Jul 31 2012

Criteria for Selecting Hydraulic Models, Part 2

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

Part 1

Skewed Roadway Alignment

Roadways should be aligned perpendicular to channel and floodplain flows. FHWA (1978) indicates that skewed crossings with angles of up to 20 degrees produced no objectionable flow patterns. The HEC-RAS Reference Manual indicates that using the projected opening is adequate for skew angles of up to 30 degrees for small flow constrictions. Two-dimensional modeling is the recommended approach for higher skew angles or moderate amounts of skew combined with moderate to high flow contraction. Not only will the flow patterns and bridge conveyance be better defined, but potential problems with backwater will also be evident. Figure 4.2 shows a crossing with an approximate 25 degree skew to the floodplain with flow from top to bottom.

This figure illustrates how floodplain impacts can vary greatly upstream of a skewed crossing. The colors represent the difference in water surface between natural (no bridge crossing) and existing conditions. The darkest color shows the greatest water surface increase and the opposite side of the embankment shows a decrease in water surface compared to natural conditions. The fact that this is also a multiple opening crossing also complicates the hydraulic conditions.

Backwater at a skewed crossingFigure 4.2. Backwater at a skewed crossing.

Road Overtopping

When computing road overtopping, the HEC-RAS model uses the total energy grade line in the cross section upstream of the bridge as the head value in the weir equation. This assumption is reasonable for many conditions. Because standard use of ineffective flow areas can trigger full floodplain flow for any amount of overtopping, HEC-RAS Applications Guide recommends comparing the road overtopping discharge to the floodplain flow and adjusting the Manning n to better maintain flow continuity. As illustrated in Figure 4.2, for roads crossing wide floodplains or skewed crossings, two-dimensional models offer a better approach. Road overtopping is still computed using the weir equation, but nodes on either side of the embankment are connected using a weir segment. The water surface and velocity at the two connected nodes are used to determine head and submergence. The head at the upstream node is used rather than the total energy grade line of the entire upstream cross section. Therefore, better estimates of the initiation of overtopping and overtopping discharges are achieved.

Upstream Controls

Two-dimensional model velocities, I-35W over Mississippi RiverFigure 4.3. Two-dimensional model velocities, I-35W over Mississippi River.

For sub-critical flow conditions, calculations progress from downstream to upstream. Locally, however, flow depth, velocity magnitude, and velocity direction can be controlled by upstream structures and obstructions. In one-dimensional modeling the usual, approximate approach is to incorporate ineffective flow areas to account for upstream obstructions. The overall flow area and conveyance are altered, but flow distribution is still based on the distribution of conveyance at the cross section. Therefore, upstream effects are not fully accounted for in one-dimensional models. Figure 4.3 shows velocity conditions at the I-35W crossing of the Mississippi River in Minnesota. This figure illustrates that two-dimensional models can be used to accurately determine whether an upstream condition impacts a downstream structure, even in sub-critical flow conditions. The I-35W Bridge is located downstream of St. Anthony Falls Lock and Dam, which concentrates the approach flow to the I-35W Bridge. During extreme events, the lock and dam could be operated with flow primarily through the three gates (as shown), or additional flow can be passed through the lock chambers. A range of upstream operating conditions was modeled for the I-35W new bridge design. For this situation flow is definitely not distributed in the downstream channel based on conveyance distribution. Another concern with this project was avoiding any adverse impact on the 10th Avenue Bridge immediately downstream. The 10th Avenue Bridge has a large pier in the center of the channel. The two-dimensional model was used to evaluate whether changes to the I-35W replacement bridge design would increase velocities approaching the 10th Avenue Bridge pier or change the flow angle of attack.

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