0901 Hooghoudt Hydraulic Conductivity-Def

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All it takes for environmental research

The Hooghoudt Ernsthydraulic conductivity method


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When

Where

How

Hooghoudt hydraulic conductivity


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Method originally developed by Prof. Hooghoudtto measure hydraulic conductivity under the

groundwater level Expressed as K in meters / day

Can be used in formulas to determine drain spacings and

drain depths Mr. Ernst, Mr. Van Beers and Mr. Van Hoorn did a

lot of complementary research



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Terminology

K value: Hydraulic conductivity factor K stands for the

thickness of a disc of water passing through soil during acertain time unit.

Isotrope soil: Has same K value in all directions

Homogeneous soil without layers

Anisotrope soil: K values differ per direction of flow.

Hooghoudt method imitates somehow flow patternaround a drain and evens out effects of various K

values.



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What does the result mean ?

K=< 5 cm/day bad permeability

K= 5-40 cm/day average permeability

K= 40-100 cm/day good permeability

K=>100 cm/day very high permeability



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Hooghoudt method: Basic method to determine K of one more or less

homogeneous layer under the groundwater level With additional measurement K of a second layer can be

determined

Method can work with or without an impermeable layer

just below the zone of interest Meant to be used until a depth of 2 m

Larger depths are possible with additional extensionrods.

Preferably use a Diver to log the water level ifmeasuring at larger depths



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Needed: Soil profile !

Make soil profile descriptions to (minimum) 2 meter!

Is there ONE or are there TWO layers that havedistinguishing permeabilities?

- If so what are the depths?

Is there an impermeable layer at less than 50 cm fromthe lowest permeable layer?

- If so; at what depth does it start

Start ofimpermeable layer



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Needed: Soil profile!

Is the profile comparable over the hole site?

-If variable Increase number of measuringlocations



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Estimate the M50 value of every layer with a sandruler and magnifier glass

K in m/day for sandy soils with less than 4% particles 0-2microns:

M50 (mu) K (m/etm)

420 >10,00



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Soil profile!

With clay soils K value depending on:

Layering

Silt and clay fraction

K-value of permeable clay soils 0,01-0,50 m/etm

With peat soils depending on: Type and structure (compressed peat fibres are

horizontally permeable but quite impermeable vertically)



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Application of the method



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Borehole (with auger 8 cm!!) is made down tominimum of 20 cm below the ACTUAL

groundwater level Preferably make bore hole a few days before

measurements.

If necessary (fluid sand) stabilize borehole withfilter screen



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Clay soils: Make bore hole a few days beforemeasurements

Allow sufficient time for water level stabilize Smearing of wall will be reduced

Determine depth of borehole

Use mounted auger as depth sensor

Mount tape holder + float



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Determine value of z in cm z = reading of tape in cm with

float floating on actualgroundwater level

Calculate S S = distance in cm between

bottom of borehole and depth ofearlier discovered impermeablelayer



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Stopwatch ready !

Remove 20-40 cm water

from borehole; generallywith a bailer Do not deepen the borehole

while removing water

Quickly lower float, startstopwatch and read floatdepth



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Note readings + time First reading on tape will be

Z + Yo at t=0 Time interval generally 5-30seconds

Try to note the time with each

1 or 2 cm of rising



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Rising speed diminishes (non -linear) when original water

level is reached Therefore only use the first 25 %

of the possible rising stretch.

Example: Removed 40 cm.Stop measuring the risingwhen water has risen 25%x40cm = 10 cm.

The last 30 cm of possible riseare not used for themeasurements.



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Calculation based on fielddata Two methods

1. Nomograms (2)

2. Equations

Two nomograms

1. For a borehole straight ONan impermeable layer

2. For a borehole withS = >1/2H

- For in between situationsthe results obtained withboth nomograms areaveraged.



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You need to know:H = depth of borehole in cm below water

levelS = distance of a (possible) impermeable

layer in cms below bottom of borehole

Y = Rising of water level in borehole

during the measuring period (= Y0-Yn)Y = (Yn + Yo) = average water level during

the measuring period

(Y0 is first measurement; Yn is last of theseries of measurements)

(Y is used in the nomogram).

t = passed time over the measuringperiod



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Measurements S, Z and H once per borehole.

