[Ord. No. 05-148 §14, 10-25-2005]
This Article IV specifies or authorizes performance and design standards to reduce the amount of sediment and other pollutants in stormwater discharges associated with the land disturbance activities as required by these regulations. The applicant's engineer shall select and design erosion and sediment controls adequate to meet those requirements.
[Ord. No. 05-148 §14, 10-25-2005]
A.
Designs
for erosion and sediment controls shall meet the following performance
standards.
1.
Compliance with all standards imposed by Missouri Department of Natural
Resources Missouri State Operating Permit MO-R100A or, if the land
disturbance area is within a valuable water resource area as determined
by the Missouri Department of Natural Resources, a Missouri State
Operating Permit MO-R109 issued in compliance with the Missouri Clean
Water Law (Chapter 644, RSMo., as amended), the Federal Water Control
Act (Public Law 95-500, 92d Congress, as amended) and Missouri and
Federal regulations pursuant thereto.
2.
Compliance with the following additional standards stated herein.
a.
No land disturbance activity shall result in the impounding of surface
water on property other than the permittee's unless the permittee
obtains easements or licenses for that purpose.
b.
Runoff into receiving streams from any area undergoing land disturbance
activities that is greater than three (3) acres shall not exceed the
six (6) month peak runoff rate for that area in its pre-developed
state.
c.
Temporary discharges into receiving streams from any area undergoing
land disturbance activities shall not result in the accelerated erosion
of those streams' channels at the point of discharge.
[Ord. No. 05-148 §14, 10-25-2005]
A.
The erosion and sediment controls and temporary facilities identified in Subsection (B)(1) shall be designed to accommodate at a minimum the runoff for the design storm specified in that Subsection, using the runoff coefficients specified in Subsection (B)(2) and calculated according to the methods defined in this Section.
B.
If installed erosion and sediment controls, designed according to this Article, fail to meet its performance standards above, the permittee shall be required to correct the deficiency in question as provided in Section 412.120.
1.
Design storm. Designs for erosion and sediment controls
and temporary facilities constructed during land disturbance activities
shall be based on the design storms shown in Exhibit 1.
|
Exhibit 1—Design Storm
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|---|---|---|---|
|
Erosion and Sediment Control and Temporary Facilities
|
Design Storm
| ||
|
Stormwater Conveyance Systems:
|
6 month
| ||
|
On-site drainage ditches and diversions
| |||
|
By-pass storm sewers and channels
|
2 year
| ||
|
Entrance road culvert
|
2 year
| ||
|
Storm Inlet Sediment Protection
|
6 month
| ||
|
Stream Crossing Structures:
| |||
|
Duration of use: 4 months or less
|
2 year
| ||
|
Duration of use: longer than 4 months
|
10 year
| ||
|
Sediment Basin:
| |||
|
Basin size
|
6 month
| ||
|
Basin overflow
|
10 year
| ||
2.
Runoff coefficient. The runoff coefficient (C) corresponds
to the effective runoff based on ground cover, ground slope and that
portion of rainfall that is lost to surface runoff by processes such
as depression storage, infiltration, interception and evaporation.
The runoff coefficients in Exhibit 2 shall be used in calculating
peak runoff rates and stormwater volumes.
|
Exhibit 2—Runoff Coefficient Table (C)
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|---|---|---|
|
Ground Cover
|
Runoff Coefficient (C)
| |
|
Pasture and unimproved areas
|
15%
| |
|
Woods
|
10%
| |
|
Lawns ≤ 6% slope
|
15%
| |
|
Lawns ≥ 6% slope
|
30%
| |
|
Graded/no vegetation ≤ 6% slope
|
50%
| |
|
Graded/no vegetation ≥ 6% slope
|
60%
| |
|
Gravel parking lot
|
75%
| |
|
Gravel road
|
80%
| |
|
Pavement, walks, buildings
|
95%
| |
|
|
The runoff coefficients shall be determined for each drainage
area to proposed erosion and sediment control and temporary facilities
based on the following criteria.
|
a.
Land disturbance areas shall be considered stripped of all vegetation
in determining runoff for erosion and sediment controls placed prior
to land disturbance activities.
b.
After cut and fill operations are completed, land disturbance areas
shall be considered stripped of all vegetation and pavement installed
in determining sediment controls, runoff conveyance systems and erosion
prevention devices.
3.
Peak runoff rate calculation method. The Rational
Method, as developed by Mulvaney in 1851, shall be used to determine
the peak (maximum) runoff rate. The Rational Method (also known as
the Rational Formula) is:
|
|
Q = C i A
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|---|---|---|---|---|
|
|
where
|
Q
|
=
|
peak runoff rate in cubic feet per second (cfs)
|
|
|
C
|
=
|
runoff coefficient (dimensionless)
| |
|
|
i
|
=
|
rainfall intensity rate in inches per hour
| |
|
|
A
|
=
|
drainage area in acres
| |
a.
