Parmeshwar L. Shrestha

Comparison of single mechanism and multi mechanism‐based approaches for kinetics of sediment removal

Abstract The process of cohesive particle sedimentation in aquatic environments is perceived as one of continuous formation of aggregates by coagulation and their separation from the water column by accumulation on the deposition bed. Over the temporal scales of practical interest for natural systems, many physical, chemical, and environmental factors govern the rate of sedimentation, hence a purely physics‐based description has not yet been developed. A new information‐based model that uses experimental information to derive governing parameters, while predicting removal through power law relationships, is proposed and tested. Analytical solutions derived for this model are shown to be applicable over large ranges of concentrations and to various fluid‐sediment environments. Comparisons with numerically derived and observed data show that the analytical model is a versatile tool that improves fundamental understanding of the physical processes involved in sedimentation. The inverse problem of identificat...

Analysis of Flood Hazards for a Residential Development

An investigation was carried out to investigate the flood potential at a proposed residential development located in Indio, California. The existing floodplain area adjacent to the project area is designated to receive flood flows originating from Thousand Palms Wash, The Indio Hills and the riverine drainage area along Interstate 10. These flood flows pass through the existing residential development via three flood control channels. A two-dimensional flood routing model (FLO-2D) was applied to study the progression of a flood flow in the interim floodplain area. Model results were analyzed to depict contour plots of the maximum flow depths in the interim floodplain for two discharge conditions that were simulated. The peak discharges entering the project area were estimated from the model results. Two flood control channels are proposed to accept these floodwaters and carry them out of the project area. The U.S. Army Corps of Engineers’ River Analysis System (HECRAS) computer program was used to compute water surface profiles in the two flood control channels for the peak discharges. A sediment transport analysis was also conducted for one of the channels using the U.S. Army Corps of Engineers HEC-6 model. Model results showed that sediment passes through the system well and is not expected to interfere with the function of the flood control channel. Maintenance measures will be in place in order to clean the sediment out of the channel after a large flood event. The design water surface elevations in the flood control channel are based on the greater of either the HEC-RAS calculations using bulked flow, or the HEC-6 computed maximum water surface elevation using the unbulked flow. All sections have adequate freeboard between the design water surface elevation and the top of pad for the design flood event. World Environmental and Water Resources Congress 2007: Restoring Our Natural Habitat

The Fire-Flood-Erosion Sequence in California: A Recipe for Disaster

Wildland fires are an inevitable component of many terrestrial ecosystems in California, where alternating episodes of heavy winter rainfall and dry autumnal winds, coupled with extended periods of drought increase the probability of major wildland fires. Factors influencing the intensity and duration of the post-fire hydrologic disturbance include the nature of the local vegetation, burn severity, the geology and topography of the burn area, and the local climate. Many parts of California are underlain by granitic bedrock. Wildfires in granitic terrain result in a common suite of hydrologic aftereffects. The Lowden Ranch Fire took place in an area of granitic bedrock and granite-derived soils in 1999. An investigation of the area in 2002 found only minor evidence of post-fire erosion and sediment transport as a probable result of generally moderate rainfall in the winter of 1999–2000 and the presence of deep, permeable granitic sand in the affected watersheds that reduced runoff. Observations made during our site investigation indicated that by 2002 the area had recovered about 70 percent of its pre-fire erosion resistance. This recovery was due mainly to the reestablishment of ground cover and the breakdown of any hydrophobic layer that had formed in the fire. Comparison of this area with granitic terrains of Southern California burned in 2002 and 2003 and affected by heavy rains in early 2005 indicate that such terrains are relatively resistant to erosion during light to moderate rainfall, but can experience catastrophic erosion during periods of heavy rainfall.

Investigation of Flood and Debris Flow Recurrence: Andreas Canyon, San Jacinto Range, Southern California

An investigation was conducted to evaluate the existing flood and debris flow hazard at the mouth of Andreas Canyon, a major watershed that drains from the rugged eastern slope of the San Jacinto Mountains, California. Unlike archetypical alluvial fans, which form as a result of streams of water spreading sediment and cutting new channels on the fan surface, the Andreas Canyon fan was constructed principally by debris flow processes. Eleven debris flows of varying ages were mapped on the fan surface, ten of which originated in Andreas Canyon. The debris flows are rather large, with a typical volume of about 10 5 m 3 . Nevertheless, hydrologic records available for the watershed suggest that storm events necessary to yield sufficient water to mobilize a debris flow of this size are not extremely uncommon, and should recur every few decades. Archeological records, however, suggest that Andreas Canyon has not experienced a major debris flow in at least 350 years. The absence of debris flows during this period suggests that their occurrence is tied to hydrologic events that are rare or absent in the current climate regime. Details of the type of hydrologic event necessary to produce a debris flow in Andreas Canyon are currently under investigation.

Flood Hazard Analysis and Protection Plan for a Residential Development

An investigation was carried out to develop a flood hazard analysis and flood protection plan for a proposed residential development located in Indio, California. The project area is part of an existing floodplain that straddles portions of the alluvial fans derived from Thousand Palms Canyon and Pushawalla Canyon. Floodwaters emanating from a channel located west of the project area impact the project site. A two-dimensional flood routing model was applied to study the progression of a flood flow in the existing floodplain. Processes simulated included overland flow, infiltration, and bottom boundary roughness. Elevations were based on USGS DEM, rough grading plans, high-resolution LiDAR survey data, and data from a field reconnaissance survey. Model results were analyzed to determine the peak discharges into the proposed development. A channel was proposed to intercept the peak flows crossing the western boundary of the proposed development and convey these floodwaters southward along the western boundary and then eastward along the southern boundary of the project. The U.S. Army Corps of Engineers’ River Analysis System (HEC-RAS) was utilized to predict water surface elevations in the proposed channel, and to establish freeboard conditions with respect to pad elevations. To account for the interception of flows along the western boundary, the flows in the proposed channel were progressively increased to the peak flows. To simulate the outflows across the southern boundary, lateral weir flow was assumed to occur. The results of the analysis show that the proposed flood control channel accept and discharge the floodwaters at the historical locations. The U.S. Army Corps of Engineers’ HEC-6 model was used to conduct sediment transport simulations in the floodplain and onsite channels to determine the potential for sediment deposition. Model results were analyzed to estimate the sediment deposition for each cross section at the peak flow of the event, and at the end of the event. The results of our model studies indicate that sediment is transported through the system well and does not interfere with the function of the proposed stormwater channel.

Macro- and Micro-scale Circulation Modeling in the Mesopotamian Marshlands of Southern Iraq

This study focuses first on the macro-scale modeling of marsh hydrodynamics. Simulation results have provided a preliminary look at overall circulation patterns arising from inflows/outflows to the system. Our analysis benefited from access to reasonably detailed topographic maps and exceptional satellite imaging products. This enabled us to include in our model most of the hydraulic structures that are responsible for causing the desiccation of the marshes. The model also provided the means to assess the effects of hypothetical water releases into the marshlands. The results of the macro-scale circulation model are now used to develop micro-scale models for pilot projects at a more detailed level. The objective is to examine the interaction of marsh conditions (e.g., bathymetric features, vegetation, etc.) and hydrodynamic forcing (freshwater inflows, meteorology, salinity and temperature gradients, the presence of hydraulic structures, etc.). With the aid of newly acquired, highly accurate topographic data, the micro-scale analysis are providing insights on circulation patterns and velocity distributions and are applied to evaluate alternative restoration scenarios and to plan/design constructed wetlands to meet water treatment needs for villages in the region.