# Soil & Water Fundamentals

Mechanics

Feb 22, 2014 (4 years and 4 months ago)

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2012 PE Review:

Soil & Water Fundamentals

Michael C.
Hirschi
, PhD, PE, D.WRE

Senior Engineer

Waterborne Environmental, Inc.

hirschim@waterborne
-
env.com

also Professor Emeritus

University of Illinois

Acknowledgements:

Rod Huffman,
past PE
Review coordinator

Daniel Yoder (2006
presenter of parts)

Rabi
Mohtar

&
Majdi

Abu
Najm

(2010
presenters of parts)

Topics

Core principles

Fluids

Soil
-
Water Basics

Soil Erosion Principles

Water Quality Principles

Sources

Environmental Soil Physics; Hillel; 1998
Hi

Soil and Water Conservation Engineering

4
th

ed. Schwab,
Fangmeier
, Elliott,
Frevert
:
S4

5
th

ed.
Fangmeier
, Elliott, Workman, Huffman, Schwab:
S5

Design Hydrology & Sedimentology for Small
Catchments;
Haan
, Barfield, Hayes:
H

Fluids Review
-

Assumptions

Water in its liquid state

Low dissolved contaminants

Low suspended contaminants

Such that
ρ

(density) = 1.0 kg/L

Incompressible

Mass is conserved

Basic nomenclature

Density is denoted as
ρ
, with units of
mass/volume (kg/L, g/mL, slugs/ft
3
, etc.)

Flow rate is usually denoted as Q, with units
of volume/time (
cfs

or ft
3
/sec,
cms

or m
3
/sec,
gallons/min, L/min, etc.)

Velocity is denoted as V, with units of
length/time (
ft
/sec, m/sec, etc.)

Area of flow is denoted as A, with units of
length
2

(ft
2
, m
2
, etc.)

Flow rate

The basic relationship between flow
rate and velocity is then:

Q = V * A

w
hich is a statement of conservation
of mass. In addition, energy is neither
created or destroyed, so an energy
balance relationship also holds…

The energy balance equation is Bernoulli’s
equation:

γ

h
= elevation of point 1

or 2 (m
or
ft
)

P
1

= pressure (Pa or psi
) at point 1

= specific weight of
fluid

v = velocity of
fluid (at 1 or 2, according to subscript)

W is energy added by a device (such as a pump)

F is energy used to overcome friction

2g
v
γ
P
h
F
W
2g
v
γ
P
h
2
2
2
2
2
1
1
1

Considering energy:

2g
v
γ
P
h
F
W
2g
v
γ
P
h
2
2
2
2
2
1
1
1

Potential Energy

Energy stored

as Pressure

Kinetic Energy

Energy input

from device

Energy output to heat
(friction)

Example use of energy balance…

Need to size pump for irrigating gardens at lot 90 feet
above Smith Mountain Lake in Virginia…

Location 1 is the lake, location 2 is a tank near the
garden.

So, h
1

= 0; h
2

= 90ft; v
1

= 0, v
2

depends upon flow
rate; P
1

= 0; P
2

is also 0 because the pipe exits to
the atmosphere above the tank; F depends upon
size of pipe and fittings (that’s another webinar),
assume F = 20 feet.

The owner wants his 500 gallon tank to fill in 2 hours
through the 0.75 inch pipe he installed.

How much energy must the pump add?

W = h
2
-
h
1

+ 1/
γ
*(P
2
-
P
1
)+ 1/2g*(v
2
2
-
v
1
2
)+F

W=90
-
0 + 1/
γ
*(0
-
0)+1/2/32.2*(3.1)
2

+ 20

So W = 90+0.15+20 = 110 feet of head

P
2

= P
1
, Q=500gal/120min/60s/min/7.48gal/ft
3
=

0.0093cfs; A= (0.75)2*3.14/4/144 = 0.003 ft
2
,
so v
2

= 3.1fps

Pump specification

So, when the owner goes shopping, he needs
to look for a pump that will deliver at least 4.2
gpm

(500 gallons in 120 minutes) against 110
feet of water head. Translating head to
pressure, there are 2.31 feet of head per psi,
so the pump needs to generate 48psi at the
pump housing while delivering 4.2
gpm
. So, a
pump that delivers 5gpm at 50psi would be
fine.

Questions on fluids basics?

Soil
-
water basics

Soil classes and particle size distributions

Soil water

Content

Potential

Flow

Basics

Soil Make Up

Mineral

Water

Air

Organic Matter

Mineral Component
-

Particles

Sand

Silt

Clay

Aggregates

Silt & Sand sizes

Less dense than primary particles

Particle Size Classifications

USDA Texture Triangle

Example

After soil sample dispersal to ensure only
primary particles are measured, a sample is
determined to be 20% clay, 30% silt and 50%
sand. What is the USDA soil texture?

