Document ID: EPA-HQ-OW-2002-0030-0028
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2002-06-24T04:00Z

Appendix
A
Evaluating
Pollutant
Loadings
from
Construction
Activities
that
Potentially
Impact
the
Environment
Environmental
Assessment
of
Construction
and
Development
Proposed
Effluent
Guidelines
June
2002
A­
1
Appendix
A
Evaluating
Pollutant
Loadings
from
Construction
Activities
that
Potentially
Impact
the
Environment
This
appendix
details
aspects
of
the
methodologies
described
in
Section
3
to
pollutant
discharges
that
result
from
construction
activities
under
two
options.
Specifically,
it
expands
on
the
discussion
presented
in
Section
3,
providing
additional
information
on
the
assumptions
used
by
EPA
in
its
assessment.

Estimates
of
Affected
Area
The
Phase
II
NPDES
storm
water
rule
economic
analysis
(USEPA,
1999)
presented
information
on
the
size
and
nature
of
construction
activities
under
the
Phase
I
and
II
storm
water
programs.
In
addition,
the
Phase
II
economic
analysis
(EA)
detailed
an
extensive
analysis
of
pollutant
loadings
for
a
range
of
site
sizes,
soil
types,
land
slopes,
and
locations.
EPA's
current
evaluation
uses
the
results
presented
in
the
Phase
II
report
to
update
its
overall
estimate
of
national
construction
site
loadings.
EPA
expects
that
new
regulation
of
the
construction
and
development
(C&
D)
category
will
augment
the
existing
state
and
Phase
I
NPDES
storm
water
programs.
In
addition,
new
regulations
will
shape
future
development
of
construction
programs
expected
under
the
Phase
II
NPDES
storm
water
program.

EPA
identified
the
array
of
potentially
affected
construction
sites
in
the
nation.
EPA's
assessment
of
construction
site
loadings
is
based
on
regulation
of
approximately
2.17
million
acres
per
year.
This
regulated
acreage
estimate
was
calculated
based
on
estimated
national
development
rates
from
the
1997
National
Resources
Inventory
(USDA,
2000),
less
the
estimated
acreage
either
occupied
by
sites
less
than
1
acre
in
size
(not
regulated)
or
sites
which
receive
Phase
II
"R"
waivers.
"R"
waivers
are
those
applied
for
and
granted
under
the
construction
general
permit
for
sites
with
very
low
erosivity.
The
Phase
II
EA
estimated
the
total
acreage
granted
"R"
waivers
to
be
approximately
33
thousand
acres
(approximately
1.8
percent
of
the
total
constructed
acreage).
Based
on
its
assessment
of
probable
construction
site
size
distribution,
EPA
estimates
that
another
1.7
percent
of
the
annual
constructed
acreage
will
be
on
sites
less
than
1
acre.
In
addition,
under
Option
1,
EPA
is
considering
removing
sites
smaller
than
5
acres.
EPA
estimates
that
approximately
18
percent
of
construction
occurs
on
sites
less
than
5
acres
in
area.

