Investigation Into The Causes Of Blocked Drainage, Canal And Culvert In Ilorin-west Local Government Area
[A CASE STUDIES OF AGBO-OBA, OJA-TUNTUN AND SAWMILL]

2.3.2      Floating Debris
Debris may be classified as floating (e.g. trees), non-floating or depositional (e.g. sediment) and urban (e.g. cars and mattresses). Floating debris can be further categorized as small, medium or large.
(a) Small floating debris:  Less than 150 mm long, can include small tree branches, sticks, leaves and refuse from yards such as litter and lawn clippings and all types of rural vegetation. This type of debris can also be introduced into a stream by earlier windstorms, bank erosion, and landmass failures or from seasonal leaf falls. It is important to note that this material is available in both urban and rural catchments, and is usually available for transportation at any time.
(b)  Medium floating debris:  Typically between 150 mm and 3 m long, consists of tree limbs and large twigs. This material is usually introduced into the flow path by channel erosion undermining riparian vegetation or through wind gusts during storms.
(c)  Large floating debris:  More than 3 m long, consists of logs or trees, typically from the same sources as for medium floating debris. Transport and storage of this material depends on discharge, channel characteristics, the size of the drift pieces relative to the channel dimensions, and the hydraulic characteristics (depth and slope) of the system. In small and intermediate size channels, this material is not easily transported and can easily become snagged mid-stream acting as a collection point for smaller material (i.e. a debris raft or log-jam). Large floating debris is usually transported during larger floods or prolonged periods of high river-stage where the floodplain is engaged and the ability of the debris to become snagged is reduced.
This type of debris can cause significant problems at bridge structures.
2.3.3       Non-Floating Debris
Non-floating debris is usually sediment of all types.
(a) Fine sediments (silt and sand):  Typically consist of particles ranging from 0.004 to 8 mm (Standards Australia, AS-1348-2002). The deposition of the finer clay-sized particles is normally a concern in tidal areas, with lower velocities. This type of debris is either transported along the streambed as bed load or within the water column as suspended load. Such material is normally sourced from sheet and rill erosion, landslip and landmass failures, and channel erosion. Yield rates for this material can be significantly influenced by the conditions of, and changes to, a catchment due to urbanization and/or rural land use practices.
(b) Gravels and cobbles:  Consist of rock typically ranging in size from 4.75 to 75 mm and 60 to 300 mm respectively (Standards Australia, AS-1348-2002). The source of this material may be from gully formation, channel erosion, landslips or land mass failure. Once mobilized, gravels and cobbles are primarily transported as bed load within high gradient streams. The deposition of cobbles can readily block the entrance of culverts or reduce the flow area under bridges.
2.3.4      Urban Debris
Urbanization of catchments introduces many different man-made materials that are less common in rural catchments and which can cause structure blockage. These include building materials, mattresses, garbage bins, large industrial containers and vehicles (Rigby and Silveri, 2001). Garbage bins can easily be washed down a street and into a stream or drainage structure, a situation made worse if a large rainfall event occurs on the same day as rubbish collection within the catchment, when bins are placed in streets for collection.
2.4     TYPES OF STRUCTURES AND DRAINAGE SYSTEMS
The types of structures or drainage elements affected by blockages can generally be grouped as follows:
(a)  Bridges and Culverts:  These cross drainage structures carry roads, railways, pipelines or other infrastructure across watercourses. These structures can be affected by a number of different types of blockage mechanisms, resulting in consequences including increased flood levels, changes to stream flow patterns, changes to erosion and deposition patterns in channels, and physical damage to the structure.