Downstream Towns Research Articles

A Comparative Study of a Typical Glacial Lake in the Himalayas before and after Engineering Management

One of the main glacier-related natural hazards that are common to alpine locations is the occurrence of glacial lake outburst floods (GLOFs), which can seriously harm downstream towns and infrastructure. GLOFs have increased in frequency in the central Himalayas in recent years as a result of global warming, and careful management of glacial lakes is a crucial step in catastrophe prevention. In this study, field surveys were conducted on 28 August 2020 and 1 August 2021 with the help of an unmanned aerial vehicle (UAV) and a boat bathymetric system on an unmanned surface vessel (USV), combined with 22 years of Landsat series imagery and Sentinel-2 MSI imagery data. Spatial analysis was then used to investigate changes in lake surface conditions, dam stability, and surrounding topography before and after an integrated project of the Jialong Co lake. The results show that: (1) from 2000 to 2020 (before engineering management), the area of the Jialong Co glacial lake increased from 0.2148 ± 0.0176 km2 to 0.5921 ± 0.0003 km2. The glacial lake expansion rate from 2000 to 2010 (0.0145 km2/a) was greater than the rate from 2011 to 2020 (6.92 × 10−6 km2/a). In 2021 (after engineering treatment), the glacial lake perimeter, area, and volume decreased by 0.6014 km, 0.1136 km2, and 1.90 × 107 m3, respectively. The amount of excavation during the project treatment was 8.13 million square meters, and the amount of filling was 1.24 million square meters. According to the results of the unmanned surface vessel (USV), the elevation of the lake surface dropped from 4331 m to 4281 m, and the water level dropped by 50 m (the designed safe water level line dropped by 30 m). (2) The results of the UAV topographic survey and geomorphological analysis showed that the engineered reinforcement of the outlet channel and surrounding dam effectively mitigated severe scouring of the foot of the final moraine at the outlet of the spillway, as well as the likelihood of glacial lake outbursts caused by ice avalanches and landslides. (3) The comprehensive engineering treatment of this typical glacial lake effectively lowered the water level and improved the stability of the moraine ridge and lake dam, providing a scientific foundation for other glacial lake outburst risk assessments and disaster mitigation and management measures. Thus, it is critical to evaluate the impact of comprehensive engineering management of key glacial lakes to support glacial lake management.

Quantitative risk assessment of two successive landslide dams in 2018 in the Jinsha River, China

Two large successive co-site landslide dams blocked the Jinsha River in the Sichuan Province of China in 2018. A quantitative risk analysis was carried out to quantify potential human and economic losses resulting from the failure of these dams, especially the interaction between the co-site landslide dams, and to further investigate the influence of digging a diversion channel on risk mitigation. Flood routing for three scenarios (i.e. after the first dam formation, when the second landslide mass was added, and with a diversion channel after failures) were simulated using HEC-RAS. The human and economic losses were evaluated using a human risk assessment model together with empirical formulations. The results show that the risk of breaching floods had increased significantly after the second co-site landslide dam formation on the pre-existing loose deposits of the first dam. This amplification effect of outburst floods was so great that the peak outflow resulting from the breaching of the second landslide dams was more important, leading to greater economic losses than those resulting from the breaching of the first dam. However, the expected loss of life caused by the breach of the two landslide dams appeared small because of the sufficient time lag provided by the long distance between the residential area and the dam site. The simulations also outline the importance of the diversion channel in decreasing the peak outflow rate and hence downstream risks. A parametric analysis shows that a deep channel with a moderate longitudinal gradient can significantly decrease the peak outflow discharge at the dam site. Flood intensity and risks to downstream towns did not change because of the relatively small attenuation rate of the peak outflow rate. However, a shorter height of the residual dam can be obtained by optimizing the diversion channel, and the amplification effects of such co-site dam can be significantly reduced in the future.