EPA's
Analysis
of
State
Programs
Table
A­
1
presents
the
results
of
EPA's
analysis
of
state
construction
programs.
EPA
focused
on
the
states
with
the
largest
annual
construction
footprint
to
estimate
the
level
of
current
control
(i.
e.,
not
all
state
regulations
were
reviewed).
As
a
result,
the
absence
of
a
"Yes"
value
in
Table
A­
1
may
indicate
that
a
construction
program
was
not
evaluated
by
EPA.
Overall,
the
results
in
Table
A­
1
were
converted
into
a
ecoregion
"score"
or
the
percent
of
developed
acreage
that
Environmental
Assessment
of
Construction
and
Development
Proposed
Effluent
Guidelines
June
2002
A­
2
would
gain
greater
management
under
EPA's
options.
Table
A­
2
indicates
the
resulting
percentage
of
construction
acreage
affected
by
the
potential
effluent
guidelines
in
each
ecoregion.
As
expected,
new
BMPs
required
under
the
options
(e.
g.,
certification
of
sediment
basins)
were
not
found
in
existing
state
regulations,
and
overall,
existing
state
requirements
require
optionlevel
BMPs
for
approximately
30­
35
percent
of
the
acreage
developed
annually.
Environmental
Assessment
of
Construction
and
Development
Proposed
Effluent
Guidelines
June
2002
A­
3
Table
A­
1.
Assessment
of
State
Construction
Control
Programs
State/
Territory
Minimum
of
3600
Cubic
Feet
per
Acre
Storage
Requirement
for
Larger
Sites
14­
Day
or
More
Inspection
Frequency
14­
Day
Cover
Required
States
with
Less
than
20
Inches
of
Precipitation
Per
Year
Alabama
Alaska
Yes
Yes
Yes
Arizona
Yes
Yes
Yes
Yes
Arkansas
California
Yes
Yes
Yes
Colorado
Yes
Connecticut
Yes
Yes
Yes
Delaware
Yes
Yes
Yes
District
of
Columbia
Florida
Georgia
Hawaii
Idaho
Yes
Illinois
Yes
Indiana
Iowa
Yes
Yes
Yes
Kansas
Kentucky
Louisiana
Maine
Maryland
Massachusetts
Yes
Yes
Yes
Michigan
Minnesota
Environmental
Assessment
of
Construction
and
Development
Proposed
Effluent
Guidelines
State/
Territory
Minimum
of
3600
Cubic
Feet
per
Acre
Storage
Requirement
for
Larger
Sites
14­
Day
or
More
Inspection
Frequency
14­
Day
Cover
Required
States
with
Less
than
20
Inches
of
Precipitation
Per
Year
June
2002
A­
4
Mississippi
Missouri
Montana
Yes
Yes
Nebraska
Nevada
Yes
New
Hampshire
Yes
Yes
Yes
New
Jersey
New
Mexico
Yes
Yes
Yes
Yes
New
York
North
Carolina
North
Dakota
Yes
Ohio
Yes
Yes
Oklahoma
Yes
Oregon
Pennsylvania
Yes
Yes
Yes
Rhode
Island
South
Carolina
Yes
Yes
Yes
South
Dakota
Yes
Yes
Yes
Yes
Tennessee
Yes
Yes
Yes
Texas
Yes
Yes
Yes
Utah
Yes
Yes
Yes
Yes
Vermont
Virginia
Yes
Yes
Yes
Washington
West
Virginia
Yes
Yes
Wisconsin
Environmental
Assessment
of
Construction
and
Development
Proposed
Effluent
Guidelines
State/
Territory
Minimum
of
3600
Cubic
Feet
per
Acre
Storage
Requirement
for
Larger
Sites
14­
Day
or
More
Inspection
Frequency
14­
Day
Cover
Required
States
with
Less
than
20
Inches
of
Precipitation
Per
Year
June
2002
A­
5
Wyoming
Yes
Yes
Environmental
Assessment
of
Construction
and
Development
Proposed
Effluent
Guidelines
June
2002
A­
6
Table
A­
2.
Percentage
of
Acreage
Developed
Without
Option
Equivalent
Requirements
Ecoregion
3600
Cubic
Feet
per
Acre
Storage
in
Sedimentation
Basins
for
Larger
Sites
(Criterion
1)
Certification
of
Sediment
Basins
(Criterion
2)
14­
Day
or
more
frequent
inspection
(Criterion
3)
14­
Day
Cover
For
Wet­
States,
or
none
required
for
dry
states
(Criterion
4)
Overall
Weighted
Percentage
of
Acres
Without
Coverage
ER
1
28.96%
0.00%
28.25%
30.72%
24.7%

