St. Charles County, MO
 
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Table of Contents
Table of Contents
[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.
3. 
If temporary facilities and erosion and sediment controls installed pursuant to approved plans fail to meet the performance standards set out herein, the Director of Development Review may require the permittee to submit modified plans as provided in Section 412.120 above.
[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
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)
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
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
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.