§ 6.8.1. Alternative New Channel Design

The following is a description of the cross-sectional characteristics of an alternative channel design to be applied at the engineer's discretion but is in no way a requirement.

A.

A pilot channel designed to carry the ten (10) year storm shall be calculated with Manning's "n" values in accordance with Tables 6-2 and 6-3. This channel is designed to separate the more frequent ten (10) year storm via an unobstructed pilot channel. Side slopes of the pilot channel shall not exceed 3:1 slope gradient and shall have a bottom width of no less than six (6) feet. The remaining cross-sectional area is designed to convey the additional storm flows up to the 100 year storm. This upper platform will accommodate vegetation with minimal maintenance requirements.

B.

The 100 year floodplain shall be contained within overbanks on each side of the pilot channel. These overbanks shall be a minimum width of ten (10) feet and have a slope gradient not to exceed 6:1. The overbanks shall be stabilized with the seeds of grasses, native wildflowers and native woody species appropriate to riparian habitat and with blanket products as described in the City of Austin Erosion and Sedimentation Control Manual. In calculating Manning's "n" values for the overbanks, reference must be made to Tables 6-2 and 6-3 with the following assumptions:

1.

Heavily wooded and brushy overbanks and

2.

Bank irregularities, which can be reasonably expected from occasional, moderate erosion.

Figure 6-4 in Appendix E of this manual depicts the conceptual idea of the alternative channel design.