GEOG 408 quiz 5

groundwater flow systems

can be local, intermediate or regional

groundwater contributes to

baseflow and event flow

baseflow

is a consistent, slowly varying contribution to the river

event flow

the surge in river flow observed during a rain event

groundwater relationship to surface water

lakes, rivers, and wetlands are typically at groundwater discharge points

hydrograph

a graph showing the rate of flow (discharge) versus time past a specific point in a river

storm hydrograph

plots precipitation and run-off/discharge over time, include baseflow with these

Runoff mechanisms: Channel precipitation

the fastest way for rain to get into the river is to fall directly

Runoff mechanisms: overland flow

rain rate exceeds inflitration rate creating ponding and overflow

Runoff mechanisms: throughflow

is flow through the unsaturated zone, once it breaks surface tension it flows with gravity downslope into the river

different hydrograph shapes

drainage basin conditions are reflected in the hydrograph shape, size, vegetation, slope, soil types

changing hydrograph

land sue changes will change the hydrograph shape, removal of vegetation, less permeable surfaces

Unit hydrograph

a tool that allows us to predict the timing and magnitude of peak flow, in response to a precipitation or snow melt event, does not include baseflow

unit hydrograph assumptions 1

two storms with equal duration and equal rainfall excess produce the same hydrograph, regardless of intensity

unit hydrograph assumptions 2

the amount of discharge is assumed to be directly proportional to amount of excess rainfall

unit hydrograph assumptions 3

the timing of runoff is the same, regardless of antecedent conditions

key reasons for constructing a unit hydrograph

flood forecasting and management, extrapolation for various storms, design of hydraulic structures, model ungauged catchments

Volumetric gauging aka Bucket method

only works for very low discharge, directly measures discharge

velocity area method

classic discharge technique, measurements of stream velocity and channel area

challenges of velocity area method

channel geometry is variable, making area estimates tricky, flow velocities can vary significantly

velocity area method drawbacks

doesn’t work well in complex channels, difficult/dangerous in large rivers and during floods, time consuming

dilution gauging

based on C1V1, dump tracer at some point upstream, time to get downstream point, measure tracer concentration at downstream point

dilution gauging considerations

need to pick an appropriate downstream point

Dilution gauging tracer considerations

readily soluble, zero or low concentration in natural system, inert, easy to detect at low conc, harmless to environment

stage measurement

river stage (or water level/water height) can be monitored easily (and continuously)

stage measurements combined with other measurements

a rating curve can be developed

stage measurement drawbacks

if streambed changes its geometry a new rating curve must be developed, it is difficult to get discharged data points for high flow (flood) situations makin the rating curve is unreliable

Weirs

modify river flow so that it travels through a known cross-sectional area with constant velocity

flumes

work similar to weirs but without the sitting pond

weirs/flumes drawbacks

generally only used on small streams, need to be sure all water is flowing through the structure, long-term weir may have sedimentation problems in the stilling pond, may have negative impacts on stream ecology