§ 4-296. Design criteria.

(a)

Street drainage. No lowering of the standard height of street crown shall be allowed for the purposes of obtaining additional hydraulic capacity.

(b)

Drainage system.

(1)

Culverts.

a.

Construction plans for proposed reinforced concrete box culverts, and related structures maybe adaptations of the current Alabama Department of Transportation (ALDOT) Standards.

b.

For culverts in residential streets, runoff from the one-hundred-year frequency flow shall not produce a headwater elevation at the roadway greater than either twelve (12) inches above the roadway crown elevation or any top of upstream curb elevation, whichever is lower.

c.

For culverts in streets other than a residential street, runoff from the one-hundred-year frequency storm shall not produce a headwater elevation at the roadway greater than six (6) inches above the roadway crown elevation or six (6) inches above any top of upstream curb elevation, whichever is lower.

(2)

Drainage facilities. All drainage facilities (including but not limited to, headwalls, open channels, storm sewers, area inlets, and detention, retention and water quality controls and their appurtenances) shall comply with the following requirements, unless otherwise noted in this section:

a.

Storm sewer inlets and gutter transitions shall be designed to avoid future driveways and to avoid conflicts with standard water and wastewater service locations. No utilities shall be allowed to cross under a storm sewer inlet.

b.

Drainage channels and detention ponds that are to be maintained by the public (city) shall be contained within dedicated easements. Adequate room for access shall be provided for drainage channels and detention ponds.

1.

Ramps no steeper than five (5) feet horizontal to one (1) foot vertical shall be provided at appropriate locations to allow access to drainage channels and detention ponds.

2.

The minimum bottom width for any channel with vegetative side slopes shall be eight (8) feet.

3.

A reinforced concrete trickle channel shall be provided in all newly constructed channels and from detention pond inlets to outlets. The area adjacent to trickle channels shall slope at a minimum of two (2) per cent.

(i)

Detention ponds shall be designed with adequate area around the perimeter for access and maintenance. Said area shall be a minimum of seven (7) feet wide for ponds with depths of five (5) feet or less (back slopes included) and a minimum of fifteen (15) feet wide for ponds over five (5) feet deep or with back slopes in excess of five (5) feet high. Said area shall not slope more than five (5) per cent.

(ii)

Rip-rap for slope protection or velocity dissipation shall be formed concrete dissipaters or mortared rip-rap.

(3)

Storm drains.

a.

Storm drains to be designed to the twenty-five-year, twenty-four-hour rainfall event.

b.

Storm drains between lots (crossing blocks) shall be avoided as much as possible. When unavoidable, such mains shall be laid along a straight alignment (absent of curves, jogs and manholes/junction boxes when traversing between lots) with manholes/junction boxes provided at each intersecting street. Storm drains along rear of residential lots (through back yards) shall be avoided. Easements shall be a minimum of fifteen (15) feet in width with an additional two (2) feet of easement for every one (1) foot of depth over eight (8) feet.

c.

All bends, wyes and pipe size changes in storm sewers shall occur at manholes/junction boxes unless otherwise approved by the city engineer.

d.

Bedding of storm sewer shall be to the top of pipe.

e.

Storm drains shall have a minimum size of fifteen (15) inches in diameter.

f.

Junction boxes and manholes shall be reinforced concrete. Junction boxes in lieu of manholes shall be provided where any pipe opening exceeds thirty-six (36) inches and where the distance from the outside surfaces of any two (2) pipes entering a manhole is less than one (1) foot, measured along the inside of the manhole.

(4)

Open channel sections.

a.

Minor collector channels shall be constructed with underground storm sewers. If it can be established by certified engineering data to the satisfaction of the city engineer that storm sewers are not physically feasible, open ditches may be used, provided that such ditches are lined properly with materials accepted by the city engineer. These structures shall be of sufficient cross-section and slope as to fully contain design flows and facilitate self-cleaning. Outfalls shall enter major collector drainage ways and major streams at grade or be designed and constructed with adequate concrete aprons, energy dissipaters or similar features to prevent erosion.

b.

