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Flood Information at Selected Bridge and Culvert Sites
PERIOD OF PROJECT: Continuous since 1966
PROJECT CHIEF: Jon Nania (jfnania@usgs.gov)
STUDY AREA: Statewide
COOPERATING AGENCY: Iowa Department of Transportation (Highway Division, Highway
Research Advisory Board)
NEED FOR STUDY: 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.
OBJECTIVES: For small drainage basins with less than 100 mi2 of drainage area:
- obtain basin characteristics and systematic flood data from a network of
representative basins in the State, and
- document peak discharges on ungaged
basins on an event basis.
METHODS: 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.
PROGRESS: Hydrologic data were collected and analyzed from 88 bridge and
culvert sites located throughout the State of Iowa during Water Year 2005 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 2005 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."
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