Long-Term Drainage from the Riprap Side Slope of a Surface Barrier
Zhuanfang Fred Zhang*
Hydrology Group, Earth Systems Science Division, Pacific Northwest National Laboratory, Richland WA 99352
Keywords: Surface Barrier; Surface Cover; Gravel; Evaporation
Received August 23, 2016; Revised April 29, 2017; Accepted May 8, 2017; Published July 1, 2017;
Available Online July 24, 2017.
Surface barriers for containing under-ground nuclear waste in place are expected to function for 1000 years. Riprap, which consists of very coarse rock fragments, has been used to protect surface barriers against wind- and water-induced erosion and damage by other natural or human activities. Although drainage from riprap could affect the hydrological performance of a surface barrier, there have been very few quantitative investigations of drainage from riprap. A riprap side slope was installed on a portion of the Prototype Hanford Barrier, which was constructed in 1994 at the Hanford Site, Washington, USA. Drainage through the riprap side slope was monitored from 1994 to 2013 under natural and enhanced precipitation conditions. Results show that, when the precipitation was lower than about 200 mm yr-1, roughly 6% of the precipitation became drainage; when the annual precipitation was higher than 200 mm yr-1, approximately 60% of the additional precipitation drained through the riprap. On average, 12.9% and 40.5% of the annual precipitation became drainage under natural and enhanced precipitation conditions, respectively. Internal evapo-ration may be a key mechanism moving water stored within the riprap side slope to the atmosphere.