View Q&A

In your study on box culverts, I have some questions:

1. Did the presents of water at the box culvert affect the accident severity?  Or did you only look at "dry" box culverts that occasionally fill with water during rain events (e.g. if Figure 6 on page 29 had always has a water depth of 5 to 10' verses 1' would that have an effect on the decision to grate/extend/shield?).
2. A 10x12 box culvert is a single cell 10' tall by 12' long box culvert.  What impact would multiple cells of the same size have on the analysis?
3. In 3R situations, project staff has made the decision to extend the box culvert only to the minimum 18'clear zone.  The 18' clear zone is from Special Report 214 Practices for Resurfacing, Restoration, and Rehabilitation (TRB).  This report is the foundation for most states 3R standards design standards.   In the report, MwRSF is using the values from the RDG for culvert extensions.  Is MwRSF indicating that for larger drainage features, such as box culverts, an agency should strive to greater than 18' clear zone recommended by Special Report 214?  If so, what is the basis for the recommendation?
4. Our structure's department is not hot on the idea of using grates because they are worried that they will "clog up with debris".  Basically, if there is a forest in the watershed they do not want to use grates.  Has there been studies on how often box culverts get clogged, or is there guidance on when it is O.K. to use grates even if there is a chance of the box being clogged?

Standard severities from RSAP were used to describe the box culvert hazard. Although these culvert severities probably were not intended to include water, all severities incorporated into RSAP have proven to be excessive. And it is reasonable to regard RSAP culvert severities to be representative of having some water in the box.

Adding two or three box culverts in a row would essentially create a longer hazard. Our sensitivity analysis showed that longer hazards create higher cost/benefit ratios for any given safety treatment. Further lengthening the hazard would make moving the culvert farther from the roadway more beneficial relative to the other two treatment options.

With regard to the clear zone offset requirements, the MsRSF had no intention to indicate that the clear zone should or should not be adjusted for 3R work relative to new construction. We chose clear zone limit its used by our sponsor, Iowa, in this study.

Many DOT's have expressed concern over culvert grates becoming clogged and leading to flooding. In a study conducted many years ago, TTI explored this problem and found no evidence of clogging. Unfortunately TxDOT chose not to publish that report. Recall that great bars are 30 inches apart and can span lengths of 20 feet or more. TTI's findings indicated that 2-1/2 foot wide by 20 foot long openings are difficult to clog, even in heavily wooded areas. Iowa has been using grades for a number of years. Perhaps you could contact them to see if clogging has been a problem.