§ 9.5.3. Development Standards

A.

Definition of Floodplain Boundaries  

1.

"A" zones, as identified in the FIS, are used to establish base flood elevations.

2.

For all streams with a drainage area of 100 acres or greater, the future-conditions flood elevations are provided by the City. If future-conditions elevation data is not available from the City, then it is determined by a licensed professional engineer using a method approved by FEMA and the City.

B.

Definition of Floodway Boundaries The width of a floodway is determined from the FIS or FEMA-approved flood study. For all streams with a drainage area of 100 acres or greater, the regulatory floodway is provided by the City. If floodway data is not available from the City, then it is determined by a licensed professional engineer using a method approved by FEMA and the City.

C.

General Standards  

1.

No development shall be allowed within any area of special flood hazard or area of future-conditions flood hazard that could result in any of the following:

a.

Raising the base flood elevation or future-conditions flood elevation equal to or more than 0.01 foot;

b.

Reducing the base flood or future-conditions regulatory flood storage capacity;

c.

Changing the flow characteristics as to the depth and velocity of the waters of the base flood or future-conditions flood as they pass both the upstream and the downstream boundaries of the property; or

d.

Creating hazardous or erosion-producing velocities, or resulting in excessive sedimentation.

2.

Any development within any area of special flood hazard or area of future-conditions flood hazard allowed under Sec. 9.5.3.C.1 shall also meet the following conditions:

a.

Compensation for storage capacity shall occur between the average groundwater table elevation and the base flood elevation for the base flood, and between the average groundwater table elevation and the future-conditions flood elevation for the future-conditions flood, and lie within the boundaries of ownership of the property being developed and is within the immediate vicinity of the location of the encroachment. Acceptable means of providing required compensation:

i.

Include lowering of natural ground elevations within the floodplain, or lowering of adjoining land areas to create additional floodplain;

ii.

Storage. In no case shall any required compensation be provided via bottom storage or by excavating below the elevation of the natural (pre-development) stream channel unless such excavation results from the widening or relocation of the stream channel;

b.

Cut areas are stabilized and graded to a slope of no less than two percent;

c.

Effective transitions are provided such that flow velocities occurring on both upstream and downstream properties are not increased or decreased;

d.

Verification of no-rise conditions (0.01 foot or less), flood storage volumes, and flow characteristics are provided via a step-backwater analysis meeting the requirements of Sec. 9.5.3.D;

e.

Public utilities and facilities, such as water, sanitary sewer, gas, and electrical systems, are located and constructed to minimize or eliminate infiltration or contamination from floodwaters; and

f.

Any significant physical changes to the base flood floodplain is submitted as a conditional letter of map revision (CLOMR) or conditional letter of map amendment (CLOMA), whichever is applicable. The CLOMR submittal is subject to approval by the Department using the FEMA community concurrence forms before forwarding the submittal package to FEMA for final approval. The responsibility for forwarding the CLOMR to FEMA and for obtaining the CLOMR approval is the responsibility of the applicant. Within six months of the completion of development, the applicant shall submit as-built surveys and plans for a final letter of map revision (LOMR).

D.

Engineering Study Requirements for Floodplain Encroachments An engineering study is required, as appropriate to the proposed development activities on the site, whenever a development proposes to disturb any land within the future-conditions floodplain, except for a residential single-lot development on streams without established base flood elevations and floodways. This study is prepared by a licensed professional engineer and made a part of the application for a permit. This information is submitted to and approved by the Department prior to the approval of any permit which would authorize the disturbance of land located within the future-conditions floodplain. Such study shall include all requirements specified in the Sandy Springs Technical Manual:

1.

Description of the extent to which any watercourse or floodplain will be altered or relocated as a result of the proposed development;

2.

Step-backwater analysis, using a FEMA-approved methodology approved by the Department. Cross sections (which may be supplemented by the applicant) and flow information will be obtained whenever available. Computations will be shown duplicating FIS results and will then be rerun with the proposed modifications to determine the new base flood profiles and future-conditions flood profiles;

3.

Floodplain storage calculations based on cross sections (at least one every 100 feet) showing existing and proposed floodplain conditions to show that base flood floodplain and future-conditions floodplain storage capacity would not be diminished by the development;

4.

The study shall include a preliminary plat, grading plan, or site plan, as appropriate, which shall clearly define all future-conditions floodplain encroachments.

E.

Floodway Encroachments Located within areas of special flood hazard are areas designated as floodways. A floodway may be an extremely hazardous area due to velocity floodwaters, debris or erosion potential. In addition, floodways must remain free of encroachment in order to allow for the discharge of the base flood without increased flood heights. Therefore, the following provisions shall apply:

1.

Encroachments are prohibited, including earthen fill, new construction, substantial improvements or other development within the regulatory floodway, except for activities specifically allowed in [subsection] (2) below.

2.

Encroachments for bridges, culverts, roadways and utilities within the regulatory floodway may be permitted provided it is demonstrated through hydrologic and hydraulic analyses performed in accordance with standard engineering practice that the encroachment will not result in any increase to the pre-project base flood elevations, floodway elevations, or floodway widths during the base flood discharge. A licensed professional engineer must provide supporting technical data and certification thereof; and

3.

If the applicant proposes to revise the floodway boundaries, no permit authorizing the encroachment into or an alteration of the floodway is issued by the City until an affirmative conditional letter of map revision (CLOMR) is issued by FEMA or a no-rise certification is approved by the Department.

F.

Maintenance Requirements The property owner is responsible for continuing maintenance as may be needed within an altered or relocated portion of a floodplain on the property so that the flood-carrying or flood storage capacity is maintained. The City may direct the property owner, at no cost to the City, to restore the flood-carrying or flood storage capacity of the floodplain if the owner has not performed maintenance as required by the approved floodplain management plan on file with the Department.

( Ord. of 4-17-2018(7) , §§ 9-51—9-53)