Sugar Hill › Development Regulations

Article 9 — Performance Guidelines

current as of 2020-12-14verified current · checked 2026-08-24Officialofficial source ↗10 sections · full chapter

§ 9.1

General

9.1.1 Purpose

The Sections enumerated in this Article are guidelines, and are intended to be benchmark indicators of what standards could be acceptable. They are further intended to allow alternate designs which could produce results similar to these performance standards and similar protection to the public. The objective of these performance standards is not to suggest a single methodological standard of acceptance exclusive of all others. Rather they establish what would otherwise be allowed in the absence of an acceptable alternative.

9.1.2 Constraints

The alternative design solutions are constrained by the Design Requirements of Article 5, the Access Requirements and Street and Right-of-Way Requirements and the Street Construction Standards of Article 6, and the Grading, Detention, Drainage Requirements of Article 8, as well as the Purpose and Intent of these Regulations.

9.1.3 Documentation Required

In the event that an alternative is suggested by the applicant, studies and reports conducted by professionals currently certified in the State of Georgia will be required to be submitted to and approved by the City. These studies and reports must clearly relate to the desired results and purposes expressed or implied in the applicable performance standard. Once an alternative has been approved by the City, it shall become a required standard applicable to the specific approved Permit only.

§ 9.2

Lots

9.2.1 Lots should be designed generally such that they are no more than four times as deep as

they are wide at the building setback line, unless excepted by the City Manager.

a.The City may require notation that a House Location Plan (HLP) is required to be approved prior to issuance of a building permit on certain lots when particular care in locating the house or other improvements will be necessary. Such lots include, but are not limited to:

(1)A lot which presents particular or unusual difficulties for a builder to meet minimum required building setbacks;

(2)A lot upon which is located an easement of unusual configuration;

(3)A lot containing floodplain but upon which no fill or other encroachment into the floodplain is anticipated at the time the Final Plat is filed;

(4)A lot upon which is located all or a part of a stormwater detention facility;

(5)A lot upon which is located a buffer which was required by the Zoning Resolution as a condition of zoning approval;

(6)All duplex lots;

(7)All lots within, or partially within, the Chattahoochee River Corridor, or containing a River Corridor Tributary Buffer Zone.

b.The City may require notation that a Residential Drainage Plan (RDP) is required to be approved prior to issuance of a building permit o certain lots where additional (site specific) engineering will be necessary to properly grade the lot or locate the building or other improvements. Such lots include, but are not limited to:

(1)A lot containing floodplain where fill or other encroachment into the floodplain is planned or reasonably expected;

(2)A lot containing severe topographic features interdicting the building site;

(3)A lot containing a drainage easement with a pipe discharge or other facilities, or flow characteristics which may adversely affect the location of a building or other site improvements.

c.The City may require notation that a Residential Drainage Study (RDS) is required to be approved prior to issuance of a building permit on certain lots where particular attention to site grading will be necessary, but formal engineering is not needed. Such an RDS is conducted in the field where the effect of the site grading must be accomplished with adequate care so as not to create a drainage problem on neighboring property.

9.2.2 Side lot lines generally should be at right angles (90 degrees) to straight street lines or

radial to curved street lines as much as practical. Side lot lines should be radial to the radius points of all cul-de-sacs. Variations of more than 10 degrees shall require approval of the City, but shall be approved when appropriate to the reasonable loading pattern of the subdivision, efficient use of the land relative to topographic conditions, or provisions of improved building sites over those which would result without variation of the side lot lines.

9.2.3 Corner lots shall be sufficiently larger so that they have the same width between

minimum side setback lines as an interior lot, but in no case shall more than 75 feet between side setback lines on a corner lot be required.

§ 9.3

Blocks

9.3.1 The lengths, widths, and shapes of blocks shall be determined with regard to:

a.Provision of adequate building sites suitable to the special needs of the type of use contemplated,

b.Applicable zoning requirements as to lot size and dimensions,

c.Needs for convenient access, circulation, control, and safety of street traffic,

d.Limitations and opportunities of topography.

9.3.2 In blocks over 1,000 feet long, the City Manager may, when existing or proposed

pedestrian circulation patterns or public gathering places so justify, require pedestrian ways or pedestrian access easements, as appropriate, through the block.

