(A) General. All open channels, whether private or public, and whether constructed on private or public land, shall conform to the design standards and other design requirements contained herein.
(B) Design storm frequencies.
(1) All open channels and swales shall accommodate, as a minimum, peak runoff from a 10-year return frequency storm. For rational method analysis, the storm duration shall be equal to the time of concentration for the drainage area. In computer-based analysis, the duration is as noted in the applicable methodology associated with the computer program.
(2) Open channels with a carrying capacity of more than 30 cfs at bank-full stage shall be capable of accommodating peak runoff for a 24-hour, 5-year return frequency storm within the drainage easement.
(3) Open channel facilities functioning as a major drainage system as defined in § 51.04 must also meet IDNR design standards. (1) The waterway for channels shall be determined using Manning’s equation, where:
(a) Q = (1.486/n) A R2/3 S 1/2;
(b) Q = Discharge in cubic feet per second (cfs); and
(c) A = Waterway area of channel in square feet.
(2) Parameters R, S and n are explained in § 51.11. (D) Channel cross-section and grade.
(1) The required channel cross-section and grade are determined by the design capacity, the material in which the channel is to be constructed and the requirements for maintenance. A minimum depth may be required to provide adequate outlets for subsurface drains, tributary ditches or streams. The channel grade shall be so that the velocity in the channel is high enough to prevent siltation but low enough to prevent erosion. Velocities less than 2 feet per second are not acceptable, as siltation will take place and ultimately reduce the channel cross-section. The maximum permissible velocities in vegetated-lined channels are shown in Table 10. In addition to existing runoffs, the channel design should incorporate increased runoff due to the proposed development.
(2) Where depth of design flow is slightly below critical depth, channels shall have freeboard adequate to cope with the effect of hydraulic jumps.
(3) Minimum swale slopes are 0.3%. All flow shall be confined to the specific easements associated with each rear and side lot swale that are part of the minor drainage system. Swales with slopes less than 0.8% shall have tile underdrains to dry the swales.
(4) If roadside ditches are used rather than storm sewers, the bottom of the ditch should be low enough to install adequately sized driveway culverts without creating "speed bumps." The driveway culvert inverts shall be designed to adequately consider upstream and downstream culvert elevations.
(5) Flow of an open channel into a closed system is prohibited, unless quantity and head loss computations demonstrate the closed conduit to be capable of carrying all the open channel flow with no reduction of velocity.
(1) Earthen channel side slopes shall be no steeper than 3 horizontal to 1 vertical (3:1). Flatter slopes may be required to prevent erosion and for ease of maintenance.
(2) Where channels will be lined, side slopes shall be no steeper than 1-1/2 horizontal to 1 vertical (1.5:1) with adequate provisions made for weep holes.
(3) Side slopes steeper than 1-1/2 horizontal to 1 vertical (1.5:1) may be used for lined channels provided that the side lining is designed and constructed as a structural retaining wall with provisions for live and dead load surcharge.
(4) When the design discharge produces a depth of greater than 3 feet in the channel, appropriate safety precautions shall be added to the design criteria based on reasonably anticipated safety needs.
(1) Characteristics of a stable channel are:
(a) It neither promotes sedimentation nor degrades the channel bottom and sides beyond tolerable limits;
(b) The channel banks do not erode to the extent that the channel cross-section is changed appreciably;
(c) Excessive sediment bars do not develop;
(d) Excessive erosion does not occur around culverts, bridges, outfalls or elsewhere; and
(e) Gullies do not form or enlarge due to the entry of uncontrolled flow to the channel.
(2) Channel stability shall be determined for an aged condition and the velocity shall be based on the design flow or the bankfull flow, whichever is greater, using an "n" value for the various channel linings as shown in Table 10 of this subchapter. In no case is it necessary to check channel stability for discharges greater than that from a 100-year frequency storm.
(3) Channel stability shall be checked for conditions immediately after construction. For this stability analysis, the velocity shall be calculated for the expected flow from a 10-year frequency storm on the watershed, or the bankfull flow, whichever is smaller, and the "n" value for the newly constructed channels in the fine-grained soils and sands may be determined in accordance with the "National Engineering Handbook 5, Supplement B, Soil Conservation Service" and shall not exceed 0.025. This reference may be obtained by contacting the National Technical Information Service in Springfield, Virginia at 703-487-4650. The allowable velocity in the newly constructed channel may be increased by a maximum of 20% to reflect the effects of vegetation to be established under the following conditions:
(a) The soil and site in which the channel is to be constructed are suitable for rapid establishment and support of erosion controlling vegetation;
(b) Species of erosion controlling vegetation adapted to the area, and proven methods of establishment are shown; and
(c) The channel design includes detailed plans for establishment of vegetation on the channel side slopes.
(G) Drainage of waterways. Vegetated waterways that are subject to low flows of long duration or where wet conditions prevail shall be drained with a tile system or by other means such as paved gutters. Tile lines may be outletted through a drop structure at the ends of the waterway or through a standard tile outlet.
(H) Appurtenant structures.
(1) The design of channels will include provisions for operation and maintenance and the proper functioning of all channels, laterals, travelways and structures associated with the project. Recessed inlets and structures needed for entry of surface and subsurface flow into channels without significant erosion or degradation shall be included in the design of channel improvements. The design will also provide for necessary flood gates, water level control devices and any other appurtenance structure affecting the functioning of the channels and the attainment of the purpose of which they are built.
(2) The effects of channel improvements on existing culverts, bridges, buried cables, pipelines and inlet structures for surface and subsurface drainage on the channel being improved and laterals thereto shall be evaluated to determine the need for modification or replacement. Culverts and bridges which are modified or added as part of the channel improvement projects shall meet reasonable standards for the type of structure, and shall have a minimum capacity equal to the design discharge or governmental agency design requirements, whichever is greater.
(I) Deposition of spoil. Spoil material resulting from clearing, grubbing and channel excavation shall be disposed of in a manner which will:
(1) Minimize overbank wash;
(2) Provide for the free flow of water between the channel and floodplain boundary unless the valley routing and water surface profiles are based on continuous dikes being installed;
(3) Not hinder the development of travelways for maintenance;
(4) Leave the right-of-way in the best condition feasible, consistent with the project purposes, for productive use by the owner; and
(5) Be approved by the IDNR or COE, whichever is applicable, if deposited in the floodway.
(1) Materials acceptable for use as channel lining are:
(e) Precast cement concrete riprap;
(h) Straw or coconut matting (only until grass is established).
(2) Other lining materials must be approved in writing by the city engineer. Materials shall comply with the latest edition of the INDOT "Standard Specifications."
(K) Maintenance responsibilities.
(1) Maintenance of open channels during construction and thereafter, shall be the responsibility of the land developer/owner.
(2) Assignment of responsibility for maintaining facilities serving 1 or more lot(s) or holding(s) shall be documented by appropriate covenants to property deeds and documented in sales agreements, unless responsibilities formally accepted by a public body.
(Ord. 01-2000, passed 7-18-2000)