A: Sandy Clay Loam

B: Sandy Loam

C: Loam

D: Clay Loam

Solution

20% Clay

30% Silt

Infiltration & soil
-
water

Infiltration is the passage of water through the soil
-
air interface into pores within the soil matrix

Movement once infiltrated can be capillary flow or
macropore flow. The latter is a direct connection
from the soil surface to lower portions of the soil
profile because of root holes, worm burrows, or
other continuous opening

Infiltrated water can reappear as surface runoff via
“interflow” and subsurface drainage

Soil, water, air

The inter
-
particle space (voids) is filled with
either water or air. The amount of voids
depends upon the soil texture and the
condition (ie. tilled, compacted, etc.).

Water (moisture) content

Special terms reflect the fraction of voids filled with
water (all vary by texture and condition):

Saturation: All voids are filled with water

Field Saturation: Natural “saturated” moisture content
which is lower than full saturation due to air that is
trapped.

Field capacity: Water that can leave pores by gravity has
done so (0.1 to 0.33 bars)

Wilting point: Water that is extractable by plant roots is
gone (15 bars)

Hygroscopic point: Water that can be removed by all usual
means is gone (but some remains, 30 bars)

Saturated (all pores filled)

Field Capacity

(Some air, some water)

Wilting point

(water too tightly held for plant use)

Plant Available Water

Soil Water Holding Capacity

(inches
-
water/foot
-
soil)

Soil Texture

Range

Average

Sand

0.4
-

1.0

0.8

Sandy Loam

1.0
-

1.5

1.3

Loam

1.0
-

2.0

1.6

Silt Loam

1.3

2.6

2.0

Clay Loam

1.3

2.6

2.0

Clay

1.4

2.4

1.8

Water States by Soil Texture

0
10
20
30
40
50
60
Sand
Sandy
Loam
Loam
Silt Loam
Clay
Loam
Clay
Volumetric Water content
Gravitational

Plant Available

Unavailable

Commentary

In a later webinar, when we discuss drainage, it is the
gravitational water that is of interest,
eg
. saturation down to
field capacity. The volume of this water, the hydraulic
characteristics of the soil in question, and the wet
-
condition
-
tolerance and value of the crop being grown dictate the
drainage system design and its feasibility.

When we consider irrigation, plant available water (AW) is
that held between field capacity and wilting point. It is this
water that we manage via irrigation to supply water to plants.
The volume of AW the soil can hold within the crop root
-
zone,
the crop value and water use, and the crop tolerance of dry
conditions dictate irrigation design and feasibility.

Moisture “release” curve

-
10cm

-
100cm

-
1000cm

-
10000cm

Any questions on general

soil and water basics?

Soil Erosion Principles

Soil erosion is a multi
-
step process:

Soil particle/aggregate detachment

Soil particle/aggregate transport

Soil particle/aggregate deposition

There must be detachment
and

transport for
erosion to occur

Deposition (sedimentation)
will

occur
somewhere downstream

A little soils refresher…

Soil primary particles:

Sand, 0.05mm to 2mm, 2.65 g/cc density

Silt, 0.002mm to 0.05mm, 2.65 g/cc

Clay, <0.002mm, 2.6 g/cc

Soil aggregates, chemically/electrically bonded sets
of primary particles:

Large, in the sand range, 1.6 g/cc

Small, in the large silt range, 1.8 g/cc

These aggregate sizes are approximately those used
in the CREAMS model (USDA
-
ARS)

Detachment

There are many sources of force and energy
required to detach soil particles & aggregates:

Raindrop impact

Shallow surface flow shear

Concentrated flow shear

Many more, at larger scales

Transportation

Many of the same processes contribute force
and energy for soil particle & aggregate
transport:

Raindrop impact

Shallow surface flow

Concentrated surface flow

Channelized flow

Others

Balancing act

Foster & Meyer (1972) proposed a balance
between detachment and transport for
flowing water:

1 = (transport load/transport capacity) +

Essentially, if the flow is using all its transport
capacity transporting sediment, there’s
nothing left to detach more. Likewise, if the
flow is detaching new sediment at
detachment capacity, there’s no capacity to
transport any sediment. Natural systems
balance out…

Example

In the 80’s and 90’s there was a successful push to
conservation tillage as a method to reduce sediment
in lakes and streams

In many situations, no improvement was seen, but
streambank erosion became more of a problem than
it was in the past

