Bridge lines should be digitized from the left overbank to the right overbank, looking in the downstream direction. Bridges and culverts are created in a similar way to the cross section cut lines. The next step is to digitize bridges, culverts and other structures along the river. By using profile tool and checking attribute table of XScutlines layer, correctness of each cross-section can be examined. Green lines in Figure11 below indicate the cross section cut lines. In order to digitize cross-sections, it is important to begin with a new Feature Class created from the Task menu and XSCutLines from the Target menu, followed with populating the attribute table of the XSCutLines feature class. This type of hydraulic structure can be determined by examining the aerial photograph. (6) Make sure there is one cross-section each on the upstream and downstream of bridge or culvert. (5) Where it is possible, maintaining two neighbour cross-sections in a consistent spacing (4) Cut lines do not intersect each other (3) always digitized from left to right (looking downstream) (2) must be extended enough to span over the entire flood extent, and it is better to end at the same elevation at both ends At certain locations, “dog-leg” shapes of cross-section are used (1) Cross section cut are digitized perpendicular to the direction of flow. Certain criteria should be followed for the appropriate drawing of cut lines (Meyer and Olivera, 2007): Consequently, digitizing an adequate number of cross-sections to generate a good representation of channel bed and floodplain is necessary. The intersection of cutlines with other RAS layers (e.g., river centreline, flowpath layer) are utilized to compute HEC-RAS attributes such as bank stations (locations that separate main channel from the floodplain), downstream reach lengths (distance between cross-sections) and Manning’s n (Ackerman, C. Cross section cutlines are utilized to extract the elevation data from the terrain model and to develop a ground profile across river flow. Then, populating all attributes of the River feature class in River feature class.Ĭross-sections are one of the most critical inputs to HEC-RAS. Before finishing the creating river centrelines, checking all the reaches created are connected is necessary. Aerial Photos used for this project provided from Open Data Catalogue of City of Chilliwack (2012).Īfter digitizing all of the reaches, each reach within a river should be assigned the corresponding name. Besides, digitizing the river centre should not only follow the visible stream on the Aerial Photo but also be in comply with the lowest evaluation path in terrain map. So, the process starts at the uppermost end of the river which is located above the intersection with Slesse Creek at a distance and ends at the Vedder Crossing (the lowermost end of the stream). In this project, Chilliwack River flowing from east to Vedder Crossing with Slesse Creek as the only tributary is considered as the study area. The digitizing procedure should be consistent with the direction of river flow. Creating the river centrelines begins with construction tool in the Create features window as shown in the figure below. The steam centreline layer is an indispensable part of the whole hydraulic analysis, as it is utilized to develop the river reach network for HEC-RAS. This is a necessary step for the later steps, as assigning physical attributes (e.g., elevation information) into all RAS layers. Then in Layer Setup of RAS Geometry Option, the TIN generated should be selected as the single TIN in the Required Surface tab. The figure below shows the TIN model of Chilliwack River above Vedder Crossing to Slesse Creek. For this project, NAD83 / UTM zone 10N was assigned as the unified coordinate system. In the meantime, unifying the same coordinate system for all the data and data frames used for this project. Utilizing Create TIN tool in ArcGIS packages converts contour lines to a TIN model required by HEC-GeoRAS in TIN development.
The DEM data (contour lines) was downloaded from Terrain Resource Information Management Program (TRIM, 2015). Since there is no existing any DTM data for Chilliwack River Watershed, pre-processing phase should start with converting available DEM data. In order to create a geometry file, it is fundamental to create a Digital Terrain Model (DTM) using ArcGIS software. Setting up Analysis Environment for HEC-GeoRAS
For the efficiency of software execution, a rigorous data preparation process is required for this project.