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STORMWATER DRAINAGE DESIGN MANUAL
CITY OF JONESBORO CRAIGHEAD COUNTY ARKANSAS
REGULATIONS DESIGNED TO LESSEN AND AVOID HAZARDS TO PERSONS AND PROPERTY CAUSED BY OBSTRUCTION TO DRAINAGE, AND TO OTHERWISE PROMOTE THE PUBLIC HEALTH, SAFETY AND GENERAL WELFARE.
1.1 Title – These regulations shall hereafter be known, cited and referred to as the “Stormwater
Management Regulations” of the City of Jonesboro, Arkansas.
1.2 Authority – These regulations are adopted pursuant to the power and authority vested
through the applicable laws and statutes of the State of Arkansas.
1.3 Applicability – The provisions of this regulation are applicable to all persons, firms, corporations, business, or other legal entity proposing to develop land in the City of Jonesboro Planning jurisdiction.
1.4 Purpose – In order to promote the public health, safety, and general welfare of the citizens
of Jonesboro, the provisions of these regulations are intended to: (1) reduce property damage and human suffering, (2) minimize the hazards of personal injury and loss of life due to flooding, and (3) protect water quality and the environment.
1.5 Definitions – For the purpose of these regulations, certain terms and words shall be used,
interpreted, and defined as set forth in this section. Unless the context clearly indicates to the contrary, words used in the present tense include the future tense; words used in the singular shall include the plural, and vice-versa; and the word, “shall,” is always mandatory.
A. Base Flood – The flood that has a one (1) percent chance of being equaled or exceeded in any given year, i.e., the 100-Year Flood.
B. Bond – The form of security for the completion or maintenance of drainage improvements.
C. Building – Any structure built for the support, shelter, or enclosures of persons, animals, chattels, or movable property of any kind.
D. Channel – Course of perceptible extent which periodically or continuously contains moving water, or which forms a connecting link between two bodies of water, and which has a definite bed and banks.
E. Compliance Letter – An acceptance letter issued by the City Engineer based upon the review of the Stormwater Management plan or construction plans as prepared and certified by the Engineer of Record for a project.
F. Conduit – Any open or closed device for conveying flowing water.
G. Critical Facilities – Include: Governmental facilities that are considered essential for the delivery of critical services and crisis management (such as data and
communication centers and key governmental complexes); facilities that are essential for the health and welfare of the whole population (such as hospitals, prisons, police and fire stations, emergency operations centers, evacuation shelters and schools); mass transportation facilities (such as airports, bus terminals, train terminals); lifeline utility systems (including potable water, wastewater, oil, natural gas, electric power and communications systems); high potential loss facilities (such as nuclear power plants or military installations); hazardous material facilities (such as industrial facilities housing or manufacturing or disposing or corrosives, explosives, flammable materials, radioactive materials and toxins).
H. Detention – The temporary detaining or storage of floodwater in reservoirs, on parking lots, on rooftops, and other areas under predetermined and controlled conditions accompanied by controlled release of the stored water.
I. Detention Basins – Any man-made area which serves as a means of controlling and temporarily storing stormwater runoff. The facility normally drains completely between spaced runoff events, e.g., parking lots, rooftops, athletic fields, dry wells, oversized storm drain pipes.
unimproved land or rehabilitates or adds improvements to an existing improvement on previously improved land.
K. Development – Any man-made change to improved or unimproved real estate, including but not limited to buildings or other structures, mining, dredging, filling, grading, paving, excavation, or drilling operations or storage of equipment or materials.
L. Differential Runoff – The volume and rate of flow of stormwater runoff discharged from a parcel of land or drainage area which is or will be greater than the volume and rate which existed prior to the development.
M. Drainage Easement – Authorization by a property owner for use by another party or parties for all or any portion of his, her, or its land for drainage purposes.
N. Engineer of Record – A professional engineer registered in the State of Arkansas who is responsible for the design and construction administration, observation, and inspection of the stormwater facilities proposed for specific development or redevelopment projects of all facilities to be dedicated to the City of Jonesboro.
O. Flooding – An overflowing of water resulting in the inundation or submergence of normally dry land.
P. Floodplain – A land area adjoining a watercourse which is likely to be flooded.
Q. Floodway – The channel of a watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without a cumulative increase of the water surface elevation more than a designated height.
Freeboard – A factor of safety expressed as the difference in elevation between the top of the detention basin dam, levees, culvert entrances and other hydraulic structures, and the design flow elevation.
Grading Permit – A permit issued by the City Engineer which allows land disturbance activities (e.g., clearing, grading, excavation, etc.) on a specific development.
Lowest Floor – Refers to the lowest floor of the lowest enclosed area (including basement). For a typical slab-on-grade construction, the elevation of the lowest floor is the top of the first floor of the house. For a typical basement foundation construction, the elevation of the lowest floor is the top of the basement floor. For a typical crawlspace foundation construction, the elevation of the lowest floor is the top of the first floor of the house. For typical split-level constructions, the elevation of the lowest floor is the top of the first living floor – the garage floor is not the lowest floor as long as there are no living areas in the garage and it is used solely for storage, parking vehicles and entry to the house. The elevation of the lowest floor of a manufactured home, however, is the bottom surface of the lowest floor joist.
Short-term Maintenance – General upkeep of the site and facilities, specifically the mowing or trimming of grasses or other vegetative cover and the removal of litter and other minor debris that could impact the functionality of the facilities or that would otherwise be considered unsightly or a nuisance.
Long-term Maintenance – Removal of sediment deposits, re-grading or shaping of embankments, drainage channels, and detention areas, and repair or replacement of piping networks, and other drainage structures.
NRCS (SCS) Method – A methodology developed by the Natural Resources Conservation Service (formerly the Soil Conservation Service) for obtaining a design hydrograph to simulate the discharge from a watercourse over a specific time period.
development site where the runoff is generated.
Peak Flow – The peak rate of flow of water at a given point in a watercourse or conduit.
Plat – A legally recorded plat of a parcel of land subdivided into lots with streets, alleys, easements, and other land lines drawn to scale.
improvement of structures or grounds, or both.
Retention Basin – A stormwater detention facility which maintains a fixed minimum water elevation between runoff events except for the lowering resulting from losses of water to infiltration or evaporation.
Special Flood Hazard Area (SFHA) – The land area covered by the floodwaters of the base flood.
Stormwater Drainage Design Manual – The set of drainage policies, analysis methods, design charts, stormwater runoff methods, and design standards used by the City of Jonesboro as the official design guidelines for drainage improvements consistent with the regulations.
Stormwater Management System – The collection of open channels, drainage swales, detention facilities, retention facilities, and enclosed conduits that comprise the overall drainage system for an area or region.
Stormwater Runoff – Water that results from precipitation which is not absorbed by the soil, evaporated into the atmosphere, or entrapped by ground surface depressions and vegetation, which flows over the ground surface.
Stormwater Runoff Management Facility – Any facility constructed to manage or otherwise control the flow of stormwater runoff from a site including but not limited to open channels, drainage swales, detention facilities, retention facilities, or enclosed stormwater conveyance systems.
Structure – Any object constructed above or below ground.
Pipes, manholes,
and certain other utility structures which exist underground may be excluded from the definition.
Sub-basin – The area that contributes stormwater runoff to a given point in the overall stormwater management system.
Substantial Damage – Damage of any origin where the cost to restore a structure to its original undamaged state would equal or exceed 50% of the market value of the structure before any damage occurred. In determining whether substantial damage has occurred, estimators must use standard contractor and materials costs. There are no exceptions for homeowners who make their own repairs or for discounted or free raw materials.
improvement to a structure with a cost equaling or exceeding 50% of the market value of the structure before any improvement. Improvements to correct indentified violations of local health, sanitary or safety Codes are not substantial improvements, regardless of the cost, as long as they are the minimum improvement necessary to bring the structure up to Code. Alterations to historical structures are also exempted, as long as the improvement does not affect the structure’s official status of “historical structure.”
Swale – A ditch or depression that is cut into the soil that allows the flow of water to pass.
Watercourse – Any surface stream, creek, brook, branch, depression, reservoir, lake, pond, river, ditch, wetland, swamp area, or drainage way in or into which stormwater runoff flows.
ARTICLE 2 STANDARD PROVISIONS
This article establishes the standard provisions of the City of Jonesboro
2.2 Stormwater Management Plan (SWMP) Required - Any person, firm, corporation,
business, or legal entity proposing to develop land within the City of Jonesboro Planning jurisdiction shall prepare and submit a SWMP to the City Engineer for review and be issued a Compliance Letter prior to commencement of construction of the improvements, except for the following:
• One new or existing single-family structure;
• One new or existing duplex family structure;
• One existing commercial or industrial structure where additional structural and site improvements are less than 2000 square feet.
corporation, business, or legal entity proposing to disturb for construction purposes (e.g., clear, grade, excavate, etc.) one (1) acre or more of total land area, or less than one (1) acre of total land area that is part of a larger common development or sale if the larger common plan will ultimately disturb one (1) acre or more, shall prepare and submit a SWPPP to the City Engineer for review and be issued a Grading Permit prior to commencement of the proposed land disturbance activities.
2.4 Drainage Alterations or Modifications Permit Required - No person, firm, corporation or business shall alter or modify any open channel, drainage swale, detention facility, enclosed stormwater conveyance system, or other watercourse either natural or artificial where any of said facility is part of the City of Jonesboro Stormwater Management System, without first submitting construction plans and supporting documentation to the City Engineer for review and being issued a Compliance Letter.
2.5 Floodplain Development Permit Required – A permit is required for all structural
development, placement of manufactured structures, clearing, grading, mining, drilling, dredging, placement of fill, excavating, watercourse alteration, drainage improvements, roadway or bridge construction, individual water or sewer installation or any other development in a Special Flood Hazard Area.
2.6 Public and Private Responsibilities
A. Public Responsibilities:
– responsibility of the City Engineer.
2. Operation and Maintenance of Publicly Owned Facilities – The City of Jonesboro Public Works Department shall be responsible, after construction, for the operation and long-term maintenance of all drainage structures and improved watercourses which are part of the City of
Jonesboro Stormwater Management System and which are not constructed and maintained by or under the jurisdiction of any State or Federal agency.
