Week 10 River Hydraulics Rehabilitating Urban Waterways (From http://hubpages.com/hub/river-bank-erosion-control-methods) See also the resources placed on Moodle. Water channels should be viewed as a resource in cities as they provide respite from the builtup environment much like green spaces. Conventional concrete waterways are ecologically undesirable as they disrupt the natural ecology. Rehabilitation involves the removal of the canals to revert to a natural soft edge. This would also require protection of the exposed bank from erosion before the establishment of protective vegetative cover.in addition, the replacement of the concretized edge to a natural edge requires measures that can accelerate this process of re-vegetation. Various options are discussed below.these low cost, low impact interventions can also be applied as general erosion control methods. 1. Coir geotextile roll Coir geotextile rolls (CGR) are the non-woven fibers of coconut husks bound within a polyethylene or coir woven mesh rope. They arrive in a roll and are rolled over the surface of the bank and provide erosion protection for newly graded slopes. Vegetation can be established over the geotextile and the roots would interlock with the fibers. Wetland plants like rooted sprigs or cuttings are used. As a biodegradable material, it provides a low impact solution to slope protection. Suitable sites for application: Perennial steams that are small to moderate in size with a relatively consistent water surface level. The water channel should not contain a lot of sediment. This would be deposited on the geotextile and kill the plants. Design considerations: The coir should be placed where it can absorb water for the plants but not inundate the vegetation. It should also not be used when the upper slope has not stabilized. The coir has to be anchored to the bank hence a stable substrate is required. Advantages: Ease of application, simple installation, biodegradable, low maintenance Disadvantages: Cost 45
2. Brush Mattress The bank is protected by a mattress formed by branches that are anchored to the ground by means of stakes. In addition to providing a natural armor, the mattress captures sediment runoff during rain. The branches used can be either dead or live branches. The drawback of such a technique is that the fascines are vulnerable to be washed away during heavy rain. Suitable sites for application: Perennial steams with sunlight to encourage establishment of woody vegetation. Design considerations: The brush mattress should be placed along the bank and within the soil so that the branches in the mattress can absorb water. The brush mattress should be placed where flood durations do not exceed the plants' tolerance. Most woody plants cannot tolerate permanent flooding and would die. Advantages: Ease of application, simple installation, biodegradable, low maintenance Disadvantages: Cost 46
3. Gabions There are three basic types of gabions - the basket, mattress, and sack. All three are wire mesh baskets filled with small rock material. The difference between a gabion basket and a gabion mattress is the thickness and the aerial extent of the basket. A sack gabion is, a mesh sack that is filled with rock material. Gabions area are used as a bank stabilization method where mattresses are inadequate (eg. at steep slopes). This is used in areas of high erosion where the rock sizes are too small to resist erosive forces. Their aesthetics are not as desirable and there is a risk of damage to the mesh. A potential infill material is the concrete rubble from the canal in urban rehabilitation projects. Suitable sites for application: Most conditions. Main issue is that the stream must be stable and not be undergoing rapid erosion. Design considerations: Primary issue is to do with foundation stability. The gabion must be design to withstand the force of the water. Advantages: Applicable in areas with serious erosion problems where other methods may be overwhelmed. Disadvantages: Aesthetics, cost of installation and maintenance. 47
4. Rootwad composites Rootwad composites are interlocking tree materials that are built at the base of the slope. In addition to stabilizing the stream bank by lowering flow velocities, the roots provide complex aquatic habitats for the establishment of aquatic wildlife. The source of material can be sourced on site off the vegetation removed during construction. Advantages: Cost effective, creates habitats for aquatic life Disadvantages: Complex implementation. 5. Tree revetment Existing large woody debris (eg. a tree) leftover from the construction works can be used. These would be anchored to the bank with the means of cable or rope. The effect of such a technique is that the revetment would dissipate flow energy. It also creates pool habitats within and increases perching sites for birds. This option is not suitable for actively eroding streams and the woody debris presents a safety hazard if it breaks free from its anchors. Advantages: Low cost, creates habitats for aquatic life Disadvantages: Maintenance is critical. Safety hazard if revetment breaks free. 48
6. A-Jacks A-jacks are interlocking cement stakes that are placed at the foot of the slope. They are effective to increase stability during high intensity storms and scour protection. As a manufactured item, they are available in a variety of sizes. Advantages: Flexibility of sizes, can be used in high erosion areas Disadvantages: Concrete not biodegradable, cost 49