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Debris Control Structures Evaluation and Countermeasures


Quiz Questions

1.____________ includes small limbs or sticks, orchard prunings, tules, leaves, and refuse. This material can be easily transported by the stream and overland flow.
Small floating debris
Medium floating debris
2. ________________________ consists of tree limbs or large sticks. The source of this material comes from trees introduced into the stream by bank erosion, mass wasting, windthrow, or collapsing of trees due to ice loading, beaver activities, or biological factors such as old age and diseases; or from erosion of emergent and riparian trees within the streams.
Small floating debris
Medium floating debris
3.Large floating debris consists of logs or trees (such large floating debris is also known as “drift”).
True
False
4. ________________________ usually occur when the channel width is slightly greater than the average tree height. They usually redirect the flow to one or both of the banks causing bed and bank erosion that could result in more trees being introduced into the river.
Deflector Jams
Dam Jams
5.Parallel Jams occur in small watersheds where the fallen trees span the channel at bank full level. The in-channel geomorphic impact associated with this type of debris jams is minimal; however, local bed scour could occur under the jam during high flows.
True
False
6._____________________ occur when the length of floating debris exceeds the effective opening between piers, resulting in the floating debris resting against two piers (Figure 2.3).
Single-pier accumulation
Span-blockage accumulations
7. ___________________ results from the displacement of water by the bridge or by the debris lodged under the bridge.
Side forces
Buoyant forces
8. Schematic Illustrating Estimate of Maximum Design Log Length is illustrated in __________________________.
Figure 3-1
Figure 3-2
9.A sheltered location is defined for the section of the bridge that includes a forest area directly upstream of the bridge that traps the transported debris and prevents it from being delivered to the bridge. This category should only be applied when the gaps between trees are much narrower than the average tree height and the width of forest along the direction of flow is more than a double line of trees. Intuitively, this category should not be applied to the upstream forest area if it is potentially subject to clearing.
True
False
10.Sketch of fully submerged pressure flow is represented in _______________________.
Figure 4-2
Figure 4-3
11. Debris Fins are open crib-type structures placed vertically over the culvert inlet in log-cabin fashion to prevent inflow of coarse bed load and light floating debris.
True
False
12. In-channel Debris Basins are structures placed across well-defined channels to form basins which impede the stream flow and provide storage space for deposits of detritus and floating debris.
True
False
13. The countermeasure matrix (Table 6.1) was developed to identify distinctive characteristics for each type of countermeasure. Four categories of countermeasure characteristics were defined to aid in the selection and implementation of the countermeasures- Debris Classification, Maintenance, Installation/Experience by State & Design Guideline References.
True
False
14.A debris sweeper is a device, generally made of polyethylene, which is attached to a vertical stainless steel cable or column affixed to the upstream side of the bridge pier. The debris sweeper travels vertically along the cable or column as the water surface rises and falls.
True
False
15. Hydrostatic forces on the bridge elements are related to the differential in water surface elevations at the upstream and downstream sides of the structure caused by the flow constriction through the bridge.
True
False