Hope/Fraser Canyon/Fraser Valley – As of July 31, The River Forecast Centre is issuing a Flood Warning for:
Fraser River downstream of Hope
Chilcotin River upstream of landslide in the vicinity of Farwell Canyon
The River Forecast Centre is issuing a Flood Watch for:
Chilcotin River downstream of the landslide in the vicinity of Farwell Canyon
Fraser River from the Chilcotin River confluence downstream to Hope
The River Forecast Centre is issuing a High Streamflow Advisory for:
Synopsis:
Overnight July 30-31, a landslide occurred on the Chilcotin River near Farwell Canyon, 18 km southeast of Hanceville, pushing debris across the river channel and blocking the flow of the Chilcotin River. This landslide has dammed the river and is creating a lake currently extending several kilometers upstream. Higher water levels are expected along the Chilcotin River upstream of the dam.
Prior to the landslide, the Chilcotin River flow at the Water Survey of Canada (WSC) Chilcotin River near Hanceville (08MB012) station was recorded at approximately 200 cubic metres per second. The near-total blockage by the landslide has caused abrupt cessation of flow downstream in the Chilcotin River, which is a major tributary of the Fraser River. A corresponding decrease in flows is apparent in the Fraser River downstream of the Chilcotin-Fraser confluence. Data from the WSC station Fraser River at Big Bar Creek (08MD013) indicate that flows declined by over 150 cubic metres per second through mid-day on July 31.
The eventual overtopping of the landslide material may lead to sudden erosion of the debris and catastrophic failure of the landslide dam. This could potentially cause an outburst flood downstream of the landslide. In this scenario, a surge of water would rapidly move down the Chilcotin River and into the Fraser River.
Water levels could quickly increase along the Chilcotin River as well as in the Fraser River immediately below the confluence. In the event of catastrophic failure, the flood surge could reach the Fraser River within hours, potentially traveling from the Chilcotin River to Hope within a day.
Further downstream in the Fraser River (below the Fraser Canyon), the flood wave would be expected to be less severe, but substantial increases in flow may extend all the way to the mouth of the river through the Lower Mainland.
