| Abstract/Description or Keywords |
Purpose: The Town of Golden, with funding and technical support provided by the British Columbia Ministry of Water, Land and Air Protection (MWLAP), commissioned this sfudy to review and refine the flood risk mapping currently in place in the Town of Golden. The flood risk mapping and development restrictions currently applied are based on a previous study in 1979 that did not consider the protection provided by the existing dike system. The present assessment is limited to the area on the south side (left bank as viewed looking downstream) of the Kicking Horse River where current development restrictions apply based on the 1979 flood risk map. Namely "the flood construction level be three (3) feet above the level of the crown of the adjacent municipal road at the point nearest the building". The combined effect of flooding from the Columbia River is incorporated into the study by assuming coincident flood peaks - potential overland flooding is primarily from the Kicking Horse River with some local backwater effect created by the Columbia River in the lower portion of the Town. Scope and Methodology: The study consists of a review and analysis of the 2}}-year return period peak discharge value - the event that is applied for flood-risk mapping in the province under the Canada-British Columbia Flood Damage Reduction Program. A visual inspection and assessment of the culrent dike system including river and potential overland flow conditions and a top of dike survey were conducted. An updated 1 m contour interval digital map of the Town was prepared. Updated hydraulic modeling of the Kicking Horse River was conducted to update and compare with the results from a 1999 study and to evaluate more extreme case scenarios to assist in assessing the risk of potential dike overtopping andlor breaching. Overland flooding scenarios were evaluated and potential flood depths were estimated based upon assumed dike overtopping and breaching scenarios. These evaluations were used to assist with developing a flood risk mapping guideline for the south (left) side of the river within the Town of Golden. Summary of Results: The 200-year return period peak discharge is estimated at 570 m3/s. However, there is significant potential for variation in this estimate. An extreme upper envelope discharge is 1,000 m3ls. During a flood the Kicking Horse River's discharge may be expected to stay near to 98Vo of the peak value for 3.5 hours and tptog5Vo ofthe peak value for as long as 10 hours. Of the modes of potential dike failure evaluated, dike overtopping and subsequent breaching is expected to be the most probable mechanism causing overland flooding. Other modes such as direct erosion ofthe riverbank or geotechnical seepage or slip failures are much less probable. River hydraulic modeling shows that the differences in water levels predicted between surveyed sections in 1997 and those in 2002 are minor and less than 10 cm at the 200-year flood level - indicating that any sedimentation impacts over this period have been minor. The existing south (left) dike freeboard at the computed 200-year flood level is greater than MWLAP's recommended minimum guideline of 0.6 m in all but three short locations where it may drop to as low as 0.5 m. These locations are at: o al40 m segment at the campground at the upstream end of Town; Ń a short segment at the sub-channel downstream of Highway 95 (final grading and top surfacing following dike upgrading work in the fall of 2003 is expected to raise the dike grade to above the minimum guideline here); and at a couple of segments upstream of the CPR bridge where the impact from overtopping is minor because flood risk levels from the Columbia River are more significant. A few similar low sections located along the right dike level are also identified. Assumed extreme case scenario evaluations show the following: The left dike may not overtop at a higher flood peak discharge of 660 m3/s (in the order of a 500-year return period event). The average increase in the 200-year flood level is O.26 m at this higher discharge. In fact, overtopping may not occur until a discharge of at least 740 m3/s, assuming that the model inputs and assumptions are correct at these high discharges' Maximum increases in assumed channel hydraulic roughness values that may account for extreme debris, sediment and turbulent flow conditions increase the predicted flood level by an average of 1.16 m. The maximum roughness values assumed in this scenario are considered extreme and unlikely. However, under this assumption, significant overtopping in the low areas along both the left and right dikes would occur. A debris jam that fully blocks the sub-channel at the Highway 95 bridge does not dramatically increase flood levels and on its own is not expected to result in overtopping of the left dike. Sediment deposition amounting to 1 m deep across the entire river bed in the reach downstream of Gould's Island increases the flood level by up to 1 m such that overtopping of the left dike may just begin to develop in the reach downstream of the island. The above are all extreme cases. Some combination of the above conditions (sediment deposition, debris and increased roughness and a flood peak exceeding 570 m3/s) may be required to cause overtopping of the dike. Ice conditions are not expected to pose as great a risk as an open water flood event. Analysis of potential dike breaching as a result of overtopping indicates that the campground area has the greatest risk for a breach and a significant breakout flow. A breach 30 m wide by 1.2 m deep may result in a breakout flow of up to 45 m3ls. Overland routing of this flow down the streets indicates that a flow depth of apprõximately 0.9 m may develop ulong the upper end of 9Ě Street. This depth of flow reduces as it spreads out downstream and may have an average depth of 0.3 m at Highway 95 with gteat...l. ponding in local low points. For such an event to occur, the following will have to occur: o a water level must occur that is at least 0.7 m higher than the estimated 200-year flood level that is computed based on a discharge of 570 m3/s; o this water level must be sustained for a period of at least 2 hours; no remedial protection measures are taken during the event; erosion occurs on the downstream side of the dike leading to a significant breach; and a o the water level in the river would need to remain at or near the top of dike level for a further period of at least 2 hours for adequate water to break out and create significant downstream flooding. Overtopping flows without dike breaching may be in the order of 5 to 15 m3/s. Downstream overland- flow depths are estimated to be in the order of 0.1 to 0.2 munder these breakout flows. Conclusion: The extreme case scenarios show that some risk of flooding still exists on the left side of the Kicking Horse River within the area protected by the dike system. Although these risks are considered to be low, property owners are to be aware of the potential risks here' The dike system is appropriately armoured and stable, with adequate freeboard in accordance with provincial guidelineslat least 0.6 m above the designated 200-year flood level equivalent to put discharge of 570 m3ls). Therefore, no flood risk development restriction guidelines are reõommended io be applied to the area protected by this dike system, provided that appropriate ongoing dike monitoring and maintenance is conducted. This includes: o locally raising the left dike by about 0.1 m in the few identified low locations to meet the 0'6 m minimum freeboard criteria, annually inspecting the dike sideslopes, the protective riprap and the dike crest, replacing /stabitizing the riprap, as required, and Ń continuing to conduct and evaluate river cross-section surveys at least every two or three years to assess sedimentation conditions' In addition, an Emergency Preparedness Plan (EPP) should be in place, updated and regularly tested, as required. |
