Redesign of a flooding culvert for a highway in Northern BC

Description

Culvert X is located in Northern British Columbia, and allows Creek Y to pass underneath
Highway Z. During flood events, Culvert X does not have the capacity to pass the peak flow and has caused water to backup and overtop the road. ______________, P.Eng, provided the project information for the purpose of an academic exercise. I created a model of Creek Y using HEC-RAS to design a new culvert that will pass the 0.5% AEP. Furthermore, I determined the flood construction level for a new house located 200m northwest of the culvert.

Culvert X is currently two side-by-side 1.8m diameter corrugated metal barrels. The inlet and outlet of the pipes are shaped to align with the sloping embankment of the highway. The culvert location is in mountainous terrain, with the typical land cover being forest and grass. The 0.5%
AEP design flood (1 in 200-year flood) has a peak flow of 40 m3/s

The modelling for this project started on ArcGIS Pro & Civil 3D. ______________, P. Eng, provided me with Lidar data of the terrain as a Digital Terrain Model. I used ArcGIS Pro to
georeference and visualize the creek bed and terrain in 3D. Next, I used Civil 3D to convert the
DTM to a surface that I exported to HEC-RAS.Once the terrain was in HEC-RAS I created a 1D model, a 1D/2D model, and a 2D model of the
terrain. I then created Culvert X in each of the three models. The 1D model consists of a river reach with cross sections. The 1D/2D model has a river reach that is connected to 2D mesh in the floodplain area, over the right bank of Creek Y. The 1D and 2D areas are connected using a
lateral structure with zero elevation so the natural riverbank acts as a levee. The 2D model is
made of mesh that covers the entire floodplain and creek area.

When I ran the plan for the 1D/2D model it had the most accurate results. It displayed flow backing up at Culvert X and running down the road for approximately 50m, and then overtopping the road. Using the 2D model I was able to see the maximum flow in Culvert X was
16.581 m3/s, which confirms that the culvert can not pass the 0.5% AEP of 40 m3/s. Furthermore,
the flood breaches the right overbank and rises to a maximum elevation of 619.2m at the location
of the new house, 0.7m higher than the terrain elevation of 618.5m

For the new culvert design, I used the 1D/2D model to iterate a new culvert size and shape. It was the most accurate because I was able to define levees, ineffective flow areas, and see the profile view of the flood through the culvert. I designed a rectangular shaped culvert because the
culvert can only be a maximum of 3m high to leave at least 1m of clearance to the road surface.

A rectangular culvert allows the culvert to be wider than it is tall. I then used hand calculations to determine the minimum rectangular culvert width which was 7.4m with outlet control governing. Using HEC-RAS I designed two side-by-side 25m x 4m x 3m (L x W x H) concrete box
culverts. After adding this culvert design to my 1D/2D HEC-RAS model the culvert was able to pass the 0.5% AEP without causing the flood to overtop the road. HEC-RAS indicated that the flood in the culvert was entirely supercritical, indicating inlet control governed. I checked the
design with hand calculations and discovered that the culvert is governed by inlet control. Furthermore, the flood does not reach the new house location, so the flood construction level is 618.5m, which is the natural terrain elevation., © Trevor Thirsk, 2021. All rights reserved. No part of this