§ 4-297. Submittal requirements and computation methods.

(a)

Survey requirements.

(1)

A boundary survey prepared by a registered land surveyor licensed in the State of Alabama that includes:

a.

All structures above and below grade.

b.

Known easements listing deed book and page where recorded.

c.

Rights-of-way listing adjacent land owners name, address, parcel number, and lot number.

d.

Existing drainage facilitates listing size, diameter, material, and entrance/exit locations.

e.

Existing utilities both underground and overhead.

f.

Blue line streams identified on USGS Quadrangle Map.

g.

Current FEMA one-hundred-year floodplain with elevation listed.

h.

Wetlands delineated.

i.

One-foot contours.

(b)

Plan requirements.

(1)

A site development plan shall be required for any site development except when:

a.

The developed area is used for gardening or agricultural purposes;

b.

The proposed work does not disturb an area greater than three thousand six hundred (3,600) square feet.

(2)

Development plan requires plans showing existing and proposed one-foot contours as they relate to the roadway, parking lot, drainage facilities, cut and fill slopes, all stormwater pipe size, material and location, identification of all areas of depression, blue-line streams, easements, erosion and sediment control measures, detention pond data including size, location, slope of bottom, outlet, invert, top elevations, spillway size and elevation, and the detention easement and an adequately sized traversable access easement. Also, catch basin location, elevation, slope, swales, ditches, and their stabilization treatment. When this site development plan includes a street to be dedicated to the city, a complete set of roadway plans must be submitted including profiles, grades, and cross-sections showing cross slope, and limits of construction. All development plans that are submitted to the building department must meet the following minimum standards:

a.

Must contain the following certification from the design engineer:

1.

The engineer of record for this project assumes full responsibility for the design shown hereon and the effects thereof. The city by reviewing this information assumes no responsibility for any unforeseen negative impacts to adjoining or downstream property owners.

b.

Stamp and signature from appropriate design professional;

c.

Construction plans;

d.

Pre- and post-developed contours;

e.

Erosion and sediment control plan;

f.

Required retaining wall calculations (if any);

g.

Owner's, and if applicable lessee's, name, address and phone number;

(3)

Plans that do not meet these minimum standards will be rejected, and will not be reviewed further until submission standards are met.

(c)

Stormwater management plan.

(1)

A stormwater management plan must be submitted with any proposed project and must address the calculations and requirements set forth in this section. The report must be prepared by a professional engineer licensed by the State of Alabama that is proficient in hydraulics and hydrology. The report must address any possible downstream impacts of the proposed development and mitigation of those impacts is required.

(2)

The following items are required elements of any stormwater management plan submitted for review. The city may require additional information if warranted by unusual site conditions.

a.

Site location. Describe the location of the subject property; locate by section/s, township, and range; identify all adjacent developments, major drainage outfalls, streets, highways, and provide a vicinity map. Include map with an aerial photo of the subject property and adjacent development and infrastructure, and a USGS Quadrangle map showing the location of the subject property.

b.

Site description. Describe the predominant existing land use and future land use, proposed development, soil types, vegetative cover, estimated pre and post developed impervious area, and provide photos of the site, existing channels, ditches, natural drains, and drainage structures.

c.

FEMA FIRMette. Indicate location of the site on a FEMA FIRMette with base flood elevation noted.

d.

Watershed map. Delineate existing drainage boundaries of off-site watersheds tributary to project area stormwater conveyance elements, indicate acreage and the exit point of each basin. The map should indicate the location of pertinent existing channels, ditches, and natural drains.

e.

Preliminary plat or site plan. The drainage study shall include a copy of the proposed developments preliminary plat or site plan.

f.

Pre-developed sub-basin map. Provide a sub-basin map for the existing condition of the site to include on and off site sub-basins tributary to the project area. All discharge points into and out of the site should be noted with ten-year, twenty-five-year, and one-hundred-year peak flow rates.

g.

Post-developed sub-basin map. Provide a sub-basin map for the proposed condition of the site to include on and off site sub-basins tributary to the project area. All discharge points into and out of the site should be noted with ten-year, twenty-five-year, and one-hundred-year peak flow rates.

h.

Floodplain mitigation considerations. Provide an estimate of the volume of fill required to be placed in special flood hazard areas and the proposed method to provide floodplain mitigation.

i.

Conclusions and recommendations. The stormwater management plan shall clearly identify the results and conclusion of the analysis and provide recommendations of any required action(s) so that surrounding properties experience no adverse impact. The conclusions should also clearly indicate how the proposed design is meeting the requirements of the City of Tuscumbia's drainage ordinance.

j.