ER
2
39.16%
0.00%
57.61%
57.61%
47.1%

ER
3
0.00%
0.00%
10.66%
10.66%
8.0%

ER
4
77.06%
0.00%
77.06%
77.06%
65.5%

ER
5
65.74%
0.00%
65.74%
65.74%
55.9%

ER
6
100.00%
0.00%
100.00%
100.00%
85.0%

ER
7
100.00%
0.00%
100.00%
100.00%
85.0%

ER
8
64.45%
0.00%
68.16%
64.45%
56.6%

ER
9
50.16%
0.00%
55.30%
42.80%
43.4%

ER
10
74.51%
0.00%
81.79%
81.79%
68.8%

ER
11
71.53%
0.00%
71.70%
71.70%
60.9%

ER
12
51.80%
0.00%
65.17%
65.17%
54.1%

ER
13
89.38%
0.00%
32.32%
89.38%
47.4%

ER
14
67.34%
0.00%
53.83%
71.01%
51.4%

ER
15
62.15%
0.00%
100.00%
100.00%
81.2%

ER
16
5.65%
0.00%
100.00%
100.00%
75.6%

ER
17
100.00%
0.00%
100.00%
100.00%
85.0%

ER
18
100.00%
0.00%
100.00%
100.00%
85.0%

ER
19
100.00%
0.00%
100.00%
100.00%
85.0%

National
Average
Weighted
by
Land
Developed
64%
0%
70%
69%
58.9%
Environmental
Assessment
of
Construction
and
Development
Proposed
Effluent
Guidelines
June
2002
A­
7
Information
in
Table
A­
2
was
converted
into
an
overall
national
"score,"
to
discount
estimated
TSS
loadings
reductions
by
accounting
for
acres
covered
by
equivalent
programs.
To
combine
the
four
analyzed
criteria,
EPA
assumed
that
the
individual
contributions
to
reductions
were
10,
15,
50,
25
percent,
respectively.
For
example,
sedimentation
basins
based
on
3,600
cubic
feet
contribute
10
percent
of
the
estimated
reduction
between
baseline
and
option
loadings.
On
a
national
basis,
EPA
estimated
that
approximately
41
percent
of
land
is
served
by
equivalent
programs,
and
would
not
be
affected
by
Option
1
or
2
requirements.
Appendix
B
Inventorying
of
Streams
Potentially
Impacted
By
Construction
Activities
Environmental
Assessment
of
Construction
and
Development
Proposed
Effluent
Guidelines
June
2002
B­
1
Appendix
B
Inventorying
of
Streams
Potentially
Impacted
By
Construction
Activities
Overview
This
appendix
describes
EPA's
effort
to
inventory
and
assess
environmental
impacts
of
construction
activities.
Specifically,
the
appendix
describes,
in
detail,
the
analytical
steps
performed
to
inventory
the
nation's
stream
system
and
provides
general
background
information
on
the
rationale
used
to
develop
the
inventory
approach.
Delineation
of
impacted
stream
environments
forms
the
basis
for
assessing
the
future
benefits
of
regulatory
controls
on
construction
and
activities.

The
objectives
of
this
appendix
are
as
follows:

°
To
describe
a
method
to
characterize
streams
by
their
hydrologic
function
based
on
regional
differences
°
To
establish
the
appropriate
map
scale
for
inventorying
streams
based
on
their
size
and
geometry
(e.
g.,
length,
slope,
dimensions).

Stream
Characterization
Many
of
the
impacts
on
streams
are
a
function
of
drainage
area
and
hydrologic
regime.
Producing
a
national
summary
of
potentially
impacted
stream
networks
is
challenging
because
the
nature
and
size
of
streams
vary
significantly
throughout
the
country.
For
example,
watersheds
that
produce
a
minimum
base
flow
of
1
cubic
foot
per
second
(cfs)
occupy
1
square
mile
in
the
eastern
United
States
but
require
100
square
miles
in
the
arid
southwest.
To
account
for
this
variation,
EPA
divided
the
country
into
19
large
hydrologic
regions
and
then
further
inventoried
the
streams
in
each
region
separately,
based
on
approximate
stream
size
categories
(i.
e.,
stream
orders).
Representative
watersheds
in
each
of
the
19
large
ecoregions
in
the
contiguous
U.
S.
(see
Figure
B­
1)
were
inventoried
to
determine
the
average
stream
density
for
the
stream
orders
that
are
the
most
likely
impacted
in
each
ecoregion.

