Construction Site Erosion and Sediment Controls

choppedspleenMechanics

Feb 22, 2014 (3 years and 5 months ago)

84 views

Construction Site Erosion and
Sediment Controls

Section 3. Hydrology and Soil Erosion

Reason for Fundamentals


The erosion and sediment control plan
included in the SWPPP is developed
before the site is disturbed based on
the best available information



Construction site erosion and sediment
control requires flexibility during
construction projects;
modifications are
often necessary

Reason for Fundamentals


Inspectors must have hydrology and
soil erosion knowledge to critically
examine erosion and sediment
controls



Basic hydrology and soil erosion
knowledge is also required to identify
erosion prone areas and to know
critical erosion time periods during
the year

Why is Hydrology Important?


Know drainage patterns and receiving
water for the site



Land development disturbs the
natural hydrologic cycle



Rainfall characteristics influence soil
erosion



Runoff controls the rate of erosion
from exposed soils

Land Development

Land cover is
changed from
undisturbed soils
and vegetation to
compacted soils
and impervious
surfaces

Land Development

Land development significantly alters
the hydrologic cycle

Land Development

Runoff volume and flow rate increases and
time of concentration decreases

Rainfall Patterns

2.0
2.4
4.1
4.5
4.5
5.0
2.8
2.9
4.7
3.4
4.4
3.2
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Average Monthly Rainfall (in)
January
February
March
April
May
June
July
August
September
October
November
December
Month
Average Monthly Rainfall
Fayetteville, AR 1971-1999
Rainfall Patterns

2.8
2.9
4.8
4.9
4.9
3.6
3.2
2.9
3.0
5.2
5.6
5.3
0.0
1.0
2.0
3.0
4.0
5.0
6.0
Average Monthly Rainfall (in)
January
February
March
April
May
June
July
August
September
October
November
December
Month
Average Monthly Rainfall
Little Rock, AR 1981-1999
Runoff

Runoff generation is dependent on many
factors including:


1.
Precipitation intensity, volume,
and spatial and temporal
distribution

2.
Watershed area and topography

3.
Ground cover and moisture
conditions

4.
Soil permeability characteristics

Time of Concentration

Most Remote

Point

Hydraulically

Outlet

Overland
Flow

Shallow
Concentrated
Flow

Stream/Pipe
Flow






pipe
shallow
overland
c
t
t
t
t
Water
Shed

Time of Concentration

Hydraulically
most remote
point

Outlet

Outlet

t
c

for top watershed is much
longer than t
c

for the bottom
watershed

Water Shed

Hydraulically
most remote
point

Soil Erosion


Erosion is a natural process
whereby soil particles are displaced
and transported by wind, rain, or
runoff



Erosion includes weathering,
dissolution, abrasion, corrosion,
and transportation, by which
material is removed from the Earth’s
surface

Soil Erosion

Soil erosion occurs in three phases:


1.
Particle detachment

2.
Sediment transport

3.
Sediment deposition


Wind
-
Generated Soil Erosion

Three types of wind
-
generated
erosion:

1.
Saltation

2.
Suspension

3.
Surface creep


Although wind
-
generated erosion is a
major concern, we are focusing this
course on water
-
generated erosion


Water
-
Generated Soil Erosion

Erosion due to water action occurs in
one of the following forms:



Splash erosion


Sheet
-
flow erosion


Rill erosion


Gully erosion


Stream erosion

Types of Erosion

(Soil Conservation Service)

Splash Erosion

The dislodging of soil particles by raindrop impact is a
primary cause of surface erosion

Raindrop Impact

(Seafriends 2001)

Sheet
-
flow and Rill Erosion

The uniform removal of soil particles by sheet
-
flow runoff

Rills are long, narrow depressions or
incisions caused by increased velocities

Rill Erosion on Slopes

Unprotected slopes will develop rills,

which will eventually form gullies

Gully Erosion

Gullies are larger and deeper depressions caused by the
higher velocities associated with concentrated flows

Gully Erosion

Gullies are very difficult to
stop once they are started

Gully Erosion

Gullies can remove up to 10 times larger volumes of soil per
unit area than sheet flows and rills

Long steep slopes are the primary place for
rills and gullies to develop

Stream Erosion

Erosion of soil by increased stream flows

Soil Erosion

Soil erosion is influenced by five
primary factors:


1.
Rainfall characteristics

2.
Soil erodibility

3.
Flow path length and slope

4.
Land cover

5.
Control measures

Rainfall Characteristics

Characteristics of the region’s
climate and rainfall have a significant
influence over soil erosion:



Rainfall patterns


Rainfall intensity


Droplet size

Soil Erodibility

The tendency of soil particles to
become detached from the soil matrix
is dependent on:



The soil texture, organic matter
content, and structure


Soil permeability


Electrostatic charges

Hierarchy of Soil Erodibility

Soil Type

Erodibility

Low
-
Plasticity Silt

Most Erodible

Silty Sand

Clayey Sand

High
-
Plasticity Silt

Low
-
Plasticity Organic Soil

The above soils are much more erodible than the following:

Low
-
Plasticity Clay

Least Erodible

High
-
Plasticity Clay

Silty Gravel

Well
-
Graded Sand

Poorly Graded Gravel

Well
-
Graded Gravel

Flow Path Length and Slope


The longer the flow path the more
potential for soil erosion



The slope of the flow path has a
significant influence over soil erosion:
higher slopes increases runoff and
erosion potential



High slopes and long flow paths should
be reduced by creating contour
diversions and benches

Land Cover


Soil erosion rates are related to the type
and amount of temporary and permanent
cover



Land covers stabilize the soil matrix,
reduce runoff velocities and volumes,
and reduce the impact of rainfall
droplets



Appropriate land cover should be
established on disturbed areas as soon
as possible after construction is
completed in the area

Control Measures


Control measures are activities
performed on the disturbed land
surface to mitigate the erosive forces
of rainfall and runoff



Control measures can be directed
toward land cover or can influence the
flow path length and steepness

Erosion Control Goals

Soil Erosion Rates:


Natural geologic rate (0.2
tons/ac/yr)


Managed forest (0.5 tons/ac/yr)


Agricultural lands (1.5 to 20
tons/ac/yr)


Construction activities (150 to
200 tons/ac/yr)

Goal
-

reduce erosion on construction
sites to 1.5 tons/ac/yr

Summary


Keep the principles of hydrology and
soil erosion in mind as you inspect
erosion and sediment controls



Verify that the controls are compatible
with site hydrology and soil types

Questions?