CODE OF THE CITY OF FLORENCE
ARTICLE VIII. STORM DRAINAGE
TABLE OF CONTENTS SECTION DESCRIPTION
Sec. 18-100 General requirements
Sec. 18-101 Drainage system standards
Sec. 18-102 Areas subject to flooding; Floodplain management administrator
Sec. 18-103 Methods of calculating streamflow and runoff
Sec. 18-104 Natural primary and/or major drainage channel requirements
Sec. 18-105 Secondary drainage channels and surface drainage requirements
Sec. 18-106 Major channel requirements
Sec. 18-107 Bridge and culvert requirements
Sec. 18-108 Closed storm sewer
Sec. 18-109 Open paved storm drainage
Sec. 18-110 Areas outside subdivision or development
Sec. 18-111 Existing open ditches
Sec. 18-112 Review by city engineer
Sec. 18-113 Application and enforcement of this article
Sec. 18-114 Interpretation and conflict
ARTICLE VIII. STORM DRAINAGE*
*Cross references – Keeping gutters free of grease, 17-12b; discharge of stormwater into sanitary sewer prohibited, 12-29' drainage onto streets, 17-7 et seq.
Sec. 18-100. General requirements
(a) Requirement for drainage system: Every proposed development must have a drainage system which shall be designed by a registered engineer and constructed by the developer to provide for the proper drainage of surface water of the development and the drainage area of which it is a part; to permit the unimpeded flow of natural watercourses, and to provide positive drainage away from on-site sewage disposal facilities. The subdivider/developer's responsibility shall include those drainage facilities to discharge such developer's storm runoff to an existing facility outside the secondary area capable of receiving said runoff with no adverse effects.
(b) Drainage study required for major developments: A drainage study and report shall be prepared by a registered engineer engaged by the developer. This report shall include both existing and proposed drainage conditions and shall include an evaluation of the ability of the proposed drainage facilities and other improvements pertaining to drainage or flood control within the development to handle the runoff which would be generated by the development. The report will also contain a section devoted to the analysis of the downstream drainage facilities and its ability to handle the existing flow and the increased flow from the development. The report shall also contain the following items:
(1) Calculated estimates of the quantity of storm water entering the development naturally; also, estimates of such water when the upper watershed area shall have been developed for the maximum land use permitted under the official land use plan.
(2) Existing conditions of the watershed that may affect the proposed development, such as subsoil type, positive drainage channels, obstructions and the like.
(3) Quantities of flow at each pickup point.
(4) Estimates of temporary erosion and pollution controls necessary while the development is under construction.
(5) Description of major, primary and secondary systems. The report shall include an evaluation of proposed facilities under both frequent and infrequent storms. (ten (10)-year, fifty (50)-year, one-hundred (100)-year). The Flood Insurance Study, City of Florence, S.C., Federal Emergency Management Agency, June 1, 1981, and Flood Plain Information, Florence, S.C., U.S. Army Corps of Engineers, October 1967, shall be considered as information and utilized where applicable.
(6) Drainage analysis for minor developments. Minor development projects as defined in section 18-1 shall also be studied in order to determine the adequacy of any existing storm drainage systems or the effect of proposed improvements. The city engineer may require as much or as little of the above information as needed to conduct an appropriate drainage analysis of the site plan, grading plan, or storm drainage analysis of the site plan, grading plan, or storm drainage plans submitted to the city as part of the requirements of the site plan needed for a zoning certificate and building permit. The city engineer may waiver the requirement that a registered professional engineer conduct the drainage study for minor developments.
(c) Avoiding runoff problems: In designing storm drainage facilities, special consideration shall be given to the avoidance of problems which may arise from concentration of storm water runoff onto adjacent developed or undeveloped properties.
(d) Handling anticipated peak discharges: Storm drainage facilities shall be designed not only to handle the anticipated peak discharge from the property being developed, but also the anticipated increase in runoff that will occur when all property at a higher elevation in the same drainage area is fully developed.
(e) Connection to underground facilities: In those instances when underground piped storm drainage facilities are reasonably accessible to the proposed development, the developer shall connect such developer's on-site drainage system to the city's system based on standard storm drainage design practices or other special design standards deemed necessary by the city engineer.
