Step1: Pre-Processing of Geometry Data
For the efficiency of software execution, a rigorous data preparation process is required for this project.
Setting up Analysis Environment for HEC-GeoRAS
In order to create a geometry file, it is fundamental to create a Digital Terrain Model (DTM) using ArcGIS software. Since there is no existing any DTM data for Chilliwack River Watershed, pre-processing phase should start with converting available DEM data. The DEM data (contour lines) was downloaded from Terrain Resource Information Management Program (TRIM, 2015). Utilizing Create TIN tool in ArcGIS packages converts contour lines to a TIN model required by HEC-GeoRAS in TIN development. In the meantime, unifying the same coordinate system for all the data and data frames used for this project. For this project, NAD83 / UTM zone 10N was assigned as the unified coordinate system. The figure below shows the TIN model of Chilliwack River above Vedder Crossing to Slesse Creek. Then in Layer Setup of RAS Geometry Option, the TIN generated should be selected as the single TIN in the Required Surface tab. This is a necessary step for the later steps, as assigning physical attributes (e.g., elevation information) into all RAS layers.
TIN Model for Chilliwack River Watershed (Vedder Crossing to Slesse Creek)
Creating RAS Layers
The main task of the pre-processing is to create the geometry data for the hydraulic analysis with HEC-RAS. The geometric data file should involve critical information about stream centrelines, flow path lines, cross-sections, hydraulic structures (e.g., bridges,culverts, levees, obstruction blocks, and ineffective areas), channel banks and other physical attributes of streams (Merwade, 2006). In HEC-GeoRAS, each physical attribute is assigned in an independent feature class which is called a RAS Layer (Merwade, 2006). Consequently, it was essential to build empty GIS layers using RAS Geometry menu on the HEC-GeoRAS extension toolbar in ArcGIS before creating any RAS layers in GIS.
Creating River Centreline
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. Creating the river centrelines begins with construction tool in the Create features window as shown in the figure below. The digitizing procedure should be consistent with the direction of river flow. In this project, Chilliwack River flowing from east to Vedder Crossing with Slesse Creek as the only tributary is considered as the study area. 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). 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. Aerial Photos used for this project provided from Open Data Catalogue of City of Chilliwack (2012).
After digitizing all of the reaches, each reach within a river should be assigned the corresponding name. Before finishing the creating river centrelines, checking all the reaches created are connected is necessary. Then, populating all attributes of the River feature class in River feature class.
Stream Centrelines on Terrain Map
Creating River Banks
Bank lines layer is utilized to distinguish the main stream channel from overbank floodplain areas. Information associated with bank location is utilized to assign different values (Manning’s n values) for each cross-section and for floodplain areas. Usually, overbank areas are assigned higher values of Manning’s n due to vegetation or existence of residential areas, which accounts for more roughness. This process is similar to the process of creating river centrelines, and bank lines can be digitized based on Aerial Photos and TIN. Red lines in figure blow represent the bank lines.
Bank Lines of Chilliwack River
Creating Flowpaths
The flowpath layer consists of a set of lines that follows the centre of mass of the water flowing down the river, during the extreme discharge event (Meyer and Olivera, 2007). The flowpath layer contains three kinds of lines: centreline, left overbank, and right overbank. Using that layer can define the over bank areas, flow direction and determine the downstream reach lengths between cross-sections in the main channel and over bank areas. As for the main channel, the flowpath centreline is basically matched the stream centrelines. When it comes to floodplains, the flowpath centrelines are drawn to define assumed water flow within the floodplain areas. Similarly, flowpath centrelines are also digitized in the upper reach to lower reach direction. Then, using the sketch tool to digitize left and right flowpaths, and utilizing the Assign Line Type button to label flowpath lines Right, Channel, Left towards downstream direction.
Creating Cross-section
Cross-sections are one of the most critical inputs to HEC-RAS. Cross section cutlines are utilized to extract the elevation data from the terrain model and to develop a ground profile across river flow. 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. T., Evans, T. A., & Brunner, G. W., 2000). Consequently, digitizing an adequate number of cross-sections to generate a good representation of channel bed and floodplain is necessary. Certain criteria should be followed for the appropriate drawing of cut lines (Meyer and Olivera, 2007):
(1) Cross section cut are digitized perpendicular to the direction of flow. At certain locations, “dog-leg” shapes of cross-section are used;
(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;
(3) always digitized from left to right (looking downstream);
(4) Cut lines do not intersect each other;
(5) Where it is possible, maintaining two neighbour cross-sections in a consistent spacing;
(6) Make sure there is one cross-section each on the upstream and downstream of bridge or culvert. This type of hydraulic structure can be determined by examining the aerial photograph.
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. Green lines in Figure11 below indicate the cross section cut lines. By using profile tool and checking attribute table of XScutlines layer, correctness of each cross-section can be examined.
Cross-Sections (Green Lines) of Chilliwack River
Creating Bridges and Culverts
The next step is to digitize bridges, culverts and other structures along the river. Bridges and culverts are created in a similar way to the cross section cut lines. Bridge lines should be digitized from the left overbank to the right overbank, looking in the downstream direction. TINs and Aerial Photo are used to identify each bridge and digitize a line along the centreline of the bridge without intersecting the cross sections. In study area of this project, there are two bridges, Vedder Bridge and Tamihi Bridge, but no known culverts. After labelling the name of each bridge, other attribute information such as station numbers and elevation should be assigned to RAS layer of the bridge. Besides, width and length of each bridge should be added in its attribute table by using measuring tool on aerial photos to determine these data.
Creating Ineffective Flow Areas
Ineffective flow areas are defined as non-conveyance areas of the floodplain. In this project, the area behind bridge abutments representing contraction and expansion zones can be considered as ineffective flow areas (Meyer, S., Olivera, F., 2007). The figure below represents the contraction reach and expansion reach in the profile of bridge cross-section. Due to there was no field investigation data, ineffective areas should be determined by examining the TIN and Aerial photos to locate the points in the channel where flow is fully expanded or contracted (Sredojevic, D., & Simonovic, S. P., 2009).
Location of bridge cross-sections (Sredojevic, D., & Simonovic, S. P., 2009)
Assigning Manning’s n values to Cross-sections
The last step before exporting the Geometry Data to HEC-RAS is assigning Manning’s value to each cross-section. Assigning the appropriate Manning’s n value is very critical for accurate computation of water surface profiles. Selecting Manning’s n values highly depends on many factors such as surface roughness, channel irregularities, channel alignment, size and the shape of channel, scour and deposition, vegetation, obstructions, stage and discharge,seasonal change, temperature, suspended materials and bed load (USACE,2002 b). The n value declines with increases in water level and discharge. When the banks are rough and grassy, the n value is probably large at high stages (Chow, 1959). There are several references available listing the typical n values. The table below indicates different types of channel and corresponding Manning’s n values.
Exporting Geometry Data to HEC-RAS
The last step of pre-processing is to export GIS input file for hydraulic analysis in HEC-RAS. Firstly, it is necessary to check which layers would be exported to HEC-RAS. The Required Surface option should have TIN model of the study area for single terrain option. The Required Layers tab should have River, XSCutLines, and XSCutLines3D for Stream Centreline, XSCutLines and XSCut Lines Profiles, respectively. Besides, in the Optional Layers tab, make sure the empty layers are set to Null. The figure below shows the setting up of Optional Layers for this project. Export of GIS Data is performed in the following way: The menu item “RAS Geometry” —> “Extract GIS DATA” is selected from the HEC-GeoRAS extension toolbar. Confirm the name export file name (default name GIS2RAS for this project) and the location, the file was saved in Maps folder.
