r. hudson - vfr research hydrology groundwater. r. hudson - vfr research groundwater topics......

R. Hudson - VFR Research

Hydrology

Groundwater

Groundwater Topics...

• General principles– Hydraulic head, fluid potential – Darcy’s Law, saturated groundwater flow

• Hydraulic conductivity K

• measurement of K

• porosity

• effects of heterogeneity on flow

• groundwater flow patterns on a slope

… relevant to Forest Hydrology

– Unsaturated groundwater flow• hydraulic properties of unsaturated soil

• drainage and infiltration

– Interflow

• Groundwater in relation to Forest Hydrology– How does forest harvesting affect groundwater – significance of those effects

Hydraulic head• Groundwater flows along an energy

gradient– there are two possible energy gradients that

affect groundwater flow: gravity and fluid pressure

z = z1

z = z2

gravity drainage

flow under fluid pressure gradient where p1 > p2

Groundwater head - energy for flow

h = z +

z = elevation head above reference elevation (datum) = pressure head (m) = P/g whereP = fluid pressure = fluid densityg = acceleration due to gravity

Groundwater headis measured usinga piezometer.

Water table well vs. piezometer

Darcy’s Law

• Groundwater flow is a function of hydraulic head gradient– total flow Q has units of volume/time

• typically m3/s or litres/sec

– specific discharge q is flow per unit area, units of length

• the negative sign indicates that flow moves in the direction of falling head

Hydraulic conductivity K• groundwater flow is driven by the

hydraulic gradient dh/dl• K is a measure of the resistance to flow, is

a property of the porous medium and the fluid

• K has units of m/s or cm/s• k is permeability, is a property of the medium related to diameter, packing, shape and roughness of grains (m2, cm2)• is the viscosity of the fluid (kg/m.s)

Range of values of KMedium K in m/s

Gravel 10-3 to 1Sand 3X10-6 to 10-2

Typical BC Forest soil 10-7 to 10-5

Bog soils 10-9 to 10-7

Marine clay 10-12 to 10-9

Basal till 10-12 to 10-10

Igneous rock, shale 10-13 to 10-10

Sandstone 10-10 to 10-6

Porosity

• Porosity is another important property of porous media that governs water flow– porosity is a measure of the capacity of the

medium to hold water

– a volume VT of soil of rock is divided up into the volume of voids Vv and volume of solids Vs

– porosity n = Vv / VT

– void ratio e = Vv / Vs

Range of values of porosityMedium Porosity (%)

gravel 25-40

sand 25-50

silt 35-50

clay 40-70

sandstone 5-30

limestone 0-20

shale 0-10

fractured basalt 5-50

fractured crystalline rock 0-10

dense crystalline rock 0-5

Relations between K and n• for soil , they are inversely proportional

– for well sorted sediments, the finer grained they are, the lower K is and the higher n is

– for poorly sorted sediments, smaller grains fill in voids between larger grains reducing K and n

• for rock, K and n are related to structure– sedimentary rock, both n and K are less than

that of parent sediments due to mineral deposition in voids

Relations between K and n...

– metamorphic and igneous rock have very low primary porosity, but K and secondary porosity are related to fracture spacing

• porosity affects velocity of flow:– the lower the porosity, the greater the flow

velocity:

v = q/n– flow velocity = specific discharge/porosity

Heterogeneity

• Geologic formations are generally not homogeneous– in BC, most forested terrain is characterized by

relatively thin (1-2 metre) coarse grained soils over basal till or igneous/metamorphic bedrock

– the contact between soil and basal layer involves a sharp discontinuity in K such that the till or bedrock interface forms an impermeable boundary

Flow in layered heterogeneity

Flow will tend to go along the zone of higher K, and acrossthe zone of lower K. Thus preferential flow occurs in high Kzones.

increasing head

Flow lines are perpendicular to equipotentials

Flow in layered heterogeneity...

– the grain size distribution of soil is generally not uniform, so there are variations in hydraulic conductivity

• zones of relatively high K in soil become preferred flow paths - they carry more flow than zones of lower K

• the distribution of K zones can be random, or K can decrease with depth in soil due to increasing clay content - the latter situation will result in more rapid groundwater flow as the water table rises

Effects of slope steepness on flow• Slope gradients affect both direction and

rate of groundwater flow– flow perpendicular to equipotentials– approx. lateral for steep slopes

Groundwater flow on a slope

Groundwater Recharge

Groundwater flowfollows hydraulicgradient: total headdecreases with depth,thus there is a downwardcomponent to the groundwater flow. Thisis groundwater recharge,and in the abcence of water input, the watertable will fall.