EPA
developed
the
boundaries
for
the
19
ecoregions
based
on
a
stream
density
assessment
that
used
EPA's
Reach
File
1
(RF1)
stream
network
and
the
76
ecoregions
developed
by
Omernik
(1987).
Figure
B­
2
shows
the
RF1
densities
in
terms
of
acres
per
stream
mile
for
each
of
the
76
ecoregions.
Combining
the
76
ecoregions
into
the
19
ecoregions
shown
in
Figure
B­
1
helps
simplify
the
analysis
while
still
capturing
a
reasonable
number
of
regions
with
similar
stream
densities
and
accounts
for
gross
changes
in
hydrology,
land
forms,
soil
types,
and
potential
natural
vegetation.
Environmental
Assessment
of
Construction
and
Development
Proposed
Effluent
Guidelines
June
2002
B­
2
In
general,
the
literature
indicates
that
environmental
sensitivity
(e.
g.,
geomorphologic
changes,
pollutant
toxicity)
is
greater
on
smaller
stream
orders,
from
the
intermittent
headwater
streams
to
small
perennial
streams.
For
most
environmental
impacts
(except
perhaps
nutrient
loadings),
the
impacts
of
the
construction
and
land
development
industry
tend
to
decrease
with
increased
stream
size,
and
the
impacts
tend
to
become
confounded
with
other
influences
(e.
g.,
other
point
and
nonpoint
source
pollutant
loads).
For
this
reason,
the
inventory
focused
on
relatively
small
watersheds
(between
2
and
7
square
miles)
to
better
assess
the
impacts
of
hydrologic
changes
on
small
streams.















	







Figure
B­
1.
Regions
for
Stream
Inventorying
Environmental
Assessment
of
Construction
and
Development
Proposed
Effluent
Guidelines
June
2002
B­
3



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
		


	

	












	
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
	



	
	

	



	


	





	

	
	






	
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	


	
	
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









Region
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
Ecoregion
Boundary
Figure
B­
2.
Stream
Densities
for
Omernik
Ecoregions
(in
units
of
acres
per
stream
mile)

Because
EPA
focused
on
small
streams,
it
was
necessary
to
select
a
method
by
which
to
characterize
streams
by
size.
Historically,
various
schemes
have
been
created
to
characterize
and
count
streams
within
a
drainage
network,
including
the
following:

°
Stream
order
is
determined
by
counting
stream
segments
starting
with
the
smallest
stream
channels
found
on
a
selected
map
scale.

°
Stream
level
is
determined
by
counting
stream
segments
starting
from
the
most
downstream
discharge
point
(ocean
or
estuary)
on
a
selected
map
scale.
Environmental
Assessment
of
Construction
and
Development
Proposed
Effluent
Guidelines
June
2002
B­
4
°
Streams
are
characterized
by
physical
descriptions
including
flow
frequency
(perennial
or
intermittent
streams),
size
(large,
medium,
or
small),
and/
or
terms
such
as
swales,
creeks,
and
rivers.

°
Watershed
size
is
based
on
the
scale
of
the
map
on
which
the
watersheds
are
just
visible.

EPA
selected
the
first
method,
stream
order
characterization,
for
use
in
this
assessment.

Map
Scale
Selection
Because
any
network
of
"streams"
identified
at
the
outset
of
a
hydrologic
inventory
is
highly
dependent
on
the
scale
of
the
map
used,
selecting
the
appropriate
scale
is
a
critical
step.
Rills
and
swales
that
are
obvious
and
identifiable
on
a
1:
2,400­
scale
map
are
completely
absent
on
a
1:
250,000­
scale
map.
Figure
B­
3
shows
the
streams
visible
on
the
following
three
scales
of
maps
for
a
typical
watershed
(10
square
miles)
in
northeastern
Maryland:

°
U.
S.
Geological
Survey
(USGS)
1:
250,000­
scale
map
or
streams
found
in
EPA's
RF1
stream
network
°
USGS
1:
100,000­
scale
map
or
streams
found
in
EPA's
Reach
File
V.
3
(RF3)
and
National
Hydrography
Dataset
(NHD)
(USGS,
2000)
stream
networks
°
USGS
1:
24,000­
scale
map.

