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Winnipeg's riverbanks consist of a combination of lacustrine clay-rich soils and alluvial deposits of layered clays, silts and sands.
Overview of Winnipeg's Riverbanks
- Lacustrine clays were deposited in proglacial Lake Agassiz between 11,500 and 8,000 years ago. Lacustrine clay deposits are the dominant shallow geological unit beneath Winnipeg, typically starting less than 2 m below ground, and extending to depth that exceed 20 m deep in some areas of the city.
- The Red River and its tributaries were formed when Lake Agassiz drained into Hudson Bay creating channels in the clay.
- Over the course of many years, rivers have deposited layers of clay, silt and sand during floods and as part of the natural process of developing meanders.
- Post-glacial erosion of the river valleys in Winnipeg resulted in steep, high riverbanks that were prone to bank failure. Lacustrine lay soils typically dominate the outside bends of the City's riverbanks while alluvial deposits are often found along the inside bends. Deposits of alluvial soil over top of lacustrine soil can often be found along straight and transition sections of the rivers.
- Clay soils are generally weak and strongly affect riverbank stability.
- The specific characteristics of each riverbank section are influenced by a range of factors, including: soil properties and stratigraphy, geometry, currents, groundwater conditions, flooding, vegetation, climate, anthropogenic influences, and time.
- Riverbanks, by nature, are dynamic as they are influenced by constantly changing environmental factors. Riverbank development and man-induced factors can also contribute to riverbank failure or affect erosion rates.
1. Lacustrine Clay Banks
- Generally weak, leading to failure-controlled slope movements.
- Slope instabilities are characteristically deep-seated failures that can sometimes extend over 15 m below ground and reach over 60 m from the river's edge while impacting long reaches of the riverbank.
- Soil groundwater and river hydraulic conditions affect bank stability.
2. Alluvial over Lacustrine Banks
- Can be susceptible to both deep and relatively shallow slope failures and high rates of erosion.
- Potential for significant loss of riverbank.
- A detailed study may be required to understand the soil conditions and stability risks in these areas of transition.
3. Alluvial Banks
- Alluvial soils generally exhibit higher strengths and therefore maintain steeper slopes.
- The less cohesive soils in alluvial banks can lead to higher rates of erosion.
- Slope instabilities are generally confined to the vicinity of the erosive face of the bank.
- Assessments need to consider bank erosion affected by river hydraulics, wave and ice action, precipitation, soil characteristics and vegetation cover.