(f) Drainage easement criteria: Drainage easements shall be provided in accordance with the following criteria:
(1) Where development is traversed by a drainage facility, adequate areas for storm drainage, including ponding, shall be allocated, conforming substantially with the lines of such drainage facility, and of sufficient width to carry off storm drainage and provide for maintenance and improvement of the drainage facility. Adequate access for maintenance and equipment will be required. Generally, for underground storm drainpipe, the minimum width of the easement shall be not less than twenty (20) feet or the outside diameters of the pipes in feet plus eight (8) feet each side of the pipe whichever is greater. Where open improved drainage channels, paved or unpaved, are permitted, the width of the easement shall be a minimum of three (3) feet on one side measured at the intersection of the existing ground and channel cut plus the width of the channel at the top or ground level, plus fifteen (15) feet on the opposite side to allow equipment to enter for maintenance operations. Depending on the design of the paved channel, this requirement may be reduced to not less than ten (10) feet on one side.
(2) The location of any surface or underground drainage facilities shall not be changed without the approval of the city engineer.
(Code 1973, App. E, 2)
Sec. 18-101. Drainage system standards.
(a) Street drainage shall serve as a basic drainage system. All streets shall be provided with an adequate storm drainage system consisting of curbs, gutters, and storm sewers.
(1) All streets shall be designed so as to carry the storm water drainage of at least the street itself and adjacent property.
(2) Curb drainage inlets shall be provided at appropriate intervals along streets with curb and gutter drainage facilities. These inlets shall connect to storm sewers and a drainage inlet structure with a protective grating shall be installed in accordance with standard specifications of the state department of highways and public transportation.
(3) Inlet spacing and capacity shall be adequate to limit the spread of water into the street and to maintain pedestrian walks and street crosswalks free of standing water.
(4) Where driveways connect to existing streets with side ditch drainage facilities, a culvert shall be provided under such driveway as required by the city engineer. The minimum pipe size is fifteen (15) inches and shall conform to standard specifications of the state department of highways and public transportation.
(5) All streets having curb and gutter on which storm water flows across intersections and/or driveways shall be provided with suitable cross-gutters at such intersections and driveways.
(b) Off-street drainage: The design of the off-street drainage system shall include the watershed affecting the subdivision and shall be extended to a watercourse or ditch adequate to receive the storm drainage and shall be designed in accordance with the following requirements:
(1) When the drainage system is outside of the street right-of-way, the subdivider shall provide all required easements in accordance with subsection (f) of section 18-100.
(2) Open ditch drainage may be used, provided that such ditches are "V" ditches with side slopes not exceeding one to three (1:3) ratio, (one vertical to three (3) horizontal) and maximum depth of two (2) feet. Where open ditches meeting these requirements are not adequate to provide satisfactory storm drainage, an underground piped system shall be installed as required by the city engineer. These open ditches shall be protected from erosion by either sodding or seeding as approved by the city engineer.
(3) No open natural major storm drainage course shall be permitted within seventy-five (75) feet of the rear or side of a building as measured from the building to the top of the edge of the drainage facility or vice versa, unless exceptional site planning opportunity is afforded and the improvement will not be jeopardized by flooding or erosion.
(4) Off-street drainage from private parking lots or other buildings/uses shall be channeled to and collected at one or more catch basins located on the private property of the development and piped underground to any reasonably accessible existing storm drainage facility. Such surface storm runoff shall not be allowed to flow over public sidewalks.
(Code 1973, App. E, 3)
Sec. 18-102. Areas subject to flooding; floodplain management administrator.
The planning and development director shall be the floodplain management administrator for the city.
(Code 1973, App. E,4)
Cross reference – See App. A, 508
Sec. 18-103. Methods of calculating streamflow and runoff.
Minimum design frequencies for calculating rainfall runoff shall be as outlined below:
Residential 5 to 10 years
Light industrial and commercial 10 to 25 years
Heavy industrial and high-value commercial 25 to 50 years
Flood protection works 50 to 100 years
Variance in design frequency shall depend upon the density of development, existing and expected; value of development; and cost effectiveness of design. Minimum design frequency may be used if approved by the city engineer. The following formula and values shall be used for calculating all streamflow and runoff for the policies and regulations established herein:
(1) Runoff from drainage areas of five hundred (500) acres or less shall be determined by the "Rational Formula":
Q = CiA
Q= Storm water runoff in cubic feet per second (cfs) at a specified point and time. C= Coefficient representing the ratio of storm water runoff to rainfall over the tributary area based on imperviousness of area, ground slope, and ground storage. Coefficients may vary from 0.15 to 0.95. Coefficients of less than 0.5 may be used if individual calculations are submitted with plans to the city engineer.
i= Average rainfall intensity in inches per hour over the tributary drainage area for the period of time equal to the time of concentration and given frequency of occurrence.
A= Equals area to be drained in acres, determined by field surveys for areas less than one hundred (100) acres, and by latest government quadrangle maps for larger areas.