The runoff coefficients (C) to be used are set out in tabular form in Subsection (B)(2), Exhibit 2 above.
b.
The rainfall intensity rates (i) were derived for
St. Charles County from the Rainfall Frequency Atlas for the Midwest,
Bulletin 71 by Huff and Angel, 1992 for a ten (10) minute rain event.
The rainfall intensity rates in Exhibit 3 shall be used.
|
Exhibit 3—Rainfall Intensity Rates (i)
| ||
|---|---|---|
|
Design Storm
|
Rainfall Intensity Rate
(inches/hour)
| |
|
6 month
|
2.86
| |
|
1 year
|
3.54
| |
|
2 year
|
4.38
| |
|
5 year
|
5.53
| |
|
10 year
|
6.62
| |
4.
Total runoff volume calculation. The total volume
of runoff for calculating sediment basin size shall be based on the
runoff coefficient times the total rainfall in a twenty-four (24)
hour period, which is:
|
|
V = P x C x A x 3630
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|---|---|---|---|---|
|
|
where
|
V
|
=
|
total runoff volume in cubic feet
|
|
|
P
|
=
|
inches of rainfall in a twenty-four (24) hour period
| |
|
|
C
|
=
|
runoff coefficient (dimensionless)
| |
|
|
A
|
=
|
drainage area in acres
| |
a.
The runoff coefficients (C) to be used are set out in tabular form in Subsection (B)(2), Exhibit 2 above.
b.
The total inches of rainfall in a twenty-four (24) hour period was
derived for St. Charles County from the Rainfall Frequency Atlas for
the Midwest, Bulletin 71 by Huff and Angel, 1992. The following exhibit
shall be used:
|
Exhibit 4
Inches of Rainfall in a 24-Hour Period
| ||
|---|---|---|
|
Design Storm
|
Total Rainfall
(inches)
| |
|
6 month
|
2.03
| |
|
1 year
|
2.50
| |
|
2 year
|
3.25
| |
|
5 year
|
4.10
| |
|
10 year
|
5.00
| |
[Ord. No. 05-148 §14, 10-25-2005]
A.
Design Manual Authorized. The Director of Development Review
is hereby authorized to develop design criteria for erosion and sediment
controls that may be employed to comply with these regulations and
to meet the performance standards set out above. The design criteria
may include specific requirements or conditions for the use of any
particular erosion and sediment control. Such design criteria shall
be included in St. Charles County's "Design Criteria for the Preparation
of Improvement Plans" (Design Manual).
B.
Use Of Design Manual And Other Guidelines. Plans required
by these regulations may include erosion and sediment controls included
in the Design Manual, but the Design Manual is not intended to preclude
use of other erosion and sediment control methods not included in
it. Engineering professionals are encouraged to design innovative
ways to address site specific conditions. In all cases, erosion and
sediment control products shall be used and installed according to
the manufacturer's specifications. In all cases, designs must be approved
by the Director of Development Review and must be in compliance with
these regulations and the terms and conditions of applicable Federal
and State permits.
[Ord. No. 05-148 §14, 10-25-2005]
A.
Surface Stabilization Techniques. Bare ground must be stabilized by vegetation, rock surfacing, erosion control blankets and netting, soil binders, structural topping, like concreting or other techniques authorized by the Design Manual or approved pursuant to Section 412.100 above. With respect to vegetation, the following provisions shall also apply.
1.
Temporary seeding shall be used if the area will be disturbed later
in the development. The area must be vegetated by permanent seeding
or sodding, when no further land disturbance will occur.
2.
Seeding, fertilizing and mulching shall be applied at the rates and
times specified in the Design Manual.
3.
Mulch can be used as temporary cover in unseeded areas to protect
against erosion over the winter or until final grading and shaping
can be accomplished. Application rates are shown in the Design Manual.
4.
Temporary seeding and mulching shall be placed on seventy percent
(70%) of the total disturbed site area according to the stabilization
schedule.
5.
Temporary seeding may be suspended in portions of the project area
which have an active building permit. Upon completion of the building
activity, the site must be permanently stabilized.
6.
Seeded areas shall be refertilized four (4) weeks after initial seeding.
The seeded area shall be inspected at that time for uniform cover
and adequate density. All areas which are bare and sparse (less than
thirty percent (30%) ground cover) shall be reseeded and mulched.
7.
Non-degradable mats shall be used only as a permanent installation
and in areas that will not be mowed.
B.