How natural flood management helps downstream urban drainage in various storm directions

The wider benefits of natural flood management are increasingly used to engage local stakeholders and justify physical implementation. Previous studies have highlighted the potential for upstream natural flood management interventions to contribute to water level management strategies by mitigating downstream water levels and promoting free discharge at surface drainage outfalls from urban areas. The study reported in this paper extended the scope of this possible benefit by examining the potential for upstream interventions to de-synchronise rural and urban responses under various storm tracks, thereby improving the performance of downstream surface drainage networks. The methodology used a coupled modelling approach. Five design events, each applied with eight different storm tracks, were used to evaluate how catchment-scale natural flood management in the upper Calder River, UK, might influence performance of surface drainage in the downstream town of Todmorden. The results suggested that all the storm tracks considered had substantial influence on whether urban rainfall occurs during the period when outfalls are inundated. While upstream natural flood management interventions create modest flow attenuation, a slight delay in rural response can significantly improve drainage performance and reduce instances of nuisance flooding.

Toddbrook: After the Incident

On 1st August 2019, Toddbrook reservoir became, overnight, the most photographed UK reservoir for the last decade. After a series of sequential flood events led to localised failure of a section of the 1970s concrete auxiliary spillway, which passes spill flows over the face of the dam, residents were evacuated from the downstream town of Whaley Bridge due to fears of dam collapse. A multi-agency response was launched, with experts working around the clock to drop the reservoir level and allow residents to return home. The news crews have now moved on and the reservoir sits nearly empty, in wait of a new spillway. Modelling and design work has continued behind the scenes to allow a detailed understanding of the response of the reservoir basin to a range of storm events. A comprehensive water level control plan has been developed, and associated temporary works installed on site, to allow the reservoir to be safely managed in its transitory state. This paper details the ‘make safe’ works installed after the incident, and the ongoing design work to bring the reservoir back into operation and ensure its future resilience.

The impact of Natural Flood Management on the performance of surface drainage systems: A case study in the Calder Valley

The evaluation of Natural Flood Management (NFM) has traditionally focused on the ability of interventions to mitigate downstream fluvial flooding by attenuating catchment response. However, the justification for the implementation of NFM projects at a local level is often supported by other benefits provided by such interventions (e.g. improvements to water quality or biodiversity). This study investigates the potential for a further, largely unrecognised, benefit. It is suggested that, by moderating water levels in downstream watercourses, upstream interventions could also help sustain free discharge from drainage outfalls, thereby improving the performance of urban surface drainage systems.A coupled modelling methodology is applied to the upper Calder Valley and the surface drainage in an area of the downstream town of Todmorden. The rural response and subsequent NFM interventions are characterised using hydrological (Dynamic TOPMODEL) and hydraulic (HEC-RAS) models. Several downstream surface drainage systems are then incorporated using an Infoworks ICM model to examine their response to changes in outfall inundation.The results suggest that catchment-scale tree planting and in-channel woody debris create modest benefits for downstream surface drainage systems. Under frequent storm events (e.g. a 1 in 10 year storm), the inundation of low-lying outfalls is completely removed. As storm severity increases (and surface flooding becomes an issue), impact from upstream NFM attenuation on outfall inundation durations diminishes significantly. However, the slight delay in rural response allows more water to escape surface systems, increasing the effective capacity of networks and reducing surface flooding. For instance, outfall inundation during an estimated 20 year event is delayed by 0.5 h, which results in up to 25% reduction in surface flood volumes.While the benefits are limited in extent, this modelling indicates that NFM can help improve downstream surface drainage performance.

Surf zone microbiological water quality following emergency beach nourishment using sediments from a catastrophic debris flow

Urban disaster response requires disposal of complex wastes. This study regards a case wherein high intensity rainfall fell over a remote mountainous area previously burned by wildfire, generating debris flows that devastated a downstream town. Sanitary sewers and homes with septic systems were damaged, releasing human waste into the debris flow field. Contaminated sediments, with their high fecal indicator bacteria (FIB) concentrations, were cleared from public rights-of-way and creek channels by local authorities, then disposed onto distant Goleta Beach for beach nourishment, causing immediate surf zone microbiological water quality exceedances. To determine potential public health threats, disposed sediments and surf zone waters were sampled and analyzed—relative to reference samples of mountain soil and raw sewage—for FIB, pathogens, human (HF183) and other host- (Gull2 TaqMan, and DogBact) associated DNA-based fecal markers, and bacterial community 16S rRNA gene sequences. Approximately 20% of disposed sediment samples contained the HF183 marker; sequencing suggested that all samples were contaminated by sewage. In an initial sediment disposal period, surf zone waters harbored intestinal bacterial sequences that were shared with disposed sediments and sewage. Yet surf zone bacterial communities returned to mostly marine clades within weeks. Taken together, multiple conventional and DNA-based analyses informed this forensic assessment of human waste contamination. In the future, similar analyses could be used earlier in disaster response to guide sediment disposal decisions towards continuously protecting beachgoer health.

