§ 25-74. Stormwater standards and criteria.
(a)
Improvement plans. The improvement plans required under section 25-30 shall be prepared in accordance with and so as to reflect the stormwater standards and criteria set forth in this section, as well as any other standards and criteria of MSD.
(b)
Drainage. All construction, details and specifications pertaining to sanitary and stormwater drainage shall be in accordance with the Metropolitan St. Louis Sewer District requirements unless otherwise noted herein. A professional engineer's seal is required on all plans submitted for approval.
(c)
Design criteria. All stormwater design is to be by the rational method, except that in certain large developments, the use of other methods of analysis such as the unit hydrograph will be considered for stormwater design. However, prior approval must be obtained from the director of public works at the preliminary design state, where use of the unit hydrograph or other methods of analysis is requested. Runoff factors based on a 25-year design frequency are generally as follows:
Average C.I. (P.I.) Factors in C.F.S. per Acre
20 Min. Inlet Time %
ImperviousC or P (Factor) 15 Yr. 25 Yr. 50 Yr. 100 Yr. 5 .37 1.7 2.00 2.18 2.37 10 .39 1.79 2.11 2.30 2.50 15 .41 1.89 2.21 2.42 2.62 20 .435 2.00 2.35 2.57 2.78 25 .455 2.09 2.46 2.68 2.91 30 .475 2.19 2.57 2.80 3.04 35 .495 2.28 2.67 2.92 3.17 40 .52 2.39 2.81 3.07 3.33 45 .54 2.48 2.92 3.19 3.46 50 .56 2.58 3.02 3.30 3.58 55 .58 2.67 3.13 3.42 3.71 60 .60 2.76 3.24 3.54 3.84 65 .625 2.88 3.38 3.69 4.00 70 .645 2.97 3.48 3.81 4.13 75 .665 3.06 3.59 3.92 4.26 80 .685 3.15 3.70 4.04 4.38 85 .705 3.24 3.81 4.16 4.51 90 .73 3.36 3.94 4.31 4.67 95 .75 3.45 4.05 4.43 4.80 100 .77 3.54 4.16 4.54 4.93 _____ Rainfall 4.60 5.40 5.90 6.40 From U.S. Weather Bureau Technical Paper No. 40, Rainfall Frequency Atlas of the United States, for durations from 30 minutes to 24 hours and return periods from one to 100 years.
(d)
Drainage structures.
(1)
All structures shall be designed for HS20-44 loading.
a.
Bridges.
1. Bridges in floodplain areas shall be designed for 100-year storm frequency and provide one foot of freeboard from bottom of superstructure to high water elevation.
2. Bridges in nonfloodplain areas shall be designed on a storm frequency of 100 years and provide two feet of freeboard between the bottom of superstructure and high water elevations.
b.
Culverts.
1. Crossroad culverts, not at low points, shall be designed on a storm frequency of 25 years, with entrance control and two feet of freeboard at the shoulder line.
2. Culverts, at or near low points in floodplains, shall be designed on a storm frequency of 100 years with entrance control and two feet of freeboard at the shoulder line.
3. Culverts, at or near low points not in floodplains, shall be designed on a storm frequency of 50 years, with entrance control and two feet of freeboard at the shoulder line. (Refer to Bureau of Public Roads Hydraulic Engineering Circular No. 5 entitled Hydraulic Charts for the Selection of Highway Culverts.)
4. All crossroad culverts shall be R.C.P. or poured-in-place reinforced concrete. Driveway culverts may be C.M.P. with the approval of the city engineer.
c.
Roadways. Whenever a watercourse is proposed or exists within proximity of or approximately parallel to an existing or proposed roadway, the low shoulder elevation of the roadway shall be established from the highwater elevation with one foot of freeboard in nonfloodplain areas and two feet of freeboard in floodplain areas.
d.
Street inlets. In certain cases, where steep grades or other unusual conditions are encountered, a grated trough may be required in lieu of multiple inlet structures upstream in order to eliminate excessive bypass.
e.
Pipes.
1. All pipes shall be reinforced concrete except for underground detention facilities. Alternate materials for underground detention facilities having an expected life comparable to concrete pipe may be substituted. Show type and specification of all pipes (e.g.,ASTM,C76,class III).
