Section 1 - Administration


Whereas the smaller streams and drainage channels serving the City of Plymouth may not have sufficient capacity to receive and convey storm water runoff, resulting when land use changes from open or agricultural use to a more urbanized use; and whereas the deposit of sediment from developments during and after construction can reduce capacities of storm sewers and drainage systems and result in damages to receiving lakes and streams; therefore, the City of Plymouth adopts this storm water drainage ordinance with the purpose of controlling urban run-off and minimizing urban flooding.


It shall be the policy of the City of Plymouth that the storage and controlled release of storm water runoff shall be required of all new development, any redevelopment, and other new construction within the City Limits. Retention facilities shall be sized to hold 25 gallons per 100 square feet of hard surface area. In addition, the release rate of storm water from developed lands shall not exceed the release rate from the land area in its present land use.

The owner, developer, or builder must submit detailed computations of runoff before and after development, redevelopment, or new construction which demonstrate that runoff will not be increased. These computations must show that the peak runoff rate after development for the 100 year return period storm of 24 hour duration must not exceed the 10 year return period pre-development peak runoff rate. (The critical duration storm is that storm duration that requires the greatest detention storage.)

Computations for Areas up to and including 200 acres may be based on the Rational Method; typical runoff coefficients are listed herein. For Areas larger than 200 acres, hydrograph techniques and/or computer drainage modeling methods may be used. Hydrograph techniques and computer modeling methods used to determine storm water runoff shall be proven methods, subject to approval of the Sanitary Board of Trustees.

Because topography and the availability and adequacy of outlets for storm runoff vary with almost every site, the requirements for storm drainage tend to be an individual matter for any project. It is recommended that each proposed project be discussed with the City Sewer Superintendent and the City Engineer, at the earliest practical time in the planning stage.


This ordinance shall become effective after its final passage, approval and publication as required by law.


The provisions of this ordinance shall be deemed as additional requirements to minimum standards required by other ordinances of the City or other governing agencies. In the case of conflicting requirements, the most restrictive shall apply.


All ordinances or parts thereof in conflict with the provisions of this ordinance are repealed.


In addition to the requirements of this ordinance, compliance with the requirements set forth in other applicable ordinances with respect to submission and approval of preliminary and final subdivision plats, improvement plans, building and zoning permits, construction inspections, appeals, and similar matters, and compliance with applicable State of Indiana statutes and regulations shall be required.


The degree of protection required by this ordinance is considered reasonable for regulatory purposes and is based on historical records, engineering, and scientific methods of study. Larger storms may occur or storm water runoff depths may be increased by man-made or natural causes. This ordinance does not imply that land uses permitted will be free from storm water damage. This ordinance shall not create liability on the part of the City of Plymouth, the Sanitary Board of Trustees, the City Sewer Superintendent, the City Engineer, or any officer or employee thereof for any damage which may result from reliance on this ordinance or on any administrative decision lawfully made thereunder.


Nothing herein contained shall prevent the City of Plymouth, the Sanitary Board of Trustees, the City Sewer Superintendent, or the City Engineer from taking such other lawful action as may be necessary to prevent or remedy any violation. All costs connected therewith shall accrue to the person or persons responsible.


I.C. 36-9-27-17 requires that a new drain connecting into a regulated drain under the jurisdiction of the Marshall County Drainage Board must have the approval of the Marshall County Surveyor.


Chapter 318 of the Acts of 1945, as amended, Sections 17 and 19, require the Natural Resources Commission approval of any construction in a floodway, and of any works for flood control. This includes bridges, dams, levees, dikes, floodwalls, wharves, piers, dolphins, booms, weirs, bulkheads, jetties, groins, excavations, fills or deposits of any kind, utility lines, or any other building, structure, or obstruction. Also, any ditch work (new construction, deepening or modification) within one half mile of a public freshwater lake of 10 acres or more in area.

The approval of the Natural Resources Commission, in writing, must be obtained before beginning construction. Applications approval should be submitted to:

Department of Natural Resources
Division of Water
2475 Directors Row
Indianapolis, IN 46241

All applications should be made on the standard application form provided by the Natural Resources Commission and should be accompanied by plans, profiles, specifications, and other data necessary for the Natural Resources Commission to determine the effect of the proposed construction upon the floodway and on flood control in the state.

Application made to and approval granted by the Natural Resources Commission does not in any way relieve the owner of the necessity of securing easements or other property rights, and permits and/or approvals from affected property owners and local, state, and federal agencies.