Incentives for Securing Water in a Himalayan town: A case from Dhulikhel, Nepal

This paper explores the negotiations and the emerging socio-political relationships and alliances that were formed to reach a series of water-sharing agreements between upstream and downstream communities, in order to secure water required for continued urbanisation of the downstream town. The research focused on the socio-political actors and users of the Dhulikhel drinking water supply system of Nepal. Primary data was collected through key informant interviews, focus group discussions and stakeholder workshops to explore the development of the negotiation process and the agreement, and the role of different actors. The qualitative data was analysed through narrative and discourse analyses. During the negotiation process, political leaders from both communities were involved in the formation and acceptance of the agreement. The long-term negotiation that started during the 1980s culminated in a series of agreements, the last of which formally introduced cash incentives to the upstream community in 2011. The downstream urban community has been paying NPR one million per annum to the upstream community for their continued role in the sustainable management of the water catchment. The paper provides insights into the shifting power relations between local rural and urban socio-political actors who play a vital role in water access negotiations, and fundamentally influence the potentials and effectiveness of incentive-based mechanisms to secure water needs.

Wind Regimes Associated with a Mountain Gap at the Northeastern Adriatic Coast

AbstractWinds through the Vratnik Pass, a mountain gap in the Dinaric Alps, Croatia, are polarized along the gap axis that extends in the northeast–southwest direction. Although stronger northeasterly wind at the Vratnik Pass is frequently related to the Adriatic bora wind, especially at the downstream town of Senj, there are many cases in which the wind at Senj is directionally decoupled from the wind at the Vratnik Pass. A cluster analysis reveals that this decoupling is sometimes related to lower wind speeds or a shallow southeasterly sirocco wind along the Adriatic, but in many cases the bora blows over a wider region, while only Senj has a different wind direction. Several mechanisms can be responsible for the latter phenomenon, including the formation of a lee wave rotor. A numerical model simulation corroborates the decoupling caused by a rotor for a single case. The prevalence of northeasterly winds at the Vratnik Pass during southeasterly sirocco episodes is another result that challenges the current understanding. It is shown that, at least in one of these episodes, this phenomenon is related to a secondary mesoscale low pressure center in the eastern lee of the Apennines that forms as a subsystem of a broader Genoa cyclone. Less frequent southwesterly winds through the gap are predominantly related to the thermal sea breeze and anabatic circulations that are sometimes superimposed on the geostrophic wind.

Simulating dam-breach flood scenarios of the Tangjiashan landslide dam induced by the Wenchuan Earthquake