2. Pipes normally should be sized using the partial full flow design; however, under special circumstances approved by the city engineer and MSD, full flow design will be allowed, provided hydraulic grade line information is shown indicating adequate freeboard.
3. For 36-inch-diameter pipes or smaller, a minimum one-percent grade or three feet per second velocity will be required. For pipes larger than 36-inch diameter, the minimum grade is the grade that will establish a self-cleaning velocity of three feet per second.
(e)
Watercourses. Where a subdivision is transversed by a watercourse, drainageway, channel or stream, there shall be provided utility easements and drainage easements and access easements of sufficient width and slope to permit access and maintenance if future expansion can reasonably be anticipated, and these easements shall conform substantially with the lines of such watercourses or drainage rights-of-way. The channel capacity shall be improved for and from a 25-year frequency storm in a manner approved by the city engineer. No structure shall be constructed closer than 20 feet to the easement of any creek, stream or other major watercourse.
(f)
Erosion control. Erosion control for drainage systems within or directly related to the right-of-way will be required in swales, ditches or creeks at inlet structures and at the discharge points of all pipes where the velocity exceeds four feet per second. Silting basins or diversion channels will be required during the construction period to insure that mud and other debris are not washed into natural watercourses, new or previously constructed storm sewers, or onto new or existing streets (public or private).
(g)
Stormwater retention. Where required by the board of aldermen, stormwater shall be retained onsite and metered out at specific rates to prevent possible flooding and erosion downstream. Detention shall be provided to retain the 25-year pre- and post-development differential runoff. The difference between the 25-year post-development and the 15-year undeveloped runoffs shall be detained. Stormwater and Detention Design Criteria and Guidelines, St. Louis County, as modified by the city and on file with the city engineer, shall dictate the design of all detention/retention basins. The simplified volume formula or an alternative acceptable to the city engineer shall be used.
In addition to any other criteria as may be imposed, the city engineer shall direct that:
(1)
All storm water detention basins with a depth, from the low water flow line to the overflow structure flow line, of four feet or more shall be fully enclosed by a fence, dividing wall or similar structure offering security from access if the side walls have a slope greater than 3:1.
(2)
All basin enclosing structures must be at least four feet tall when measured on the side facing away from the detention area, and must have a lockable gate large enough to provide access for maintenance equipment.
(3)
All enclosures must be placed at or above the overflow elevation of the basin.
(4)
Any basin that is not required to have an enclosure must have side slopes no steeper than 6:1.
(5)
All basins must be final graded and sodded to a level two feet above the overflow elevation immediately upon the completion of the storm water pipe installation. An extension to this provision may be granted in writing by the city engineer due to weather or other mitigating conditions for a period of not more than 180 days.
(6)
Reserved.
(7)
All enclosing structures must be compatible in design and style with the surrounding neighborhood and may contain no openings large enough to allow the passage of a six-inch diameter sphere.
(h)
Plans and calculations. Plans shall be any scale from one inch equals ten feet through one inch equals 100 feet, so long as the scale is an increment of ten feet. Plans shall be prepared on exhibits 24 inches by 36 inches. All calculations submitted shall be prepared in a legible manner. Plans and calculations shall include the following information:
(1)
Site drainage map. A drainage map is required and should be developed from a base reproduction of the site plan or grading plan. The map must include the following information:
a.
The existing and proposed contours at one- or two-foot intervals, for the subject property, extending offsite as required for proper design of proposed improvements. Contour intervals other than the above as determined by the topography must be approved by the city engineer or city planner. Use U.S.G.S. datum only. Locations and elevations of benchmark references are available from the county department of highways and traffic and the Metropolitan St. Louis Sewer District.
b.
The location of existing and proposed property lines, streets, sinkholes, area within the tract subject to inundation by stormwater, railroads, easements of record, and other significant natural features such as wooded areas and rock formations, etc.
c.
All existing and proposed stormwater facilities, such as inlets, manholes, pipes, culverts, bridges, channels, etc.
d.
All existing and proposed improvements where required for proper design review, which may include, but not be limited to, pavement, curbs, sidewalks, buildings, utilities, etc.
e.
The existing waterways, including names; if no name, an indication of creek or river to which the waterways are tributary.
f.