§ 9.4

Access

A maximum number of 200 residential dwelling units shall be allowed to be constructed with only one street outlet to an existing public street. If a second access to an existing public road is not available or, in the opinion of the City Manager, could induce non-residential traffic through the development, a single entrance may be allowed if designed with a traffic signal and/or sufficient right-of-way and improvements to provide a protected left-turn lane.

§ 9.5

Roadway Design

9.5.1 Street Grades and Design Speeds

a.Minimum grade for all local and minor collector streets shall be 1.5%. Minimum grades for all major collector and arterial streets shall conform to Georgia D.O.T. practice.

b.Minimum grade of less than 1.5% on a local street may be approved by the City, based on adequate engineering designs, where at least 1.5% cannot reasonably be achieved due to topographical limitations imposed by the land. In such cases, a Record Drawing and such computations as necessary shall be provided after construction to establish that the street will drain in accordance with these Regulations. Street sections where unacceptable pooling, excessive spread at catch basins, or other hazardous conditions occur shall be reconstructed or otherwise improved to eliminate such conditions.

c.Minimum vehicle design speeds and maximum grades allowable in City of Sugar Hill by street classification shall be as shown in Table 9-A.

Table 9-A Minimum Design Speeds and Maximum Grades.

Street Category	Maximum Grade	Design Speed
Principal Arterial	6%	60 MPH
Major Arterial	8%	50 MPH
Minor Arterial	10%	50 MPH
Major Collector	10%	40 MPH
Minor Collector	10%	30 MPH
Local	15%*	20 MPH
* Grades between 12% and 14% shall not exceed a length of one hundred and fifty feet (150') and shall require an "as graded" survey prior to the installation of the curb or utilities. The distance shall be measured as the tangent length between points of curvature.

Table 9-B. Constant (K) Values for Vertical Curves

	CREST CURVES		SAG CURVES
STREET CATEGORY	MINIMUM	DESIRABLE	MINIMUM	DESIRABLE
Principal Arterial	200	320	125	155
Major Arterial	100	170	80	110
Minor Arterial	55	80	55	70
Major Collector	55	80	55	70
Minor Collector	30	30	35	35
Local	10	10	20	20

a.All new streets shall adhere to the following standards governing horizontal curvature and superelevation:

Table 9-C Horizontal Curves

Street Category	Minimum Radius (FT)	Maximum Superelevation
Principal Arterial	1333	0.06
Major Arterial	833	0.06
Minor Arterial	560	0.06
Major Collector	560	0.04
Minor Collector	300	0.04
Local	120	0.00
* No superelevation will be allowed on Minor Collectors internal to residential S/D

R= v 15 (e + f) f = side friction factor, as follows:

Vehicle Design Speed (v) 30 40 50 60 Side Friction Factor (f) .16 .15 .14 .12

c.Widening section along existing streets shall be designed reflecting existing curvature and superelevation, if any, unless the existing street has been included in a specific design by Gwinnett County or Georgia D.O.T. which calls for different standards, in which case the project will be coordinated with the overall design.

d.Superelevation Runoff Roadway edge curves shall be provided for tangent runout (bringing edge from a normal crown to centerline elevation) and superelevation runoff (from the end of tangent runout to the point of design superelevation) in accordance with design standards of the Georgia Department of Transportation or other professional engineering standards.

e.Tangents and Compound Curves Between reverse horizontal curves there shall be not less than the minimum centerline tangents shown in Table 9-D unless otherwise specified by the Georgia Department of width.

Table 9-D Tangents

Street Category	Minimum Tangent Length	Desireable Tangent Length
Principal Arterial	150	180 Feet
Major Arterial	125	150 Feet
Minor Arterial	100	120 Feet
Major Collector	100	120 Feet
Minor Collector	75	90 Feet
Local	50	60 Feet
NOTE: Minimum tangents are based on the distance traveled in 1.7 seconds at the design speed for each category of street. Desirable length is based on distance travelled in 2.0 seconds.

Reading grid · rebuilt for display · verify at source (p. 79) ↗

§ 9.6

Street Intersections.

9.6.1 Angle of Intersection

Intersections shall generally be at right angles and shall not be at an angle of less than 85 degrees unless approved by the City, nor less than 80 degrees unless the intersection is signalized in which case the angle of the intersection may be reduced subject to the review and approval of the Gwinnett County Traffic Engineer.

