Hilly areas are characterized with high terrain slope. In case of a storm, the runoff gushes down the hill at very high velocities causing erosion of soil along drains/slopes
Impact of Erosion and Sedimentation on performance of storm water drains
On steeper slopes, water moves faster as compared to flatter slopes and this increased flow velocity aided by a lack of significant vegetative cover results in transportation of larger amount of sediments. The increasing urbanization of hilly areas increases the paved surfaces and results in increased surface runoff, further aiding the transport of eroded sediments.
Additional Design considerations for drainage systems
i. Storm drains should be constructed on both sides of the road and connected with cross drains across the road at suitable intervals having gratings to collect rainwater from the surface of the road during rains
ii. Roof water drains should be connected to these drains so that the rainwater may not spill over the slopes
iii. The collected storm water conveyed through drains should be disposed off in the valley stream through existing natural channels or constructed c hannels along the slope at a suitable location
Iv Natural channels should be engineered either by constructing a stepped channel or chute (design guidelines may be seen in chapter 5). The width of such an engineered channel should never be reduced from its existing natural width
v. Valley stream bank at the point of outfall should be protected by revetment against erosion
vi. To drain out the increased discharge through the natural channel in a valley, the bank of such channel should be protected by retaining wall made of rock block or gabion box, depending on the steepness of the side slope
vii. Sufficient weep holes should be provided in case of concrete/masonry retaining walls. Weep holes shall be provided in cement stone masonry walls at a spacing of about 1.5 m centre-to-centre in either direction. The size of weep holes shall be 100 mm to 150 mm connected with PVC (flexible) pipes embedded at 100 down from the horizontal towards the valley.
viii. For a channel carrying debris and having a moderate slope (say 10° < S < 30°) intermediate sill projecting from the bed can be constructed to reduce the flow velocity
ix. For a channel having a thick natural cover of boulders (which is found in most of the natural channels), no additional measures are necessary for protection against scouring.
x. In absence of a boulder bed in a moderate slope, a discrete concrete block may be placed to prevent scouring under the impact of high streamflow velocity, while keeping the bed permeable to allow infiltration
xi. The road must have adequate cross slope or camber as per Clause 5 of IRC, SP-42 (1994) for quick disposal of storm water runoff laterally to the roadside drain/drains
xii. A minimum longitudinal grade of 0.5 % should be provided to the road, wherever possible in order to facilitate surface drainage
xiii. Intercepting drain, as in a hilly terrain sloping towards the road, should have adequate size and be connected properly with well-designed culverts/bridges
xiv. In case width of terrain contributing flow to the drain is very large, intercepting drain at higher elevation should be provided
Temporary erosion and sedimentation control practices
The temporary erosion and sedimentation control measures to be implemented in the catchment area and the drainage systems are elaborated below:
a) Mulching
Mulching refers to the application of plant residues or other suitable materials to the soil surface to prevent erosion and reduce surface flow velocities. Mulching also fosters plant growth by increasing available moisture and providing insulation against extreme heat or cold.
b) Temporary/permanent seeding
Temporary seeding and permanent seeding are two types of vegetative controls. Temporary seeding is applied in areas that will be dormant for 15 days or more whereas, permanent seeding is applied in areas that will be dormant for one year or more. Selection of vegetation types depends on the season, site conditions and costs.
c) Sediment Basins
A sediment basin is a constructed embankment of compacted soil across a drainage way which detains sediment-laden runoff. The basin allows runoff to pond and sediment to settle down. They are generally used where the construction area is disturbed in 2 ha or more. Accumulated sediment within the basin should be removed as necessary.
Check Dams Check dams are small temporary dams, constructed across a drainage ditch to reduce erosive runoff velocities of concentrated flows. Check dams are limited to use on small open channels draining 4 ha (10 ac) or less.
Sediments should be removed when it reaches approximately half the height of the dam. Check dams should be spaced in the channel so that the crest of the downstream dam is at the elevation of the toe of the upstream dam.
Silt Fences
A silt fence is the most widely used temporary sediment barrier. The fence consists of a filter fabric supported by wooden posts or wire mesh. It is placed across or at the toe of a slope to intercept and detain sediment and reduce flow velocities. The maximum effective life of a silt fence is approximately six months. Proper maintenance of a silt fence requires removal of sediment deposits when necessary. Silt fences which decompose or become ineffective prior to the end of the expected useable life should be replaced immediately.
Brush Barrier
A brush barrier is a temporary sediment barrier composed of materials (such as weeds, vines, root mats, soil, rock, etc.) pushed together at the perimeter of a given site and at the toe of fills. Maintenance measures include inspection following each rainfall and removal of sediment deposits when they reach half of the barrier height.
Diversion Dike
A diversion dike is constructed of compacted soil and is used to divert runoff to an acceptable location. They are placed either at the top of a disturbed area to divert site runoff, or at the bottom to deflect sediment-laden runoff to a sediment trapping structure. Dikes should be inspected weekly and after rainfall events and repairs made as necessary.
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Urban Infrastructure and Network Study notes for M. plan Sem-III
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