The runoff details are required, showing flows tributary to each proposed and existing structure and cumulative flows in pipes and gutters. Include "Q" and area.
g.
All existing bodies of water (such as lakes or ponds) indicating surface area and elevation.
(2)
Supplemental drainage map. A supplemental map may be required and should be developed from a portion of U.S.G.S. quadrangle map. Map must include the following information:
a.
The site boundary.
b.
The watershed tributary to the project area.
(i)
Surface drainage. For areas not in floodplain. The following shall be included and/or observed:
(1)
No sheet flow will normally be allowed over retaining walls or terraces that are steeper than a 3:1 slope and/or greater than five feet in height. Berms and/or swales shall be provided to collect the flow at the top of the terrace and carry it to a drainage structure. Total accumulation inside berms and/or swales shall be a maximum of four cubic feet per second.
(2)
Provide critical cross sections, profiles, and hydraulic computations for ditches and swales with flows in excess of one c.f.s. and creeks with flows in excess of four c.f.s.
(j)
Drainage structures. The lateral location of all drainage structures shall be shown by street centerline stationing and perpendicular offset distance, in addition to the following requirements:
(1)
Inlets shall be appropriately sized and spaced, taking into consideration bypass, depth of flow and width of spread of flow in gutters. Width of spread shall not exceed six feet.
(2)
For inlets in vertical sag curves, the center of the main chambers shall be placed within one foot of the exact mathematical low point.
(3)
All manholes shall be located outside of proposed roadway pavements.
(4)
Inlets shall be located as follows:
a.
Inlet face one foot from back of roadway curbing along driving lanes.
b.
Inlet face flush with back of roadway curbing along parking shoulders.
c.
Area inlets in roadway ditches to be compatible with ultimate improvements.
d.
Inlets shall be placed at intersections such that no part of the inlet structure or sump is in the curb rounding.
(5)
Bypass shall be minimized at inlet structures and shall be shown on the hydraulic computations and site drainage map.
(6)
Where termination of pavement occurs, erosion protection in the form of asphalt curb, gutters, additional structures, paving and/or revetment may be required.
(7)
Data required on plans and/or profiles shall include:
a.
Type of structure (single, double, grated, etc.).
b.
Flowline and top elevations.
c.
"Q" in upstream gutter line and within pipe reach.
d.
The hydraulic grade line at the structure shall be shown on the plan, profile and computation sheets.
e.
Hydraulic computations shall be included as part of the site improvement plans.
(8)
For nonstandard structures two additional sets of plans shall be submitted for review.
(k)
Pipes.
(1)
The following information shall be shown on the plans and/or profiles:
a.
Location of pipes and length.
b.
Grade.
c.
Alignment.
d.
Curve data for radius pipes.
e.
Material and class.
f.
Limits of granular backfill.
(2)
At intake structures, such as flared end section or headwalls, entrance control shall be applied. Two-foot minimum freeboard is required. Check for inundation upstream. Cutoff walls are required to be three feet deep on headwalls. Concrete headwalls are required at the end of the flared end section and at the joint between the pipe and flared end section.
(3)
Outfall termination shall conform to the following:
a.
All concrete outfall pipes shall have a reinforced concrete headwall or a flared end section with cutoff wall and revetment or paving.
b.
Any corrugated metal outfall pipe shall be installed with a cutoff wall. Revetment or paving may be required for erosion control.
c.
If discharge is at a property line, an attempt will be made to acquire a discharge easement on the adjacent property, prior to approval. A guarantee will be required for future extension of the outfall to the property line.
(4)
Open trenching of roadway pavements will be permitted only with the prior approval of the city engineer.
(5)
Road closings will not be permitted without prior approval of the board of aldermen. Contractors will be required to maintain one lane open to traffic at all times.
(l)
Sanitary sewers. All construction details and specifications shall be in accordance with the applicable requirements for drainage structures and pipes within the road rights-of-way and other MSD requirements.
(m)
Bridges and culverts. The following shall be included and/or observed:
(1)
Submit two additional sets of plans for bridges and poured-in-place concrete culverts.
(2)
Structure widths shall be designed for the ultimate roadway width. Guardrail and fencing shall be required at appropriate locations.
