Water Resources Planning Branch, WRA, MOEAWater Resources Planning Branch, WRA, MOEA

Riverbed material investigation and application

The main purpose of the riverbed material investigation is to obtain the grain size distribution of the armor layer and floodplain material in the riverbed. The results of the investigation can be used to judge and analyze river scouring and silting conditions, the roughness setting of the river channel, for hydraulic and sediment model simulation and used as a reference for river channel maintenance mainagement.

Sampling process

The methods of riverbed material investigation are generally divided into surface sampling and volumetric sampling. The surface sampling is to acquire the information on the pebbles on the surface layer which is more suitable for floodplain and near shallow water areas. The volumetric sampling is to acquire the vertical distribution of the materials in surface layer and armor layer below, which is usually opted in practice, as it is more objective and easier. The flow chart of riverbed material investigation is shown in Figure 1, and in-situ test is shown in Figure 2 and 3.

1. Excavate manually a square pit of volume of 1 cubic meter for the test.
2. In-situ sieving the excavated specimen through the 3”, 1.5”, 3/4” and 3/8” sieves and record their weights, including those larger than 3” which should be measured and weighed togehter.
3. Split the sample which pass throuth the 3/8” into four parts and take back to laboratory for analysis.
4. Using indoor sieve analysis with #4, #8, #16, #30, #50, #100 and #200 sieves, records and combine with the in-situ results to find the distribution of grain sizes as well as the average grain size (mm).

Figure.1 Flow chart of riverbed material investigation

Figure.2 In-situ volumetric sampling (excavation of test pit)

Figure.3 In-situ volumetric sampling (sieve analysis)

Recommendation

Riverbed materials change are sensitive to topography, geology or river development; different sites need different consideration requirements, it is necessary to gather and realise existed information (e.g., historical data of cross-section and riverbed material, geological features and orthophoto) to effectively choosethe best investigation methods.