Groundwater discharge

At riparian sites, ground-water discharge often occurs.In this case, head increases with depth, resulting in anupward component to ground-water flow. In the exampleshown, under high flow condi-tions the water table risesto the surface near the stream,groundwater discharges out of the soil and enters the stream by overland flow.

Later that year...

...at the same site, under lowflow conditions, the water tableand the stream stage havedropped. Groundwater is stilldischarging to the streamchannel, but not at the soilsurface. Total head is nowindependent of depth within thesoil. There is no longer an upward component to ground-water flow. Discharge to the channel is essentially horizontal.

Occurrence of groundwater

• Saturated vs. unsaturated– Define as water content of soil– Saturated: all the void spaces are filled with

water: s = n

– Unsaturated: void spaces are only partially filled with water: < n

– K is reduced because cross sectional area for flow is less than saturated cross section: K is now a function of moisture content

Saturated vs. unsaturated flow

Saturated: void spacesfilled with water: = n, > 0

Unsaturated: voids partiallyoccupied by air: < n, < 0.K is reduced; K = K() or K()

below water table above water table

Soil drainage and infiltration

• If pressure head increases with depth, then why does soil drain?– recall, there are two components of head:

pressure head and gravity head– soil drains under gravity when elevation

gradient (dz/dl) > pressure gradient (d/dl)– drainage will continue until equilibrium is

reached– equilibrium may never occur

Infiltration• Initially, moisture content at the surface is

low, hence K is low• When water is supplied to a dry soil,

initially the water is absorbed, raising the moisture content and hence increasing

• this creates a head gradient that drives water down towards the water table.

• water moves down under large head gradient at the wetting front , overcoming the fact that K is low for dry soil

Infiltration rates

• Over time, infiltration rate will tend towards the saturated hydraulic conductivity of the soil

• Initial infiltration rates for dry soil can be up to 5 times Ks for very dry soil

• typical infiltration rates for forest soil are in the range of 50 to 300 mm/hr depending on the soil and its moisture content

Macropore flow

• It is generally accepted among forest hydrologists (if not hydrogeologists) that macropore flow is a significant component of runoff from forested catchments– Darcy’s Law does not describe macropore flow– difficulty is in defining a representative

dimension for a macropore– macropores can form a large interconnected

network

Macropore flow (interflow)– they are formed from the rotting out of dead tree

roots, aminal burrows, cracks in soil resulting from blocky structure, etc.

• how to define a representative dimension for such a feature?

– subsurface flow through macropore networks is much faster than soil matrix flow

– often called interflow

– we still do not know how to describe it mathematically

Forest harvesting and groundwater• Forest harvesting alters groundwater

levels (thus, groundwater flow) by altering the water balance– increase in water available for infiltration due

to decreased interception, increased snowmelt– decrease in extraction of water from the soil

due to decreased evapotranspiration

• Related activities can also alter soil structure

Effects of ground skidding and roads

– ground based yarding can result in soil compaction, thereby reducing infiltration capacities

• exessive access roads

• ground skidding

– these effects would tend to result in increased runoff, hence reduced infiltration

• Road cuts on steep terrain can interrupt subsurface flows

Effect of road cut on groundwater flow

Before: ground-water flow ontreed slope

After: potentially increased flow, inter-ception by road cut,conversion toditch flow

– intercepted flows can either be routed to the stream channel thereby altering streamflow hydrograph, or can be routed back onto slope below the road by way of culverts or cross ditches

– in many cases, poorly placed culverts and inadequate culvert density have resulted in concentration of ditch flows onto unstable slopes, resulting in landslides

Improper culvert placementA plan view schematic of a road cut showing water flow pattern

road surface

hillside above road

Too few culverts and poor placement results in flow disruption

cut bank

Landslides and pore pressure– Increased pore pressures at the failure plane of

potential instability results in reduced frictional contact between soil grains

– this results in a reduction in the forces that keep the soil on the hillside.

r. hudson - vfr research hydrology groundwater. r. hudson - vfr research groundwater topics......

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