ACEC _ Sediment Channels - Division of Mine Permits

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22 févr. 2014 (il y a 3 années et 3 mois)

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Sediment Channels

Kentucky Department for Natural
Resources

Division of Mine Permits

Sept. 6 and 14, Oct. 4 and 18, 2012

Sediment Channels


There are a variety of different approaches to sediment channel
design currently being used.



Above Ground


Incised


Single Spillway


Multiple Spillways



Other approaches will be considered on a case by case basis but may
require Department approval of the method per TRM #13.



The most problem encountered difficulties we encounter in reviewing
these proposals include;


Horizontal and Vertical control


Spillway Placement


Flood Routing/Internal baffles


Closure/Backfill and Grading


Sediment Channels

Topics for consideration in this presentation



Design


Compliance Demonstration


Construction


Removal


Sediment Channels

Design Considerations

Under the regulations, the design must provide


Sediment storage volume

commensurate with the watershed runoff,


This can be accomplished by detailed modeling or an disturbed acreage
-
volume
ratio.



Typically volume does not seem to be a design problem with sediment channels


Adequate detention time

to provide for settling of sediment



A function of flood routing, in that sufficient detention time for settling will also
serve to eliminate flooding downstream


Flood routing capacity

sufficient to pass the 25 year storm without overtopping, or
increasing flooding potential (due to increased disturbed area)





The

sediment

storage

volume

required

by

regulation

is

given

in

405

KAR

16
:
090

Section

2
.


Section

2
.

Sediment

storage

volume
.

Sedimentation

ponds

shall

provide

adequate

sediment

storage

volume

as

approved

on

a

case
-
by
-
case

basis

by

the

cabinet

based

upon

the

anticipated

volume

of

sediment

to

be

collected

and

a

feasible

plan

for

clean
-
out

operations
.

The

plan

shall

include

a

time

schedule

or

clean
-
out

elevations,

or

an

appropriate

combination,

that

shall

provide

periodic

sediment

removal

sufficient

to

maintain

adequate

volume

for

the

sediment

to

be

collected

during

the

design

precipitation

event

under

Section

3

of

this

administrative

regulation
.

The

proposed

clean
-
out

plan

shall

be

included

in

the

design

and

shall

be

approved

if

the

department

determines

that

the

proposed

plan

is

feasible
.



There

must

be

an

anticipated

volume

of

sediment
.

The

most

commonly

used

modeling

program,

SEDCAD

(and

it’s

predecessors)

predict

a

storm

volume

of

sediment

that

can

be

translated

into

an

annual

discharge
.



There

are

also

conservative

industry

standards

that

can

be

used

in

formulating

an

appropriate

sediment

storage

volume
.


Sediment Channels

Design
-

Sediment Storage Volume


Total

Sediment

Storage

at

the

spillway


0
.
125

acre
-
feet

per

disturbed

acreage
.

This

is

a

commonly

accepted

value,

with

it’s

origins

in

SCS

and

BLM

research
.



If

a

ratio

less

than

0
.
125

acre
-
feet

per

disturbed

acreage

is

used,

it

will

need

to

be

modeled

to

determine

if

effluent

standards

are

met
.

The

0
.
125

acre
-
feet

per

disturbed

acreage

has

been

previously

tested
.




The

sediment

will

be

cleaned

out

once

it

reaches

60
%

capacity
.

Both

of

these

elevations

need

to

be

labeled

on

the

stage
-
storage

curve
.

Sediment Channels

Design
-

Sediment Storage Volume

Volume of Cells


If the volume of the sediment channel is calculated at a 0%
slope, the channel must be excavated to a point where the
channel has no slope.


If the channel is long (i.e. > 300’
-
500’) the applicant will need to
discuss how they are going to establish vertical control.





The regional coal dip slope can be used to calculate volume of
the channel.



If the sediment channel has a slope but the volume calculations
were based upon a channel with a 0% slope, the volume must
be reduced by an appropriate factor.

Sediment Channels

Design
-

Sediment Storage Volume

Sediment Channels

Design
-

Sediment Storage Volume


The channel can be constructed in segments as mining progresses or the
entire channel can be constructed.



As described in the regulatory guidance document
On
-
Bench Ponds without
Off
-
Bench Sediment Control;

originally approved May 15, 2009, “In
-
pit retention
is allowed for no more than 2 acres of disturbance in a design.”



Due to this restriction, it is only feasible to construct approximately 500’
-
600’ of
the channel before certifying if the channel is built in segments.



If the channel is constructed in its entirety, the channel and spillway will have
to built and certified before disturbance of more than 2 acres.

Sediment Channels

Design
-

Sediment Storage Volume


Each

channel

will

have

a

15


undisturbed

natural

barrier
.

This

barrier

will

serve

as

the

controlling

portion

of

the

Sediment

Channel

if

not

incised
.




