ABSTRACT: Waterfront erosion beachmark study, City of Victoria, B.C.

CGRG Bibliography of Canadian Geomorphology
Search Results

Author :

Date :

Title :

Publication :

Issue :

Page(s) :

Abstract

An investigation of the rate of recession of coastal bluffs along the shoreline of the City of Victoria was conducted in 1996-1997. The scope of work consisted of: a) the determination of bluff recession rates using historical air photos and suitable map data, as well as any changes which could be determined from "as built" engineering drawings; b) an assessment of the effects of existing engineering structures on erosion rates of the shoreline bluffs and beach levels; c) an examination of the current state of the shoreline bluffs, beaches and engineering structures; and d) the establishment of survey benchmarks for use in future shoreline monitoring studies. The study area presently comprises a variety of shoreline types, both natural and man-made. Natural shoretypes include steep south facing bluffs of unconsolidated glacial and glacial-marine deposits fronted by mixed sand and gravel beaches and/or bedrock outcrops. Man-made shoretypes include vertical concrete seawalls (James Bay seawall), concrete and asphalt sloping revetments (Holland Point seawall), and sloping riprap revetments (Clover Point, Horseshoe Bay). For most of the man-made shoretypes, beaches fronting the structures tend to be narrow and may be entirely submerged by normal high tides. The rate of recession of coastal bluffs was primarily determined from 1958 and 1991 ground survey data provided by the City Engineering Department. The survey data comprised bluff crest and bluff toe positions extending along the shoreline. Changes in the position of the bluff crest and toe were determined using computer assisted analysis of digital drawing files. Based upon the analysis of this data, the average rates of recession of the bluff crest and toe for the shoreline are about 0.01 m/yr. A series of digitally orthorectified air photos from 1928, 1954, 1977 and 1992 were also analyzed for bluff crest position changes. Although the analysis is considered less accurate than the ground survey data, the air photos also provide confirmation of the relatively low rates of bluff recession. Attempts to use "as-built" drawings of engineering structures (such as seawalls, revetments, drainage outlets, stairways, etc) to measure shoreline changes were largely unsuccessful. Any drawings of engineering structures which were located were "design" drawings and did not show the adjacent beach features or bluff slope positions. An assessment of the effects of existing engineering structures on the shoreline bluffs and beaches was also inconclusive for the same reasons. An examination of the current state of the shoreline indicates that bluff recession is due primarily to mass movement slope transport processes such as shallow slides, small rotational slumps, mudflows and gullying processes (both natural and human induced). These processes are associated with seasonally high ground water levels and surface water flowing into and onto bluff crest sediments. Slope processes erode material from the upper and middle portions of the bluff slope and deposit material at the toe of the slope. By contrast, marine processes (wave action) which cause erosion of the toe of the slope occur less frequently. In general, marine processes are not removing the supporting foundation of the bluffs and are not initiating bluff slope erosion. It is recommended that the results of this study serve as a benchmark or baseline for a routine survey monitoring program of bluff recession rates and processes, and beach profile changes. Based upon the results of existing 1958 and 1991 ground surveys, bluff crest and bluff toe position surveys should be conducted every five years and the first survey should be conducted as soon as possible. In addition, "as-built" surveys should be conducted of the major coastal engineering structures to establish their present condition and relationship to bluff slope positions and beach levels. Data from the monitoring program would also be valuable to the assessment of existing shoreline conditions and provide data necessary to determining appropriate and cost-effective solutions to shoreline management objectives.

Bibliography of Canadian Geomorphology