(a)   The stormwater management criteria contained in this policy sets forth the minimum technical criteria for the analysis and design of stormwater detention facilities in Garnett, Kansas. The purpose and intent of these criteria are to provide for the proper design of stormwater appurtenances and to enhance the harmonious development and redevelopment of the city of Garnett.

(b)   These criteria are applicable to all residential, commercial, institutional and industrial development or redevelopment undertaken in the city of Garnett. Stormwater detention facilities shall be required where hydraulic calculations indicate an increase in stormwater runoff rate leaving the site due to proposed development or redevelopment.

(Code 1961, 4-15-1; Ord. 3861; Code 2023)

An engineering study as outlined in section 4-1007 is hereby required for all locations to determine the impact of the proposed development on the existing drainage system and to determine the need for detention. The study shall be submitted to the city of Garnett with the preliminary plat or site plan for proposed development. The timing of the submittal will be determined by the city of Garnett depending upon the scope and nature of the project. The study shall be prepared under the direct supervision of a professionally licensed engineer.

(a)   Detention will not be required when the engineering study indicates:

(1)   Construction of a detention facility will increase the downstream system’s peak discharge by delaying the peak from the proposed development so it coincides with the peak discharge from upstream area.

(2)   Where site discharge occurs within the limits of the special flood hazard area inundated by the one percent (1%) annual chance (100-year) flood as defined by the federal insurance study current at the time the development is proposed.

(3)   When the drainage study, provided by the developer and prepared by a registered professional engineer, quantifies the problems and adequately demonstrates to the city engineer that a waiver of a specific requirement is appropriate.

(b)   Detention will not be required in any event, and therefore no engineering study is required:

(1)   For additions to, improvement and repair of existing single-family and duplex dwellings.

(2)   For developments of less than one-half (1/2) acre in area.

(3)   For construction of any one, new single-family or duplex dwelling unit.

(Code 1961, 4-15-2; Ord. 3861; Code 2023)

The hydrological method and general parameters to be used for computations of stormwater runoff and peak rates for stormwater detention shall be as follows:

(a)   Hydrograph Method: The application of a hydrograph method is required for all detention facilities. Computer models or manual methods are permissible as approved by the city engineer. The following is a list of acceptable computer models:

(1)   NRCS technical release number 55 (TR-55) - “Urban Hydrology For Small Watersheds”.

(2)   NRCS technical release number 20 - “Project Formulation - Hydrology”.

(3)   U.S. Army Corps of Engineers, Hydrologic Engineering Center - “Hec-1 Flood Hydrograph Package”.

(4)   U.S. Environmental Protection Agency “Storm Water Management Model” (SWMM).

(5)   Calculations generated in accredited engineering software programs utilizing one or a combination of the above methods will be accepted.

(b)   Design Storm: The rainfall intensities used shall be from the current Kansas department of transportation rainfall intensity tables for Anderson County. The following is a general guide to be used to determine the appropriate design storm for a given watershed:

(c)   Runoff Coefficients: Stormwater runoff coefficients or curve numbers shall be determined for proposed land use in modeling. Standard coefficients from an accredited engineering publication will be accepted for use in analysis. Existing runoff coefficients shall be determined but in no case shall existing unpaved areas be analyzed with a curve number exceeding 69, 79 and 84 for hydrologic soil groups B, C and D, respectively.

(d)   3.4 Time Of Concentration: The time of concentration shall be calculated as the sum of overland flow time, the shallow concentrated flow time and the concentrated system flow time. Accredited engineering guides utilized shall be provided for support of analysis.

(Code 1961, 4-15-3; Ord. 3861; Code 2023)

In all new developments, developers shall dedicate public easements for all public drainage system components. In all existing developments, easements shall be acquired from the property owners at no cost to the city of Garnett before drainage system improvements are made.

(Code 1961, 4-15-4; Ord. 3861; Code 2023)

(a)   The city of Garnett will only be responsible for maintenance of enclosed drainage system components located within public street rights of way. Maintenance of all improved or natural channels, all overflow channels, detention facilities and enclosed drainage system components within public easements and all easements associated with drainage system components shall be the responsibility of the individual property owner or development association.

