Most counties have bridges that are no longer adequate and, therefore, are faced with a large capital expenditure if the same type replacement structure is proposed. Because a low water stream crossing (LWSC) may be an attractive low-cost alternative to replacing a costly bridge, a manual has been developed to design LWSCs for use in Iowa. The purpose of the manual is to provide consistent guidelines for county engineers and consultants designing these crossings. An LWSC is defined as an unvented ford, a vented ford (one having some number of pipes), a low water bridge, or other structure that is designed so that its hydraulic capacity will be insufficient one or more times during a year of normal rainfall. The use of unvented fords is discouraged in Iowa, and locations where vented fords are permissible have been narrowly defined. Because local social, economic, and political conditions vary from county to county, no hard and fast rules have been set down as to where LWSCs can be used; nevertheless, once the decision to use an LWSC has been made, the manual contains a simple design procedure for these crossings. This procedure includes the following phases: hydrology, hydraulics, roadway geometrics, and material selection. Discharges are estimated from equations that include drainage area, return period, and flow duration. Three methods are included to select the material used to protect the crossing from washing out, the first two of which are based on geomorphic relationships developed from Iowa stream gauging station records.

Media Info

  • Media Type: Print
  • Features: Figures; Maps; References; Tables;
  • Pagination: pp 35-42
  • Serial:

Subject/Index Terms

Filing Info

  • Accession Number: 00450685
  • Record Type: Publication
  • ISBN: 0309038049
  • Report/Paper Numbers: HS-038 995
  • Files: TRIS, TRB
  • Created Date: Feb 28 1986 12:00AM