Major collector channels drainage ways, detention ponds and related structures may utilize either existing natural open sections which may be modified, or newly constructed facilities. If modified or newly constructed facilities are utilized, they shall be lined with permanent materials including, but not limited to, concrete or vegetation.

c.

Vegetated channels shall have sufficient grade but with velocities that will not be so great as to create erosion. Side slopes shall not be steeper than three (3) (horizontal) to one (1) (vertical) for channels four feet or less in depth and no steeper than 4:1, in all other channels to allow for future growth and to promote slope stability. All slopes shall be hydromulched, sodded or seeded with approved grass, grass mixtures or groundcover suitable to the area and season in which they are applied.

d.

Discharge from storm sewer outfalls shall not cause channel, or stream bank erosion. If the storm drain discharges to an open drainage facility (as determined by the city), the applicant must show acceptable nonerosive conveyance to that drainage facility, appropriate energy dissipation at the outfall and a stable headwall.

e.

No area within the limits of construction of the development shall allow stormwater to become stagnant. Maximum retention or "draw-down" time for detention ponds shall not exceed twenty-four (24) hours from the time of peak storage to the time of complete emptying of the pond, as determined by hydrograph routing or other calculations acceptable to the city. This requirement does not apply to facilities in which retention or "draw-down" time is required to be greater than twenty-four (24) hours.

(c)

Computations.

(1)

Computations to support all drainage designs shall be submitted to the city engineer for review. The computations shall be in such form as to allow for timely and consistent review and also to be made a part of the permanent city record for future reference. Computations shall demonstrate that as a result of the proposed development there will not be any adverse impact to downstream structures adjacent to the drainage for design storms up to the one-hundred-year storm. All computations submitted shall be from a licensed professional engineer with expertise in the area of hydraulics and hydrology.

(d)

Development within FEMA floodplains.

(1)

Federal Emergency Management Agency.

a.

The Federal Emergency Management Agency (FEMA) maintains flood insurance rate maps (FIRMs) that depict floodplain and floodway boundaries based on existing conditions of development in the contributing area.

b.

FEMA revises or amends FIRMs by issuing of a letter of map amendment (LOMA) or letter of map revision (LOMR). FEMA establishes the process and fee schedule for review of LOMA or LOMR requests.

(2)

Coordination of City of Tuscumbia and FEMA floodplain delineations.

a.

If a LOMR is to be requested due to land development activities that alter existing conditions, then the following requirements are applicable:

•

The property owner must enter into a letter agreement with the city assuring that the proposed activities are consistent with existing plans and improvements of the city, that the city will request its consultant to prepare any studies, plans, proposals or applications to FEMA and that the city will incur no cost, expense or liability from the project

•

The property owner must provide all information necessary for city review and submission to its consultation for preparation of the request and pay all costs, fees and expenses associated with the request.

b.

If a LOMR-F is to be requested, due to land development activities that alter existing conditions, then the following requirements are applicable:

•

The property owner must complete an application for permit to "develop in a special flood hazard area" and an application for a "site development permit". Both permits require submission of an elevation certificate based on construction drawings and completed by a registered land surveyor or licensed engineer.

•

A second elevation certificate based on existing construction conditions must be submitted to the city: (1) for slab construction - after the slab form is set but prior to pouring; or (2) for crawl space or stem wall construction - after the floor is framed or the form set but prior to erecting any walls, to ensure compliance with floor elevation requirements of the flood damage prevention ordinance. The slab for all equipment servicing the building must also comply with elevation requirements of the ordinance.

•

A third elevation certificate based on finished construction must be submitted to the city to ensure compliance with flood damage prevention ordinance and prior to issuance of a certificate of occupancy. At the time the third elevation certificate is completed, the application to FEMA for a LOMR-F to remove the structure from the floodplain and avoid paying flood insurance can begin.