The
three
map
scales,
respectively,
permit
successively
finer
viewing
of
stream
sizes:
(1)
large
perennial
streams,
(2)
medium
perennial
to
intermittent
streams,
and
(3)
larger
swales
and
intermittent
streams.
Although
not
shown
in
Figure
B­
3,
an
even
finer
detail
stream
network—
one
based
on
1:
2,400­
scale
maps
(a
scale
commonly
used
by
local
governments)
that
includes
the
smallest
swales—
can
be
visualized
by
increasing
the
number
of
1:
24,000­
scale
streams
threefold
(i.
e.,
delineation
of
watersheds
as
small
as
2
acres).
Figure
B­
3
illustrates
the
importance
of
map
scale
selection:

°
Inventorying
stream
networks
based
on
1:
24,000­
scale
will
include
many
more
streams
than
a
1:
250,000­
scale
inventory;

°
The
stream
order
assigned
to
any
stream
will
be
different
based
on
the
map
scale;
and
°
Direct
evaluation
using
only
EPA's
RF1
and
RF3
hydrologic
stream
coverages
would
grossly
undercount
the
number
of
streams
potentially
impacted.
Environmental
Assessment
of
Construction
and
Development
Proposed
Effluent
Guidelines
June
2002
B­
5
0.
9
0
0.
9
1.
8
2.
7
Miles
S
N
E
W
Swal
e
=
1:
24,
0
00
EPA
R
F3
=
1:
100,
000
Stream
Network
Scale
EPA
R
F1
=
1:
250,
000
Figure
B­
3.
Stream
Networks
for
1:
250,000­,
1:
100,000­,
and
1:
24,000­
Scale
Maps
Note:
The
1:
24,000­
stream
network
shown
contains
more
streams
than
the
USGS
identified
on
its
7.5­
minute
quadrangle
maps
using
typical
blue
or
dashed
blue
lines.
This
figure
includes
all
swales
that
can
be
drawn
based
on
contour
lines
given
on
the
1:
24,000
map,
resulting
in
an
enhancement
that
shows
two
to
three
times
more
"streams"
than
are
shown
on
the
original
map
(down
to
watersheds
approximately
10
acres
in
size).

Interpretation
of
contour
lines
defines
a
stream
network
based
on
land
forms
as
the
contours
are
present
because
streams/
swales
have
created
them.
This
contour­
based
enhancement
defines
a
"stream"
based
on
topography,
regardless
of
whether
or
not
the
stream
is
actually
drawn
on
the
map.

Because
using
an
increased
detail
of
stream
network
(smaller
map
scale)
requires
increased
effort
levels,
EPA
developed
a
method
that
was
both
practical
and
depicted
the
appropriate
stream
level
for
this
assessment.
The
amount
of
stream
data
available
is
extensive;
the
national
coverage
for
RF1
contains
100
megabytes
of
data,
while
RF3
contains
7,400
megabytes.
All
of
RF1
(data
on
just
the
largest
rivers
in
the
nation)
can
reside
and
be
analyzed
on
a
single
microcomputer.
However,
the
RF3
network
and
the
similar,
newer
NHD
are
so
large
they
can
be
analyzed
in
a
Environmental
Assessment
of
Construction
and
Development
Proposed
Effluent
Guidelines
June
2002
B­
6
microcomputer
environment
only
when
divided
into
20
separate
parts.
Therefore,
EPA
assumed
that
a
national
dataset
containing
all
streams
and
swales
identifiable
from
1:
2,400­
scale
maps
would
be
unworkable
within
the
current
limits
of
any
microcomputer.