B= Runoff from drainage areas greater than five hundred (500) acres will be determined by use of hydrograph or other engineering methods as approved by the city engineer.
(2) The size of closed storm sewers, open channels, culverts, and bridges shall be determined by using the "Manning Formula" which may be modified for use with runoff determined by the "Rational Formula" to:
Q = 1.486 R S A
Q= Discharge in cubic feet per second.
A= Cross-sectional area of water in conduit in square feet.
R= Hydraulic radius of water in conduit.
S= Mean slope of hydraulic gradient in feet of vertical rise per foot of horizontal distance.
N= Roughness coefficient.
(Code 1973, App. E. 5)
Sec. 18-104. Natural primary and/or major drainage channel requirements.
All natural primary and/or major drainage channels which are located within or along the property line of an improvement, development or subdivision shall be protected and improved by the developer as follows:
(1) The existing channel lying within or along the property line of the subdivision or parcel of land proposed for development or redevelopment shall be cleaned to provide for the free flow of water, and the channel shall be straightened, widened, and improved to the extent to prevent overflow resulting from a fifty (50)-year frequency rainfall, beyond the limits of the dedicated drainage easement provided for in subsection (f) of section 18-100.
(2) Site improvement shall provide for the grading of all building pads to an elevation where all building pads will not be subject to overflow from one hundred (100)-year frequency flood and in a manner that will provide for a rapid runoff of storm water.
(3) Whenever channel improvements are carried out, sodding, backsloping, cribbing, and other bank protection shall be designed and constructed to control erosion for the anticipated conditions and flow resulting from a fifty (50)-year frequency rainfall.
(4) An existing natural drainage channel shall not be located in a street easement unless it is placed in an enclosed storm sewer except under the following conditions:
(a) Where a paved street surface at least two (2) lanes wide is provided on both sides of a paved channel so as to provide access to abutting properties.
(b) When a condition exists as outlined above, adequate space shall be dedicated as right-of-way to provide for maintenance of the paved drainage channel.
(5) Culverts, bridges and other drainage structures shall be constructed in accordance with the specifications and design criteria of the city when the city shall have present or future maintenance responsibility.
(6) No open natural storm drainage course shall be permitted within seventy-five (75) feet of the rear or side of a building as measured from the building to the top of the edge of the drainage facility or vice versa, unless exceptional site planning opportunity is afforded and the improvement will not be jeopardized by flooding or erosion.
(Code 1973, App. E, 6)
Sec. 18-105. Secondary drainage channels and surface drainage requirements.
All secondary drainage channels which are within, or immediately adjacent to, an improvement, development or subdivision shall be protected and improved by the developer as follows:
(1) Secondary drainage channels which have a primary function of collecting surface water from adjacent properties or intercepting and diverting side hill drainage shall be underground and piped.
(a) In single-family residential, duplex or apartment/townhouse/condominium development, site-grading shall be carried out in such a manner that surface water from each dwelling lot will flow directly to a storm sewer, sodded swale, or paved street with storm drainage without crossing more than four (4) adjacent lots in overland flow and with no adverse effects to adjacent property.
(b) In commercial, industrial and institutional development, roofs, paved area, yards, courts and courtyards shall be drained into a storm sewer system.
(3) Surface water collected on streets shall be diverted to storm drains at satisfactory intervals to prevent overflow of six (6)-inch high curbs during a ten (10)-year frequency rain for the area and grades involved. Design frequency may vary with the classification of street, highway, or land use in the area. Drainage area allowed for surface flow on streets at point of diversion shall not exceed twenty (20) acres, regardless of flow.
(4) Drainage easement of satisfactory width to provide working room for construction and maintenance shall be provided for all storm sewers as detailed in subsection (f) of section 18-100.
(Code 1973, App. E, 7)
Sec. 18-106. Major channel requirements.
All major channels which are located outside the floodplain area as defined by the federal emergency management agency's flood insurance study and maps, or the U.S. Army Corps of Engineers, whichever is most appropriate to the situation, and which are located within or immediately adjacent to an improvement or subdivision shall be protected and improved by the developer as follows:
(1) The existing channel shall be cleaned to provide free flow of water, straightened, widened, leveed or diked, or otherwise improved to the extent required to prevent overflow form a fifty (50)-year frequency flood.
(2) Site improvements shall be in accordance with the provisions of section 508 of Appendix A (the city zoning ordinance).
(Code 1973, App. E, 8)
Sec. 18-107. Bridge and culvert requirements.
All flow of water across continuous streets or alleys shall be through culverts or bridges. Bridges and culverts shall be sized to accommodate a fifty (50)-year frequency rain. Design of bridges and culverts shall conform to city and construction specifications of the state department of highways and public transportation, and approved by the city engineer.
