Item #12

Council directed Staff to explore ways to protect citizens who live adjacent or near to proposed development from stormwater run-off. As a result Staff looked at various “stormwater detention” ordinances across the state and nation. What resulted was a Down Stream Impact Analysis Ordinance where the developer/engineer is required to analyze all off-site discharge points and determine the impact of the run-off on surrounding properties and infrastructure. Once the off-site impact(s) has been determined, ways to mitigate those impacts are proposed through off-site improvements, on-site mitigation or through Council approved payment in-lieu.

7.3.3     Peak Runoff Control

There shall be no net increase in peak stormwater run-off flow leaving a development from pre-development conditions for the one (1)-year design storm. New developments are required to minimize damage to subject streams caused by storm flows.

(A)        Calculation of Peak Flow

Acceptable methodologies for computing pre- and post-development conditions for the one (1)-year design storm include:

(1)        The Rational Method (used when the drainage area is two hundred (200) acres or less)

(2)        The Peak Discharge Method as described in USDA Technical Release Number 55 (TR-55) (drainage area is 2,000 acres or less)

(3)        The Putnam Method (drainage area is greater than 2,000 acres)

(4)        USGS Regression Equations, where applicable

The same method must be used for both the pre- and post-development conditions.

(B)        Exceptions to Peak Flow Control

Developments with less than ten (10) percent net increase in peak flow of the one (1)-year design storm are not required to control peak flow from the site. However, if the net increase in peak flow from the new development is greater than ten (10) percent, the entire net increase from pre-development one (1)-year design storm peak flow must be controlled.

(C)        Downstream Impact Analysis

A Downstream Impact Analysis shall be supplied for all new proposed developments that require site/subdivision plan approval. Each development plan shall:

(1)        Calculate the pre- and post-development discharges for each discharge point from the site.

If the post-development calculated discharge is less than a ten (10) percent increase for the two (2), five (5) and ten (10)-year peak discharges at each discharge point, then no further analysis is needed.

If the post-development peak discharge at any discharge point is greater than forty (40) cubic feet per second (cfs) then further analysis is needed and shall be treated as in Section 7.3.3(C)(2) below. In lieu of the ten (10) percent increase in off-site run-off the engineer shall limit run-off to the extent that downstream conveyance systems can accommodate the run-off increase without causing any property damage.

(2)        If the post-development discharge for the two (2), five (5) and ten (10)-year discharges at any discharge point is greater than ten (10) percent of the pre-development discharge, then additional analysis is needed for that discharge point.

Identify the point downstream where the impacts become less than a ten (10) percent increase.

Analyze the impacts of the increase on the stormwater conveyance system(s) (pipes, culverts, ditches, swales, etc.) between the discharge point and the point where the impact becomes less than ten (10) percent.

Identify negative impacts on all improvements (businesses, homes, lawns, streets, pipes, ditches, swales, etc.) through the use of inlet and outlet control calculations and backwater analyses for culverts, channel capacity calculations for swales and ditches.

Construct on-site detention, off-site improvements, or make a payment-in-lieu for inadequate downstream conveyance systems to fully mitigate all impacts created by the proposed development. Requests for payment-in-lieu must be approved by Town Council.

Payment-in-lieu shall be based on the sealed, approved estimate of one hundred (100) percent of the costs to construct the appropriate mitigation facility. Those costs shall include design, land costs, construction, and a fifty (50)-year cycle of maintenance. The chosen facility has to be a facility that is capable of obtaining all permits required by all appropriate state and federal agencies.

(3)        If on-site detention is the chosen mitigation, limit the peak discharge at all discharge points to what would be expected from an R-40 zoning condition for the two (2), five (5) and ten (10)-year storm events, not the exceed a ten (10) percent increase over the pre-development conditions.