(A) Types. The four different types of basic drainage features are as follows:
(1) Closed systems
(2) Reinforced concrete-lined open channels.
(3) Natural channels.
(4) Detention facilities.
(B) Closed systems.
(1) Stormwater runoff shall be carried in storm sewer pipe when either of the following apply:
(a) The runoff can be carried in a pipe of 60 inches in diameter or smaller; or
(b) Where it necessary for the protection of adjacent facilities that the stormwater be carried in an enclosed facility.
(2) Headwalls shall be constructed at the outfall of all storm sewer systems. Adequate velocity control measures shall be designed to control the upstream and downstream velocities to prevent erosion. When it is mutually agreeable to both the city and the owner, a concrete lined channel with sufficient access easement may be used in lieu of a closed pipe.
(C) Reinforced concrete-lined open channels. Reinforced concrete-lined open channels should be used when the criteria outlined in division (B)(1) is exceeded.
(1) Reinforced concrete lined channels shall conform to the following:
(a) Channels draining an area with a “CA” factor of 250 or less shall be lined with reinforced concrete in a manner which will contain the design frequency storm, plus one foot of freeboard within the concrete lining. Where “C” is equal to the runoff coefficient, and “A” is equal to the drainage area in acres.
(b) Channels draining an area with a “CA” factor of more than 250 but less than 500 shall be concrete lined to contain the runoff from a ten-year return frequency storm with balance of the required design frequency storm contained within grassed slopes no steeper than three horizontal to one vertical and with a minimum of one foot freeboard.
(c) Channels draining an area with a “CA” factor of more than 500 but less than 2,000 shall be constructed with a reinforced concrete pilot channel not less than 20 feet in width and having at least an 18-inch vertical side depth, a six-inch depressed invert, and an appropriate edge protection as accepted by the City Engineer or his authorized designee. The remainder of the channel shall consist of earthen side slopes with proper vegetative cover on slopes not steeper than three to one, nor flatter than 1.5%.
(d) In lieu of the improvements of a channel draining an area with a “CA” factor in excess of 500, the City Council may elect to accept the dedication of all land within the natural 100-year floodplain of the existing drainage channel as a permanent drainage right-of-way.
(e) That the criteria for drainage improvements as hereinabove set forth in divisions (a) through (d) above shall be applicable to publicly owned lands solely at the discretion of the city.
(2) The maximum permissible velocities in channels shall conform to the velocities as shown in Table I set forth in division (E) below, unless otherwise approved in writing by the City Engineer or his authorized designee.
(3) The minimum roughness coefficients for various channels and floodplains shall conform to the values as shown in Table I set forth in division (E) below, unless approved in writing by the City Engineer or his authorized designee.
(D) Natural channels. Natural channels may be preserved when the criteria in division (C)(1)(d) above are met or when the developer desires to preserve natural channels within his addition for purposes of aesthetics and/or open space. The following criteria shall apply when natural channels are to be preserved:
(1) An application for preservation of a natural channel shall be submitted to and approved by the Director of Public Works prior to approval of the preliminary plat. This application shall contain the following information furnished by the developer:
(a) Topographic, hydrologic and hydraulic information sufficient to properly evaluate the proposal and showing that:
1. Before any site work has begun, all land having elevation below the 50-year return frequency flood elevation is contained within an easement dedicated to the public for the purpose of providing drainage;
2. The channel easement has a minimum hydraulic capacity to accommodate a 50-year return frequency storm based on a fully developed watershed;
3. That all minor channel improvements, such as reshaping, realignment, and the like, are protected with sodding, backsloping, cribbing, or other bank protection that is designed and constructed to control an analysis shall also be made to determine the effects of the 100-year flood as required by § 154.124(G)(3). Any change to a regulatory floodway shall comply with § 154.122(B)(3).
(2) When the natural channel to be preserved is one which has had a floodplain information report prepared on it by FEMA, the 100-year return frequency storm shall be as shown in that particular report unless otherwise directed by the City Engineer or his authorized designee.
(E) Detention facilities. Detention facilities will be required when the downstream drainage system is not adequate to carry the increased flow from the development. The facilities shall be designed to restrict the rate of the 100-year storm flow to the predevelopment conditions. If there is additional capacity in the downstream drainage system, the additional capacity will be prorated based on the area of the contributing drainage areas. The detention system must be approved by the Director of Public Works for design and method of operation and maintenance.
(F) Table I: minimum roughness coefficients and maximum permissible velocities for channels.
Table I Minimum Roughness Coefficients and Maximum Permissible Velocities for Channels | ||
Channel Description | Coefficient of Roughness "n" | Maximum Permissible Mean Velocity fps |
Table I Minimum Roughness Coefficients and Maximum Permissible Velocities for Channels | ||
Channel Description | Coefficient of Roughness "n" | Maximum Permissible Mean Velocity fps |
I. Minor natural streams. | ||
A. Fairly regular section. | ||
(1) Some grass and weeds; little or no brush | 0.030 | 6 |
(2) Dense growth of weeds, depth of flow materially greater than weed height | 0.035 | 8 |
(3) Some weeds, light brush on banks | 0.035 | 7 |
(4) Some weeds, heavy brush on bank | 0.050 | 8 |
(5) Some weeds, dense willows on banks | 0.060 | 8 |
(6) For trees within channels with branches submerged at high stage, increase all values above by | 0.010 | |
B. Irregular section with pools, slight channel meander, use A.(1) to (5) above and increase all values by | 0.010 | |
C. Floodplain pasture. | ||
(1) Short grass | 0.030 | 8 |
(2) Tall grass | 0.035 | 8 |
D. Floodplain cultivated areas. | ||
(1) No crop | 0.030 | 6 |
(2) Mature row crops | 0.035 | 6 |
Table I Minimum Roughness Coefficients and Maximum Permissible Velocities for Channels | ||
Channel Description | Coefficient of Roughness "n" | Maximum Permissible Mean Velocity fps |
Table I Minimum Roughness Coefficients and Maximum Permissible Velocities for Channels | ||
Channel Description | Coefficient of Roughness "n" | Maximum Permissible Mean Velocity fps |
E. Floodplain uncleared. | ||
(1) Heavy weeds, scattered brush | 0.050 | 8 |
(2) Wooded | 0.120 | 8 |
II. Major natural streams. Roughness coefficient is usually less than for minor streams of similar description on account of less effective resistance offered by irregular banks or vegetation on banks. Values of "n" for channels or larger streams of mostly regular sections, with no boulders or brush may be in the range from 0.028 to 0.033. Overbanks will be classed as flood plain with "n" values as shown in divisions I. C., D. and E. above. | ||
III. Unlined vegetated channels. | ||
Clays (Bermuda grass) | 0.035 | 8 |
Sandy and silty soils | 0.035 | 5 |
IV. Unlined nonvegetated channels. | ||
Sandy soils | 0.030 | 2.5 |
Silts | 0.030 | 1.5 |
Sandy silts | 0.030 | 3.0 |
Clays | 0.030 | 5.0 |
Coarse gravels | 0.030 | 8.0 |
Shale | 0.030 | 10.0 |
Rock | 0.020 | 15 |
V. Lined channels | ||
Neat concrete | 0.015 | 15 |
Riprap (i.e., broken concrete and rubble) | 0.030 | 15 |
('68 Code, App. B, § 7.04) (Ord. 8-1989-34, passed 8-8-89)