Surface Stabilization Schedule. Land disturbance activities
shall be scheduled as provide in the table below:
|
Exhibit 5—Soil Stabilization Schedule
| ||
|---|---|---|
|
Soil Disturbance Activity or Condition
|
Required Stabilization Time
| |
|
Soil disturbance has ceased in areas greater than 2,000 square
feet.
|
14 days
| |
|
After construction of dikes, swales, diversions and other concentrated
flow areas.
|
5 days
| |
|
When slopes are steeper than 3 horizontal to 1 vertical.
|
7 days
| |
|
When slopes are greater than 3% and longer than 150 feet.
|
14 days
| |
|
Perimeter controls around soil stockpiles.
|
End of workday
| |
|
Stabilization or covering of inactive stockpiles.
|
30 days
| |
|
When land disturbance is completed, permanent soil stabilization
must be installed.
|
30 days
| |
C.
Land Disturbance Phasing. Land disturbance activities should
be scheduled in stages of development, so that only the areas that
are actively being developed are exposed. Land disturbance areas exceeding
ten (10) acres may require phasing, if the Director of Development
Review determines that runoff from the land disturbance area may adversely
affect other property.
[Ord. No. 05-148 §14, 10-25-2005]
A.
Surface
water shall be diverted from the face of all cut and fill slopes exceeding
eight (8) feet in vertical elevation.
B.
Slope
breaks shall be provided whenever the vertical elevation of any slope
exceeds twenty (20) feet.
C.
Diversion
berms shall not exceed eight hundred (800) feet in length.
D.
No
excavation shall be made so close to the property line to endanger
any adjoining public or private street without supporting and protecting
such public or private street or property from settling, cracking
or other damage.
E.
No
fill material shall be placed so as to cause or to allow the same
to be deposited upon or to flow onto another property without written
consent of the owner.
F.
No
fill material shall be placed so as to cause or to allow the same
to be deposited upon or to flow onto any public street, walk, place
or way, nor so close to the top of a bank of a channel as to create
the possibility of bank failure.
G.
Materials
for fills shall consist of material obtained from excavation of banks,
borrow pits or other approved source. Material shall be free of vegetative
matter and deleterious material and shall not contain large rocks
or lumps except as certified by a geotechnical engineer to be acceptable
fill material.
H.
No
cut or fill slope shall be made steeper in slope than three (3) horizontal
to one (1) vertical without a geotechnical report and approval by
the Director of Development Review.
I.
Individual
and isolated slopes, rock dikes, undisturbed natural slopes and slopes
blending with the natural terrain may be steeper than the requirements
as approved by the Director of Development Review.
J.
All
fills and trench backfills shall be compacted to the minimums as defined
in the Design Manual. Compaction of fills and backfills must be certified
by a geotechnical engineer.
K.
Solid
rock, shale, tree stumps, masonry and other obstructions shall be
removed to a depth of two (2) feet below finished grade or pavement
subgrade.
[Ord. No. 05-148 §14, 10-25-2005]
A.
Temporary
conveyance of stormwater during land disturbance activities depends
upon the peak runoff for the design storm and a suitable method to
prevent erosion after construction. The requirements listed below
shall be used for temporary conveyance of stormwater.
1.
All drainage shall be designed to transport surface waters to the
nearest practical storm drain, natural watercourse or street as approved
by the Director of Development Review.
2.
Diversion channels and ditches are to be designed to a non-erosive velocity as defined in the Design Manual or the geotechnical report required by Section 412.060 (2)(a). Diversion ditch length shall not exceed eight hundred (800) lineal feet.
3.
A rock outlet is required at all pipe and improved channel discharges
to open watercourses. The maximum design velocity shall be ten (10)
feet per second (fps). If the discharge velocity exceeds ten (10)
fps, an engineered energy dissipater may be required as determined
by the Director of Development Review.
[Ord. No. 05-148 §14, 10-25-2005]
A.
A temporary
construction vehicle wash-off pad is required to avoid tracking mud
onto public roads and must be located where construction traffic leaves
the site. The permittee shall remove any mud, sediment or debris tracked
onto public roads by sweeping or other mechanical means.
B.
Sediment
basins shall be used to meet water quality discharge requirements
and pre-developed runoff rates during land disturbance activities.
Sediment basins shall be designed for the following criteria.
1.
Sediment volume shall be determined from the Natural Resources Conservation
Service's Revised Universal Soil Loss Equation (RUSLE).
2.
Wet volume shall contain the total runoff produced from the six (6)
month twenty-four (24) hour storm.
3.
Dewatering time shall be twenty-four (24) hours for the total volume
of wet storage of the basin.
4.
The outlet must be designed to convey the peak 10-year runoff with
a minimum one (1) foot freeboard between the water surface of the
outlet and the top of the basin embankment.
5.
Other sizing requirements are described in the Design Manual.
