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
– 4th ed. Schwab, Fangmeier, Elliott, Frevert: S4
– 5th 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/ft3, etc.)
• Flow rate is usually denoted as Q, with units
of volume/time (cfs or ft3/sec, cms or m3/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
length2 (ft2, m2, etc.)
Flow rate
The basic relationship between flow
rate and velocity is then:
Q=V*A
which 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:
P1 v12
P2
v 22
h1 

 W  F  h2 

γ
2g
γ
2g
h = elevation of point 1 or 2 (m or ft)
P1 = pressure (Pa or psi) at point 1
γ = 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
Considering energy:
Energy input
from device
Energy output to heat
(friction)
2
1
2
2
P1 v
P2
v
h1 

 W  F  h2 

γ
2g
γ
2g
Energy stored
as Pressure
Potential Energy
Kinetic Energy
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, h1 = 0; h2 = 90ft; v1 = 0, v2 depends upon flow
rate; P1 = 0; P2 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 = h2-h1 + 1/γ*(P2-P1)+ 1/2g*(v22-v12)+F
P2 = P1, Q=500gal/120min/60s/min/7.48gal/ft3
= 0.0093cfs; A= (0.75)2*3.14/4/144 = 0.003 ft2,
so v2 = 3.1fps
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
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
Answer: C, Loam
30% Silt
20% Clay
Infiltration & soil-water
• Infiltration is the passage of water through the soilair 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
Sand
Sandy Loam
Loam
Silt Loam
Clay Loam
Clay
Range
0.4 - 1.0
1.0 - 1.5
1.0 - 2.0
1.3 – 2.6
1.3 – 2.6
1.4 – 2.4
Average
0.8
1.3
1.6
2.0
2.0
1.8
Water States by Soil Texture
Volumetric Water content
60
50
40
Gravitational
30
Plant Available
20
Unavailable
10
0
Sand
Sandy
Loam
Loam
Silt Loam
Clay
Loam
Clay
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-conditiontolerance 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
-10000cm
-1000cm
-100cm
-10cm
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) +
(detachment load/detachment 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
•
•
•
•
•

1 d 2g
SG  1
Vs  
18  

Vs = 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 68oF
• d in mm, Vs in fps
Vs  2.81 d
2
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
– Vs,clay = 1.12*10-4 fps = 0.04 ft/hr = 0.97 ft/day
– Vs,silt = 0.11 fps = 405 ft/hr = 1.83 mi/day
– Vs,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?
• Vs = [(1 ft)/(1 hr)](1 hr/3600s) = 2.778*10-4 fps
• d = (Vs/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
• Load
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…
Adding units:
1000 ug/L (ppb)
1 mg/L (ppm)
0.001 g/L (pp(th))
how about another:
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)
Load versus concentration
• 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!