Supporting calculations. Provide in appendices pertinent calculations referenced in the study. Provide supporting documentation for all design parameters used in calculation and modeling flows (e.g. include TOC flow paths, CN tables, all routing model input and output, detention volume calculations, etc.).

(d)

Hydrology methods.

(1)

Consideration of peak runoff rates for design conditions is generally adequate for conveyance systems such as storm sewers or open channels. However, if the design includes flood routing, detention ponds, retention ponds, etc., a flood hydrograph is usually required.

(2)

Rational method. The rational method is generally acceptable for the determination of peak flows from watersheds smaller than fifty (50) acres. This method is not acceptable for detention/retention pond sizing, or evaluation.

(3)

NRCS unit hydrograph method.

a.

The NRCS unit hydrograph method is specifically cited for drainage computations, using twenty-four-hour Type II rainfall distribution and AMC II soil conditions. The NRCS method shall be used to compute a peak flow for sizing all stormwater conveyances or to generate a hydrograph for the purposes of detention/retention routing. The NRCS unit hydrograph method shall be used for all design calculations, but other methods may be consulted for sizing stormwater conveyances (particularly if conservative values and assumptions are used).

b.

The NRCS was formerly called the soil conservation service (SCS), part of the United States Department of Agriculture. The TR-55 publication (Urban Hydrology for Small Watersheds) is the principal technical reference to be downloaded from NRCS: http://www.wsi.nrcs.usda.gov/products/w2g/h&h/docs/other/TR55documentation.pdf .

(4)

The maximum sheet flow length to be used shall be less than or equal to 100 feet.

(e)

Functional design of stormwater drainage systems. In selecting the design frequency storm, the following criteria (listed in the order of being progressively more restrictive) will be used:

(1)

Longitudinal side drains shall be designed for a ten-year frequency flood, providing that no residential or commercial structures are flooded by a one-hundred-year flood.

(2)

Roadway cross-drains for all local streets and collector streets shall be designed for a twenty-five-year frequency flood, providing that no structures are flooded by a one-hundred-year flood.

(f)

Design of open channels. Manning's equation is the principal means for determining flow capacity and velocity in open channels. Open channels shall be designed according to the "Design of Roadside Channels with Flexible Linings Hydraulic Engineering Circular Number 15, Third Edition (HEC 15). This guide can be downloaded at the following address: http://www.fhwa.dot.gov/engineering/hydraulics/pubs/05114/index.cfm .

(g)

Design of curb and grate inlets.

(1)

Use of the City of Tuscumbia standard inlets (see City of Tuscumbia Construction Specifications) or ALDOT standard inlets is required within all public rights-of-way or drainage easements. Use of standard inlets on private property is encouraged for reasons of structural reliability, ease of maintenance, common availability and standardized installation methods. The designer must locate street inlets to quickly drain stormwater from paved surfaces, keeping streets passable and safe for vehicular traffic. Street inlets must be spaced and located in a manner to carefully balance vehicle safety, drainage system capacity, economics and efficiency. Maximum inlet spacing is generally three hundred (300) feet unless proven otherwise by computations. Inlets should be located at uphill corners of each street intersection to prevent sheet flow of stormwater. The basic geometry of stormwater flow along curbs is a thin shallow triangular cross-sectional area. If the section contains curb and gutter, then the stormwater flow is a composite shape formed by both concrete and asphalt surfaces, for which Manning's equation is still applicable. Based upon the longitudinal slope of the gutter and the cross slope of the street, the gutter flow will spread across the street. The spread impacts vehicular traffic in a negative way, causing vehicles to hydroplane or to pull in one (1) direction. Basic references for computing spreads, inlet capacities, and interception rates for curb and grate inlets are FHWA Hydraulic Engineering Circular No. 12, Drainage of Highway Pavements (March 1984), or FHWA Hydraulic Engineering Circular No. 22, Urban Drainage Design Manual (November 1996). Both references can be downloaded in Acrobat format at the FHWA website: http://www.fhwa.dot.gov/engineering/hydraulics/highwaydrain/index.cfm .

(2)

Detailed inlet computations are usually not required for local residential streets and alleys, except at sag locations where potentially inadequate inlets could flood nearby houses and buildings. Slow design speeds on local streets usually minimize the impact of spread and hydroplaning, although local streets do tend to have steeper approach slopes for intersections. Typical considerations for inlet design include:

a.