9.6.2 Maximum Grade

Street intersections should be designed with a flat grade wherever possible, but in no case should the grade exceed 2% in normal situations (or 4% in topographical hardship situations on local streets).

9.6.3 Intersection Approaches:

Horizontal Alignment

a.New local streets which approach an intersection with a street in a category higher than itself on a horizontal curve having a centerline radius less than 240 feet shall provide a tangent section of roadway at least 30 feet long. Minor collectors approaching an intersection with a major thoroughfare on a horizontal curve having a centerline radius of less than 550 feet shall also provide the 30 foot tangent section. The tangent length shall be measured along the centerline of the street, from the right-of-way line of the intersecting street, extended, to the point of tangency with the centerline of the curve section.

b.New major thoroughfares shall provide tangent sections at intersections with streets in equal or higher categories as needed to provide adequate stopping distances at their design speeds.

9.6.4 Intersection Approaches:

Vertical Alignment

a.For intersections with local or minor collector streets, a leveling of the street at a grade not exceeding 2 percent shall be provided but no level approach distance is required for streets approaching at less than 7 percent, and a minimum 25 foot level approach distance shall be provided for streets approaching at a grade of 7 percent or more. (See Standard Drawings).

b.As a street approaches an intersection with a major thoroughfare, there shall be a suitable leveling of the street at a grade not exceeding 2 percent and for a distance not less than the following minimums:

TABLE 9-E

APPROACH DISTANCES AT MAJOR INTERSECTIONS

APPROACHING MINIMUM APPROACH

STREET CATEGORY DISTANCE

Principal Arterial 100 Feet Major Arterial 100 Feet Minor Arterial 100 Feet Major Collector 75 Feet Minor Collector 75 Feet Local 50 Feet

*Distance of the approach is measured from edge of pavement of the intersecting street to the point of curvature in the approaching street.

9.6.5 Intersection Radii

Intersection radii for roadways measured at back of curb and for the right-of-way lines shall be as follows. For intersecting streets of difference classification, the larger radii shall be provided. In all cases, adequate right-of-way shall be provided to maintain minimum of 11 feet from back-of-curb. Larger radii may be required for streets intersecting at angles less than 90 degrees.

TABLE 9-F

INTERSECTION RADII

ROADWAY R-O-W

Table or figure — shown as printedverify at source (p. 83) ↗

STREET CATEGORY                                       RADII             RADII*

Arterial                                              40 Feet           20 Feet

Major Collector                                       40 Feet           20 Feet

Minor Collector-Residential                           25 Feet                  9 Feet

Minor Collector-Nonresidential 40 Feet 20 Feet

Table or figure — shown as printedverify at source (p. 83) ↗

Local-Residential                                     20 Feet                  9 Feet

Local-Commercial or Office                            25 Feet           11 Feet

Local-Industrial                                      40 Feet           25 Feet

*Intersecting right-of-way lines may be joined by an arc having the minimum radius shown, or by a miter which cuts across right-of-way lines connecting the points where the required radius would have otherwise been tangent.

9.6.6 Islands

Islands in street intersections shall conform to the design requirements of the standard drawings. In no case shall anything in an island extend more than 3 feet above the street grade within the right-of-way, except traffic regulatory devices and other infrastructure erected or approved by The City of Sugar Hill. No island shall be approved which contains less than 100 square feet.

9.6.7 Intersection Corner Sight Distance

a.Intersections shall be designed with adequate corner sight distance for each street which approaches a street in an equal or higher street category (except an intersection of two local streets). Where necessary, backslopes shall be flattened and horizontal or vertical curves lengthened to provide the minimum required sight distance.

b.The minimum corner sight distance from the approaching street shall be equal to or exceed 10 times the regulated speed of the intersecting street, as measured from the center of the approaching street in both directions along the right-of-way line of the intersecting street. As an alternative, the minimum corner sight distance requirement may be calculated using AASHTO "Policy on Geometric Design of Highways and Streets," Chapter 9 (at-grade intersections), latest edition. The sight distance shall provide clear visibility of an object 4 feet above the intersecting street viewed from the centerline of the approaching street at the right-of-way line of the intersecting street, at a height of 3.5 feet above the ground.