The

channel

will

be

further

defined

as

mine

backfill

is

placed

forming

the

interior

side

of

the

sediment

channel

unless

the

channel

is

incised
.

A

statement

can

be

included

that

the

15


undisturbed

barrier

can

be

re
-
constructed

if

necessary
.



Sediment

channels

constructed

on

a

pre
-
law

bench

needs

to

be

built

on

the

solid

portion

of

the

bench
.



Backfill

stability

analysis

needs

to

include

the

sediment

channel

in

place
.




Stability

analysis

needs

to

account

for

water

impounding

on

the

backfill

if

the

channel

is

not

completely

incised
.



Breach Considerations



Due to the potential high volumes that could
be stored in the channel, consideration of
potential breach needs to be evaluated for
each channel. This is especially important if
there are no interior baffles.



To

be

in

compliance

with

405

KAR

16
:
090

Section

3
,

a

25
-
year

pre
-
mining

and

during

mining

hydraulic

analysis

needs

to

be

submitted

to

ensure

the

open

channel

spillway

is

correctly

sized

and

the

channel

meets

the

peak

discharge

standards
.



If

a

single

spillway

is

proposed

with

sequential

construction,

construction

standards

need

to

address

how

the

existing

spillway

will

be

closed

after

the

new

spillway

is

constructed
.


Sediment Channels

Design
-

Detention Time

Short
-
Circuiting



Special consideration needs to be made to
ensure the spillways are not short
-
circuited.




The most straightforward way to achieve this is to
place a small berm to ensure water does not flow
directly into the spillway


A 10
-
year, 24
-
hour storm event will model the entire length of
the channel with one spillway
.



The volume will be reduced by an appropriate percentage to
account for changes in vertical elevation if the slope of the dip
is not taken into account.



A 10
-
year, 24
-
hour storm event with one watershed will be
modeled as the “worst
-
case” with one spillway.



The channel will be considered in compliance with effluent
standards at all times during construction if the single
spillway/worst
-
case model and the entire channel model “pass”
unless specific conditions require further analysis.


Sediment Channels

Design
-

Detention Time/Flood Routing



The

worst
-
case

watershed

is

dependent

on

several

issues
.

Some

include

the

amount

of

disturbance,

slope

and

size

of

the

watershed
.



As

described

in

the

regulatory

guidance

document

On
-
Bench

Ponds

without

Off
-
Bench

Sediment

Control
;

originally

approved

May

15
,

2009
,

“In
-
pit

retention

is

allowed

for

no

more

than

2

acres

of

disturbance

in

a

design
.


Therefore,

the

initial

disturbance

needs

to

be

looked

at

as

a

possibility

of

being

the

worst
-
case
.


Sediment Channels

Compliance Demonstration
-

Effluent


A 25
-
year, 6
-
hour (or 24
-
hour) pre
-
mining and during
mining model for the worst
-
case watershed with one
spillway needs to be submitted to ensure each outfall
is within 10% of the pre
-
mining peak discharge.

Sediment Channels

Compliance Demonstration
-

Effluent

Multiple Spillways:

Rock
-
Check Dams


There

is

an

approved

method

for

calculating

the

discharge

through

the

rock
-
check

dams

that

separate

each

cell
.

Please

see

Dick

Rohlf

or

Whitney

Faulkner

if

an

application

accounts

for

the

flow

through

a

rock
-
check

dam

to

verify

the

method

is

acceptable
.



Discharge

through

a

porous

rock
-
check

dam

can

start

at

the

sediment

cleanout

elevation

as

long

as

specific

design

assessment

addresses

such

flow
.



The

application

needs

to

address

the

porosity

(i
.
e
.

solid

barrier

or

porous)

of

the

rock
-
check

dams
.

If

the

channel

has

a

fairly

low

slope

and

the

top

of

all

check

dams

are

lower

than

the

associated

spillways,

the

porosity

of

the

dams

is

not

as

essential
.

Multiple Spillways:

Sediment Channel Design


To

allow

flexibility

in

placement

of

the

spillway

outfalls,

each

sediment

channel

will

be

designed

with

a

minimum

and

maximum

number

of

spillways

with

spacing

limitations
.



The

application

will

need

to

specifically

state

the

minimum

and

maximum

number

of

spillways

as

well

as

the

minimum

spacing

between

spillways
.



As

long

as

the

total

number

of

spillways

built

is

between

the

minimum

and

maximum

with

the

appropriate

spacing,

the

channel

will

be

considered

within

compliance
.




By

allowing

a

minimum

and

maximum

number

of

outfall

spillways,

the

spillways

can

be

constructed

in

the

localized

low

points

along

the

coal

seams
.



If

the

number

of

spillways

needed/constructed

exceeds

the

number

permitted,

a

revision

will

be

necessary
.



