment transport is far less common than supply limited –occurs when the sediment supply to the stream is in excess of the ability of the stream to transport it occurs in braided and anastamosed rivers where sediment supply is high –as a result, more sediment moves on the falling limb than the rising limb of the hydrograph –hysteresis occurs where sediment concentration for a given flow is greater for falling than rising streamflow –large rivers can exhibit both types of transport behaviour

Dual behaviour of large rivers u Large rivers tend to be transport limited during freshet periods –sediment concentrations tend to be high during spring freshet because flow is low - sediment is “concentrated” sediment production occurs as a result of supply of sediment accumulated over winter that enters the system all at once u As flows increase, they become supply limited –sediment transport capability increases

Fraser River at Mission, 1983

Transport vs. supply limited Fraser River 1983 transport limited April supply limited May - June rising falling rising falling

Columbia River 1976 somewhere south of Golden transport limited event

May 1976 Columbia River Rising Falling

Measurement of suspended sed. u Direct method: sample analysis –a sample of stream water is collected using a manual DH48 depth integrated sampler or an automatic sampler such as ISCO 3700 –the sample is filtered through a 2  m filter to determine sediment concentration in mg/l u Indirect method: turbidity is measured and interpreted as SS conc. by field and lab calibration

Sediment production processes u rain splash erosion –sheet wash erosion - surface runoff u freeze-thaw erosion –expansion and contraction of erodible rock surfaces –frost expansion –needle ice development u animal movement u ravelling of loose sediment surfaces u undercutting and slumping of loose surfaces u mass wasting

Factors controlling sediment production u Cause of runoff –rainfall produces more sediment than an equivalent volume of snowmelt rain erosion on exposed sediment sources vs. water yield from snowpack that blankets the exposed sed. rainfall produces larger peaks than streamflow, hence greater sediment transport and channel scour u Size of storm –larger storms generally produce more sediment larger yield, and greater peak concentration

Controlling factors u Watershed morphology –lithology: softer more erodible bedrock (e.g., basalt - Catherine Cr.) produces more sediment than harder more resistant bedrock (e.g., granite - Russell Creek) –type and connectivity of sediment sources debris flow deposits are big producers hillside sediment sources –landslide scars, road cuts –slope stability

Controlling factors u Land use, e.g. logging and related activities –related to other factors such as slope stability and surface erosion from road surfaces and cut and fill slopes increase in mass wasting processes following logging yarding disturbance –potential increase in streamflow after logging increase in sediment transport capability increase in stream channel and bank erosion

Sediment Production at Russell Cr.

How does logging affect sediment production? u Studies show that increased sediment after logging is related to logging roads –surface erosion from road surfaces –road related landslides u Open slope failures increase after logging u If soil disturbance is minimized, clear-cuts are not sediment sources –burning –ground skidding