9.6.8 Obstructing Visibility at Intersections

On any corner lot, within an area formed by the lot lines on the street sides of such lot and a line (miter) joining points on such lot lines located at a distance of 20 feet from the point of their intersection, the following shall apply:

a.There shall be no fence or wall or hedge higher than three feet.

b.Except standard street signs erected by Gwinnett County or the City of Sugar Hill, there shall be no obstruction to vision, other than a post, column or tree exceeding one (1) foot in greatest cross-sectional dimension, between a height of three (3) feet and a height of fifteen (15) feet above the established grade of either of the intersecting streets.

9.6.9 Turning Lanes at Intersections

Left turning lanes shall be provided on all new internal project streets, classified as a minor collector or major thoroughfare, intersecting a major thoroughfare, and may be required in other locations to meet traffic demand and safe operations. Right turning lanes may be required to meet traffic demands or safety concerns. When provided, turning lanes shall meet the following criteria:

a.Storage length - A minimum of 150 feet of storage length for turning lanes on any arterial roadway shall be used. A minimum of 100 feet of storage length for turning lanes on all collectors shall be used.

b.Taper Length - The minimum taper length shall be 50 feet.

c.Left turning lanes from arterial roads shall be subject to longer storage lengths and tapers, as determined on a case by case basis.

§ 9.7

DRIVEWAY INTERSECTIONS

9.7.1 Angle and Improvements

Driveways shall generally intersect streets at right angles. The portion of a driveway located within a public right-of-way shall be paved, if any. Driveways providing access to parking lots which contain six or more spaces shall be paved in accordance with the parking lot requirements of the Zoning Resolution.

9.7.2 Driveway Design Standards

a.Driveways serving single-family detached or duplex residences may be no less than ten feet wide at the right-of-way line and shall provide a radius to the back of curb or edge of pavement of the roadway of no less than five feet. All other driveway curb cuts on public streets shall conform to the standards shown on the driveway details contained in the Standard Drawings, by land use type as follows:

(1)Driveway Detail 1 (32' Width, 25' Radius) for:

(a)Service Stations;

(b)Commercial Sites (over 80,000 Square Feet);

(c)Office/Institutional Complexes (Over 100,000 Square Feet);

(d)Apartment/Condo Complexes (Over 200 Units); and;

(e)Mobile Home Complexes (Over 200 Lots).

(2)Driveway Detail 2 (28' Width, 25' Radius) for:

(a)Commercial Sites (80,000 Square Feet or Less);

(b)Office/Institutional Complexes (100,000 Square Feet or Less);

(c)Apartment/Condo Complexes (200 Units or Fewer); and,

(d)Mobile Home Complexes (200 Lots or Fewer)

(3)Driveway Detail 3 (32' Width, 40' Radius) for:

(a)Industrial Sites

(4)Driveway Detail 4 (Optional Design with Island) for:

(a)Private Commercial/Office Street Entrances;

(b)Private Entrances to Apartment/Condo Complexes (Over 200 Units);

and,

(c)Private Entrances to Mobile Home Complexes (Over 200 Lots).

b.All driveways and driveway curb cuts on State highways shall conform to Georgia DOT standards.

9.7.3 Auxiliary Lanes

Along any major thoroughfare, a deceleration lane, acceleration lane, larger turning radius, traffic islands, or other devices or designs may be required to avoid specific traffic hazards which would otherwise be created by the proposed driveway location.

9.7.4 Corner Sight Distance

All driveways approaching a minor collector or major thoroughfare shall provide adequate corner sight distance. The minimum corner sight distance from the driveway shall be equal to or exceed 10 times the regulated speed of the intersecting street, as measured from the center of the driveway in both directions along the right-of-way line of the intersecting street. As an alternative, the minimum corner sight distance requirement may be calculated using AASHTO "Policy on Geometric Design of Highways and Streets," Chapter (at-grade intersections), latest edition. The sight distance shall provide clear visibility of an object 4 feet above the intersecting street viewed from the centerline of the driveway at the right-of-way line of the intersecting street, at a height of 3.5 feet above the ground.

9.7.5 Separation, and Spacing

All driveways except those serving residential units on individual lots shall be recommended to meet the following criteria:

a.Minimum separation from a street intersection: 100' from centerline of driveway to nearest right-of-way line of the intersecting street, extended. For any driveway on a major thoroughfare having a centerline between 100' and 200' from the intersecting street right-of-way line, access restriction may be imposed to avoid traffic hazards.

