§ 26-178. Procedure for the submission, review, and approval of stormwater runoff management plans.

No land development shall increase stormwater runoff over previously existing conditions except through an approved stormwater management plan as provided for in this Code. The plans shall be prepared by a licensed professional engineer in the state in accordance with the Kansas City Metropolitan Chapter of the American Public Works Association "Standard Specifications and Design Criteria" section 5600 and as provided for in this Code. No grading or construction permits will be issued prior to its approval by the community development director.

(1)

Preliminary stormwater management plan. A preliminary stormwater management plan shall accompany all initial applications for land development. The plan shall include the following:

a.

A drainage map based on the data available through the Johnson County, Kansas "Automated Information Mapping System (AIMS)" showing existing improvements, proposed improvements, and topographical features including contours of the development site and adjoining land. The map shall show the preliminary drainage system layout; the areas of the drainage basins and sub-basins tributary to each collection element; the runoff coefficient (C) for each drainage basin; local and system time of concentration (Tc) for each collection element; design and 100-year storm intensity for both the local and system Tc; design and 100-year storm antecedent moisture coefficient (K); design and 100-year stormwater discharges (Q) for each system element at both the local and system Tc; the location of proposed detention facilities; and the 100-year flood boundary if the site is in or adjacent to a FEMA regulated floodplain.

b.

A preliminary stormwater management plan and general layout drawing or drawings showing existing improvements, topographical features, and contours; and proposed improvements including structures, paving, the drainage system, detention facilities, and grading concepts. The drawings shall also indicate preliminary collection and conveyance element locations and sizes; and anticipated building low entry and 100-year flood water surface elevations.

c.

A written stormwater management summary or report including descriptions of the site and surrounding watershed; existing and proposed land use status; existing topography and surface conditions and proposed changes. The report will also discuss stormwater detention requirements; 100-year storm protective level constraints; the location and capacity of existing downstream stormwater management facilities; and any other issues or considerations which may affect the design.

(2)

Final stormwater management plan. Upon approval of the preliminary stormwater management plan and report, a final stormwater management plan shall be submitted for review and approval with the final development plan application. The final plan submittal shall include the following:

a.

Final drainage map, stormwater management plan/general layout and stormwater management report incorporating preliminary plan review comments and final design development. Drainage easements will be delineated on the stormwater management plan/general layout or on separate ownership/right-of-way drawings if necessary for clarity.

b.

Plan and profile drawings (or separate profile drawings when overall plan views of the project are provided) showing collection and conveyance element locations and stationing; sizes and lengths; invert elevations and conduit slopes; existing and proposed surface grade; ties to baselines and property lines; subdivision names, lot numbers and ownerships; and material types and specifications (e.g., conduit shape, RCP class - wall type, CSP corrugation dimensions - wall thickness, Manning's n, etc.); existing and proposed utility crossings and alignments; and geotechnical test borings. The design storm and 100-year storm hydraulic grade lines in conduits and structures and at surface collection and conveyance elements shall be shown on the profile view. The 100-year water surface elevation in streets shall be shown on the profile view at all road crossings and on the plan view in sag vertical curve low points. The low entry elevation of all structures shall also be shown on the plan view.

c.

Cross sections at 50 feet maximum intervals and at other locations as needed to convey the intent of the design shall be provided for all open channels and significant overflow channels in yards (generally where the design of the overflow channel is critical to meeting the 100-year flood protective level requirement).

d.

Drawings showing details and schedules of all special structures which do not conform to city standards.

e.

Design storm and 100-year storm hydraulics information for all collection elements shall be shown on the plan and profile drawings or in a schedule on the stormwater management/general layout drawings, including:

1.

Discharge - Q _{(design year)} and Q ₁₀₀ (cfs).

2.

Method of calculation (e.g., weir formula, orifice formula, nomographs, etc.).

3.

Bypassed Flow - Q _bypass (cfs).

4.

Depth or Head (H _weir or h _orifice , H or HW - culvert outlet or inlet control, etc.).

5.

Velocity - V (fps) where applicable.

6.

Velocity Head - V /2g (ft.) where applicable.

7.

Discharge coefficient "C" or "K" where applicable.

f.

Design storm and 100-year storm hydraulics information for all conveyance elements may be shown on the plan and profile drawings or in a schedule on the stormwater management/general layout drawings, including:

1.

Full flow capacity - Q _full (cfs).

2.

Full flow velocity - V _full (fps).

3.

Discharge - Q _{(design year)} and Q ₁₀₀ (cfs).

4.

Normal depth - d _n (ft.) for open channel flow conditions.

5.

Froude number - F _r for open channel flow conditions.

6.

Fiction slope - S _f (%) for pressurized flow conditions.

7.

Pressurized flow friction head loss through conduit reach - h _f (ft).

8.

Velocity - V (fps).

9.

Velocity head - V /2g (ft).

10.

Velocity head loss coefficient at structure - "K."

11.

Head loss at structure - h _L (ft).

g.

Detention basin design drawings shall include the following information:

1.

Elevation-area-volume curves for the storage facility including notation of the storage volumes allocated to runoff, sediment and permanent residual water storage for other uses (wet basins only).

2.

Inflow hydrographs for the ten-year and 100-year recurrence interval design storms.

3.

Stage-discharge rating curves for each spillway and for combined spillway discharges.

4.

Routing curves for the ten-year and 100-year recurrence interval design storms with time plotted as the abscissa and the following plotted as ordinances:

(i)

Cumulative inflow volume.

(ii)

Cumulative discharge.

(iii)

Stage elevation.

(iv)

Cumulative storage.

5.

Provisions shall be made to permit emergency access to the detention basin and for the access of auxiliary equipment for emptying, cleaning and maintenance purposes.

6.

Underground detention facilities shall be provided with access for maintenance and sediment removal, and shall have positive gravity outlets. Venting shall be provided to prevent gas accumulation.

(3)

Construction specifications. Stormwater management construction materials and installation shall meet or exceed the provisions of sections 2100 "Grading and Site Preparation," 2400 "Seeding and Sodding," 2600 "Storm Sewers," and 2700 "Structures" of the APWA Standard Specifications and Design Criteria. Stormwater management structures shall be designed in accordance with the provisions of section 2604 "Structures" of section 2700, and 5710.3 "Curb Inlets, Field Basins and Junction Boxes" and 5710.5 "Reinforced Concrete Box Culverts" of section 5700 "Structures."

(Code 1967, ch. 7, art. I, § 7-9; Ord. No. 1689, § 7(7-9), 7-9-2012; Ord. No. 1729, § 5, 1-12-2015 )