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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.

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Hydrology l.jpg

Hydrology

Groundwater

R. Hudson - VFR Research


Groundwater topics l.jpg
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

R. Hudson - VFR Research


Relevant to forest hydrology l.jpg
… 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

  • R. Hudson - VFR Research


    Hydraulic head l.jpg
    Hydraulic head

    • Groundwater flows along an energy gradient

      • there are two possible energy gradients that affect groundwater flow: gravity and fluid pressure

    z = z1

    p1

    p2

    flow under fluid pressure

    gradient where p1 > p2

    z = z2

    gravity drainage

    R. Hudson - VFR Research


    Groundwater head energy for flow l.jpg
    Groundwater head - energy for flow

    z = elevation head

    above reference

    elevation (datum)

    Y = pressure head (m)

    = P/rg

    where

    P = fluid pressure

    r = fluid density

    g = acceleration

    due to gravity

    Groundwater head

    is measured using

    a piezometer.

    Y

    h = z +Y

    z

    datum

    R. Hudson - VFR Research


    Water table well vs piezometer l.jpg
    Water table well vs. piezometer

    R. Hudson - VFR Research


    Darcy s law l.jpg
    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

    R. Hudson - VFR Research


    Hydraulic conductivity k l.jpg
    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)

    • m is the viscosity of the fluid (kg/m.s)

    R. Hudson - VFR Research


    Range of values of k l.jpg
    Range of values of K

    R. Hudson - VFR Research


    Porosity l.jpg
    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

    R. Hudson - VFR Research


    Range of values of porosity l.jpg
    Range of values of porosity

    R. Hudson - VFR Research


    Relations between k and n l.jpg
    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

    R. Hudson - VFR Research


    Relations between k and n13 l.jpg
    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

  • R. Hudson - VFR Research


    Heterogeneity l.jpg
    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

    R. Hudson - VFR Research


    Flow in layered heterogeneity l.jpg
    Flow in layered heterogeneity

    Flow lines are perpendicular to equipotentials

    increasing head

    sand

    clay

    Flow will tend to go along the zone of higher K, and across

    the zone of lower K. Thus preferential flow occurs in high K

    zones.

    R. Hudson - VFR Research


    Flow in layered heterogeneity16 l.jpg
    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

    R. Hudson - VFR Research


    Effects of slope steepness on flow l.jpg
    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

    dh/dl

    dh/dl

    R. Hudson - VFR Research


    Groundwater flow on a slope l.jpg
    Groundwater flow on a slope

    R. Hudson - VFR Research


    Slide19 l.jpg

    Groundwater Recharge

    Groundwater flow

    follows hydraulic

    gradient: total head

    decreases with depth,

    thus there is a downward

    component to the

    groundwater flow. This

    is groundwater recharge,

    and in the abcence of

    water input, the water

    table will fall.


    Slide20 l.jpg

    Groundwater discharge

    At riparian sites, ground-

    water discharge often occurs.

    In this case, head increases

    with depth, resulting in an

    upward component to ground-

    water flow. In the example

    shown, under high flow condi-

    tions the water table rises

    to the surface near the stream,

    groundwater discharges out

    of the soil and enters the

    stream by overland flow.


    Slide21 l.jpg

    Later that year...

    ...at the same site, under low

    flow conditions, the water table

    and the stream stage have

    dropped. Groundwater is still

    discharging to the stream

    channel, but not at the soil

    surface. Total head is now

    independent of depth within the

    soil. There is no longer an

    upward component to ground-

    water flow. Discharge to the

    channel is essentially horizontal.


    Occurrence of groundwater l.jpg
    Occurrence of groundwater

    • Saturated vs. unsaturated

      • Define q as water content of soil

      • Saturated: all the void spaces are filled with water: qs = n

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

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

    R. Hudson - VFR Research


    Saturated vs unsaturated flow l.jpg
    Saturated vs. unsaturated flow

    below water table above water table

    Saturated: void spaces

    filled with water: q = n,

    Y> 0

    Unsaturated: voids partially

    occupied by air: q < n,Y < 0.

    K is reduced; K = K(Y) or K(q)

    R. Hudson - VFR Research


    Soil drainage and infiltration l.jpg
    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 (dY/dl)

      • drainage will continue until equilibrium is reached

      • equilibrium may never occur

    R. Hudson - VFR Research


    Infiltration l.jpg
    Infiltration

    • Initially, moisturecontent 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 Y

    • 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

    R. Hudson - VFR Research


    Infiltration rates l.jpg
    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

    R. Hudson - VFR Research


    Macropore flow l.jpg
    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

    R. Hudson - VFR Research


    Macropore flow interflow l.jpg
    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

    R. Hudson - VFR Research


    Forest harvesting and groundwater l.jpg
    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

    R. Hudson - VFR Research


    Effects of ground skidding and roads l.jpg
    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

  • R. Hudson - VFR Research


    Effect of road cut on groundwater flow l.jpg
    Effect of road cut on groundwater flow

    Before: ground-

    water flow on

    treed slope

    After: potentially

    increased flow, inter-

    ception by road cut,

    conversion to

    ditch flow

    R. Hudson - VFR Research


    Slide32 l.jpg

    • 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

    R. Hudson - VFR Research


    Improper culvert placement l.jpg
    Improper culvert placement thereby altering streamflow hydrograph, or can be routed back onto slope below the road by way of culverts or cross ditches

    A plan view schematic of a road cut showing water flow pattern

    hillside above road

    cut bank

    road surface

    Too few culverts and poor placement results in flow disruption


    Landslides and pore pressure l.jpg
    Landslides and pore pressure thereby altering streamflow hydrograph, or can be routed back onto slope below the road by way of culverts or cross ditches

    • 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.