Greater separation may be required for safe operation of a free-right lane, acceleration or deceleration lane, etc.

b.Minimum separation between driveways along the same side of a major thoroughfare: 100' between centerline as measured along the roadway edge or back of curb.

c.Whenever possible, proposed driveways along one side of a street shall coincide with existing or proposed driveways on the opposite side of such street.

d.Maximum number of driveways serving a single project: one for each 400' of property frontage, or fraction thereof per street, along a major thoroughfare. This is not meant to be a spacing standard but only an expression of the total number of driveways that are permitted serving a single project.

§ 9.8

STORMWATER DETENTION GUILDELINES

9.8.1 General

a.Stormwater detention facilities shall be designed so that their peak release rates, when combined with those of all detention bypass areas in the same basin, produce peak flowrates and flow velocities at the site's boundary line no greater than those which occurred at the same location for pre-developed conditions.

b.The positive effects of stormwater management via on-site detention facilities diminish rapidly as the distance downstream from the point of distance increases, and the smaller the facility's contribution is, as a percentage of the total runoff contributing to downstream flow, the shorter the distance downstream that the benefits are realized. Because of these limitations, on-site detention is effective at controlling flooding only when flow from the facility is a significant percentage of the total flow at the point of interest, and only if the point of interest is "immediately" downstream. The concepts of "immediately downstream" and "significant percentage of total flow are inseparable." The portion of a receiving watercourse (one which receives and conveys runoff from a site) which lies within a flow distance of one-half mile (2,640 linear feet) downstream from the site, shall generally be considered to constitute that portion of the watercourse which is "immediately" downstream. However, the total flow in the receiving watercourse may become very large, relative to the flow contributed by the project site, within a much shorter distance For this reason, the "substantial percentage" test must also always be applied. For purposes of these Regulations, the flow from a site represents a "significant percentage" of the total flow in a watercourse only when the ratio of the peak flow rate from the site to the peak flowrate in the watercourse (including the contribution from the project site) is greater than 5 percent.

c.Peak flowrate control shall normally be provided only for the 2-year, 5-year, 10-year, and 25-year frequency storm events. However, under certain conditions, the 100-year event must also be detained to the pre-developed rate. Such control of the 100-year event shall be provided when failure to do so would result in flooding of other habitable dwellings, property damage, or public access and/or utility interruption.

d.For any storm water analysis, the composite “C” (Rational Method) or CN (SCS Method) used for analysis of pre-development conditions shall not exceed 0.15 or 55, respectively, unless prior approval has been obtained from the Department. A pre-design conference between the design engineer and appropriate Department personnel, which may in certain straightforward cases be conducted via the telephone, is required.

e.Rational Method runoff coefficients used for analysis of pre- and post-development conditions shall be consistent with those shown in the Storm Water Design Manual..

9.8.2 Dam Design and Construction Criteria

a.Detention facilities which take the form of normally-dry basins, ponds, or lakes usually are created by damming a drainageway or watercourse. Such dams can take a variety of different forms, the most common being earthen embankments and reinforced concrete walls. Each type of dam has different characteristics, and the selection of the most appropriate type for a particular site should be made by a Professional Engineer and based on the physical features of the dam site, the purpose of the dam, the type of impoundment, safety, and maintenance requirements.

b.For purposes of these Regulations, dams will be addressed separately for each of the three most frequently encountered types of detention facilities: normally-dry basins, ponds, and lakes. A normally-dry basin is one designed to impound stormwater runoff for only a brief period of time following a storm event. The vast majority of the time the basin will be completely dry except for any normal stream flows which pass through unimpeded. Lakes and ponds, on the other hand, are designed to impound a body of water at least several feet in depth on a more-or-less permanent basis. Lakes and ponds vary from one another only in terms of magnitude. The magnitude of a lake is determined primarily from the height of its dam, the size of its contributing drainage area, and the volume of water it is capable of impounding. For purposes of these Regulation, a pond is any lake having a dam height of less than 20 feet, a drainage area of less than 100 acres, and which is incapable of impounding more than 10 acre-feet of water.

c.All dam design is to be certified by a Professional Engineer currently registered in the State of Georgia.

d.Dams for normally-dry detention basins shall conform to the following:

(1)Dams for normally-dry detention basins may be constructed of earth, reinforced concrete, mortared rubble, or other suitable materials.