To
maintain
a
relatively
small
map
scale,
EPA
performed
an
inventory
of
streams
and
swales
identifiable
based
on
1:
24,000­
scale
maps
(where
swales
are
added
manually)
by
first
sampling
representative
watersheds
or
areas.
(An
actual
inventory
of
individual
swales
and
streams
on
a
1:
24,000­
scale
for
specific
acreage
developed
in
any
given
state
in
any
given
year
is
beyond
current
computational
capabilities
and
the
limits
of
available
data,
requiring
some
type
of
approximation
or
sampling
technique).
EPA
used
digital
elevation
maps
(DEMs),
which
allowed
EPA
to
process
contour
data,
enhancing
the
original
stream
network
to
provide
data
on
the
larger
intermittent
streams
(typically
streams
draining
less
than
30
acres).
Because
EPA's
assessment
of
the
construction
industry
indicates
that
a
medium­
sized
construction
start
is
approximately
20
acres,
this
approach
is
refined
enough
to
inventory
the
number
and
size
of
streams
potentially
impacted
by
construction
and
land
development
activities.
The
number
and
length
of
streams
in
a
larger
area
were
then
estimated
by
using
the
stream
density
found
in
the
sampled
watershed/
area.
Appendix
C
Impacts
of
Construction
Activities
on
Hydrology
Environmental
Assessment
of
Construction
and
Development
Proposed
Effluent
Guidelines
1
The
Soil
Conservation
Service
(SCS)
is
the
former
name
of
the
Natural
Resources
Conservation
Service
(NRCS).

June
2002
C­
1
Appendix
C
Impacts
of
Construction
Activities
on
Hydrology
Overview
This
appendix
describes
hydrologic
changes
that
result
from
construction
and
post­
development
activities,
and
focuses
primarily
on
changes
in
runoff
rates
and
soil
infiltration.
The
general
hydrologic
changes
caused
by
these
industries
have
environmental
and
economic
impacts.

The
objectives
of
this
appendix
are:

°
To
demonstrate
the
variation
in
runoff
rate
for
a
10­
acre
site
as
it
changes
from
a
forested
condition
into
a
construction
condition.

°
To
describe
the
environmental
benefits
of
current
BMPs
primarily
designed
to
limit
discharge
from
construction
sites.

Methodology
A
simple
hydrologic
model
was
developed
to
depict
the
hydrologic
changes
that
result
from
construction
and
land
development
activities
on
a
(10­
acre)
site.
The
size
of
10­
acres
was
chosen
because
it
represents
the
typical
size
for
a
construction
site.
In
addition,
the
hydrologic
changes
are
believed
to
be
similar
to
changes
that
result
on
larger
sites
such
as
100­
acre
sites
and
1000­
acre
sites.

Investigation
of
hydrologic
changes
was
performed
by
using
two
hydrologic
models:
TR­
55
and
TR­
20.
These
models
use
data
developed
over
many
years
by
USDA/
Natural
Resources
Conservation
Service
(NRCS),
and
are
among
the
most
often
employed
models
for
the
hydrologic
design
of
hydraulic
structures,
such
as
storm
drainage
systems
(USDA,
2002).

The
10­
acre
watershed
was
assumed
to
have
a
50/
50
mix
of
soils
in
the
type
B
and
C
hydrologic
soil
classification,
with
an
average
ground
slope
of
7
percent.
Time
of
concentration
was
derived
based
on
standard
TR­
55
worksheets
that
analyze
sheet
flow,
shallow
concentrated
flow,
and
pipe
flow.
For
the
analysis,
the
2­
year
24­
hour
SCS
1
type
II
rainfall
event,
totaling
3.2
inches
of
rainfall,
was
used
to
conservatively
estimate
the
runoff
hydrographs.

Multiple
land
use
conditions
(Table
C­
1)
were
evaluated
to
help
assess
the
hydrologic
impacts
for
the
small
10­
acre
site.
EPA
notes
that
most
construction
sites
occupying
10
acres
are
Environmental
Assessment
of
Construction
and
Development
Proposed
Effluent
Guidelines
June
2002
C­
2
equipped
with
a
sedimentation
pond,
intended
to
minimize
sediment
discharge
from
the
site.
Although
sediment
ponds
are
not
designed
specifically
shave
the
peak
runoff
rate
(i.
e.,
limit
the
construction
site
peak
discharge
rate
to
be
equal
to
or
less
than
the
peak
runoff
from
the
forested
site),
these
structures
inherently
have
some
capability
of
peak­
shaving
depending
on
the
site
conditions.
In
addition,
sedimentation
ponds
can
be
built
to
increase
its
peak­
shaving
capability.
For
the
purposes
of
this
assessment,
EPA
assumed
that
a
sedimentation
pond
(Condition
3)
shaves
the
peak
completely,
as
shown
in
Figure
C­
1.