Place inlets at all sag locations and other depressed areas to ensure positive drainage. Ensure that ponded water does not flood nearby structures, buildings, or houses. Flanking inlets, at an offset distance of twenty-five (25) feet or fifty (50) feet, are desirable in sag locations with large flow rates.

b.

Place inlets at street intersections to prevent stormwater from flowing across a street or entrance. This is particularly important wherever a local street intersects a larger street, such as a collector or arterial. Valley gutters across street intersections are not encouraged, unless specifically used for very short streets or cul-de-sac.

c.

Maintain a minimum curb and gutter longitudinal slope of 0.5 per cent to keep positive drainage. When designing a flat stretch of street, the street designer may incorporate a gently rolling vertical profile to maintain positive drainage (along with placement of additional inlets). The designer is cautioned that the use of long vertical curves is discouraged in areas with minimum slopes.

(h)

Design of storm drainage systems. Manning's equation is typically used to compute nonpressurized flow in pipes and storm drainage systems where inlets and headwalls are closely spaced to allow atmospheric pressure throughout the entire system. Computations for each pipe should be performed systematically (such as in a table) and include the drainage area, design flow, velocity, capacity, diameter or size, slope, length, construction material, upstream and downstream inlets, etc. Computations should also include one (1) or more maps and drawings to show drainage areas, impervious surfaces, slopes, land cover, paths for computing time of concentration, and any off-site areas that contribute flow. Minimum size diameter of storm drainage pipes is fifteen (15) inches. For allowable types of pipe see the City of Tuscumbia Construction Specifications. Computation of the hydraulic grade line (HGL) may be required by the engineer, particularly if pipes are designed without excess capacity, pipes are placed at steep slopes with high velocities, the outfall is submerged, or if there are excessive deflection angles in the stormwater drainage system. Excessive velocities should be avoided to prevent HGL problems and the potential for erosion. Minimum design velocities should be at least three (3) feet per second to ensure that a storm drainage system has some capability for self-cleaning.

(i)

Design of culverts. A culvert is a single drainage pipe, not part of an enclosed system, which has a pipe or box opening as the inlet condition. Allowable flow within culverts are subject to inlet control, outlet control, or some combination of the two (2) controls. Culvert design is performed using FHWA Hydraulic Design Series No. 5, Hydraulic Design of Highway Culverts (September 1985), which can be downloaded at http://www.fhwa.dot.gov/engineering/hydraulics/library arc.cfm?pub number=7&id=13 as an Adobe Acrobat document. Considerations in culvert design include analysis of open channels at both ends of the culvert, potential for storage or channel routing, and design of energy dissipaters and outlet protection. Head loss can be reduced by using headwalls, wing walls, mitered slopes, and tapered inlets; refer to FHWA Hydraulic Design Series No. 5 for more details concerning culvert design. Considerations for determining the allowable headwater are potential for upstream property damage, road overtopping, erosion potential, human safety, and whether wing walls and headwalls are designed as part of the culvert. Minimum size diameter for culverts is fifteen (15) inches.

(j)

Hydraulic grade line computations. Where the hydraulic grade line (HGL) is deemed to be critical by the city engineer or his representative, the HGL shall be computed. HGL computations must be performed by a registered engineer using principles of hydrology and hydraulics, and basic formulas such as conservation of momentum and energy, continuity of flow, and types of flow classification.

(k)

Analysis of downstream system.

(1)

Discharge from a developed site (typically a stormwater detention or retention basin) must be routed to an existing natural or manmade stormwater pipe or channel with adequate capacity. Calculations must be submitted that show the capacity of the receiving stormwater pipe or channel to handle the design storms. The first reason for analysis of the downstream system is to ensure that known flooding problems are not exacerbated. Stormwater detention basins are always designed so that the peak flow discharge is not increased. This means that the immediate downstream receiving channel, if it currently has adequate capacity, will continue to be adequate. However, if the stormwater detention basin causes a longer duration for peak or near-peak flows, then flooding could occur in locations where it did not occur before. The second reason for analysis of the downstream system is to determine any backwater effects on the detention outlet structure and embankment. In most situations, the design engineer assumes inlet control conditions for the detention basin control structure, which must be verified to ensure that the detention basin operates as designed.

(2)

If no existing natural or manmade stormwater channel exists downstream the discharge must be returned to a sheet flow condition before it is discharged. The location of the discharge onto the downstream property must be in a similar location as the predevelopment discharge.

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