(2)The design of any concrete or rubble wall over 5 feet in height shall be certified by a Structural Engineer currently registered as a Professional Engineer in the State of Georgia, and the structural design shall be based on soil tests certified by a Geotechnical Engineer currently registered as a Professional Engineer in the State of Georgia.

(3)Any non-earthen structure shall be designed to prevent piping failure through its subgrade and abutments.

(4)The construction of walls over 5 feet in height shall be monitored and approved by a qualified materials testing company.

(5)Earthen dams for normally-dry detention basins shall have a top width of no less than 8 feet, and slopes on both faces no steeper than 2 (horizontal) to 1 (vertical).

(6)For earthen dams for normally-dry detention basins, there shall be at least 1 1/2 feet of vertical separation between the 100-year ponding elevation in the basin and the low point on the top of the dam. One foot of this distance is to provide a margin of safety against overtopping of the dam and the other 6 inches is to allow for settlement. No separation is required for a nonearthen dam, if it has been designed to overtop safely.

(7)More stringent design and construction criteria shall be used for dams for normally-dry detention basins whenever the probable consequences of dam failure are severe.

e.Dams for ponds shall conform to the following:

(1)Any engineer responsible for the design of a dam for a pond is expected to be knowledgeable of the criteria contained within the Georgia Safe Dams Act, Georgia Department of Natural Resources "Rules for Dam Safety"

publication, and the U.S.D.A. Soil Conservation Service's Technical Release No. 60 "Earth Dams and Reservoirs." The provisions of each are to be applied wherever applicable. Applicability shall be determined based upon site-specific constraints and downstream conditions. Consultation with appropriate City personnel both prior to and throughout the design process is encouraged.

f.Dams for lakes shall conform to the following:

(1)Any engineer responsible for the design of a dam for a lake is expected to be thoroughly familiar with the criteria contained within the Georgia Safe Dams Act, Georgia Department of Natural Resources "Rules for Dam Safety"

publication, and the U.S.D.A. Soil Conservation Service's Technical Release No. 60 "Earth Dams and Reservoirs." All design is to be in accordance with the applicable requirements contained in each of the above referenced publications.

9.8.3. Detention Facility Outlet Devices

a.Because of the variables that may be associated with the choice of an outlet device for any given condition, the design consultant is responsible for the selection of the device, subject to the review and approval of the City.

b.The City will include in its consideration the ease of maintenance, longevity of the system, freedom from congestion, practicality, and aesthetics in its review of the outlet device. The consultant should be guided by the preference of vertical weir designs since they have proven to generally meet most of the considerations expressed herein.

c.No orifice shall be smaller than 3 inches in diameter. An orifice smaller than 15 inches in diameter shall be protected by a trash rack. A trash rack protecting an orifice shall have surface area of at least 10 square feet. Design shall be in accordance with the Storm Water Design Manual. No opening in the trash rack shall have an area more than one-half the size of the area of the orifice being protected. Two-stage trash racks, or screens having progressively smaller openings placed in series, are suggested. To facilitate outlet operation, curved or inclined trash racks designed to allow debris to rise with the water level are preferred. In all cases, trash racks shall be either hinged or removable to facilitate maintenance operations.

d.If the primary detention facility outlet is a conduit through a dam, and there is not an orifice, weir-box, or other flow-control device affixed to the upstream end, then the conduit shall be analyzed for both inlet and outlet control conditions. If an orifice or weir-box is affixed, then the conduit shall be analyzed to determine if any flows will occur for which outlet control conditions of the conduit, rather than the hydraulic characteristics of the flow-control structure, will determine the total flows occurring.

In any case where the conduit through the dam is less than 15 inches in diameter, the trash rack provisions of "c" above shall be followed.

e.Unless the 100-year maximum flow velocity in a conduit through a dam forming a pond or a lake is less than 10 feet per second, and the hydraulic grade line for the 100-year condition is at or below the crown of the conduit for at least 90 percent of its length, the conduit must be equal or superior to Class V reinforced concrete pipe in its structural characteristics.

