Technical Studies

River sediment data is essentialfor watershed management, flood control engineering, and river ecology. A newsuspended sediment concentration (SSC) measurement method based on time domainreflectometry (TDR) technique had been developed from 2006-2009 under projectssupported by Water Resource Agency (WRA). Unlike optical and acoustic methods,TDR SSC method is not affected by the soil particle size or type, and it ismore economical, easy to be maintained, and applicable for high SSC monitoring.In a project from 2011~2012, the technique continued to mature to overcome somepractical problems, including suppression of fouling, modulization ofdetachable probe, and digitization of TDR sampler to avoid temperature effect.A SSC data acquisition and information management system was also developed. Tofurther implement the TDR SSC monitoring, this study continued to maintain andupdate existing monitoring stations and added two more demonstration monitoringstations. Full runoff events were monitored to provide full event sedigraph andevaluate field performance.

Inaddition to the field monitoring, several attempts were conducted to furthersuppress the temperature effect and increase measurement accuracy. It was foundthat the new coating (PEEK) proposed in the previous study to reduce foulingcauses additional temperature effect on SSC measurement. The coating thicknessshould be reduced or some temperature-independent coating material should beused. The experimental results also show that thermal couple sensor and thecorresponding type of A/D module significantly reduce temperature effect on thewater temperature measurement, which is required to compensate the effect ofwater temperature on SSC. A heat dissipating mechanism in the enclosure is alsoproposed and added in the drafted in technical manual. At this point, the TDRmethod has a resolution of 1000 ppm and practically unlimited measurementrange. The SSC data acquisition and information management system was furtherimproved in this study. In addition, the TDR SSC data dissemination was enabledbased on WRA’s WRISP hydrological information exchange platform. The techniquedeveloped in this study was applied in another project dealing with monitoringof density current in Shihman reservoir, successfully providing detailedsediment transport information for sediment sluicing operation.

Thisstudy also examined hydrological stations of WRA and prioritized stations inwhich TDR SSC monitoring can be added. However, some practical problems (e.g.interference of debris and sedimentation in the protection pipe) were stillobserved in the field monitoring program. Further improvement is suggestedbefore adding more new monitoring stations. It was recommended to continueimproving the field construction method and investigate SSC variation in thecross section. Further study is also required to obtain thedischarge-representative average SSC.