Flood Information at Selected Bridge and Culvert Sites

PERIOD OF PROJECT: Continuous since 1966
PROJECT CHIEF: Jon Nania Email Jon Nania
STUDY AREA: Statewide
COOPERATING AGENCY: Iowa Department of Transportation (Highway Division, Highway Research Advisory Board)


Systematic flood information is needed for the proper hydraulic design of new bridges, culverts, and other flow structures (especially on small drainage basins of less than 100 mi2) and for the evaluation of existing structures. There also is a need to analyze the hydrology and hydraulics of proposed sites with little available data and to document outstanding floods at ungaged sites on an event basis. Because of the large number of small basins in the State, relatively few will have specific flood data available. Therefore, flood discharges are estimated from numerical models that are calibrated using data collected at these sites. To define and calibrate these models for basins in Iowa, flood data must be available from basins with a variety of characteristics, such as drainage area, topography, soil type, shape, and land use.


For small drainage basins with less than 100 mi2 of drainage area:

  1. Obtain basin characteristics and systematic flood data from a network of representative basins in the State.
  2. Document peak discharges on ungaged basins on an event basis.


A crest-stage gage (CSG) is a 2-inch diameter galvanized steel pipe attached vertically to a bridge pier or culvert wingwall. The length of the pipe varies from a few feet to over thirty feet depending upon the design of the culvert or bridge pier. The pipe has a locking cap on the top end, and a bottom end-cap with holes. Within this pipe is a sanded wooden stick with a granular cork reservoir on the lowest end. The holes in the end-cap allow water to enter into the pipe and leave high water cork marks on the wooden stick. Since the pipe is permanently attached to the structure and the structure has a known elevation from geographic surveys, a relative gage datum can be developed for the locking cap and end-cap. Thus any mark from a high water event left on the wooden stick will have a relative gage height associated with it based on the addition of or subtraction from the end-cap or locking cap respectively. Each site has a unique stage-discharge relationship rating. These rating are either developed from a series of direct discharge measurements or from the elevation survey, to produce a theorectical rating. Using the high water marks, a peak discharge is obtained.


Hydrologic data were collected and analyzed from 90 bridge and culvert sites located throughout the State of Iowa during Water Year 2010 and these data are included in the Annual Water Resources Data Iowa Report. This report is available on-line. Data for water years 1998 to 2010 can be found in the on-line Annual Reports under Publications. The annual data report shows one maximum gage-height peak, discharge, site location, drainage area and date of peak for the year for each site. In addition, some sites have updates to their historical data bases and are labeled as "Revised Records."