David R. Rounce

Identification of Hazard and Risk for Glacial Lakes in the Nepal Himalaya Using Satellite Imagery from 2000–2015

Glacial lakes in the Nepal Himalaya can threaten downstream communities and have large socio-economic consequences if an outburst flood occurs. This study identified 131 glacial lakes in Nepal in 2015 that are greater than 0.1 km2 and performed a first-pass hazard and risk assessment for each lake. The hazard assessment included mass entering the lake, the moraine stability, and how lake expansion will alter the lake’s hazard in the next 15–30 years. A geometric flood model was used to quantify potential hydropower systems, buildings, agricultural land, and bridges that could be affected by a glacial lake outburst flood. The hazard and downstream impacts were combined to classify the risk associated with each lake. 11 lakes were classified as very high risk and 31 as high risk. The potential flood volume was also estimated and used to prioritize the glacial lakes that are the highest risk, which included Phoksundo Tal, Tsho Rolpa, Chamlang North Tsho, Chamlang South Tsho, and Lumding Tsho. These results are intended to assist stakeholders and decision makers in making well-informed decisions with respect to the glacial lakes that should be the focus of future field studies, modeling efforts, and risk-mitigation actions.

Reducing Turbidity of Construction Site Runoff: Factors Affecting Particle Destabilization with Polyacrylamide

The U.S. Environmental Protection Agency is in the process of developing a nationwide standard for turbidity in construction site runoff. It is widely expected that this standard cannot be met with conventional erosion and sediment control measures; consequently, innovative practices for managing sediment on construction sites must be developed. One emerging practice is the use of polyacrylamide (PAM) to improve sediment control by promoting flocculation of particles in runoff. For this practice to be used efficiently, it is essential to understand how soil and PAM properties affect flocculation. A protocol for creating modified synthetic stormwater runoff from soil samples was developed and used on soils from six Texas construction sites. Flocculation tests were performed on these synthetic stormwaters with PAM doses from 0.03 to 10 mg/L. The polymers used ranged from anionic PAMs of 0% to 50% charge density and 0.2 to 14 Mg/mol molecular weight as well as one co-polymer. A neutral PAM and the co-polymer were found to be most effective in reducing the turbidity of all the modified synthetic stormwater below 200 NTU at doses of 10 mg/L. Hardness and electrophoretic mobility tests indicated interparticle bridging to be the bonding mechanism. The high molecular weight (HMW) anionic PAMs were effective on only two of the six modified synthetic runoffs. As the charge density of these HMW PAMs increased, their effectiveness decreased.

Reducing Turbidity of Construction Site Runoff: Coagulation with Polyacrylamide

The U.S. Environmental Protection Agency is developing a nationwide standard for turbidity in construction site runoff. It is widely expected that conventional measures for erosion and sediment control cannot meet this standard; consequently, innovative practices for managing sediment on construction sites must be developed. One emerging practice is the use of polyacrylamide (PAM) to improve sediment control by promoting flocculation of particles in runoff. Effective use of this practice requires an understanding of how the properties of soil and PAM affect flocculation. The objective of this study was to develop such an understanding. A protocol for creating modified synthetic stormwater runoff from soil samples was developed, and synthetic runoff suspensions were created with soil from six Texas construction sites. Particle size distribution was used to compare the synthetic runoff suspensions with grab samples of stormwater from one site. Flocculation tests were performed on the synthetic runoff suspensions with PAM doses ranging from 0.03 to 10 mg/L. The polymers used included anionic PAMs of 0% to 50% charge density and 0.2 to 14 Mg/mol molecular weight and one copolymer. A neutral PAM and the copolymer were the most effective in reducing the turbidity of all the synthetic runoff suspensions below 200 nephelometric turbidity units at doses of 10 mg/L. Hardness tests indicated interparticle bridging to be the bonding mechanism. The high-molecular-weight (HMW) anionic PAMs were effective on only two of the six synthetic runoff suspensions. As the charge density of these HMW PAMs increased, their effectiveness decreased.