Private Responsibilities:
Each developer of land has responsibility to provide on the developer’s property Stormwater Runoff Management Facilities to ensure proper drainage and control of stormwater on and from the developer’s property;
Each developer of land has a responsibility to properly maintain any on- site Stormwater Runoff Management Facility. Such responsibility is to be transmitted to subsequent owners through appropriate covenants to be recorded with or in the deed, and;
Each developer of land has a responsibility both during and after construction to provide, install, and maintain appropriate erosion control measures and other stormwater Best Management Practices (BMPs) as needed to minimize any adverse impact to water quality or the local environment.
ARTICLE 3 APPLICATION FOR PERMITS
development projects within the Jonesboro Planning jurisdiction.
3.2 Stormwater Management Plans (SWMP)
3.2.1 Preparation – The SWMP shall be prepared in accordance with the provisions set
forth in these regulations and shall be sealed by the Engineer of Record for the project.
3.2.2 Submission – The Developer shall submit two (2) copies of the SWMP and all
supporting documentation to the City Planning Office along with the review fee as established by the City Council. Upon receipt, the Planning Office will forward the submittals to the City Engineer for review.
3.2.3 Plan Review – The City Engineer or his designated representative shall review the
submittals for conformance to the City of Jonesboro minimum design standards as established in these regulations and in the Stormwater Drainage Design Manual.
The City Engineer will review and comment or approve the project submittals within fifteen (15) business days upon receipt. Failure of the City Engineer to meet this timeline shall not be considered approval of the proposed work.
3.2.4 Rejection – If it is determined that the proposed development will not control stormwater runoff in accordance with these regulations, a comment letter listing the deficiencies shall be issued to the Developer for response. Re-submittals shall be made to the City Planning Office.
If needed, an informal meeting between the Developer and the City Engineer or his designee may be scheduled by either party to review the overall concepts included in the plan. The purpose of this meeting may vary, but generally shall be to jointly agree upon an overall stormwater management concept for the proposed development and to review criteria and design parameters which shall apply to the project.
3.2.5 Acceptance – If it is determined that the proposed development will control
stormwater runoff in accordance with these regulations, a Compliance Letter shall be issued to the Developer stipulating the conditions of approval which are:
1. The Engineering Department relied upon statements and representations
made in the engineer’s report, plans, and specification. In case any statement or representation in the aforementioned documents is found to be incorrect, then the approval may be revoked;
2. There shall be no deviations from the approved plans and specifications
unless revised plans and specifications have been first submitted for review and written consent given amending the Compliance Letter;
3. The review of the plans and specifications are for conformance to City of
Jonesboro minimum design standards and in no way constitute an analysis of the hydraulic or structural design;
4. If construction of this project is not started within one-year from the date of
approval, then the approval is terminated, and;
5. Construction inspection for the work related to the Stormwater Management
plan shall be the responsibility of the Engineer of Record.
Other conditions of approval may be added by the City Engineer as needed, as long as the added conditions are consistent with these regulations.
Should the original Engineer of Record be prevented from completing the project, the Developer shall employ another qualified engineer and immediately notify the City Engineer. The Developer shall also provide the name, address and telephone number of the new Engineer of Record.
3.3 Stormwater Pollution Prevention Plan (SWPPP)
3.3.1 Preparation – The SWPPP shall be prepared in accordance with the provisions
set forth in these regulations. All SWPPPs shall be prepared and sealed by the Engineer of Record for the project except those for:
• One new or existing single-family structure that is part of a larger common development;
• One new or existing duplex family structure that is part of a larger common development, or;
• One existing commercial or industrial structure where additional structural or site improvements are less than 2000 square feet and that is part of a larger common development.
SWPPPs for the above exempt sites can be prepared by the Developer, provided that the Developer is knowledgeable with the applicable local, state, and federal requirements for SWPPP preparation.
3.3.2 Submission – The Developer shall submit two (2) signed copies of the SWPPP to
the City Planning Office along with the review fee as established by the City Council. Upon receipt, the Planning Office will forward the submittals to the City Engineer for review.
3.3.3 Plan Review – The City Engineer or his designated representative shall review the
submittals for conformance to the City of Jonesboro minimum design standards as established herein.
3.3.4 Rejection – If it is determined that the proposed development will not control
stormwater runoff in accordance with these regulations, a comment letter listing the deficiencies shall be issued to the Developer for response. Re-submittals shall be made to the City Planning Office.
3.3.5 Acceptance – If it is determined that the proposed development will control
stormwater runoff in accordance with these regulations, a Grading Permit shall be issued to the Developer.
3.4 Drainage Alterations/Modifications
3.4.1 Preparation – Construction plans for alterations or modifications to the City of
Jonesboro Stormwater Management System shall be prepared in accordance with the provisions set forth in these regulations and shall be sealed by the Engineer of Record for the project.
3.4.2 Submission – The Developer shall submit two (2) copies of the construction plans
and all supporting documentation to the City Planning Office along with the review fee as established by City Council. Upon receipt, the Planning Office will forward the submittals to the City Engineer for review.
3.4.3 Plan Review – The City Engineer or his designated representative shall review the
submittals for conformance to the City of Jonesboro minimum design standards as established by these regulations and the Stormwater Drainage Design Manual.
3.4.4 Rejection – If it is determined that the construction plans do not comply with these
regulations, a comment letter listing the deficiencies shall be issued to the Developer for response. Re-submittals shall be made to the City Planning Office.
3.4.5 Acceptance – If it is determined that the proposal complies with these regulations,
a Compliance Letter shall be issued to the Engineer of Record stipulating the conditions of approval which are:
made in the engineer’s report, plans, and specification. In case any statement or representation in the aforementioned documents is found to be incorrect, then this approval may be revoked;
Jonesboro minimum design standards and in no way constitute an analysis of the
hydraulic or structural design;
5. Construction inspection for the work related to the alterations or modifications to the City of Jonesboro Stormwater Management System is the responsibility of the Engineer of Record.
Should the original Engineer of Record be prevented from completing the project, the Developer shall employ another qualified engineer, and immediately notify the City Engineer. The Developer shall also provide the name, address and telephone number of the new Engineer of Record.
3.5 Floodplain Development Permit
3.5.1 Preparation – Application for a Floodplain Development Permit shall be made by
the Developer on forms prescribed by the City Engineer and made available through the City Planning Office.
3.5.2 Submission – The Developer shall submit two (2) copies of the application and all
supporting documentation to the City Planning Office along with the review fee as
established by the City Council. Upon receipt, the Planning Office will forward the submittals to the City Engineer for review.
3.5.3 Plan Review – The City Engineer or his designated representative shall review the
3.5.4 Rejection – If it is determined that the proposed development is not in accordance
with these regulations, a comment letter listing the deficiencies shall be issued to the Developer for response. Re-submittals shall be made to the City Planning Office.
3.5.5 Acceptance – If it is determined that the proposed development is in accordance
with these regulations, a Compliance Letter shall be issued to the Developer stipulating the conditions of approval which are:
made in the Floodplain Development Application. In case any statement or representation in the aforementioned documents is found to be incorrect, then
the approval may be revoked;
Jonesboro minimum design standards and in no way constitute an analysis of the hydraulic or structural design, and;
approval, then the approval is terminated.
ARTICLE 4 DESIGN CRITERIA AND PERFORMANCE STANDARDS
4.1 Design Criteria – The City of Jonesboro’s Stormwater Drainage Design Manual (Drainage
Manual) latest version shall be the accepted design document.
Developer and the Engineer of Record to verify that they are using the latest version of the Drainage Manual in the design of their proposed development. Unless otherwise provided, the following rules shall govern the design and improvements with respect to managing stormwater runoff:
A. Method of Determining Stormwater Runoff – Developments where the upstream drainage area contributing runoff is less than 100 acres should be designed using the SCS, Type II 24-hour distribution, TR-55 Hydrograph Method for calculating runoff. Developments where the area contributing runoff is greater than 100 acres shall use the City of Jonesboro basin analysis for calculating runoff. If not available, the FEMA Data shall be used for calculating runoff. If neither the basin analysis nor the FEMA Data is available, the SCS TR-55 Hydrograph Method for calculating runoff shall be used.
B. Differential Runoff – The total volume, peak flow rate, and velocity of stormwater runoff from a site, post-development, should to the extent practicable approximate that of the site prior to the development.
C. Development Design – Developments shall be located and laid out in such a manner as to minimize the velocity of overland flow, allow maximum opportunity for infiltration of stormwater into the ground, preserve and utilize existing and planned streams, channels, detention basins, retention basins, and include wherever possible, streams and floodplains within parks and other public grounds.
Streets, curbs and gutters, parking areas, enclosed conveyance systems, detention basins, retention basins, and other generally accepted practices and methods for stormwater control may be a part of the overall stormwater runoff management systems for a particular site. To the maximum extent possible, these facilities shall be concurrently designed to effectively manage stormwater runoff in accordance with these regulations.
D. Enclosed Systems and Open Channels – Enclosed systems and open channels shall be designed in accordance with the Drainage Manual.
E. Evaluation of Downstream Flooding – The Engineer of Record shall evaluate whether the proposed plan will cause or increase downstream flooding conditions within the drainage sub-basin in which the project is located or if it will otherwise increase peak flows from the drainage sub-basin. This evaluation shall be made on the basis of existing downstream development and an analysis of stormwater runoff with and without the proposed development. When it is determined that the proposed development will cause or increase downstream flooding conditions, provisions to correct such cause or increase shall be included in the overall design of the stormwater management system improvements. Such provisions may include downstream improvements or detention of stormwater runoff and its regulated discharge to the downstream storm drainage system, or both.
When it is determined that a localized flooding condition exist downstream of a proposed development, the Engineer of Record for the project shall notify the City Engineer of this discovery and include sufficient documentation in the project
submittals to assist in the delineation of the identified Special Flood Hazard Area (SFHA).
