Basic Hydrology & Hydraulics: DES 601 Module 18 Channel Analysis and Design - II

Sectional Shapes Engineered sections partially full are treated as open channels Engineered and natural channels are often simple shapes (rectangular, trapezoidal) Module 18

Natural channels The three main natural channel patterns are straight, meandering, and braided Module 18

Straight streams A stream is classified as straight when the ratio of the length of the thalweg (path of deepest flow) to the length of the valley is less than 1.05. This ratio is known as the sinuosity of the stream. Module 18

Braided streams Braiding is caused by bank caving and by large quantities of sediment load that the stream is unable to transport. Deposition occurs when the supply of sediment exceeds the stream’s transport capacity. Module 18

Meandering streams A meandering stream consists of alternating S-shaped bends. In alluvial streams, the channel is subject to both lateral and longitudinal movement through the formation and destruction of bends. Bends are formed by the process of erosion and sloughing of the banks on the outside of bends and by the corresponding deposition of bed load on the inside of bends to form point bars. Module 18

Degree of sinuosity The sinuosity (ratio of channel to valley length) is used to classify stream types Module 18

Natural Channel Analysis Similar in scope to engineered channel analysis. Cross section orientation (to flow path) and stationing need to reflect stream distance (along curvilinear flow path) Stream “type” can change with stage (braided to meandering) Module 18

Natural Channel Analysis Channel roughness (i.e. Manning’s n) can vary across a cross section. HDM provides guidance for how to account for such change (6-12 through 6-17) Computer programs (WSPRO, HEC-RAS, etc.) are recommended for such analysis. Module 18

Linings Channel lining may be desirable or necessary for: reducing maintenance frequency Resisting erosive forces Improve hydraulic efficiency (conveyance) fit channel size for right-of-way or safety Anticipated flow volumes, topography, and soil conditions influence the selection of channel lining material Module 18

Rigid vs. flexible linings Engineers may design roadside channels with rigid or flexible linings. Flexible linings in channels conform better to a changing channel shape than rigid linings. A rigid lining may resist an erosive force of high magnitude better than a flexible one. The following types of rigid linings are common: cast-in-place concrete, soil cement, fabric form work systems for concrete, and grouted riprap. The following are classified as permanent flexible linings: rock riprap, vegetative lining, and gabions The following are classified as permanent flexible linings: curled wood mat, straw with net, and jute. Module 18

Rigid linings The following types of rigid linings are common: cast-in-place concrete, soil cement, fabric form work systems for concrete, and grouted riprap. Module 18

Flexible linings Permanent flexible linings include : rock riprap, vegetative lining, and gabions Module 18

Flexible linings The following are classified as temporary (transitional) flexible linings: curled wood mat, straw with net, and jute. Module 18

Lining Design Guidance FHWA-NHI-05-114 for flexible (and rigid) lining Main analysis issue is roughness (Manning’s n) – the document has extensive tables and design guidance for the resistance term Module 18