Aug 02 2012

Criteria for Selecting Hydraulic Models, Part 3

Continuing with our coverage of the new HDS 7 manual from FHWA.

Part 1, Part 2


Bends, Confluences and Angle of Attack

Highly sinuous rivers are, by definition, not one-dimensional, especially during floods when water in the floodplain flows more directly down valley and moves in and out of the channel. One-dimensional models must consider different channel and floodplain flow distances between cross sections and compute a discharge-weighted flow length. Two-dimensional models do not make any simplifying assumptions related to channel versus floodplain flow distance because the two-dimensional network directly incorporates flow paths. Flow conditions at confluences also vary depending on the proportion of flow in the main stem and tributary. With a one-dimensional model, determining the angle of attack for pier scour calculations is highly subjective in these situations and can be difficult for many other conditions. Two-dimensional models provide improved estimates of angle of attack because velocity direction is computed directly.

Channel Network, Altamaha Sound, GeorgiaFigure 4.4  Channel Network, Altamaha Sound, Georgia.

Multiple Channels

Anabranched and braided rivers have multiple channels and flow paths that complicate hydraulic calculations. Figure 4.4 shows an extreme example of multiple channels at Altamaha Sound in Georgia. The figure depicts channels in blue, flood-prone areas in green, and roadway alignments in red. The area is subject to riverine and tidal flooding. Not only are there nine crossings (five on I-95 and four on SR 17), but there are more than 20 individual channel segments, or reaches that would need to be included in a HEC-RAS split-flow model. The hydraulic engineer would also have to decide the amount of adjacent floodplain to assign to each channel segment and may well need to allow for lateral flow between floodplain segments. Two-dimensional models, while still a significant challenge, clearly have numerous advantages in this situation. Although many multiple channel situations are well simulated with the split-flow options in HEC-RAS, the effort in developing a two-dimensional model for these conditions may be less than an equivalent one-dimensional model.

Tidal Conditions and Wind Simulation. Figure 4.4 is an example of the complex channel and hydraulic conditions that occur more frequently in tidal waterways than in upland rivers. Tidal waterways include inlets, estuaries, bays, and passages. Many bays and estuaries are crossed by causeways with multiple bridge openings and the potential for overtopping and wave attack. The HEC-25 manuals include information and guidance on tidal and coastal conditions, including tides, storm surges, and wind, that impact transportation structures. Some coastal hydrodynamic conditions are dominated by wind-driven currents. Many two-dimensional models include wind stress acting on the water surface as a boundary condition. Therefore, two-dimensional models need to be used for many coastal bridge hydraulic analyses.

Two-Dimensional flow within a bridge openingFigure 4.5.  Two-dimensional flow within a bridge opening.

Flow Distribution at Bridges

The HEC-18 manual establishes scour evaluation procedures recommended by FHWA. Flow and velocity distributions are required within the bridge opening to calculate contraction, pier and abutment scour. One-dimensional models estimate flow and velocity distribution based on the incremental conveyance within a cross section (see Section 5.4). This assumption requires that each point in the cross section also have the same water surface elevation and energy slope. Figure 4.5 shows water surface and velocity vectors from a two-dimensional model. The model represents a relatively simple situation, but does not meet the one-dimensional criteria described above. In this figure, the thin lines indicate the channel banks and embankment. The water surface is relatively uniform along the upstream face of the bridge, varying by less than 0.3 ft (0.1 m), but the velocity vectors in the overbank areas in the bridge opening are not perpendicular to the bridge face. Although these are indicators that the flow is not one-dimensional, the most significant departure from one-dimensional assumptions is the velocity in the overbank areas under the bridge. A one-dimensional model would estimate much lower velocity in the overbanks based on conveyance and equal energy slope at the bridge cross section. The average energy slope in the overbank areas under the bridge is over five times the energy slope of the channel area, resulting in velocities more than twice what is computed from one-dimensional conveyance-weighted calculations.

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