F. Detention – The following design criteria shall govern the design of detention facilities:
1. Release Rate – The release rate from any detention facility shall be equal to or less than that of the site prior to the proposed development.
2. Freeboard – Detention storage areas shall have adequate capacity to contain the storage volume of tributary stormwater runoff with freeboard in accordance with the Drainage Manual.
3. Outlet Control Works
Size and hydraulic characteristics shall be such that all water and detention storage is released to the downstream stormwater conveyance systems within 24 hours after the end of the design rainfall. Normal time for discharge ranges from 3 to 24 hours.
4. Spillway – Emergency spillways shall be provided in accordance with the Drainage Manual.
5. Design Data Submittal – In addition to complete plans, all design data shall be submitted as required in the detention design data submittal section of the Drainage Manual.
alternatives selected by the Engineer of Record, the design criteria for detention shall follow those given in the Drainage Manual.
G. Alternatives to On-site Detention
1. Alternative Methods – Where on-site detention is deemed inappropriate, alternatives to on-site detention shall be permitted. The methods may include off-site detention or improvements.
Determinations regarding the appropriateness of on-site detention shall be made by the City Engineer based upon the impact of the proposed development on existing drainage networks and the location of the project in relation to existing floodplains, regional detention facilities, and other planned drainage or channel improvements. Disputes, if any, shall be resolved by the Stormwater Management Board.
Determinations regarding the appropriateness of off-site detention or comparable improvements shall be made by the City Engineer
based upon the impact of the proposed development on existing drainage networks and the location of the project in relation to existing floodplains, regional detention facilities, and other planned drainage or channel improvements. Disputes, if any, shall be resolved by the Stormwater Management Board.
2. Excess Stormwater Storage Credit – A Developer may receive credit for excess stormwater storage (in acre-ft) created on one site that may be applied to another site within the same watershed. The transfer of storage volume credit (in acre-ft) shall not be allowed when the site where credited storage is proposed to be transferred has an existing flooding condition downstream or the proposed development will cause downstream flooding.
A. Stormwater Channel Location – Generally acceptable locations of stormwater runoff channels may include but not be limited to the following:
1. In a depressed median of a double roadway, street or parkway provided the median is wide enough to permit maximum three (3) to one (1) side slopes;
2. Along the roadway, street, or parking; or,
3. Located along lot lines within the rear yards of lots or parcels. Stormwater runoff channels located along front and side lot lines shall be enclosed with conduits sized in accordance with the Drainage Manual. Channels along rear lot lines can be open ditch. In all cases, provisions (site grading, berms, dikes, swales, etc.) shall be provided for the safe containment or conveyance of the base flood along these routes.
B. Easements – Drainage easements for access, maintenance, detention and conveyance of stormwater shall be provided and shown on the final plat. Easements shall be required for all drainage ways serving two or more lots or parcels and shall be designated for drainage only. Minimum required dimensions for drainage easements are included in the Drainage Manual (Figures 1 and 2; Appendix 2), but in no case shall the prescribed easements be less than that required to convey or detain the 100-yr runoff. All drainage easements shall have a maintenance agreement approved before final acceptance of the improvements.
C. Storm Sewer Outfall – The storm sewer outfall shall be designed in accordance with the Drainage Manual so as to provide adequate protection against downstream erosion and scouring.
D. Lot Lines – Whenever the plans call for the passage or storage of floodwater, surface runoff, or stormwater along lot lines, grading of all such lots shall be prescribed and established for the safe passage or storage of the waters, and no structures including fences may be erected, shrubbery or trees planted, or
changes made to the prescribed grades and contours of the specified floodways which will obstruct the flow of stormwater.
E. Manholes – All sanitary sewer manholes constructed in a floodplain or in an area designed for the storage or passage of flood or stormwater, shall be provided with either a watertight manhole cover or be constructed with a rim elevation of a minimum one (1) foot above the high water elevation of the base flood, whichever is applicable to the specific area. The Engineer of Record shall identify which sanitary sewer manholes within the project limits are subject to this requirement, and coordinate with City Water and Light regarding the appropriate remedy.
F. Floor Elevations – The lowest floor of any occupied residence or commercial building in a local or FEMA designated Special Flood Hazard Area (SFHA) shall be established by a licensed Professional Engineer registered in the State of Arkansas, using good engineering practices. Any proposed lot or lots located in a local or FEMA Special Flood Hazard Area (SFHA) shall be identified and the Finished Floor Elevation (FFE) shown on the record plat. Finished Floor Elevations shall be referenced to the applicable engineering study.
ARTICLE 5 BONDS, MAINTENACE ASSURANCE, CERTIFICATIONS, NOTIFICATIONS AND FINAL ACCEPTANCE
5.0 Maintenance Agreement – A maintenance agreement assuring perpetual maintenance of
stormwater management improvements and drainage easements to be dedicated to the City shall be agreed upon by the City and the Developer prior to final acceptance of the improvements. The maintenance agreement shall be the responsibility of the Developer and shall be in a form acceptable to the City Engineer and the City Attorney.
5.1 Performance Bond – The Developer shall, before the sale of any lot or lots, either complete
the improvements or provide a Performance Bond to guarantee the completion of the improvements in a timely manner. The bond shall be in a form and in an amount acceptable to the City Engineer and the City Attorney, and shall assure that the prescribed work will be well and truly performed and complete in good, sufficient, and workmanlike manner within an agreed time period, not to exceed six (6) months, and shall indemnify and hold harmless the City of Jonesboro from and against all claims, loss, damages, injury or death, court costs, litigation expense, reasonable attorneys’ fees, and expenses whatsoever which may arise because of or resulting from the Developer’s operation.
All companies furnishing bonds shall be on the U.S. Treasury Department’s most current list of approved surety companies (Circular 570, as amended) and be authorized to transact business in the State of Arkansas.
5.2 Maintenance Bond – A one-year maintenance bond against defects in workmanship shall
be required for any portions of the stormwater management improvements proposed to be dedicated to the City of Jonesboro. The maintenance bond shall be in a form acceptable to the City Engineer and the City Attorney, and shall indemnify and hold harmless the City from and against all claims, loss, costs, damages, injury or death, court costs, litigation expense, reasonable attorneys’ fees, and expenses whatsoever which it may suffer or be compelled to
suffer to pay by reason of failure of the Developer to keep said work in repair or against any and all defects of workmanship or materials. No maintenance bond shall be required for improvements located on private property which is not to be dedicated to the City.
5.3 Certifications – Prior to issuance of any occupancy permit or entrance into the one-year
probationary period for subdivision acceptance, the Engineer of Record shall submit to the City Engineer certification that the stormwater management system is complete and functional in accordance with the plans approved by the City Engineer. Any deviations from the approved plans shall be noted. To insure the adequacy of stormwater quantity detention facilities and stormwater management practices, this certification shall, at a minimum include “record drawings” showing final topographic features of all facilities and updated hydrologic and hydraulic computations for the as-built conditions.
Prior to the issuance of any occupancy permit subject to minimum floor elevation requirements, a registered engineer or registered land surveyor, shall submit to the City Engineer certification of the elevation of the lowest floor (including basement); or if the structure has been flood proofed, the elevation to which the structure is flood proofed. This information must be provided on a FEMA Elevation Certificate.
5.4 Notifications – Developers having been issued a Grading Permit shall notify the City
Engineer upon final stabilization of the disturbed lands and request that the grading permit be terminated.
5.5 Final Acceptance – Upon expiration of the approved one-year maintenance bond and
correction of all deficiencies noted in the eleventh month anniversary inspection report prepared by the City Engineer or his designated representative and presented to the Developer, the City Engineer shall issue or cause to be issued a letter to the Engineer of Record for the project accepting the improvements into the City’s long-term maintenance program.
As-builts of the project shall be provided to the City of Jonesboro in ESRI shapefile and PDF formats in conformance with provisions of this manual.
For all shapefiles, a separate spatial data file should be used for each structure and specific attributes should be recorded on site at the time of collection. The fields that should be used for recording these attributes along with all feature attribute input criteria are located in Appendix 6-“GIS Database Submittals” of the Stormwater Drainage Design Manual.
Final Acceptance by the City of Jonesboro will not be given until as-builts are provided to the City.
6.1 General – It shall be the duty of the City Engineer to bring to the attention of the City
Attorney any violation or lack of compliance of these regulations.
6.2 Violations and Penalties – Any person, firm, corporation, or other legal entity who fails to
comply with or violates these regulations shall be guilty of a misdemeanor and upon conviction thereof shall be fined not less than $100 per day and not more than $500 per day for so long as the violation or violations exist. Each day shall constitute a separate offense.
6.3 Inspection – The City Engineer may make or cause to be made the inspection of any tract
of land for each of the following:
A. Permit – Before a Compliance Letter or Grading Permit is issued, the City Engineer may examine or cause to examine any tract of land for which an application has been received.
B. Construction – The City Engineer may inspect or cause to be inspected at various intervals during the course of construction including but not limited to:
1. Prior to the initiation of the project after temporary or permanent erosion prevention and sediment control practices have been installed;
2. After the completion of rough grading and installation of stormwater management structures, and;
3. Upon completion of the project.
The City Engineer shall issue or cause to be issued a Notice of Violation (NOV) to the Developer of the project where the violations are observed. The NOV shall identify the violation or violations found and request that a Corrective Action Plan (CAP) with timeline be developed, submitted to the City Engineer for approval, and implemented to ensure future compliance with these regulations. Failure to respond to the NOV within 30 calendar days from the date shown shall result in the NOV and all supporting documents being forwarded to the City Attorney for appropriate enforcement action and may result in the issuance of a Stop Work Order.
6.4 Revocation; Stop Work Orders – The City Engineer may revoke any permit issued under
the provisions of these regulations when informed of any false statement misrepresentation of facts in the application or plans. Any non-permitted stormwater management system, or construction, or fill located within a floodplain shall, upon written notice, be removed at the expense of the Developer.
When it is found that any provisions of these regulations are being violated, the City Engineer may issue a stop work order. The stop work order shall be served upon the Developer and the Engineer of Record for the project, and by posting of the stop work order at the site of the violation.
