The MN for Test series one, Test series two and Test series three tests were calculated by applying MN Equation 57. The water depths, bed slopes in Appendix D, plus the riprap and water properties in Tables 15, 17 and 19 were used as input values for Equation 57 to compute the MN for all the tests.
Test series one, Test series two and Test series three tests were analysed individually. However, the analysis procedure was similar with minor differences regarding the median stone size and the region of failure as well as the bed state of movable/non-movable conditions. Test series one tested for the failure of 0.038 m median stone size and the incipient failure occurred only on the bed area. Test series two tested for the 0.075 m median stone size and the failure of riprap occurred in the steep bed area only. However, Test series three tested for the 0.075 m median stone size and the failure region was on the side bank.
In Test series one, it was found that the riprap critical hydraulic incipient failure condition was defined by a MN value of 0.119. In Test series two it was found that the riprap critical hydraulic incipient failure condition was defined by a MN of 0.127. As anticipated, the two critical hydraulic incipient failure MN were similar and close to Rooseboom (1992) 0.12 MN value criteria. However, in Test series three it was found that the critical hydraulic incipient failure condition for angular riprap dumped on steep side slopes of 0.4 was defined by a MN of 0.227.
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Chapter 6 : Evaluation of HEC-RAS Ability to Predict
Riprap Incipient Failure Conditions.
From the analysis in Chapter 5, it was found that there was a unique MN defining the incipient failure conditions of angular riprap dumped on steep beds as well as for angular riprap dumped on steep side bank slopes. The two critical MN were found to be 0.119 and 0.227, respectively. The two critical MN were determined with an exceedance probability of 95% with respect to the tests performed at the hydraulic laboratory.
HEC-RAS is one of the widely used hydraulic engineering design software. The software is generally used to perform hydraulic flow analysis on rivers. Based on the simulation results and the type of project, flood lines may be determined, or river protection structures may be designed using the output results from HEC-RAS.
Some of the reasons why HEC-RAS is generally favourable among river hydraulic design engineers are:
• HEC-RAS has quick computation times.
• It is relatively easy to learn to use the HEC-RAS software • The software is based on fundamental hydraulic principles. • The HEC-RAS is available to download for free.
• It produces reliable surface water elevations for one directional flow problems.
With respect to abovementioned reasons, some engineers may opt to use the HEC-RAS software to design stable riprap protection in steep slopes but overlooking the fact that HEC- RAS may not be able to appropriately simulate water depths its limitations in application.
Thus, it was critical that the thesis evaluates the capability of the HEC-RAS to determine the critical MN that defines the critical incipient failure conditions of angular riprap placed in steep bed slopes and steep side bank slopes.
The following limitations associated with the simulation of hydraulic flow conditions using a HEC-RAS further necessitate the evaluation of the ability of HEC-RAS to accurately determine the critical MN incipient failure conditions:
6-136 • HEC-RAS does not account for the effects of bed porosity, turbulence and wave action
produced by riprap rocks.
• The bed is assumed to be fixed (immovable bed) in HEC-RAS. • HEC-RAS models the top of the riprap as the bed level.
• Moreover, in a 1-D surface water profile simulation, only the flow in one direction is simulated. Thus, the flow behaviour and direction in a 1-D HEC-RAS modelling simulation does not simulate 3-D flow effects.
As a result of the above, the water depth may not be accurately determined when simulating open channel flows with porous bed problems.
When calculating the MN in Chapter 5, it was evident that the water depth was the main sensitive input parameter in Equation 57. Thus, the ability of the HEC-RAS software to simulate water depths that are representative of the physically determined water depths will determine its ability to simulate the riprap incipient failure conditions.
In this chapter, 1-D HEC-RAS numerical hydraulic models were prepared to simulate the hydraulic condition of the physical laboratory models that were tested.
This chapter describes the basic theory underpinning the 1-D HEC-RAS hydraulic numerical modelling software. Not all the theory was covered, but the main principles are highlighted. The main stages that were followed during the preparation of the hydraulic HEC-RAS models are described in this chapter. The stages comprise the modelling of the geometrical cross- sections of the laboratory model, the input boundary conditions, the recorded flow data for the models and the roughness coefficient determination. MN determined with the HEC-RAS model were calculated and summarised. A comparison of the HEC-RAS MN and physical model MN results was performed. Based on the comparison, correction factors were recommended for application onto HEC-RAS produced incipient motion MN analysis.