Abstract. Floods from failures of landslide dams can pose a hazard to people and property downstream, which have to be rapidly assessed and mitigated in order to reduce the potential risk. The Tangjiashan landslide dam induced by the Mw = 7.9 2008 Wenchuan earthquake had impounded the largest lake in the earthquake affected area with an estimated volume of 3 × 108 m3, and the potential catastrophic dam breach posed a serious threat to more than 2.5 million people in downstream towns and Mianyang city, located 85 km downstream. Chinese authorities had to evacuate parts of the city until the Tangjiashan landslide dam was artificially breached by a spillway, and the lake was drained. We propose an integrated approach to simulate the dam-breach floods for a number of possible scenarios, to evaluate the severity of the threat to Mianyang city. Firstly, the physically-based BREACH model was applied to predict the flood hydrographs at the dam location, which were calibrated with observational data of the flood resulting from the artificial breaching. The output hydrographs from this model were inputted into the 1-D–2-D SOBEK hydrodynamic model to simulate the spatial variations in flood parameters. The simulated flood hydrograph, peak discharge and peak arrival time at the downstream towns fit the observations. Thus this approach is capable of providing reliable predictions for the decision makers to determine the mitigation plans. The sensitivity analysis of the BREACH model input parameters reveals that the average grain size, the unit weight and porosity of the dam materials are the most sensitive parameters. The variability of the dam material properties causes a large uncertainty in the estimation of the peak flood discharge and peak arrival time, but has little influence on the flood inundation area and flood depth downstream. The effect of cascading breaches of smaller dams downstream of the Tangjiashan dam was insignificant, due to their rather small volumes, which were only 2% of the volume of the Tangjiashan lake. The construction of the spillway was proven to have played a crucial role in reducing the dam-breach flood, because all the other natural breach scenarios would have caused the flooding of the downstream towns and parts of Mianyang city. However, in retrospect improvements on the spillway design and the evacuation planning would have been possible. The dam-break flood risk will be better controlled by reducing the spillway channel gradient and the porosity of the coating of the channel bottom. The experience and lessons we learned from the Tangjiashan case will contribute to improving the hazard mitigation and risk management planning of similar events in future.

Efficient dam break flood simulation methods for developing a preliminary evacuation plan after the Wenchuan Earthquake

Abstract. The Xiaojiaqiao barrier lake, which was the second largest barrier lake formed by the Wenchuan Earthquake had seriously threatened the lives and property of the population downstream. The lake was finally dredged successfully on 7 June 2008. Because of the limited time available to conduct an inundation potential analysis and make an evacuation plan, barrier lake information extraction and real-time dam break flood simulation should be carried out quickly, integrating remote sensing and geographic information system (GIS) techniques with hydrologic/hydraulic analysis. In this paper, a technical framework and several key techniques for this real-time preliminary evacuation planning are introduced. An object-oriented method was used to extract hydrological information on the barrier lake from unmanned aerial vehicle (UAV) remote sensing images. The real-time flood routine was calculated by using shallow-water equations, which were solved by means of a finite volume scheme on multiblock structured grids. The results of the hydraulic computations are visualized and analyzed in a 3-D geographic information system for inundation potential analysis, and an emergency response plan is made. The results show that if either a full-break or a half-break situation had occurred for the Chapinghe barrier lake on 19 May 2008, then the Xiaoba Town region and the Sangzao Town region would have been affected, but the downstream towns would have been less influenced. Preliminary evacuation plans under different dam break situations can be effectively made using these methods.

Rehabilitation of a debris-flow prone mountain stream in southwestern China – Strategies, effects and implications

Rehabilitation of Shengou Creek, a small, steep mountain stream in southwestern China that is prone to debris flows, started more than 30. years ago through an integrated program of engineering applications (check dams and guiding dikes), biological measures (reforestation), and social measures (reducing human disturbance). Small and medium-sized check dams and guiding dikes were constructed on key upper and middle sections of the creek to stabilize hillslopes and channel bed. Meanwhile, Leucaena leucocephala, a drought-tolerant, fast-growing, and highly adaptive plant species, was introduced to promote vegetation recovery in the watershed. The collective community structure of tree, shrub, and herb assemblages in the artificial L. leucocephala forest, which developed after 7. years, enhanced soil structure and drastically reduced soil erosion on hillslopes. Cultivation of steep land was strictly controlled in the basin, and some inhabitants were encouraged to move from upstream areas to downstream towns to reduce disturbance. These integrated measures reduced sediment supply from both hillslopes and upstream channels, preventing sediment-related hazards. The development of natural streambed resistance structures (mainly step-pool systems) and luxuriant riparian vegetation aided channel stability, diversity of stream habitat, and ecological maintenance in the creek. These findings are compared with Jiangjia and Xiaobaini Ravines, two adjacent non-rehabilitated debris-flow streams which have climate and geomorphologic conditions similar to Shengou Creek. Habitat diversity indices, taxa richness, biodiversity, and bio-community indices are much higher in Shengou Creek relative to Jiangjia and Xiaobaini Ravines, attesting to the effectiveness of rehabilitation measures. 2011 Elsevier B.V.