A hearing to appeal the stop work order may be requested by the Developer upon whom an order has been served. An appeal must be requested in writing to the Secretary of the Stormwater Management Board or his or her designee within 30 days after service of the stop work order. The Stormwater Management Board shall hold an appeal hearing within 31 days after receipt of appeal. If no appeal is requested within 30 days after service of the stop work order, the stop work order shall remain in effect until compliance with the appropriate regulations can be demonstrated to the reasonable satisfaction of the City Engineer.
7.1 Interpretation, Conflict and Severability Interpretations
A. Interpretation – In their interpretation and application, the provisions of these regulations shall be held to be the minimum requirements for the promotion of the public health, safety and general welfare.
B. Conflict with Public and Private Provisions – These regulations are not intended to interfere with, abrogate, or annul any other ordinance, rule or regulation statute or other provision of law. Where any provision of these regulations imposes restrictions different from those imposed by any other provision of any other ordinance, rule or regulation, or other provision or law, whichever provisions are more restrictive or impose higher standards, shall control.
C. Private Provisions – These regulations are not intended to abrogate any easement, covenant or any other private agreement or restriction, provided that where the provision of these regulations are more restrictive or impose highest standards or regulations that such easement, covenant or other private agreement or restriction, the requirements of these regulations shall govern. Where the provisions of easement, covenant or private agreement or restriction imposed duties and obligations more restrictive, or higher standards than the requirements of these regulations, and regulations or determinations there under, then such private provisions shall be operative and supplemental to these regulations and determinations made hereunder.
D. Severability – If any part of provision of these regulations or application thereof to any person of circumstances is adjudged invalid by any court or competent jurisdiction, such judgment shall be confined in its operation to that part, provision, or application directly involved in the controversy in which such judgment shall have rendered and shall not affect or impair the validity of the remainder of these regulations or the application to other persons or circumstances. The governing body hereby declares that it would be enacted the remainder of these regulations even without any such part, provision or application found to be unlawful or invalid.
7.2 Saving Provision – These regulations shall not be construed as abating any action now
pending under, or by virtue of, prior existing regulations, or as discontinuing, abating, modifying, or altering any penalty accruing or about to accrue, or as effecting the liability of any person, firm or corporation, or as waiving any right to the City of Jonesboro under any section or provision existing at the time of adoption of these regulations, or as vacating or annulling any rights obtained by any person, firm, or corporation by lawful action of the City, except as shall be expressly provided for in these regulations.
7.3 Amendments – For the purpose of providing for the public health, safety and general
welfare, the governing body may, from time to time, amend the provisions of these regulations. The Public Works Department has the responsibility for updating, on a continuing basis, the Stormwater Drainage Manual.
Only the Developer may appeal an adverse decision of the City of Jonesboro regarding stormwater development issues, including, but not limited to, stormwater runoff quantity and quality, floodplain impact, stop work orders, and impact to neighboring properties, to the Stormwater Management Board.
All appeals and variance requests must be complete and filed on the form provided by the Secretary of the Board and shall include:
2. The name of the Developer’s representative, if any;
3. The case number, map number, and parcel number, if any;
4. The interpretation that is claimed;
5. The decision of the City Engineer or his agent;
6. The location of the property;
7. The stormwater drainage plans which were accepted, and the deviation from the stormwater drainage plan that is being requested;
8. The specific action requested of the Board, and;
9. The reasons justifying such action.
All appeals and variance requests must be filed within thirty (30) days after an adverse decision of the office of the City Engineer for the City of Jonesboro regarding stormwater development issues, including, but not limited to, stormwater runoff quantity or quality, or both, floodplain impact, stop work orders and impact to neighboring properties. The required items must be submitted ten (10) business days prior to the regular monthly Stormwater Management Board meeting for the appeal or variance to be heard at that meeting. A filing fee as established by the City Council shall be charged to each appellant and shall be payable to the City of Jonesboro. Appellant shall also be responsible for any and all publication fees.
All appeals and variance requests will be filed with the Secretary of the Board. Secretary of the Board shall:
1. Accept all appeals and variance requests on behalf of the Board;
2. Assign each appeal or variance request a number;
3. Number each appeal or variance request consecutively in order of receipt (beginning on January 1 of each year), preceded by a hyphen and the year of filing;
4. Ensure that appeals or variance requests are heard in the order that they appear
on the calendar;
5. Prepare an agenda and distribute it to each Board member at least five (5) business days before each meeting;
6. Send a copy of the agenda to the City of Jonesboro Public Works Committee, the Metropolitan Area Planning Commission, the Mayor, the City Clerk, the City Attorney, the Public Works Director, the City Engineer, and the City Floodplain Administrator, and;
7. Include on the agenda each appeal or variance request to be heard.
Variance Considerations
1. In passing variances for applications, the Stormwater Management Board shall consider all technical evaluations, all relevant factors, all applicable local ordinances and regulations, and:
e. The necessity of the ancillary facility;
f. The availability of alternative locations that is for the proposed facility, not subject to flooding or erosion damage;
g. The relationship of the proposed development or improvement plan to the master drainage plans for that area;
h. The safety of access to the property in times of flood for ordinary and emergency vehicles;
i. The expected heights, velocity, duration, rate of rise, and sediment transport of the floodwaters expected at the site;
j. The costs of providing governmental services during and after flood conditions including maintenance and repair of public utilities and facilities such as sewer, gas, electrical, water systems, streets, and bridges, and;
k. Any other relevant facts that pertain to compliance with City Ordinances and Regulations or are mandated by Federal or State laws, rules, or regulations.
2. Upon consideration of the factors listed above, and the objectives of these regulations, the Stormwater Management Board may attach such conditions to the granting of
variances as it deems necessary to further the objectives of these regulations.
3. Conditions for variances:
a. Variances shall only be issued upon a determination that the variance is the minimum necessary, considering the flood hazard, to afford relief; and in the instance of a historical building, a determination that the variance is the minimum necessary so as not to destroy the historic character and design of the building;
b. Variances shall only be issued upon (i) a showing of good and sufficient cause, (ii) a determination that failure to grant the variance would result in exceptional hardship, and (iii) a determination that the granting of a variance will not result in any of the following: (a) increased flood heights; (b) additional threats to public safety or extraordinary public expense; (c) create a public or private nuisance; (d) cause fraud on or victimization of the public; (e) or conflict with existing Federal or State laws, rules, regulations;
c. The Secretary of the Stormwater Management Board shall maintain the records of all appeal actions, and;
d. The City Floodplain Administrator shall report any variances to the Federal
4. Variances may be issued for the reconstruction, rehabilitation, or restoration of structures listed on the National Register of Historic Places or the State Inventory of Historic Places without regard to the procedures set forth in this section, except for Items 3a, 3c, and 3d above, and provided the proposed reconstruction, rehabilitation, or restoration will not result in the structure losing its historical designation.
The Board shall meet at regular
monthly intervals with the day and time to be determined by the chairman.
Notice of such regular or special meetings shall be provided to the media as required by the Arkansas Freedom of Information Act by the Secretary of the Board who shall notify the City Clerk and all notices of meetings shall be posted on the City of Jonesboro web site.
The Board shall provide fifteen (15) minutes at the conclusion of each regular Board meeting for public comment on non-agenda items. Each individual is required to limit his or her comments to five (5) minutes. The Board reserves the right to suspend the rules and allow additional time if necessary.
1. Members and alternates of the Board shall receive applicant’s documents from the Secretary no less than five (5) business days before the appeal hearing date;
2. Following the introduction of the case, the Board will be given two (2) minutes to review documents pertinent to the appeal hearing;
The Developer or Developer’s agent will be allowed ten (10) minutes to present the case to the Board;
4. City of Jonesboro technical staff shall be given ten (10) minutes for questions and comments. Technical staff shall include, but not be limited to, the City Engineer, the Assistant City Engineer, the City Floodplain Administrator, the City Public Works Director, or any designated city staff with pertinent technical information related to the appeal;
5. The Mayor or City Council member(s), or both, shall be given two (2) minutes to speak if he or she requests time to comment;
6. A public representative, for or against the requested variance, may have two (2) minutes to comment, with a maximum of three persons representing each side. Persons wishing to address the Board shall register with the Secretary of the Board or his/her designee, including their name, address and a brief description of their concern, prior to the beginning of the appeal hearing;
7. Rebuttal will be limited to five (5) minutes per each public representative;
8. The Developer, City of Jonesboro technical staff, or others present will then respond to questions from the Board;
9. Upon a motion by any Board member specifically stating the amount of additional time requested and approval by five (5) members of the Board, additional time may be granted to a speaker, and;
10. Once a motion and a second to the motion are made, further discussion is limited to members of the Board unless they have additional questions for the applicant or others. In the absence of procedures included in the Board’s enabling legislation or this document, the most recent edition of the Robert’s Rules of Order shall apply.
Conduct During Appeal Hearing
Any person other than a Board member shall only address their comments to the Board, shall respond to the questions asked, and shall accord the utmost courtesy to the Board and the other participants. The chairman reserves the right to remove any participant from the hearing upon repeated rude or derogatory remarks, abusive comments and unsubstantiated statements as to motives or personalities, or both.
At the conclusion of all of the evidence in all cases heard at that hearing session, the Board shall discuss the cases and render decisions on that date or defer decisions for no longer than thirty-one (31) days thereafter. The Board shall have the authority to table, approve or deny a variance or appeal.
The Developer may withdraw his/her appeal one time. The appeal shall be heard at the next regularly scheduled Board meeting. If applicant withdraws an appeal a second time, the Developer must wait 180 days before requesting that the appeal be heard by the Board.
Any action taken by the Board shall be by motion which shall state the reason or reasons for the action taken with particularity. All the decisions of the Board shall be in writing and must indicate the vote of the Board upon the decision.
A quorum of the Board must be present to render any decisions. Five (5) Board members are considered a quorum and there must be five (5) votes to take any affirmative action.
The vote of an alternate member of the Board shall be counted in the tabulation of the result only if she or he is substituting for a regular member. If the alternate member is not substituting for a regular member, the vote shall be recorded but not counted in the decision of the Board.