Upon

certification

of

the

spillway

segments,

it

is

our

understanding

that

DOW

will

re
-
issue

General

Permit

Coverage

to

reflect

the

actual

number

of

spillways

and

their

locations

Multiple Spillways:

Sediment Channel Design



Spillways

will

be

designed

for

a

minimum

distance,

usually

around

300

-
500

.

Spillways

can

be

locate

closer

as

long

as

the

channel

has

been

specifically

designed

for

the

closer

spacing
.



In

Item

17
.
8
,

the

applicant

will

list

the

minimum

number

of

spillways

for

KPDES

outfalls

with

a

specific

ID

number
.


Multiple Spillways:

Rock
-
Check Dams


There

has

to

be

a

minimum

1
.
0


of

freeboard

between

the

top

of

the

rock
-
check

dam

and

the

top

of

the

sediment

channel
.



The

maximum

distance

between

rock
-
check

dams

needs

to

be

stated

in

the

application
.

A

statement

that

the

rock
-
check

dams

can

be

located

closer

in

areas

of

greater

grade

can

be

included

as

well
.

Multiple Spillways: Construction and
Certification of Channels


If

it

is

proposed

to

mine

multiple

seams,

each

reviewer

needs

to

verify

the

worse
-
case

stability

is

analyzed
.

The

worst
-
case

could

be

an

upper

seam

with

the

channel

in

place
.



If

the

channel

is

constructed

in

segments,

the

sediment

channel

will

be

certified

“to

date”

as

mining

progresses
.




The

channel

will

be

certified

every

500


or

quarterly

which

ever

is

less
.



Multiple Spillways: Construction and
Certification of Channels


Each spillway will be surveyed and certified at time
of construction.



Each certification will show the actual location of
numbered spillways, any additional spillways that are
required by field conditions and any spillways
removed. The latitude and longitude of each
spillway will be specified along with the MRP map to
show the location.



Each certification will be submitted to DMRE and
DOW.


Multiple Spillways: Construction and
Certification of Channels


Certifications

can

be

submitted

for

the

entire

channel

or

in

stages

with

the

initial

stage

certified

to

date

in

500


segments

or

quarterly

which

ever

is

less

and

the

final

stage

certified

30

days

after

completion

of

backfilling

and

grading



As

discussed

in

the

regulatory

guidance

document

On
-
Bench

Ponds

without

Off
-
Bench

Sediment

Control
;

originally

approved

May

15
,

2009
,

prior

to

disturbance

of

more

than

2

acres,

either

the

entire

channel

must

be

certified

or

the

initial

stage

must

be

certified

to

date
.

Each

stage

certified

to

date

must

have

a

certified

spillway
.



Criteria for Approval for As
-
Built
Certifications


Constructed

as
-
built

volume

at

the

spillway

approximates

or

is

greater

than

design
.



Slope



If

the

channel

is

constructed

at

the

designed

slope

or

a

flatter

slope
.



Reconstructed

berm

height

of

5


or

less
.

Slopes

on

reconstructed

berm

2

to

1

or

less
.



Spillway

bottom

width

and

elevations

in

compliance

with

Directive

29
.

Criteria for Approval for As
-
Built
Certifications (continued)


The number of spillways and the minimum spacing
of spillways are in compliance with the permit
standards.


The spacing between rock checks or berms are in
compliance with permit standards.


Appropriate drainage features are used to prevent
short circuiting.


Latitude and longitude must be shown for each
spillway.


Criteria for Approval for As
-
Built
Certifications (continued)


The

location

of

each

spillway

must

be

indicated

on

the

MRP

Map

or

a

portion

of

the

MRP

Map
.



The

porosity

of

the

rock
-
check

dams

is

in

compliance

with

the

permit
.



Sediment Channels

General Notes


Removal

Plans



We

tend

to

get

removal

plans

that

are,

in

essence,

boilerplate

from

dugouts

and

even

embankment

type

structures
.

Our

people

will

be

looking

at

these

much

more

closely

in

the

future

to

ensure

their

appropriateness
.


Backfill

Stability



To

repeat,

the

worst

case

backfill

stability

must

consider

the

channel

in

place,

and

account

for

high

moisture

levels

at

pool

level
.


Plans

and

Submittals
-

We

have

been

asked

repeatedly

by

both

permit

and

enforcement

staff

to

request

more

detail

on

the

design

drawings
.

Of

particular

interest

is

the

cross
-
section,

which

should

show

the

constructed

channel,

coal

seams

to

be

mined,

berms,

etc
.

in

detail
.


Maximum

Depth



While

it

does

not

happen

commonly,

we’ve

received

proposals

for

excessively

deep

channel

(
10
+

ft
.
,

up

to

18

ft!)
.

This

is

a

safety

and

logistic

concern
.

At

a

minimum,

if

such

a

channel

is

proposed,

outlet

should

be

provided

at

regular

intervals

to

allow

ingress

and

egress
.



Questions???