9.8.4 Emergency Overflow Requirements

a.For every type of detention facility, a planned safe flowpath must be provided for conveyance of flows of water in excess of those for which the detention facility was designed. In many instances, this function can be provided through installation of an emergency spillway. Emergency spillways are usually excavated open channels, either vegetated or paved with reinforced concrete.

b.Every earthen dam shall be provided with an open-channel emergency spillway, unless all of the following apply:

(1)The principal spillway is a closed conduit having a cross-sectional area that can pass 125 percent of the 100-year storm routed peak discharge.

(2)The principal spillway is a closed conduit having a cross-sectional area of at least one square foot per each three acres of drainage area, or a maximum of twenty square feet of surface area, whichever is less.

(3)The inlet is a reinforced concrete box structure having an interior width equal to the width of the conduit.

(4)The principal spillway capacity is at least equal to the capacity required for an open-channel emergency spillway.

(5)The low point of the dam crest in not in a fill section.

(6)A trash rack or other debris protection is provided on the outlet control.

c.Any portion of any emergency spillway excavated into a dam embankment or other fill section must be paved. Pavement material shall be either reinforced concrete or asphalt, as dictated by the design life of the dam and the potential consequences of its failure. Any portion of any emergency spillway excavated into natural ground shall be vegetated in accordance with the practices described in the "Manual for Erosion and Sediment Control in Georgia."

d.In determining the necessary dimensions of an open-channel spillway for a normally-dry basin, a pond, or a lake, either the methodology contained in the "Earth Urgency Spillway Design Data" section of the "Manual for Erosion and Sediment Control in Georgia".

e.Emergency spillway capacity for earthen dams shall be as follows:

(1)For normally-dry detention basins, ponds, and lakes, having a dam height of less than 9 feet, a drainage area of less than 200 acres, and which are incapable of impounding more than 20 acre-feet of water, and for which the probable sequences of dam failure are not severe, the emergency spillway shall be placed at the 25-year ponding elevation or higher. Its capacity shall be at least equal to the difference between the 100-year peak flow into the detention facility and the 25-year peak release rate from the facility.

(2)For normally-dry detention basins, pond, and lakes, which do not meet all of the magnitude limitations in "1" above, the emergency spillway shall be placed no lower than the 100-year ponding elevation, and its capacity shall be at least equal to the lesser of either the full 100-year peak flowrate into the facility, or the routed one-third PMF hydrography. In those cases where State or Federal regulations may require greater spillway capacity, those more stringent regulations shall govern.

f.Emergency overflow for non-earthen dams may take the form of planned structure overtopping. In such cases, however, care must be taken to prevent flows from eroding supporting soils along the toe of or immediately downstream from the dam so as to cause it to be undermined. The profile of the top of the dam shall be so designed as to prevent flows along the ends of the structure which might result in abutment erosion.

9.8.5 Parking Lot Detention Facilities

a.Parking lot detention facilities shall generally be of one of the two following types:

(1)Depressed areas of pavement at drop inlet locations; and,

(2)Ponding areas along sections of raised curbing. The curbing in these areas is usually higher than a standard curbed section.

b.The detention methodology utilized for all parking lot detention facility design shall conform to the Storm Water Design Manual.

c.Parking lot detention areas shall be located so as to restrict ponding to areas other than parking spaces near buildings, and to not encroach upon entrance drives.

d.The maximum depth of detention ponding in a parking lot, except at a flow control structure, shall be 6 inches for a 10-year storm, and 9 inches for a 100-year storm.

The maximum depth of ponding at a flow control structure shall be 12 inches for a 100-year storm.

e.In truck parking areas, the maximum depth of ponding shall be 12 inches for the 10-year storm.

f.Detention ponding areas are to be drained within 30 minutes after the peak inflow occurs.

g.Parking lot detention areas shall have a minimum surface slope of 1 percent, and a maximum slope of 5 percent.

9.8.6. Underground and Rooftop Detention Facilities

The design of underground or rooftop detention facilities shall be in accordance with current engineering standard practice, and shall conform to the general spirit and intent of this Article. In the case of rooftop detention, permissible structural loads and weatherproofing shall be governed by the Georgia State Building Code.