The decision of the Board on each appeal shall be promptly entered on the minutes of the meeting of the Board by the Secretary and filed in the City Clerk’s office.
No re-hearing of a decision by the Board shall be held except on motion to reconsider that vote by a member of the majority of the Board on the preceding vote, or on a motion or written request to reconsider a prior decision receiving five (5) votes of a quorum of the Board.
If the request for a re-hearing is granted, the case shall be put on the calendar for a re- hearing. In all cases, the request for a re-hearing shall be by the Developer in writing, reciting the reasons for the request, and shall be duly verified and accompanied by the necessary data and diagrams. The Developer requesting the re-hearing shall be notified to appear before the Board on a date to be set by the Board. The notification shall be by the secretary.
No member of the Board shall act on any case in which he/she has a personal interest, whether it is a direct or indirect financial interest in the property itself, or by virtue of family relationship with the Developer pursuant to the City of Jonesboro’s Code of Ethics (Code Section 2.20.14).
Any person who has a conflict shall notify the chairman or secretary immediately upon realizing a conflict may exist. A Board member who has a conflict may not participate as a Board member on the item in which he or she has a conflict of interest.
An alternate member shall replace any member who has a conflict of interest or who is unable to attend due to an illness or an extended absence from the metropolitan area. The Secretary of the Board shall be responsible for contacting the Board members prior to the meeting to determine the need for alternate members. If an alternate member is needed, the secretary shall be responsible for contacting the alternate member.
The Chairman or Vice-Chairman may call special meetings at a time and place of their choosing. Whenever such a special meeting is called, the public shall be notified by appropriate means in accordance with the Arkansas Freedom of Information Act.
Public announcements including, but not limited to, Board decisions, Board recommendations or other Board actions shall be released to the public upon approval by the Board or in the interim upon approval by the Chair, or in his absence, the Vice-Chair.
7.8 Appeals from Decisions of the Board
Any aggrieved party may appeal any decision of the Board to the Craighead County Circuit Court as provided in ARK. CODE ANN. §14-56-425.
8.1 Disclaimer of Liability – The performance standards and design criteria set forth herein and in the Drainage Manual establish minimum requirements which must be implemented with good engineering practice and workmanship. Use of the requirements contained herein shall not constitute a representation, guarantee, or warranty of any kind by the City of Jonesboro, or its officers and employees of the adequacy or safety of any Stormwater Management plan imply that the land uses permitted will be free from damages caused by stormwater runoff. The degree of protection required by these regulations is considered reasonable for regulatory purposes and is based on historical records, engineering and scientific methods of study. Larger storms may occur or stormwater runoff heights may be increased by man-made or natural causes. These regulations, therefore, shall not create liability on the part of the City of Jonesboro or any officer or employee with respect to any legislative or administrative decision lawfully made hereunder.
1.3 Drainage Policy
2.4 Design Guidelines and Checklists for Storm Drainage Plans
2.4.1 Drainage Area Map
2.4.2 Storm Sewer Design Guidelines
2.4.3 Laterals
2.4.4 Inlets and Intakes
2.4.5 Plan and Profile Sheets
2.4.6 Detention
2.4.7 Bridges
2.4.8 Open Channels
3.2 Intentionally Deleted
3.3 SCS, Type II 24-hour Distribution, TR-55 Hydrograph Method
3.3.1 Accumulated Rainfall (p) and Rainfall Distribution
3.3.2 Curve Number (CN) Factors
3.3.3 Hydrologic Soil Groups
3.3.4 Cover Type
3.3.6 Hydrologic Condition
3.3.7 Antecedent runoff condition (ARC)
3.3.8 Ponding and Swamp Adjustment Factor (F p )
3.4 Computer Hydrograph Methods
STORM DRAINAGE APPURTENANCES
4.2 Design Frequencies
4.3 Runoff Calculations
4.4 Street Flow
4.4.2 Calculation of Flow in Streets
4.4.3 Uniform Gutter Sections
4.4.4 Composite Gutter Sections
4.4.5 Parabolic Street Sections
4.5 Drainage Inlet Design
4.5.1 Inlet Types
4.5.2 Interception Capacity of Inlets on Grade
4.5.2.1 Grate Inlets
4.5.2.2 Curb-opening Inlets
4.5.2.3 Combination Inlets
4.5.3 Interception Capacity of Inlets in Sag Locations
4.5.3.1 Grate Inlets in Sags
4.5.3.2 Curb-Opening Inlets
4.5.3.3 Combination Inlets
4.5.4 Inlet Locations
4.5.4.1 Geometric Controls
4.5.4.2 Inlet Spacing on Continuous Grades
4.6 Hydraulic Design of Closed Conduits
4.6.1 Velocity in Closed Conduits
4.6.2 Roughness Coefficients for Closed Conduits
4.6.3 Minor Head Losses in Closed Conduits
5.2 Cross Sections
5.3 Roughness Coefficients
5.4 Velocity Requirements
5.5 Channel Drop Structures
Sloping Drop Structures
6.2 Culverts Flowing with Inlet Control
6.3 Culverts Flowing with Outlet Control
8.2 No Adverse Impact Policy
8.3 Detention Reservoir Routing
8.3.1 Inflow Hydrograph
8.3.2 Stage-Storage Curve
8.3.2.1 Stage-Storage Calculations
8.3.3 Stage-Discharge Curve
8.3.3.1 Stage-Discharge Calculations
8.3.4 Dry Reservoirs (Ponds)
8.3.5 Open Channels
8.3.6 Wet Reservoirs (Ponds)
8.3.7 Parking Lots
8.3.8 Control Structures
8.3.9 Emergency Spillways
9.0 FLOODPLAIN GUIDELINES
9.1 General Standards
9.2 Risk Zone Specific Standards
9.3 Hydrology
9.4 Hydraulics
9.5 SFHA Submittals
10.0 EROSION CONTROL
10.2 Environment Protection Agency BMP’S
Street Capacity Nomographs
Culvert Nomographs
GIS Database Submittals
Permit No. ARR150000
The purpose of this manual is to establish standard principles and practices for the design and construction of storm water drainage facilities within Jonesboro, Arkansas. In addition floodplain compliance guidelines will be presented to insure base flood elevations will not rise because of the construction or modification of structures or land alteration within the floodplain. The design factors, formula, graphs, and procedures are intended for use as engineering guides in the solution of drainage problems involving determination of the quantity, rate of flow, and conveyance of storm water. The procedures defined herein shall be applied by experienced professional engineers licensed to practice in the State of Arkansas. Also, ultimate responsibility for the design of storm drainage structures lies with the engineer of record. As such, prudent engineering judgment should be used in the design of any facility within Jonesboro.
Methods of design other than those indicated herein may be considered in difficult cases where experience clearly indicates they are preferable. However, there should be no extensive variations from the practices established herein without express approval from the City of Jonesboro.
This manual presents various applications of accepted principles of surface drainage engineering and is a working supplement to the information obtained from standard drainage handbooks and other publications on drainage. It is presented in a format that gives logical development of solutions to problems of storm water drainage design and floodplain management.
This manual is intended to be used by the City of Jonesboro, consulting engineers contracted with the City, and for private development within the planning jurisdiction of the City. This manual applies to storm drainage conditions, which are generally relative to the City of Jonesboro and the immediate geographical area. Accepted engineering principles, applied to the City of Jonesboro’s storm drainage requirements, are detailed within this manual.
The basic objective of the City of Jonesboro is to construct and maintain facilities intended to minimize the threat of flooding to all areas of the City and comply with the requirements of the National Flood Insurance Program. Drainage facilities are defined as all channels, pipes or other structures which handle public water. Additionally, it is the City’s intent to insure that adequate facilities are constructed to accommodate new development such that existing property will not be subjected to additional flooding and so as not to increase the limits of the floodplains as shown on the flood insurance rate maps (FIRM’s) for the City of Jonesboro and other entities (County, Levee Improvement Districts, and Municipal Utility Districts).
It is not economically feasible to construct storm sewer facilities, which are large enough to keep the street systems from becoming inundated during severe storm events. City policies as defined in the governing drainage ordinance and standards in this manual are
designed to minimize the impacts (depth and duration) of storm events and insure that the lowest floor elevation of any structures not used exclusively for storage, access, or parking are, at a minimum, twelve (12) inches above the 100-year flood elevation. The intent of this policy is that there should not be any street ponding for minor storm events, minor street ponding for larger events, and major ponding for the 100-year event storms but without water inundating building structures. Every attempt will be made to design major thoroughfares so that they are passable during severe storm events.
The City of Jonesboro has included in this manual criteria covering the design of storm water systems to serve both existing and new developments.
The criterion is considered a minimum for the City of Jonesboro. Approval from other applicable agencies may be required. Ultimate approval for any variance of the criteria contained in this manual must be given by the City of Jonesboro.
2.0 CONSTRUCTION PLAN PREPARATION
This section covers the preparation of drainage construction plans for the City of Jonesboro.
Plans shall be submitted in accordance with the City of Jonesboro’s Checklist for Storm Drainage Plans. The first engineering plan set submission shall be complete, and in sufficient detail to allow review by the City of Jonesboro. All topographic surveys should be furnished to allow establishment of alignment, grades, and rights-of-way requirements.
The hydraulic design of the proposed facilities shall be accomplished based on the procedures and criteria outlined in this manual. Calculations shall be submitted as part of the plan set. These plans shall show the alignment, drainage areas, size of facilities, and grades.
Storm drainage plans shall include at a minimum, a drainage area map, plan-profile sheets, and channel cross-sections, if applicable.
Survey control performed for the project shall reference two reference marks established by the City of Jonesboro. A copy of the reference marks can be obtained from the City Engineer.
Survey control for the project shall conform to the following requirements:
• Vertical control will be NAVD 88, Third Order Vertical
• Horizontal control will be NAD 83, Third Order Class 1, Arkansas State Plane North Zone
All drawings shall be prepared on bond paper with a minimum sheet size of 24” x 36” , to a standard engineering scale, and shall be clearly legible when sheets are reduced to half scale. All drawings shall be signed and sealed by a Professional Engineer registered in the State of Arkansas. After each review, all review comments shall be addressed, additional data incorporated, and drafting of plans completed. Each plan- profile sheet shall have a benchmark shown.