(e)

Site grading considerations.

(1)

A comprehensive grading plan shall be included with subdivision or site construction plans.

(2)

The grading plan shall be designed to ensure all lots will adequately drain upon completion of the improvements.

(3)

Where practical, all lots shall be graded from rear to front at which point the drainage shall be intercepted by the street. Alternate grading schemes may be utilized if it can be demonstrated to the satisfaction of the city engineer that grading from rear to front would be detrimental to trees or other natural features; or it would not be reasonably adaptable to the existing topography because of excessive cuts and fills, or future lot development (i.e., commercial, industrial or multifamily lots).

(4)

All lots shall be graded at a minimum of one (1) per cent. Grading of lots with existing slopes of one (1) per cent or greater will not be required provided the conditions under subsection (3) above have been satisfied and it is demonstrated to the satisfaction of the city engineer that there are no existing or proposed features that will prevent the lots from adequately draining.

(5)

Unless otherwise accepted by the city engineer, surface swales shall be designed and provided along lot lines when more than two (2) lots will be contributing to stormwater runoff at any given point. Side slopes for swales shall not exceed 10:1 (horizontal: vertical) unless otherwise accepted by the city engineer.

(6)

Minimum finished floor elevations shall be shown for all lots. Such elevations shall be as required by the City of Tuscumbia Flood Damage Prevention Ordinance (Ordinance No. 1421-10).

(7)

Blue tops shall be set at lot corners and other points to ensure grading is accomplished in accordance with the plan.

(8)

Following final grading, all exposed areas shall be permanently stabilized. Earthen areas shall be seeded or sodded and erosion controls shall remain in place until grass growth reaches one and one-half (1½) inches; is of a density where it can be reasonably expected to be self-sustaining; and there are no bare areas in excess of ten (10) square feet.

(f)

Erosion control.

(1)

Silt fences, sedimentation basins, stabilized construction entrances/exits and similar recognized techniques shall be employed during and after construction to prevent point source sedimentation loading of downstream facilities. Such installations shall be to the satisfaction of the city engineer. Additional measures may be required during and after construction if, in the opinion of the city engineer, they are warranted.

(2)

All disturbed and exposed areas due to construction shall be permanently stabilized. All such areas shall be dressed with topsoil and vegetated by seeding or sodding as appropriate. Where the city engineer determines that future maintenance is materially impaired or erosion is a distinct possibility, the developer shall be required to use concrete or similar permanent cover in lieu of vegetation. Erosion control matting (either pre-seeded or seeded after placement) may also be required if the city engineer determines that such protection of slopes is required to ensure that seeding or soil will not wash off of slopes.

(3)

The developer of a proposed development shall submit to the Tuscumbia Planning Commission with the construction plans four (4) copies of a plan to control erosion on the site of the proposed subdivision. Said plan shall be prepared by the subdivider's engineer according to the best available practices of sediment and erosion control and shall consist of a map(s) and a description of the premises setting forth the proposed: (a) improvements to be constructed; (b) changes to be made in the contours of the site; and (c) removal or destruction of the natural topsoil, trees, or other natural vegetation on the said described premises.

(4)

The city engineer shall review said sediment and erosion control plan and submit written comments to the Tuscumbia Planning Commission prior to presentation of the construction plans for approval. Written comments shall specifically state the acceptability of the plan, nonacceptability, or any necessary changes to insure adequate erosion control. The city engineer shall also submit written comments to the Tuscumbia Planning Commission, prior to presentation of a final plat for approval, stating that the sediment and erosion control plan has been carried out in substantial compliance with this section or that the subdivider has failed to comply. Where the city engineer's comments or other validated evidence indicates that the subdivider has not carried out the approved sediment and erosion control plan the Tuscumbia Planning Commission shall not grant final approval of the subdivision.

(Ord. No. 15-0817A, 8-17-2015)