9.8.7 Sediment Basins

a.Stormwater management and sediment trapping functions should be separated whenever possible. Every erosion control design should seek to: first, prevent erosion from occurring; second, trap sediments as close to their sources as possible, and: third, provide a second-tier or backup line of defense against sediments leaving the project site. This backup defense will usually consist of check dams/and or sediment basins.

b.Whenever a sediment basin and a detention facility are both required on the same watercourse, the sediment basin should be located immediately upstream of the detention facility.

c.In unusual cases where a normally-dry detention basin is planned to be used to trap sediment as well as provide stormwater control, the basin may be undercut to accommodate the sediment so that the required detention characteristics, particularly volume, will be maintained.

d.The design of sediment basins shall be in accordance with Appendix C of the "Manual for Erosion and Sediment Control in Georgia."

9.8.8 Ponds and Lakes Not Used for Detention

In such cases where a pond or lake is provided as part of a development, but is not planned to function as a stormwater detention facility, the same general and specific criteria contained in these Regulations shall apply, but may be modified in instances where a specific requirement is clearly detention oriented rather than safety-based.

§ 9.9

CULVERTS AND PIPE COLLECTION SYSTEM GUIDELINES

9.9.1 Culverts

a.Single barrel or single cell culvert structures are less prone to clogging and require less maintenance than multi-barrel or multi-cell installations and should therefore be used whenever feasible.

b.The maximum velocity in a corrugated metal culvert for the 100-year flow shall be 15 fps (feet per second). Velocities over 10 fps shall be considered a special design with particular attention required to pipe or structure invert protection and to fill slope, stream bed, and stream bank stability.

c.The minimum allowable slope shall be in accordance with the Storm Water Design Manual.

9.9.2 Piped Collection Systems

a.The maximum velocity in a corrugated metal piped system for the design flow shall be 15 fps. Velocities over 10 fps shall be considered a special design with particular attention required to pipe invert protection and the ability of the receiving waterway or detention facility to accept the flow without damage.

b.The minimum allowable slope shall be in accordance with the Storm Water Design Manual.

c.The maximum allowable slope for concrete pipe shall be 10 percent, corrugated metal pipe shall be 14 percent, and for a HDPE pipe shall be 14. Greater slopes may be approved if installation is in accordance with manufacturer's recommendations. In cases where the slope is in excess of 10 percent, anchor collars may be required.

d.A minimum pipe cover of one foot shall be required.

9.9.3 Outlet Location - Culverts and Pipe Systems

a.Outlet structures (such as headwalls) shall not be located closer to the project site's property line with an adjoining property than a flow distance equal to 6 pipe diameters. For non-circular conduits, this distance shall be six times the rise dimension of the conduit.

b.The invert elevation of a culvert or pipe outlet shall be no more than 2 feet above the elevation of the bottom of the receiving watercourse at the outlet.

9.9.4 Energy Dissipation

The maximum developed condition flow velocity at the project site's downstream property line with an adjoining tract shall not exceed the maximum predeveloped condition velocity.

Calculations may be required to support this velocity standard on a case-by-case basis.

9.9.5 Discharge of Concentrated Flows

a.The discharge of concentrated flows of stormwater into public roadways shall be avoided. In no case shall such concentrated flows, including flows from swales, ditches, draws, driveways, or piped systems, exceed the allowable peak flowrates in Table 9-G, below.

TABLE 9-G

MAXIMUM FLOWS INTO STREETS

ALLOWABLE PEAK FLOWRATE

STREET CLASSIFICATION FOR A 2-YEAR STORM Local 2.0 cfs Minor Collector 1.0 cfs Other 0.5 cfs

b.In residential subdivisions, the drainage area contributing to the peak flow along any property line between lots within 50 feet of the building setback line for either lot shall not exceed 2 acres, unless contained within a piped drainage system or maintained in a natural watercourse. The stormwater conveyance shall be in a drainage easement.

§ 9.10

Site Lighting Guidelines

All site lighing shall be designed and maintained to prevent light trespass onto nearby residential properties in excess of two hundredths (.2) of a foot candle (fc) as measured at a six (6’) foot height perpendicular to the ground. All lamps must be shielded to prevent glare from any point along the property line so that no direct light source is visible from a sight line established at a distance between four (4’) feet and six (6’) feet perpendiculars from the ground. The lighting plan must be approved by the City’s Planning Director prior to receiving any land disturbance and building permit.

Article 10. Plan and Plat Specifications.