Design Guidelines and Checklists for Storm Drainage Plans
The drainage area map shall be to a standard engineering scale, and show the street
Scale shall be selected to adequately depict drainage areas, flow paths,
Section 2 Construction Plan Preparation
When calculating runoff, the drainage area map shall show the boundary of the drainage area contributing runoff into the proposed system. The area shall be further divided into sub-areas, sequentially numbered, to determine flow concentration points or inlet locations. The centerline of all streets will normally be a boundary of a drainage area, to insure that inlets are sized and positioned to fill the need without depending on storm water crossing over the street crown for proper drainage.
Direction of flow within streets, alleys, natural and man-made drainage ways, and at all system intersections, shall be clearly shown on the drainage area map and/or paving plans. Existing and proposed drainage inlets, storm sewer pipe systems, and drainage channels shall also be clearly shown and identified.
The following items/information shall be included:
Use design criteria as outlined in drainage regulations and prescribed in this manual;
Standard Engineering Scale. Show match lines between any two (2) or more maps. Show graphic bar scale;
Show drainage areas including acres, land use description, hydrologic soil group, and inlet time for each area;
Show existing sub-areas for alley, street, and off-site areas;
Indicate contours on map for on- and off-site, not to exceed two (2) foot contour. For large drainage areas show contours at intervals appropriate to indicate drainage patterns;
Location of all existing and proposed drainage structures on the project site;
Show local and FEMA designated SFHA and floodways if they exist. If not, note that none exists;
Indicate city zoning on drainage area. Identify land use for adjacent properties;
Show points of concentration and their designations;
Inlets, their size and location, the bypass flow for each, the direction of flow as indicated by flow arrows, the station for the centerline;
Indicate runoff at all inlets, dead-end streets and alleys, or to and from adjacent additions or acreage;
For cumulative runoff, show calculations;
Show north arrow. Orient sheet such that the direction of north is to top page or to the left;
Location of existing and proposed drainage structures;
A table depicting runoff computations;
Flow arrows to indicate all crests, sags, and street and alley intersections; and,
Street names shall be indicated.
Storm Sewer Design Guidelines
General guidelines for the design of closed conduit systems, storm sewers, are outlined below.
Diversion of flow from one natural drainage area to another will not be allowed;
Show plan and profile of all storm sewers;
Pipe Material in City rights-of-way or easements shall conform to the following minimum requirements:
• Roadway Crossings RCP Class III or Class IV ASTM C-76 ASTM C-506 ASTM C-507 Box Culverts and Small Bridges ASTM C-1433 ASTM C-507
• Mains and Laterals Aluminized Steel Type 2 Corrugated Steel Pipe ASTM A-929 ASTM A-760 RCP Class III or Class IV ASTM C-76 ASTM C-506 ASTM C-507 Box Culverts and Small Bridges ASTM C-1433 ASTM C-507
Alternative pipe materials for use outside City rights-of-way or easements shall be approved by the City Engineer.
In areas where pipes cross beneath railroads, areas of deep fill and areas subjected to heavy loads the engineer shall select the pipe material that is adequate for the design load and provide documentation of such analysis;
The minimum allowable concrete strength for concrete pipe is 3,500 psi. Specify concrete strength for all structures;
Provide inlets where street capacity is exceeded. Provide inlets where addition of alley runoff to street exceeds intersecting street capacity;
Storm water flow from streets into alleys is to be avoided.
may approve this type of drainage flow when it is not possible to direct flow anywhere else;
Maximum discharge velocity of eight (8) feet per second (fps) is allowed at the pipe outfall. Velocities that exceed 8 fps must be approved by the City Engineer and must include adequate provisions for erosion control considering the soil conditions at the outfall;
As it relates to erosion control, discharge flow lines of storm sewers shall be a
maximum of two (2) feet above the natural flow line of the channel, unless channel lining is present. Energy dissipation shall be provided when specified by the City Engineer; Where fill is proposed for trench cuts in creeks or outfall ditches, compaction shall be 95% of the maximum density as determined by ASTM D 698; and,
Any off-site drainage work or discharge to downstream property will require an easement. Easement shall be sized such that the developed flows can be conveyed within the easement.
Laterals are defined as minor storm sewer lines that serve the purpose of connecting a single inlet to a larger storm sewer main line. The following is a list of requirements that apply to laterals.
Show laterals on trunk profile with stations;
Provide lateral profiles for laterals exceeding thirty (30) feet in length. Potential conflicts with existing utilities (i.e. sanitary sewer, etc.) should be shown in profile;
Laterals shall be placed in profile such that the hydraulic grade line is not less than one foot from the curb flow line, unless utilities or storm sewer depth requires otherwise;
Laterals shall not enter the corners or bottoms of inlets;
In general, the angle of confluence between main line and lateral shall not exceed ninety (90) degrees. Situations where angles exceed this requirement must be supported by calculations that show that the connection will not create adverse flow conditions in the connecting pipe;
Longitudinal centers should intersect;
At junction structures, downstream pipe crown elevations should not be above upstream pipe crown elevations; and,
Minimum pipe diameter within City rights-of-way or easements shall be eighteen (18) inches unless otherwise approved by the City Engineer.
Inlets shall be provided at the following locations as a minimum:
• At locations on grade where the design flows exceeds the depth and spread criteria;
• At all low points (sag points) in gutters;
• Immediately upgrade of median breaks and street intersections;
• Immediately upgrade of roadway cross slope reversals;
• Upstream and downstream of bridge locations where applicable; and,
• Behind curbs and sidewalks as necessary to drain low areas
Inlets shall be given the same number designation as the area or sub-area contributing runoff to the inlet. The inlet number designation shall be shown opposite the inlet. At intersections, where possible, the end of the inlet shall be ten (10) feet from the curb return or Point of Tangency, and the inlet location shall also provide minimum interference with the use of adjacent property. Inlets in residential areas should be located in streets and alleys so that driveway access is not prohibited to the lots. Drainage from abutting properties shall not be impaired, and shall be designed into the storm drainage system.
Data opposite each inlet shall include paving or storm sewer stationing at centerline of inlet, size and type of inlet, number or designation, top of curb elevation and flow line of inlet as shown on the construction plans.
Indicate direction of flow and the design flow (Q) entering the inlet. Identify capacity of inlet and any bypass flow that may result;
On plan view, indicate inlet designation number, location of inlet (station and offset), size of inlet, sizes of pipes entering and exiting the inlet with associated flow lines, and top-of-curb elevations; and,
Use standard curb inlets in City rights-of-way or easements as presented in the City of Jonesboro Street Improvement Program Manual, latest edition.
In the plan view, the storm sewer designation, size of pipe, and length of each size pipe shall be shown adjacent to the storm sewer. The main line sewer plan shall be stationed at one hundred (100) feet intervals.
This data shall consist of pipe diameter in inches, the design storm discharge in cubic feet per second, slope of hydraulic gradient in percent, Manning capacity of the pipe flowing full in cubic feet per second, velocity in feet per second, and V 2 /2g. Also, the head loss at each interception point shall be shown.
Stationing and flow line elevations shall also be shown at all pipe grade changes, pipe size changes, lateral connections, manholes, and wye connections. Crown elevations should conform to 2.4.3(7).
The recommended scale for storm sewer plan and profile sheets is 1”= 50’ minimum and 1”= 100’ maximum;
Indicate property lines and lot lines along storm sewers, and show easements with dimensions;
Provide separate plan and profile of storm sewers. The storm drain lines should also be shown on paving plans with a dashed line. Full pipe sections should be shown on sanitary sewer profiles at crossings;
Show pipe sizes in plan and profile;
Show hydraulics on each segment of pipe profile to include: Q (Design Flow), C (Manning full flow capacity); S (Slope), V (Velocity), V 2 /2g (Velocity Head);
Show all existing utilities in plan and profile. Show sanitary sewer profiles at all crossings of storm drain profiles;
Indicate existing and proposed ground line on all street, alley, and storm sewer profiles;
Show future streets and grades where applicable;
When connections are made to existing systems, computations must be provided to show the capacity of the existing system to accept flows. Hydrologic Grade Line (HGL) will be calculated from the outfall to the connection point including the designed flows of the added system;
Indicate flow line elevations of storm sewers on profile, show pipe slope (percent grade). Match pipe soffits at all junction boxes or inlets;
In general, the angle of confluence between main line and lateral shall not exceed ninety (90) degrees;
Show details of all non-standard structures such as junction boxes, headwalls, storm sewers, flumes, and manholes;
Pipe deflections for directional changes shall be placed at the manufacturers recommendations. Deflections exceeding the manufacturer’s recommendation will not be tolerated;
Bends in pipes may be used in unusual circumstances with approval by the City. No bend at one location may exceed thirty (30) degrees;
Show water surface elevation of the outfall for design year event (i.e. Q 25 );
On all dead-end streets and alleys where water exits at the dead-end, show grade out to “daylight” for drainage on the profiles and provide erosion control when necessary. Show typical section and slope of “daylight” drainage;
At sags in pavement, provide a positive overflow (swale) to act as a safety path for failure of the storm drain system. Also, provide minimum lowest floor elevations along this overflow rout; and,
Provide sections for road, railroad, and other ditches with profiles. Show design water surface on profile.
The profile portion of the storm sewer plan-profile sheet shall include:
The existing and proposed ground profile along the centerline of the proposed sewer;
The hydraulic gradient of the sewer;
The profile of the proposed storm sewer line;
Profile view of utilities that cross the proposed sewer alignment;
Locations where laterals intersect the main line;
Text identifying the pipe size, elevations at 50’ intervals, physical grade in percent, stations where laterals intersect main line; and,
Hydraulic data for each length of storm sewer between interception points shall be shown on the profile.
