VII. - CITY FACILITIES (Major ditches)

The proper hydraulic design of channels is of primary importance to insuring that nuisance drainage conditions, flooding, sedimentation and erosion problems do not occur or the frequency of their occurrence is at an acceptably low rate. For facilities to be operated or maintained as City facilities, the following minimum design standards shall be applied to construction of new or reconstruction of existing facilities.

Design Frequency

New City facilities shall be designed and constructed to contain and safely convey runoff from the 100-year frequency storm when at all feasible to do so. Consideration must be made for the capacity of existing channels downstream, and no improvement shall be made that increase the frequency of downstream flooding.

Design Flow Velocities

Excessive flow velocity can cause erosion problems, may pose a threat to bank stability and may create safety problems. Additionally, velocities that are too low may allow sediment deposition resulting in loss of channel capacity. Generally, design flow velocities in unlined open channels (for 100-year flows) should be between 3 and 5 fps. Flow velocities in concrete lined channels may increase to be between 5 and 8 fps.

City Ditch Width and Ditch Depth

The minimum width shall be adequate to provide maintenance areas, maintainable side slopes, and minimum bottom widths in accordance with City criteria for the ultimate channel section. Ditch depth shall be what is required to provide the required capacity at a minimum.

City Ditch Channel Slope

City ditches shall have a minimum constructed channel slope of 0.05% to provide for the minimum velocities noted earlier. Excessive slopes may unnecessarily increase the potential for erosion of banks and undermining of bridge and culvert structures, therefore maximum slopes should generally not exceed 0.50%. In areas of steep topography, channel drop structures may be required to limit channel invert slopes.

City Ditch Side Slopes

In grass lined channels, maximum side slopes shall be 4:1 (horizontal:vertical). Variance from this criterion may be granted by the City to accommodate site specific issues, but 3:1 slopes should be the steepest unlined slope proposed. Side slopes for concrete lined channels shall be based on field conditions and shall be site specific.

City Ditch Bottom Width

The bottom width for City ditches should generally be no less than six feet. A larger bottom width may be required to meet other issues including ditch capacity, design velocity, etc.

City Ditch Horizontal Curves

In general, centerline curves for grass channels should be as gradual as possible and should have a radius greater than three times the ultimate ditch top width. Smaller curvature radii can be allowed with adequate slope paving as approved by the City.

City Ditch Confluences

The angle of intersection between the tributary and main channel should be between 15° and 45° (with an optimal value of 30°). Angles in excess of 90° will not be permitted.

City Ditch Transitions

Expansions and contractions should be designed to create minimal flow disturbance and thus minimal energy loss. Design consideration must be given to reducing erosion potential and turbulent flow characteristics at ditch transitions.

City Ditch Drop Structures

When introducing flow into ditch main channel from shallow surface swales, the designer must include drop pipes to reduce the erosion potential at the confluence. Drop pipes shall be appropriately sized for the area being served. Drop pipe structures shall be HDPE pipe (or approved equal), properly bedded with a discharge elevation of 12" above the main channel flowline. These drop pipes shall provide for a continuous maintenance berm along the main channel and shall include erosion protection at upstream and downstream ends.

City Concrete Lined Channels

As field conditions necessitate, concrete lined channels may be required to provide adequate capacity or erosion protection for less than optimum drainage easement widths. Design of concrete lined channels will be considered by the City on a case-by-case basis.