Investigation of System Design Criteria and the Capital Cost of Varying Design Return Periods for Soil and Water Conservation Structures

Highlights Very few sediment yield events contribute to annual sediment yield. Any rainfall, runoff, and peak discharge event has the potential to generate the most extreme sediment yield event. Twenty year return period recommended for design of conservation structures. Abstract . Design of conservation structures includes both hydrologic and hydraulic designs. Hydrologic design involves estimation of design floods which are required for the sizing of the hydraulic structures. The minimum recommended return period for the design of conservation structures is 10 years but due to the projected levels of risk, and the fact that a few large events are likely to be responsible for the majority of the erosion, the 10-year return period currently recommended may be inadequate. This study investigated system design criteria and the capital cost of varying design return periods for soil and water conservation structures in the sugar industry of South Africa. Observed rainfall data and results of runoff, peak discharge and sediment yield simulated using the Agricultural Catchments Research Unit (ACRU) model were utilized in this study. Relationships between extreme events of sediment yield and the rainfall, runoff and peak discharge events associated with them were analyzed and the capital cost of varying design return periods was also investigated. The results showed that only 0.2% of sediment yield events contributed up to 95% of the annual sediment yield simulated in the sugar production areas in South Africa and that any event of rainfall, runoff and peak discharge had the potential to generate an extreme sediment yield event provided the soil surface was not adequately protected. Based on a sustainable soil loss of 5 t ha-1, the 20-year return period was recommended for the design of soil and water conservation structures. Furthermore, the capital cost implication of varying design return periods from the minimum 10-year return period ranged from an increase of 16% to 35% across the sugar industry. Therefore, given that soil erosion is associated with adverse effects on sustainable crop production and also increases in costs of replanting destroyed crops, the 20-year return period is recommended for the design of soil and water conservation structures in the sugar industry in South Africa. Keywords: Capital cost, Design criteria, Erosion, Return perioW, Risk, Soil and water conservation.

Stochastic Analysis Method in Application of Longshore Sedimentation

Huanghua is one of the important ports in China. During its constructing stage, Huanghua port encountered the heavy sedimentation after each typhoon or other storm weather processes. In the feasibility study, the predicted yearly amount of sedimentation is far less than that observed. In order to obtain the reasonable design amount of sedimentation, the stochastic analysis methods are involved in this paper to predict the total sedimentation of different design return periods with corresponding confidence intervals. For the prediction of typhoon or storm (wind speed and duration) induced sedimentation, the trivariate joint probability of wind speed, duration and corresponding sedimentation can be obtained by stochastic simulation technique based on the importance sampling procedure.

DESIGN OF STORMWATER INFILTRATION FOR REDUCTION OF COMBINED SEWER OVERFLOW (CSO)

Stormwater infiltration is a significant tool for combined sewer overflow abatement because it involves a decrease in the impervious area connected to the sewer system. When allowing the infiltration structures to overflow into the existing sewer it is shown that for a required reduction in CSO-volume there exists an unambiguous relation between the infiltration structure volume and the size of impervious area connected to infiltration. Further, the presence of an optimal solution minimizing the total trench volume is pointed out. For a Danish sewer system with a travel time of 30 min and an interceptor capacity of 0.2 μm/s the optimal solution for reducing the CSO-volumes by 40 percent involves connecting 65 percent of the impervious area to infiltration trenches with a total storage volume of 3.6 mm. This corresponds to designing the trenches according to an exceedence return period of 0.04 yrs compared to the commonly applied design return periods of 2 to 10 yrs.