Provide drainage area map and show all computations for runoff affecting the detention basin;
Provide a plot plan with existing and proposed contours for the detention basin and plan for structural measures;
Where earth embankment is proposed for impoundment, furnish a typical embankment section and specifications for fill including profile for the structural outflow structure and Geotechnical report;
Provide structural details and calculations for any item that is not a standard detail;
Provide detention basin volume calculations and elevation versus storage curve;
Provide detention calculations for volume by elevation-area, outflow using orifice and/or weir, and reservoir routing; and,
(7) Provide SCS, Type II 24-hour distribution, determination of storage requirements, (permitted for areas of 100 acres or less). Areas greater than 100 acres shall use City of Jonesboro FEMA data (if available).
Show Geotechnical soil boring information on plans;
Provide channel cross sections of the water surface elevations for the design storm immediately upstream and downstream of the structure;
Provide hydraulic calculations on all sections;
Provide structural/standard details and calculations;
Provide vertical and horizontal alignment;
Show soil erosion protection measures;
The pre- and post- construction SFHA and regulatory floodway if defined shall be delineated on the plans; and,
Additional requirements for Bridges may be found in Section 7.0 and Section 9.0 .
Plan view of channel showing existing and proposed alignment including creek centerline stationing, north arrow, and scale;
Profile of existing and proposed creek centerline;
Profile of the 25-year and 100-year water surface elevation;
Typical cross sections showing dimensions, and the station limits for which they apply;
Velocities and discharges for the 25-year and 100-year storms;
Limits of temporary erosion protection associated with the construction of the channel needs to be indicated in plan view;
Indicate property lines and lot lines along with existing utilities and show easements with dimensions; and,
Include on the construction plans or as in a separate report, the computations performed in developing the water surface profile.
The planning, design, and construction of drainage facilities are based on the determination of one or more aspects of storm runoff.
Continuous long-term records of rainfall and resulting storm runoff in an area provide the best data source from which to base the design of storm drainage and flood control systems in that area. However, it is not possible to obtain such records in sufficient quantities for all locations requiring storm runoff computations. Therefore, the accepted practice is to relate storm runoff to rainfall, thereby providing a means of estimating the rates, timing and volume of runoff expected within local watersheds at various recurrence intervals.
It is generally accepted that urban development has a pronounced effect on the rate and volume of runoff from a given rainfall. Urbanization generally alters the hydrology of a watershed by improving its hydraulic efficiency, reducing its surface infiltration, and reducing its storage capacity.
For certain small drainage areas (generally less than 100 acres in size), the widely used NRCS (formerly SCS) graphical peak discharge (TR55) provides a useful means of determining peak discharges. If the engineer wishes to use an alternative design technique, it is recommended that the City Engineer be consulted prior to design. If the area is larger than 100 acres and has FEMA hydrology determined for it, the FEMA hydrology model shall be used.
3.2 (Intentionally Deleted)
The Soil Conservation Service (SCS now Natural Resources Conservation Service – NRCS) hydrologic method has been widely used by engineers and hydrologists for analyses of small urban watersheds.
This method results from extensive analytical work using a wide range of statistical data concerning storm patterns, rainfall-runoff characteristics and many hydrologic observations in the United States. The SCS method can be applied to urban drainage areas of any size. Major parameters required to calculate the hydrograph include the rainfall distribution, runoff curve numbers, time of concentration, and drainage area.
The runoff equation used by the SCS is a relationship between accumulated rainfall and accumulated runoff derived from experimental plots for numerous soils and vegetation. The SCS Runoff Curve Number (CN) method is described in detail in NEH-4 (SCS 1985). The SCS runoff equation is:
accumulated direct runoff, (in)
accumulated rainfall (potential maximum runoff), (in)
potential maximum retention after runoff begins, (in)
initial abstraction (including surface storage, interception, and infiltration prior to runoff), (in)
Initial abstraction (I a ) is all losses before runoff begins. It includes water retained in surface depressions, water intercepted by vegetation, evaporation, and infiltration. I a is highly variable but generally is correlated with soil and cover parameters. Through studies of many small agricultural watersheds, I a was found to be approximated by the following empirical equation:
By removing I a as an independent parameter, this approximation allows use of a combination of S and P to produce a unique runoff amount. Substituting equation (3.3) into equation (3.2) gives:
Graphical peak discharge was developed from TR-20. The peak discharge equation is
q p = peak discharge, (cfs) q u = unit peak discharge, (cfs/sq mi/in)
A m = drainage area, (mi 2 )
Q = runoff, (in)
F p = pond and swamp adjustment factor (necessary if spread throughout the watershed and not considered in Tc computation)
q u A m QF p
The following are the steps necessary to use the SCS Method for peak discharge computations:
1. Determine the drainage area;
2. Determine the soil classification based on runoff potential (Group A, B, C, or D). See Section 3.3.3 for detailed information. One approach for a general classification is to determine the soil name and type from SCS (NRCS) soils map
or report for Craighead County;
3. Determine the antecedent soil moisture conditions (AMC). For design purposes,
the AMC will be “average” or II;
4. Classify cover type and hydrologic condition of the soil-cover complex as good, fair, or poor. For additional information see Tables 2-2a, b, and c in Appendix 1;
5. Determine the Curve Number (CN) for the AMC II soil classification. If necessary, determine a weighted value by dividing the sum of the products of the subarea sizes and CNs by the total area. (See Section 3.3.2 for details about CN);
6. Estimate the watershed time of concentration in hours (Tc) using TR-55 with max sheet flow of 100 ft.;
7. Determine the potential maximum storage (S). Use Equation 3.5 to calculate the potential maximum storage;
8. Determine the initial abstraction (I a ). Use CN to determine I a using Table 4-1 from TR-55, as shown below. See if I a is greater than P;
Table 4-1. I a values for runoff curve numbers
9. Use information in Section 3.3.1 to determine the total rainfall for watershed for the design frequency;
10. Determine the accumulated direct runoff (Q) using equation 3.4 (or solution of runoff equation TR-55 Figure 3.1);
11. Determine I a /P ratio;
12. Using I a /P ratio and Tc determine unit peak discharge (qu). Use TR-55 Exhibit 4-II Unit peak discharge (q u ) for NRCS (SCS) type II rainfall distribution;
13. Determine the unit peak discharge using graphical methods of the Ia/P ratio and the Tc (hours) and using Worksheet 4 in Appendix 1;
14. Determine the pond and swamp adjustment factor (Fp); and,
15. Compute peak discharge using equation 3.6
Appendix 1 has worksheets from the TR-55 manual to determine peak discharge.
Jonesboro is located in the SCS Type II hypothetical storm area. The design storm duration for drainage work is the 24-hour duration at the relevant frequency. The rainfall depth vs. frequency is shown in Table 3-1.
Table 3-1. Rainfall Depth vs. Frequency for Jonesboro, AR from TP-40 and HYDRO-35
The major factors affecting CN determination are hydrologic soil groups (HSG), cover type, treatment, hydrologic condition, and antecedent runoff condition (ARC). The CN varies according to the factors below. Tables 2-2a, b and c in Appendix 1 provide details for CN selection. If a watershed subarea has multiple land uses, the CN can be weighted for the subarea.
Soil properties influence the relationship between rainfall and runoff by affecting the rate of infiltration. NRCS divides soils into four hydrologic soil groups based on infiltration rates (Groups A-D). Urbanization also impacts soil groups as well.
Group A - Group A soils have low runoff potential due to high infiltration rates even when saturated. These soils primarily consist of deep sands, deep loess, and aggregated silts.
Group B - Group B soils have moderate infiltration rates when saturated. These soils primarily consist of moderately deep to deep, moderately well-drained to well-drained soils with moderately fine to moderately coarse textures (shallow loess, sandy loam).
Group C - Group C soils have slow infiltration rates and a moderately high runoff potential. These soils when saturated usually have a layer near the surface that impedes downward movement of water. These soils are moderately fine to fine in texture and examples include clay loams, shallow sandy loams, soils low in organic content, and soils usually high in clay.
Group D - Group D soils have high runoff potential (very slow infiltration rates) when saturated. These soils are predominantly clay soils with a high swelling potential, soils
with a permanent high water table, soils with a claypan or clay layer at or near the surface, and shallow soils over nearly impervious material.
The SCS has published a county soil survey book for Craighead County. Use of the soil survey book and TR-55 table of soils and their hydrologic soil group is necessary to determine the Curve Number (CN).
Cover types can include vegetation, bare soil, and impervious surfaces. Cover type can be determined by reconnaissance, aerial photography, and land use maps. Tables 2-2a,
b and c in Appendix 1, addresses most cover types.
Treatment is a cover modifier that describes management of cultivated agricultural lands. Table 2-2b in Appendix 1 is used with agricultural areas that are cultivated.
Hydrologic condition is generally estimated from plant density and residue cover to account for the effects of cover and treatment in infiltration and runoff. Hydrologic condition only is a factor in CN for agricultural lands. Details about good, fair, and poor conditions are found in Table 2-2b and Table 2-2c in Appendix 1.
For design purposes, the antecedent runoff conditions (ARC) will be average, or Type II. The CN values assume medium ARC (or Type II) conditions.
If ponds and swamp areas are spread throughout the watershed and not considered in
the Tc computation, an adjustment is needed. Table 4-2 includes the adjustment factor for percent of area of the subbasin that is pond or swamp.
TR55-Table 4.2. Adjustment factor (Fp) for pond and swamp areas that are spread throughout the watershed
The Corps of Engineers have developed HEC-HMS for determining hydrology. For large areas, HEC-HMS provides flow estimates that can account for valley storage and routing which SCS methods do not. HEC-HMS can be obtained from the Corps of Engineers. It can be downloaded from their website at http://www.hec.usace.army.mil/. For a Conditional Letter of Map Revision (CLOMR) or Letter of Map Revision (LOMR), a FEMA approved hydrology model must be used. Currently (May 2006), HEC-HMS is approved by FEMA.
NRCS has also developed a computer version of TR-55. It can be downloaded from their website at http://www.wcc.nrcs.usda.gov/hydro/hydro-tools-models-tr55.html or
http://www.wcc.nrcs.usda.gov/hydro/hydro-tools-models-wintr55.html.