Use of a hydrodynamic model to forecast floods of Kalu River in Sri Lanka

AbstractKalu River, the third longest river in Sri Lanka, discharges the largest amount of water into the ocean while causing floods along its route from the most upstream major town, Ratnapura, to the most downstream town, Kalutara. It has become necessary to either totally control these floods or instruct people to adjust their activities to the rhythm of the river and prepared them to live with floods with minimum damages. This paper presents a model developed to determine water levels along the river from Ratnapura to 79 km downstream Kalutara using the HEC‐RAS hydrodynamic model. The model was calibrated and verified for both steady and unsteady flow conditions. It provides water levels and inundation areas along the river for different discharges. A set of tables, which could be used by people with less technical knowledge, were prepared to predict flood levels at downstream locations based on observed water levels at upstream locations.

One- and Two-Dimensional Coupled Hydrodynamics Model for Dam Break Flow

1-D and 2-D mathematical models for dam break flow were established and verified with the measured data in laboratory. The 1-D and 2-D models were then coupled, and used to simulate the dam break flow from the reservoir tail to the dam site, the propagation of dam break waves in the downstream channel, and the submergence of dam break flow in the downstream town with the hydrodynamics method. As a numerical example, the presented model was employed to simulate dam break flow of a hydropower station under construction. In simulation, different dam-break durations, upstream flows and water levels in front of dam were considered, and these influencing factors of dam break flow were analyzed, which could be referenced in planning and designing hydropower stations.

Mathematical optimisation of drainage and economic land use for target water and salt yields*

Land managers in upper catchments are being asked to make expensive changes in land use, such as by planting trees, to attain environmental service targets, including reduced salt loads in rivers, to meet needs of downstream towns, farms and natural habitats. End‐of‐valley targets for salt loads have sometimes been set without a quantitative model of cause and effect regarding impacts on water yields, economic efficiency or distribution of costs and benefits among stakeholders. This paper presents a method for calculating a ‘menu’ of technically feasible options for changes from current to future mean water yields and salt loads from upstream catchments having local groundwater flow systems, and the land‐use changes to attain each of these options at minimum cost. It sets the economic stage for upstream landholders to negotiate with downstream parties future water‐yield and salt‐load targets, on the basis of what it will cost to supply these ecosystem services.

High-altitude glacial lake hazard assessment and mitigation: a Himalayan perspective

Abstract Glaciers throughout the Himalayas have been receding rapidly over the last few years. In the areas vacated by the ice snouts lakes have formed behind large moraine dams. As with their Andean counterparts, these lakes can pose significant threats to downstream towns, roads and power schemes. For a developing country like Nepal, for example, the prospective loss of economically vital infrastructure and food-producing land can be devastating, in addition to the human misery. The Tsho Rolpa glacier lake has formed by the retreat of Trakarding Glacier at the head of the Rolwaling Valley in northern Nepal. The lake is now over 3 km long and contains an estimated volume of 80 x 10 6 m 3 . The snout of Trakarding Glacier terminates in the lake which, at its western end, is dammed by an ice-cored moraine. It is feared that the lake could be the source of a major glacier lake outburst flood (known locally as a ‘GLOF’) which could destroy the nearby villages of Na and Beding, as well as trekking routes, vital bridges, etc. , further downstream. At Khimti, 80 km downstream from Tsho Rolpa, a new hydroelectric power scheme is being built and could potentially be at risk. It is known that the local glaciers in the Rolwaling Valley have been the source of many GLOFs in historical as well as contemporary times. The last GLOF destroyed several houses and valuable farm land at Beding in July 1991. Following an initial GLOF-risk assessment in September 1994, a trial siphon was installed at Tsho Rolpa in May 1995. This is the first time that such measures have been used in the Nepalese Himalayas. Since then further remediation measures have been proposed and are awaiting funding. The nature of the glacial hazards at Tsho Rolpa and the engineering mitigation measures being proposed are described. Furthermore, it is concluded that an integrated national policy on glacial hazards should be developed urgently.