(Code 1973, App. E. 9)
Sec. 18-108. Closed storm sewer.
Closed storm sewers shall be constructed of precast, prefabricated pipe, or built in place of closed box design to conform with city and construction specifications of the state department of highways and public transportation. Sizing shall be calculated by the "Manning Formula". However storm sewers carrying runoff from streets may be designed to serve the design frequency rainfall for the drainage area involved, provided that overflow from a one-hundred (100)-year frequency rainfall can reach a suitable outlet without inundating any building pad.
(Code 1973, App. E, 10)
Sec. 18-109. Open paved storm drainage.
Open paved storm drainage channels shall be constructed in accordance with city specifications. Side slopes above the paved section shall be shaped and sodded on a slope of three (3) horizontal to one (1) vertical or flatter. Fences shall not be located any more than one foot (measured horizontally) from the right-of-way/easement line as provided for in subsection (f) of section 18-100.
(Code 1973, App. E, 11)
Sec. 18-110. Areas outside subdivision or development.
City reserves the right to require improvements to preclude any backup of tail water inundating any areas outside of the dedicated drainage easements in the subdivision or development as a result of a fifty (50)-year frequency flood.
(Code 1973, App. E, 12)
Sec. 18-111. Existing open ditches.
Any existing man-made waterway over two (2) feet in depth shall be piped at the time of development.
(Coded 1973, App. E, 13)
Sec. 18-112. Review by city engineer.
Prior to authorization of any building permit by the codes enforcement division, the city engineer shall review and approve all such streamflow, runoff calculations, and drainage plans as such engineer may require of a developer under the terms of this article, and the city engineer shall have final authority of engineer interpretations of all required fifty (50)-year and one-hundred (100)-year flood elevations necessary to this article and shall report the findings to the city's floodplain management administrator for appropriate action.
(Code 1973, App. E, 14)
Sec. 18-113. Application and enforcement of this article.
(a) Inspections: Sufficient inspections shall be made to insure compliance with the specifications set forth in this article. A registered engineer, employed by the developer and approved by the city engineer, may certify in writing to the city engineer that such engineer has inspected each phase of the construction of the storm drainage improvements required in this article and said inspection certification shall meet the terms of this article. The city engineer, however, shall make a final inspection of said improvements before accepting said improvements for dedication to the city for permanent maintenance.
(b) Variations and exceptions to this article:
(1) Whenever strict compliance with these regulations would result in extraordinary hardship or injustice to the subdivider because of unusual topography, unusual size or shape of the property, or unusual conditions in surrounding property or development, the planning commission, acting only upon the written concurrence of the city engineer, may modify, vary or waive such regulations in order that the subdivider or developer may subdivide or develop the property in a reasonable manner, provided that such modification, variation or waiver will not nullify the intent or purpose of this article and that the public welfare, interest of the city and the surrounding area shall be protected. Any such variance, together with reasons therefore, shall be entered upon the minutes of the planning commission.
(2) In granting modification, variations or waivers, the planning commission may impose such other reasonable conditions as well, in its judgment, in order to justify such modification, variation or waiver and still maintain the objectives of these regulations.
(3) Each modification, variation or waiver of these regulations sought by a subdivider or developer shall be applied for specifically on forms supplied by the planning and development division, copies of which shall be forwarded to the city engineer for such engineer's review and comments and an evaluation of such submitted to the planning and development division and the city planning commission.
(c) Amendments to this article. The city council may amend the regulations or provisions of this article after study and a written report by the planning commission and the holding of a public hearing as required by law.
(d) Enforcement. No zoning certificate or building permit shall be issued for any new building or development by the zoning secretary or codes enforcement division without a written statement from the city engineering division stating that the storm drainage facilities for said property and building meet all of the requirements of this article.
(e) Penalty. Any violation of these regulations will be a misdemeanor and upon conviction is punishable as provided by law.
(Code 1973, App. E, 15)
Sec. 18-114. Interpretation and conflict.
In interpreting and applying the provisions of this article, the provisions shall be held to be minimum requirements necessary to uphold the purpose of this article. It is not intended by this article to interfere with or abrogate or annul any easements, covenants, or other agreements between parties; provided, however, where this article imposes greater restriction on the subdivision and/or use of buildings or land, or requires more open spaces or more stringent development standards than required by other resolutions, ordinances, rules or regulations, or by easements, covenants, or agreements, the provisions of this article shall govern. When the provisions of any other statute require more restrictive standards than are required by the regulations of this article, the provisions of such statute shall govern.
(Code 1973, App. E, 17)