4.0 STORM DRAIN AND DRAINAGE APPURTENANCES
This section contains storm drainage design criteria and demonstrates the design procedures to be employed on drainage projects within the City of Jonesboro. All drainage design calculations and assumptions shall submitted with permit applications.
Table 7 in Appendix 1 shows the appropriate design frequencies to be used for storm drain designs in the City of Jonesboro.
To begin design of a storm drainage system, it is necessary to compute the amount of runoff that accumulates upstream of the intake structures. For basins less than 100 acres, the SCS Method should be used for computing runoff. The equation is:
runoff, (in)
rainfall, (in)
The next step in the design of the storm drain system is to calculate the flow within the streets.
The following street classifications will provide clarity in discussing the requirements and methodology to calculate the flow in streets:
Principal Arterials: Serve the major centers of activity Minor Arterials: Intended to provide land access Collectors: Connect local streets in residential neighborhoods
Section 4 Storm Drain and Drainage Appurtenances
Locals: Provide access to various public and private properties
The following descriptions relate to the shape of the cross section of the roadway:
Straight Crown - A constant slope from one gutter flow line across the street to the other gutter flow line.
Parabolic Crown - A pavement surface shaped in a parabolic from one gutter flow line to the other.
Vertical Displacement Between Gutter Flow Line - Due to topography, it will be necessary at times that the curbs on a street be placed at different elevations.
The calculation of flow in streets is dependent on street width and shape. Generally, there are two shapes for streets: straight crown and parabolic crown. The straight crowned street can be further subdivided into two types of gutters: uniform and composite. The following discussion covers the methodology used to compute the flow in the street.
Table 7 in Appendix 1 shows the requirements for the design of the roadway drainage.
The runoff in the gutter is generally treated as open channel flow. Therefore, Manning’s Equation can be used to calculate the flow or spread in the road section. The following formula is a modified version of the Manning’s Equation. It incorporates the geometry of the uniform roadway section.
S X 1.67 S L 0.5 T 2.67
K C = 0.56
n = Manning’s roughness coefficient (0.013 for concrete) S L = Longitudinal slope of road, (ft/ft)
S X = Pavement (road) cross-slope, (ft/ft) =
= Total width of flow or spread
= Total discharge, (cfs)
S L = Longitudinal slope of road
This equation assumes that the depth of flow, d, is small when compared to the overall spread and therefore the spread is assumed to be equal to the wetted perimeter. Also, the friction along the curb height is assumed to be negligible when compared to the friction along the spread.
The roadway should be designed such that the spread will be maximized just upstream of the inlet. See design spread criteria in Table 7 in Appendix 1.
W = Flow in depressed section, (ft 3 /s)
S = Side flow, (cfs)
S W = Gutter Cross Slope, (ft/ft)
S X = Pavement (road) cross-slope, (ft/ft)
W = Width of depressed gutter, (ft)
T S = Width of side flow, (ft)
T = Total width of flow, (ft)
a = Continuous gutter depression, (in)
In order to calculate the flow in a composite section the ratio of frontal flow to total gutter flow, E o , can be calculated using Formula (4.4) in conjunction with Formula (4.3).
T / W − 1
Parabolic Street Sections
For residential streets, parabolic sections are often used because they provide a flatter driving surface than uniform sections. However, the flow capacity is less for the parabolic section than the uniform section. The following formulas can be used to calculate the flows and associated spread in a parabolic section.
= ((Q/S 0.5 ) C2 ) /C1
Q = (y*C1) (1/C2) *S 0.5
= B – (C3-C4y) 0.5
= (T x y) / 3
= Flow depth in gutter for one side of street, (ft)
= Gutter discharge for one side of the street, (cfs)
= Spread for one side of the street, (ft)
= Cross sectional area of flow, (ft 2 )
= Velocity of flow, (ft/ S )
= 1/2 of the street width
Table 4.1: Parabolic Roadway Coefficients
385.7143
555.4286
* 8”
829.7143
* These crown heights shall be used for new developments
Note: These constants were derived for a Manning’s n of 0.016.
Alternatively, the nomographs included in Appendix 4 can be used as aids in designing parabolic roadway drainage.
The hydraulic capacity of a storm drain inlet depends upon its geometry as well as the characteristics of the gutter flow. Inlet capacity governs both the rate of water removal from the gutter and the amount of water that can enter the storm drainage system. Inadequate inlet capacity or poor inlet location may cause flooding on the roadway resulting in a hazard to the traveling public.
In general inlets should be placed to meet the spread requirements summarized in Table 7 in Appendix 1. In addition, inlets should be spaced at a maximum distance of 600 feet apart or before intersecting street.
Storm drain inlets are used to collect runoff and discharge it to an underground storm drainage system. Inlets are typically located in gutter sections, paved medians, and roadside and median ditches. Inlets used for the drainage of highway surfaces can be divided into the following three classes:
1. Grate inlets;
2. Curb-opening inlets; and,
3. Combination inlets
Grate inlets consist of an opening in the gutter or ditch covered by a grate. Curb- opening inlets are vertical openings in the curb covered by a top slab. Combination inlets consist of both a curb-opening inlet and a grate inlet placed in a side-by-side configuration, but the curb opening may be located in part upstream of the grate.
Inlet interception capacity, Q j , is the flow intercepted by an inlet under a given set of conditions. The efficiency of an inlet, E, is the percent of total flow that the inlet will intercept for those conditions. The efficiency of an inlet changes with changes in cross slope, longitudinal slope, total gutter flow, and, to a lesser extent, pavement roughness.
In mathematical form, efficiency, E, is defined by the following equation:
= Inlet Efficiency (ft 3 /s)
= Total Gutter Flow
Q j = Intercepted F=low, (ft 3 /s)
Flow that is not intercepted by an inlet is termed carryover or bypass and is defined as follows:
= Q - Q j
Q b = bypass flow, (ft 3 /s)
In Appendix 4, design charts for inlets on grade and procedures for using the charts are
presented for the various inlet configurations. Remember that for locally depressed inlets, the quantity of flow reaching the inlet would be dependent on the upstream gutter section geometry and not the depressed section geometry.
Charts for grate inlet interception have been made and are applicable to all grate inlets tested for the Federal Highway Administration. The chart for frontal flow interception is based on test results which show that grates intercept all of the frontal flow until a velocity is reached at which water begins to splash over the grate. At velocities greater
than “Splash-over” velocity, grate efficiency in intercepting frontal flow is diminished. Grates also intercept a portion of the flow along the length of the grate, or the side flow.
A chart in Appendix 4 is provided to determine side-flow interception. Chart 5B in
Appendix 4 determines the “splash-over” velocity.
A procedure for determining the interception capacity of combination inlets is also
Grate inlets perform satisfactory over a wide range of gutter grades. The capacity of an inlet depends on its geometry and the cross slope, longitudinal slope, total gutter flow, depth of flow and pavement roughness. The depth of water next to the curb is a major
factor in the interception capacity of both gutter inlets and grate inlets. At low velocities
all the water flowing in the gutter adjacent to a grate is intercepted. On steep slopes
only a portion of the frontal flow will be intercepted if the velocity is high or the grate is short and splash over occurs. For grates less than two (2) feet long intercepted flow is small.
A parallel bar grate inlet is the most efficient type of curb inlet; however when crossbars
are added for bicycle safety the efficiency is greatly reduced. Where bicycle traffic is a
design consideration, the curved vane grate and tilt bar grate are recommended for both hydraulic features and safety.
Where debris is a problem, consideration should be given to debris handling efficiency rankings. Table 4-2 presents the results of tests for debris handling efficiency. This table should be used for relative comparisons only.
Table 4-2. Average Debris Handling Efficiencies of Grates Tested
30º - 85 Tilt Bar
45º - 85 Tilt Bar
– 50x100
45º - 60 Tilt Bar
When the velocity approaching the grate is less than the “splash-over” velocity, the grate
will intercept essentially all of the frontal flow. Conversely, when the gutter flow velocity exceeds the “splash-over” velocity for the grate, only part of the flow will be intercepted.
A part of the flow along the side of the grate will be intercepted, dependent on the cross
slope of the pavement, the length of the grate, and flow velocity.
The ratio of frontal flow to total gutter flow, E o for a uniform cross slope is expressed by equation 4.12:
Q w = 1 -
Q= total gutter flow, (ft 3 /s)
Q w = flow in width W, (ft 3 /s)
W= width of depressed gutter or grate, (ft)
T= total spread of water, (ft)
Chart 2B in Appendix 4 provides solutions of E o for either uniform cross slopes or composite gutter sections.
The ratio of side flow, Q s , to total gutter flow is:
= 1 - E o
The ratio of frontal flow intercepted to total frontal flow, R f , is expressed by equation
R f = 1 – K u (V – V o )
K u = 0.09 in English Units
V= velocity of flow in the gutter, (ft/s)
V o = gutter velocity where “splash-over” first occurs, (ft/s)
(Note: R f cannot exceed 1.0)
This ratio is equivalent to frontal flow interception efficiency. Chart 5B in Appendix 4 provides a solution for equation 4.14 which takes into account grate length, bar configuration, and gutter velocity at which splash-over occurs. The average gutter velocity (total gutter flow divided by the area of flow) is needed to use Chart 5B in Appendix 4. This velocity can also be obtained from Chart 4B in Appendix 4.
The ratio of side flow intercepted to total side flow, R s , is side flow interception efficiency, is expressed by equation 4.15. Chart 6B in Appendix 4 provides a solution to equation
⎛ ⎜ ⎜ 1 +
R s = 1 /
⎝ S L
u = 0.15 in English Units
= length of grate along gutter, (ft)
x = roadway cross slope
The efficiency, E, of a grate is expressed as provided in equation 4.16:
E = R f E o + R s (1 - E o )
The first term on the right side of equation 4.16 is the ratio of intercepted frontal flow to total gutter flow, and the second term is the ratio of intercepted side flow to total side flow. The second term is insignificant with high velocities and short grates. The interception capacity of a grate inlet on grade is equal to the efficiency of the grate multiplied by the total gutter flow as represented in equation 4.17.
Q j = E Q = Q [R f E o + R s (1 - E o )]