The engineering staff of the Division of Water is available to discuss and offer suggestions regarding requirements in the design of structures in floodways. High water marks have been set on many of the streams in the state, and information is available from the Division of Water on actual and/or potential flooding. Information regarding bench marks set to Mean Sea Level Datum, General Adjustment of 1929, is available from the Division of Water, Surveying and Mapping Section.

Applications are considered by the Natural Resources Commission at regular meetings usually held each month. After the application and plans have been approved by the Natural Resources Commission, a certificate of approval is forwarded to the applicant.

A fee is charged by the Natural Resources Commission for approvals under the Flood Control Act. Unless stated otherwise in the approval, construction is considered to be a permanent development, and no renewals of the approvals are granted for temporary construction. The right is reserved to require additional data where necessary.


For the purpose of this ordinance, the following definitions shall apply:

A. Capacity of a Storm Drainage Facility - The maximum flow that can be conveyed or stored by a storm drainage facility without causing damage to public or private property.

B. Channel - A natural or artificial watercourse which periodically or continuously contains moving water, or which forms a connecting link between two bodies of water. It has a defined bed and banks which serve to confine the water.

C. City Plan Director - The person or entity serving in the capacity of Plan Director or Plan Consultant for the City of Plymouth.

D. Compensatory Storage - An artificial volume of storage within a floodplain used to balance the loss of natural flood storage capacity when artificial fill or structures are placed within the floodplain.

E. Contiguous - Adjoining or in actual contact with.

F. Culvert - A closed conduit used for the passage of surface drainage water under a roadway, railroad, canal, or other impediment.

G. Detention - The amount of excess storm water accumulated on the site that may be released at a restricted runoff release rate.

H. Detention Basin - A facility constructed or modified to restrict the flow of storm water to a prescribed maximum rate, and to detain concurrently the excess waters that accumulate behind the outlet.

I. Detention Storage - The temporary detaining or storage of storm water in storage basins, on rooftops, in streets, parking lots, school yards, parks, open spaces, or other Areas under predetermined and controlled conditions, with the rate of drainage therefrom regulated by appropriately installed devices.

J. Drainage Area - The area from which water is carried off by a drainage system; a watershed or catchment area.

K. Drop Manhole - A manhole having a vertical drop pipe connecting the inlet pipe to the outlet pipe. The vertical drop pipe shall be located immediately outside the manhole.

L. Dry Bottom Detention Basin - A basin designed to be completely dewatered after having provided its planned detention of runoff during a storm event.

M. Duration - The time period of a rainfall event.

N. Erosion - Wearing away of the land by running water, waves, temperature changes, ice or wind.

O. Flood Elevation - The elevation at all locations delineating the maximum level of high waters for a flood of given return period and rainfall duration.

P. Flood or Flood Waters - The water of any watercourse which is above the banks of the watercourse. It also means the water of any lake which is above and outside the banks thereof.

Q. Flood Hazard Area - Any flood plain floodway, floodway fringe, or any combination thereof which is subject to inundation by the regulatory flood; or any flood plain as delineated by Zone A of a Flood Hazard Boundary Map.

R. Flood Plain - The area adjoining the river or stream which has been or may hereafter be covered by flood waters.

S. Flood Protection Grade - The elevation of the lowest floor of a building. If a basement is included, the basement floor is considered the lowest floor.

T. Floodway - See Regulatory Floodway

U. Floodway Fringe - That portion of the flood plain lying outside the floodway, which is inundated by the regulatory flood.

V. Footing Drain - A drain pipe installed around the exterior of a basement wall foundation to relieve water pressure caused by high groundwater elevation.

W. Grade - The inclination or slope of a channel, canal, conduit, etc., or natural ground surface usually expressed in terms of the percentage the vertical rise (or fall) bears to the corresponding horizontal distance.

X. Impact Areas - Areas defined and mapped by the City of Plymouth are unlikely to be easily drained because of one or more factors including but not limited to any of the following: soil type, topography, land where there is not adequate outlet, a floodway or floodplain, land within 75 feet of each bank of any regulated drain or within 75 feet from the centerline of any regulated tile ditch.

Y. Impervious - A term applied to material through which water cannot pass, or through which water passes with difficulty.

Z. Inlet - An opening into a storm sewer system for the entrance of surface storm water runoff, more completely described as a storm sewer inlet.

AA. Junction Chamber - A converging section of conduit, usually large enough for a person to enter, used to facilitate the flow from one or more conduits into a main conduit.

BB. Lateral Storm Sewer - A sewer that has inlets connected to it but has no other storm sewer connected.