(3)
Reinforced concrete approach slabs will be required for all bridges.
(4)
The following information shall be shown for all culverts:
a.
Size, type, grade, alignment, minimum cover for the loading, wing-walls, and/or erosion protection in the form of paving or revetment or floodwall, if required, cutoff footings, etc.
b.
Any connections to culverts shall be shown with a detail; no utilities to run through any culvert. Alignment and changes of grades shall be, as previously mentioned, for pipes. Slopes from culverts to roadways shall not be steeper than 3:1 and shall be sodded or protected with paving or revetment where required.
(n)
Roadway dams. The following shall be included and/or observed:
(1)
A soil engineering design report shall be submitted indicating the adequacy of the proposed dam construction.
(2)
Any dam design shall be supported by a soils report prepared by a professional engineer registered in the state. In no case shall side slopes be steeper than 3:1.
(3)
Adequate top width for pavement and sidewalks shall be provided on dams where the roadway fill serves as the dam. In such cases, the dam shall be designed to store a 100-year 20-minute storm with one foot of freeboard, assuming a blocked overflow structure exists. Berms, guardrail, fence, etc., may also be required.
(4)
The relief line from the spillway structure or the overflow structure in the dam shall be sized on the total upstream tributary area, using the ultimate runoff factor.
(5)
An adequate overflow structure shall be provided which will be of reinforced concrete construction, with the openings covered by grates. Such system shall be designed to maintain the required freeboard and storage and shall also be equipped with a drain valve so that the water level may be lowered for maintenance purposes.
(6)
Erosion protection will be required where the stability of the structure may be endangered.
(7)
The dam design and soils report may require review by the Corps of Engineers or other agencies designated by the city engineer.
(o)
Sinkholes. The following shall be included and/or observed:
(1)
An engineering report shall be submitted indicating the existing sinkhole conditions, intended use and proposed treatment to be accomplished as a part of the development.
(2)
A sinkhole can be used as a point of drainage, provided that an adequately sized outfall pipe is provided to an existing natural discharge point such as a ditch, creek, river, etc., in case the sinkhole becomes inadequate to discharge at the required rate.
(3)
Where required sinkholes within a development shall be treated by approved methods.
(p)
Floodplain study. In order to develop or improve any land subject to flooding and inundation during a 100-year storm (floodplain area), or any land adjacent to a ditch, creek or other drainageway, a property owner or user shall submit a floodplain study to the board of aldermen, MSD, the U.S. Corps of Engineers, state department of natural resources and FEMA, which plan shall show the provisions for adequate flood protection against high water.
(1)
The floodplain study must be prepared by a registered professional engineer of demonstrated competence in hydrology and must show the protection against high water for the proposed improvements, the effect of proposed improvements on other properties, and such other hydrologic problems as may result from the improvements.
(2)
Roadway shoulders and low window or doorsill openings of proposed buildings shall be set two feet above the determined high-water elevation.
(3)
The following information is to be included in the floodplain study:
a.
Profile of creek or ditch.
b.
Profile of top of bank.
c.
Cross sections of critical points on the stream.
d.
U.S.G.S. map of watershed.
e.
Computations establishing the "Q" and the high water elevations for use in determining low sill elevations, etc.
(4)
The above hydraulic computation shall include the average mean velocities. Where velocities in improved channels exceed five feet per second, erosion protection must be provided.
(5)
Where any portion of the floodplain is filled, a volume equal to the filled volume must be excavated to provide for equivalent storage in the floodplain area.
(6)
Two copies of the floodplain study shall be submitted for review.
(q)
Creeks. All creeks within the property being developed within 100 feet of a lot line shall be improved to handle a 25-year storm, and such channel shall be protected from erosion by a means acceptable to the city engineer. The existing 500 feet of downstream channel shall be analyzed as to capacity after development. At the discretion of the board of aldermen, a cash payment to the city in the amount of 110 percent of the estimated cost may be made in lieu of such improvements.
(Code 1973, § 23-50; Ord. No. 2393, § I, 7-11-94; Ord. No. 2493, § 1, 12-11-95; Ord. No. 2575, §§ 1, 2, 2-24-97; Ord. No. 02-32, § 1, 6-24-02)