(A) Manning Equation. Open channel design shall be based on the Manning Equation.
(B) Minimum size. 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 such that the velocity in the channel is high enough to prevent siltation but low enough to prevent erosion. Velocities less than one and one-half feet per second should be avoided because siltation will take place and ultimately reduce the channel cross-section. Developments through which the channel is to be constructed must be considered in design of the channel section.
(C) Side slopes. Side slopes for grass-lied channels shall be no steeper than three to one. Side slopes for rock riprap-lined channels shall be no steeper than one and one-half to one. Channels with vertical walls may be constructed, with approval of the City Engineer.
(D) Drainage of vegetated waterways. Vegetated waterways that have less than 1.5% slope, are subject to low flows of long duration or where wet conditions prevail shall be drained with an underdrain or low flow structure. Lines may be outletted through a drop structure at the end of the waterway, or though a standard tile outlet.
(E) Longitudinal slopes. Longitudinal slopes shall be grass-lined and, for channel length of 400 feet or less, the minimum standard longitudinal slope shall be 1.3%. If the slope is less than 1.3% but at least 1%, then an underdrain is required. If the slope is less than 1%, a paved concrete swale as approved by the City Engineer shall be required. For channel length greater than 400 feet, the minimum standard longitudinal slope shall be 1%. For slopes between 1% and 1.3%, an underdrain is required.
(F) Alternative swale/ditch treatments. Alternative open channel treatments shall be subject to approval of the City Engineer.
(G) Channel stability. All swales shall be blanketed or fiber mulched, so erosion control extends completely to the top of the bank.
(H) Effects of channel improvements. The effect 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 there to shall be evaluated to determine the need for modification or replacement. Culverts and bridges which are modified or added as part of 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) Disposition of spoil. Spoil material resulting from clearing, grubbing and channel excavation shall not stay on the site unless it is to be used as part of the approved grading plan. In no case shall spoil be deposited in the floodway. Excavated materials, when stored temporarily on-site, must be handled in accordance with the Rule 5 Erosion Control Plan and the erosion control measures included in the construction plans.
(Ord. 2-2010, passed 3-8-10)