I contend that now that the streams are receiving
cleaner water (because of less upland erosion), but
at similar rates, from farm fields, the stream uses less
of its erosive energy to transport load it receives
from runoff water, so it has capacity to undercut
banks and scour the streambed

Multi
-
stage erosion

Sediment transport

Settling (H.204
-
209)

Stokes’ Law

V
s

= settling velocity

d = particle diameter

g = accel due to gravity

SG = particle specific gravity

ν

= kinematic viscosity

Simplified Stokes’ Law

SG = 2.65

Quiescent water at 68
o
F

d in mm, V
s

in fps

1
18
1
2
SG
g
d
V
s

2
81
.
2
d
V
s

Example: Settling Velocities

Given:

ISSS soil particle size classification

Find:

Settling velocities of largest sand, silt, and clay
particles

ISSS classification

Largest particles size

Clay = 0.002mm

Silt = 0.2mm

Sand = 2mm

V
s,clay

= 1.12*10
-
4

fps = 0.04 ft/hr = 0.97 ft/day

V
s,silt

= 0.11 fps = 405 ft/hr = 1.83 mi/day

V
s,sand

= 11.24 fps = 7.66 mph = 184 mi/day

Another example…

Given:

Stokes’ Law settling

Find: particles larger than what size can be
assumed to settle 1
ft

in one hour?

V
s

= [(1
ft
)/(1
hr
)](1
hr
/3600s) = 2.778*10
-
4

fps

d = (
V
s
/2.81)
1/2

= 0.00994mm (in the silt range)

Application of process knowledge to
control

Limit individual parts to limit whole

Limit detachment

Limit transport

Enhance deposition strategically

Where damage is minimal

Where cleanup is possible

Control of Soil Erosion by Water

Detachment limiting strategies

Reduce raindrop impact

Reduce runoff

Reduce detachment capacity of runoff

Increase soil resistance to erosive forces

Transport limiting strategies

Reduce runoff volume

Reduce runoff transport capacity

Example

No
-
Till

Detachment

Raindrop impact detachment is very low due to high
surface cover percentage

Flow shear detachment is low due to low velocities caused
by tortuous flow path

Soil is resistant to erosion because of low disturbance

Transport

Raindrop transport is limited by surface residue

Flow transport is limited by increased infiltration, lessening
runoff

Flow transport is further limited by small dams created by
surface residue

Example

Mulch on newly seeded
area

Detachment

Raindrop impact detachment is very low due to high
surface cover percentage

Flow shear detachment is low due to low velocities caused
by tortuous flow path

Transport

Raindrop transport is limited by surface residue

Flow transport is limited by increased infiltration, lessening
runoff

Flow transport is further limited by small dams created by
surface residue

Comparison of no
-
till vs. mulch

Detachment

Likely higher with mulch for same surface cover fraction
because of higher soil disturbance for seedbed preparation

Likely higher for no
-
till following dry years because amount
of residue cover is dictated by prior year crop growth

Transport

Likely higher for mulch, unless “cut” in because no
-
till
residue is effectively “cut” in during planting, at least for a
small area, hopefully across slope

Likely higher for mulch situation because seedbed prep
likely reduced average aggregate diameter

Control of Sediment in Runoff

Reduce transport capacity of flow

Enhance deposition of sediment

Reduce transport capacity

Reduce velocity

Barriers

Must let water pass, though slowly

Must be flow
-
stable, even after use

Must be where maintenance is possible

Reduce slope steepness

Channel must be of adequate capacity

Increase infiltration

Enhance deposition of sediment

Use flocculant to increase sedimentation

Usually in sedimentation ponds when other
methods are not adequate

Expensive…

Questions on erosion principles?

Water Quality Principles

Concentration

Concentration

Concentration can be defined different ways:

Mass of contaminant per mass of material

Mass of contaminant per volume of material

Units might be
ppt
, ppm, ppb, mg/L, µg/L,
ng
/L

Concentration in water

Consider the following concentrations of a
herbicide in well water:

1000

1

0.001

Which would you prefer to find in your well?

Yes, it is a trick question…

1000
ug
/L (ppb)

1 mg/L (ppm)

0.001 g/L (
pp
(
th
))

1,000,000
ng
/L (
pp
(
tr
))

Water only!

Please remember that ppm = mg/L ONLY for
liquids with a density of 1.0 mg/L (water)

As we have seen, concentration is the mass of
a contaminant in a given volume of water

Load is the rate of contaminant mass
movement, equal to the concentration times
the flow rate, C (mass/volume) * Q
(volume/time)

Regulations

Maximum Contaminant Level (MCL) is a
concentration above which public drinking
water systems must

Total Maximum Daily Load (TMDL) is a
contaminant load in kg/day or some other
unit.

Questions?

Thank you!