City of Gresham
Hart Crowser has held an on-call geotechnical contract with the City of Gresham for nearly 10 years. The City is located within an area of significant topography and a number of small streams. The streams are actively eroding into the hillside topography, often resulting in stability problems. Under our on-call contract, the City asked us to evaluate two such problem areas: Royal Court and El Camino Avenue.
The Royal Court project is a stormwater outfall that traverses a steep slope adjacent to Johnson Creek. Johnson Creek is actively eroding the hillside below the outfall, undermining support of the pipe. Hart Crowser evaluated the stability of the slope utilizing rope-supported rappels down the slope due to the steepness and presence of the stream at the toe. We found that the slope was subject to mostly shallow failures, but that could still lead to failure of the piping. Hart Crowser provided recommendations to increase the stability of the slope on a short term basis, and a new alignment in the long term.
The El Camino project is a home and roadway adjacent to a small stream. The homeowner noted progressive loss of the stream bank adjacent to his home and the culvert beneath the roadway. Hart Crowser was asked to evaluate the stability of the streambank, estimate the future performance of the bank, and provide recommendations to address resulting concerns. We completed a field reconnaissance, took measurements of significant features, collected anecdotal information, and completed a background review of geology, soils and development history. From our evaluation we noted a number of factors contributing to the problem:
- Fill and bank over-steepening from development
- Stream flow not aligned with the culvert
- Poorly constructed armoring at the culvert
- Removal of bank adjacent vegetation
- Increased upslope development that increased peak flows above natural conditions
- Hart Crowser ranked the different factors allowing the City to understand their potential liability for the stream bank loss and also to evaluate and select the most cost-effective measures to address the problem