Field assessment of scour at the outlet of selected road culverts

Abstract

Scour downstream of culverts outlet is a common problem that can lead to damage to the culvert structure and bank erosion to the adjacent channel. Traditional scour prediction equations are applicable only to a limited range of situations, although offering the engineer some guidance on the likely magnitude of maximum scour depth. A general prediction equation of scour that applicable to all circumstances is not presently available. Moreover, there is a need for an assessment of applicability of available equations based on field data for a particular type of structure like a culvert. Field data can be collected to verify the equation used to estimate the design scour of the selected culverts. Four culverts on different type of road located at Sonargaon Upazila under Narayangonj district have been selected for assessment of field scour at culvert outlets. At the immediate down stream of the culvert an area of 18 m x 18m was taken for field measurement of scour. This measurement has been done at an interval of 1m x1m grid spacing. Pre and Post monsoon conditions have also been considered for scour measurements in same location. Scour depth measured in the field has been compared with anticipated maximum depth of scour based on different available formula. In this work, the Lacey, the Blench, the Ahmed and the Chitale equations are used to calculate the maximum scour depth. Flow velocity through the culverts is measured in field and the flow rate has been estimated through culvert. In addition, rational formula has been applied to estimate the maximum runoff discharge through the culverts and compared with the measured flow rate by using area velocity method. The sediment samples are also collected and analyzed to obtain representative size of bed material. The contour maps have been plotted and analyzed to investigate the extent and location of maximum scour hole. Predictive capacity of equations used is also compared with result obtained from field assessment. It is found that the depth of maximum scour found in the field is relatively closer to predicted depth calculated with the Lacey equation.