Table
C­
1.
Evaluated
Hydrologic
Conditions
for
a
Typical
10­
Acre
Site
Land
Use
Condition
Description
1
Pre­
development:
a
forested
land
use
2
Construction:
cleared
and
grubbed
soil
surface
with
no
vegetation
and
without
construction
runoff
BMPs
(No
sedimentation
ponds)

3
Construction:
cleared
and
grubbed
soil
surface
with
no
vegetation
with
storm
water
BMPs
(a
sedimentation
pond
that
also
shaves
the
peak
runoff
to
match
the
predevelopment
peak
flow)

The
results
of
the
analysis
are
presented
below
for
each
of
these
land
use
conditions.

Discussion
of
Runoff
Results
for
Modeled
Land
Use
Conditions
Figure
C­
1
compares
the
predicted
runoff
hydrographs
for
Land
Use
Conditions
1
through
3.
The
hydrographs
in
the
figure
show
the
large
increase
in
runoff
volume
and
peak
runoff
rate
that
occurs
for
construction
sites
with
or
without
storm
water
BMPs
that
limit
the
peak
runoff
rates.
This
increase
is
caused
by
the
removal
of
existing
vegetation
and
compaction
of
site
soils
with
earth
moving
equipment,
which
greatly
diminishes
the
site's
ability
to
absorb
rainfall
and
limit
discharge.
In
fact,
NRCS
data
strongly
suggest
that
a
fully­
constructed
site
(e.
g.,
a
residential
neighborhood)
produces
less
runoff
than
a
denuded
site
under
construction,
even
though
impervious
surfaces
(e.
g.,
driveways,
roofs)
have
not
yet
been
installed.
Environmental
Assessment
of
Construction
and
Development
Proposed
Effluent
Guidelines
June
2002
C­
3
Comparison
of
Various
Construction
Conditions
for
A
Ten
Acre
Construction
Site
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
10
12
14
16
18
20
Time
In
Hours
Flow
Discharged
(cubic
feet
per
second)
Forested
Construction
Site
Without
BMPs
Construction
Site
With
BMPs
Figure
C­
1.
Runoff
Hydrographs
for
a
10­
Acre
Construction
Site
Although
the
implementation
of
peak­
shaving
BMPs
minimizes
some
of
the
flooding
downstream
of
a
construction
site
due
to
high
peak
flows,
it
does
not
eliminate
the
potential
for
enhanced
flooding
that
is
caused
by
longer
durations
of
high­
flow
discharges.
Table
C­
2
indicates
that
the
construction
site
produces
high
flows
for
a
much
greater
duration
than
flows
originally
released
from
the
forested
site.
In
fact,
the
10­
acre
site
that
once
produced
a
flow
rate
equal
to
or
greater
than
3
cubic
feet
per
second
(cfs)
for
only
0.2
hours
will
produce
more
than
3
Environmental
Assessment
of
Construction
and
Development
Proposed
Effluent
Guidelines
June
2002
C­
4
cfs
for
3.2
hours
when
peak­
shaving
BMPs
are
employed
during
construction.
Should
a
2­
year
storm
occur
during
the
construction
period,
the
longer
flow
duration
increases
the
chances
that
the
discharge
will
be
combined
with
downstream
peak
flows
from
other
developing/
developed
locations
to
produce
a
flooding
condition.

Table
C­
2.
Comparison
of
Durations
of
High
Flow
Rates
for
Different
Land
Use
Conditions
Land
Use
Condition
Hours
of
flow
equal
to
or
greater
than:

3
cfs
2
cfs
1
cfs
Forested
0.2
0.3
0.8
Construction
site
without
peak
shaving
BMPs
0.9
1.4
3.3
Construction
site
with
peak
shaving
BMPs
3.2
4
5.7
cfs
=
cubic
feet
per
second