(b)   The city of Garnett will conduct periodic inspections of the above systems. In the event maintenance concerns are identified but left unresolved by the responsible property owner or association, the city of Garnett shall resolve those concerns and seek compensation from the responsible property owner or association.

(Code 1961, 4-15-5; Ord. 3861; Code 2023)

(a)   Detention facilities shall be sized to provide for no increase in runoff rate for the proposed development conditions. Proposed detention shall provide control of increased runoff rates for the 2, 10, and 100-year frequency storms.

(b)   Public and private detention may be accomplished with either wet or dry bottom basin facilities. Public safety shall be addressed in the study if joint uses, such as parking or recreation, are proposed.

(c)   Wet bottom basins shall require additional storage volume to accommodate five (5) years of sediment and storage volume necessary to maintain a minimum water depth of three feet (3’).

(d)   Erosion control for flows into and out of the proposed detention facilities shall be addressed in the study and provided as necessary.

(Code 1961, 4-15-6; Ord. 3861; Code 2023)

Stormwater drainage studies shall contain the following general information, supporting calculations and drainage area map. The report shall contain:

(a)   Names and addresses of the development team including, but not limited to, the landowner, developer, architect and engineer.

(b)   Date of submittal.

(c)   A list of all permits required by local, state and federal agencies.

(d)   General summary discussion regarding the proposed development or redevelopment land use and facilities proposed for stormwater management.

(e)   General discussion regarding the impact of proposed stormwater flow to adjacent properties and the downstream system.

(f)   Any special consideration items addressed in the study.

(g)   Supporting calculations and design guides utilized in analysis including, but not limited to:

(1)   A contour map indicating on and off site drainage areas to the development used in analysis along with a summary of area, curve number and flow rates.

(2)   Proposed location and type of detention facility to be provided along with stage storage and stage discharge curves.

(3)   Proposed size and type of control structure along with discharge velocity calculations and erosion control proposed.

(4)   Existing and proposed 2, 10 and 100-year hydrographs for each sub watershed along with discharge flow rates.

(5)   A summary table tabulating the total discharge for the site, existing and proposed, for the 2, 10, and 100-year storms.

(Code 1961, 4-15-7; Ord. 3861; Code 2023)

Complete notice of intent (NOI) to request coverage under the requirements and conditions of the Kansas stormwater runoff from construction activities general permit shall be submitted to Kansas department of health and environment prior to construction. The stormwater pollution prevention plan (SWPPP) shall meet the known requirements of the national and Kansas general permit. A copy of the signed and dated copy of the authorized NOI shall be provided to the city of Garnett prior to any disturbance of soil on the construction site. The Kansas department of health and environment does not review or approve the SWPPP. The owner and operator of the site should satisfy themselves that the requirements of the national and Kansas general permit and the NOI are being met by the erosion and sediment controls that are being placed on the site. The developer shall be responsible for maintenance of stormwater pollution prevention/control measures according to standards current at the time of development.

(Code 1961, 4-15-7; Ord. 3861; Code 2023)

It shall be a violation of this article and the policy herein stated:

(a)   To proceed with any development or redevelopment without having completed an engineering study, unless specifically excepted herein;

(b)   To fail to obtain and dedicate to the public use appropriate easements for all public drainage system components;

(c)   To fail to make the project available for inspection or to call for such an inspection by the city of Garnett when directed or requested by the city of Garnett at such time as any specified stage of completion is reached; or

(d)   To fail to address and resolve to the satisfaction of the city of Garnett within a reasonable time any maintenance concern; provided, however, nothing in this subsection shall abridge the city of Garnett’s right to resolve such concerns and then seek compensation.

Any person convicted of violation of this article shall be punished as provided in the general penalty provisions of this code.

Any unpaid costs, fines or fees may be collected in the manner provided by K.S.A. 12-1,115 and any amendments thereto or shall be assessed and charged against the relevant lot or parcel of ground.

(Code 1961, 4-15-8; Ord. 3861; Code 2023)

The provisions of this article are intended to supplement installation and maintenance requirements, or other general provisions, of the international plumbing code, as from time to time adopted and in force in the city of Garnett, insofar as such code addresses stormwater drainage matters.

(Code 1961, 4-15-10; Ord. 3861; Code 2023)