CC. Manhole - Storm sewer structure through which a person may enter to gain access to an underground storm sewer or enclosed structure.

DD. Major Drainage Systems - Drainage systems carrying runoff from an area of one or more square miles.

EE. Minor Drainage Systems - Drainage systems having an area of less than one square mile.

FF. Off-Site - Everything not on site.

GG. On-Site - Located within the controlled area where runoff

HH. Outfall - The point or location where storm runoff discharges from a sewer or drain. Also applies to the outfall sewer or channel which carries the storm runoff to the point of outfall.

II. Peak Flow - The maximum rate of flow of water at a given point in a channel or conduit resulting from a particular storm or flood.

JJ. Plan Commission - The City of Plymouth Planning Commission.

KK. Radius of Curvature - Length of radius of a circle used to define a curve.

LL. Rainfall Intensity - The cumulative depth of rainfall occurring over a given duration, normally expressed in inches per hour.

MM. Reach - Any length of river, channel or storm sewer.

NN. Regulated Area - All of the incorporated land within the City of Plymouth and the area located within the two-mile zoning boundary of the City of Plymouth.

OO. Regulatory Flood - That flood having a peak discharge which can be equaled or exceeded on the average of once in a one hundred (100) year period, as calculated by a method and procedure which is acceptable to City of Plymouth. If a permit from the Natural Resources Commission for construction in the floodway is required (see Section 1, X., page 3), then the regulatory flood peak discharge should be calculated by a method acceptable to the City and the Natural Resources Commission. This regulatory flood is equivalent to a flood having a probability of occurrence of one percent (1%) in any given year.

PP. Regulatory Floodway - The channel of a river or stream and those portions of the floodplains adjoining the channel which are reasonably required to carry and discharge the peak flow of the regulatory flood of any river or stream.

QQ. Release Rate - The amount of storm water release from a storm water control facility per unit of time.

RR. Retention - The amount of storm water to be kept on the site. This storm water is utilized to recharge the ground water table through drywells, percolation, etc.

SS. Retention Basin - A facitlity constructed or modified to be used as a ground water recharge area. This basin does not release water off the site.

TT. Return Period - The average interval of time within which a given rainfall event will be equalled or exceeded once. A flood
having a return period of 100 years has a one percent probability of being equalled or exceeded in any one year.

UU. Runoff Coefficient - A decimal fraction relating the amount of rain which appears as runoff and reaches the storm drainage system to the total amount of rain falling. A coefficient of 0.5 implies that 50 percent of the rain falling on a given surface appears as storm water runoff.

VV. Sanitary Board of Trustees - That governing body, or their appointed representative, designated by the City of Plymouth to regulate the storm water system within the corporate limits (including all inlets, manholes, piping, and basins that convey or contain storm water), and to review and approve drainage plans within the two-mile zoning boundary.

WW. Sediment - Material of soil and rock origin, transported, carried or deposited by water.

XX. Siphon - A closed conduit or portion of which lies above the hydraulic grade line, resulting in a pressure less than atmospheric and requiring a vacuum within the conduit to start flow. A siphon utilizes atmospheric pressure to effect or increase the flow of water through a conduit. An inverted siphon is used to carry storm water flow under an obstruction such as a sanitary sewer.

YY. Spillway - A waterway in or about a hydraulic structure, for the escape of excess water.

ZZ. Stilling Basin - A basin used to slow water down or dissipate its energy.

AAA. Storage Basin - A generic term used to identify Detention Basin and/or Retention Basin.

BBB. Storage Duration - The length of time that water may be stored in any storm water control facility, computed from the time water first begins to be stored.

CCC. Storm Sewer - A closed conduit for conveying collected storm water.

DDD. Storm Water Drainage System - All means, natural or man-made, used for conducting storm water to, through or from a drainage area to any of the following: conduits and appurtenant features, canals, channels, ditches, streams, culverts, streets and pumping stations.

EEE. Storm Water Runoff - The water derived from rains falling within a tributary basin, flowing over the surface of the ground or collected in channels or conduits.

FFF. Tributary - Contributing storm water from upstream land areas.

GGG. Urbanization - The development, change or improvement of any parcel of land consisting of one or more lots for residential, commercial, industrial, institutional, recreational or public utility purposes.

HHH. Watercourse - Any river, stream, creek, brook, branch, natural or man-made drainageway in or into which storm water runoff or floodwaters flow either regularly or intermittently.

III. Watershed - See Drainage Area.

JJJ. Wet Bottom Detention Basin - A basin designed to retain a permanent pool of water after having provided its planned detention of runoff during a storm event.