Except in cases where rapid changes can be expected

(near high and low tide) Determining the rising speed should be repeated

3-5 times

Leave first (or other) results out if they are strange Average the Y, Y en Vs values of the successful

measurements

Repeat procedure on other locations. Use one of following tables to input values at one

location



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Measuringmethod (a)

Time needed for each 2 cm ofrise. YcmYcm Zcm Hcm Scm Above field surface(cm)

1 2 3 4 5 6


h d h d l d


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Measuringmethod (b)Number of cms rise per seconds. Ycm Ycm Zcm Hcm Scm Above soil surface(cm)

1 2 3 4 5 6


H h d h d li d i i


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Rising speed is influenced by: Surface of the wet part of the borehole (wall +

bottom) (H)

Hydraulic conductivity of the wet section of theborehole

The created average pressure difference (Y) during the

measuring period Because the deeper the water level is lowered the morerapid the rising will initially be!

H and Y are used in the nomograms to determine C

Input C in in Vs = K / C (or K= Vs x C) The resulting K is in meters / day


H h d h d li d i i


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Start withdetermining H

on the X-axis.Go upward andstop at thecalculated value

Y. From therego left to readC.

If H = 100 cmand Y = 50 cmthenC = 4,8


H h dt h d li d ti it


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Then determine Vs=Y/t. Which is the rise (Y0-Yn) divided by the time stretcht.

Imagine (at H=100 cm and Y=50 cm) that Y0-Yn = 20cm and that the measuring time was 480 seconds Vs=Y/t is dan 0,042 cm/sec

Input C in K = C. Vs

K = 4,8 x 0,042 = 0,20

K is in meters / day so K= 0,20 m/day (averagepermeability) Nomogram two is only used for situations with a borehole

directly on the impermeable layer.




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Another exampleand now step bystep:

D = 185 cm

Z measured withtape + float: 65 cm

H = 185 65 5 cm= 115 cm The 5 cm is height

of float!!

H and Z are nowknown!

D

Z

Reference level (field surface)

Undisturbed water level

H

Float height




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Tape reading at t0 = 175 cm

Tape reading at tn = 150 cm

Then Y0 = 175 Z = 110 cm And Yn = 150 Z = 85 cm

D

Z



H

Float height

t0

tn




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Y = Y0 - Yn then becomes 110 85 = 25 cm

Y = (Y0 Yn)/2 = (110+85)/2= 97,5 cm

Say measuring time between t0 en tn was 480 seconds = t

Rising-speed = s = Y / t

Vs = 25 / 480 = 0,052 cm/sec

D

Z



H

Float height

t0

tn




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Now with H and Y available C can be determinedusing one of the nomograms.

For this example C = 2.8

Then input C in equation K = C. Vs K = 2.8 x 0,052 = 0,145 meter / day

Instead of using nomograms you can use equationsto calculate C (see manual) Less accurate than the nomograms

Easier when using a calculator or computer

Also when diameter of borehole differs from 8 cm




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Extra possibilities Determination of K

factors of two layers Measure K1 inonly layer I. Thenmeasure K1+2 inlayers I+II.

The effect onK1+2 of K1 iscalculatedresulting in K2

Further info inmanual




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Extra possibilities

Inversed auger hole method

Water is added into theborehole.

There should be no influence ofnearby groundwater orimpermeable layer

- Groundwater level orimpermeable layer at leastat 20 cm (better 50 cm)below borehole

Speed of lowering is determined

HH

t

hh0

ht




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Inversed auger hole method

Take care because

Initially no saturation around the borehole Non linear lowering water pressure complicates matters

A measurement with a so called constant head wouldbe better because

You can check if saturation level is OK (stable water flowinto soil)

No change in length of soil column that is wetted duringmeasurement (improved accuracy; better defined layer for

which resulting K counts).




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More info in the manual and the literaturementioned there


Documents

0901 Hooghoudt Hydraulic Conductivity-Def