§ 35-29.7. Acceptable downstream conditions.

(a)

As part of the development, the downstream impacts of developments must be carefully evaluated for the two (2) design focus areas of streambank protection and flood mitigation, as provided in Table 35.29B. The purpose of the downstream assessment is to protect downstream properties from increased flooding and downstream channels from increased erosion potential due to upstream development. The importance of the downstream assessment is particularly evident for larger sites or developments that have the potential to dramatically impact downstream areas. The cumulative effect of smaller sites, however, can be just as dramatic and, as such, the NCTCOG-iSWM design focus area requirements must be followed for both small sites and large sites.

(b)

The downstream assessment shall extend from the outfall of a proposed development to a point downstream where the discharge from a proposed development no longer has a significant impact, in terms of flooding increase or allowable velocities, on the receiving stream or storm drainage system. The city shall be consulted to obtain records and maps related to the National Flood Insurance Program and availability of flood insurance studies and flood insurance rate maps (FIRMs) which will be helpful in this assessment. The downstream assessment shall be a part of the preliminary and final plans, and must include all of the following information and analyses:

(1)

Hydrologic analysis of the pre- and post-development on-site conditions;

(2)

Drainage path that defines extent of the analysis;

(3)

Capacity analysis of all existing constraints points along the drainage path, such as existing floodplain developments, underground storm drainage systems culverts, bridges, tributary confluences, or channels;

(4)

Off-site undeveloped areas considered as "full build-out" for both the pre- and post-development analyses;

(5)

Evaluation of peak discharges and velocities for three (3) 24-hour storm events, streambank protection storm; conveyance storm; and flood mitigation storm; and

(6)

Separate analysis for each major outfall from the proposed development.

(c)

Once the analysis is complete, the owner must provide the city engineer with answers to the following four (4) questions at each determined junction downstream:

(1)

Are the post-development discharges greater than the pre-development discharges?

(2)

Are the post-development velocities greater than the pre-development velocities?

(3)

Are the post-development velocities greater than the velocities allowed for the receiving system?

(4)

What are the post-development flood heights compared to the predevelopment flood heights?

These questions shall be answered for each of the three (3) storm events. The answers to these questions and if they negatively impact property or the public safety and welfare as determined by the city engineer will determine the necessity, type, and size of non-structural and structural controls to be placed on-site or downstream of the proposed development.

(Ord. No. 2017-10008, § II, 10-26-17)