MESOSCALE PERSPECTIVE OF ENVIRONMENTAL FLOW ASSESSMENT IN THE MIDDLE REACH OF SEKAMPUNG RIVER, INDONESIA

The wetted perimeter method (WPM) is used in hydrology and hydraulics to calculate instream flows. The WPM requires few data. It requires only the values of the wetted perimeter, flow and water level, which can be obtained from the hydrologic stations of the river in question. In addition, the WPM is not limited by the impacts of human activities on the river runoff. Therefore, this method is generally suitable for the current conditions in China. However, the process of applying the WPM involves two key aspects: how to plot the curve describing the relationship between the wetted perimeter and the discharge and how to confirm the breakpoint of the wetted perimeter-discharge curve. The traditional method is to calculate the curvature or the slope of the wetted perimeter-discharge curve to obtain the minimum flow. According to this method, the minimum flow corresponds to the point of maximum curvature or to the point at which the slope of the curve is equal to 1. The wetted perimeter-discharge curve of a natural river is only part of the complete curve. Thus, the instream flow calculated by the traditional method is the minimum or maximum discharge. The new criterion for defining the breakpoint of the wetted perimeter-discharge curve is that the slope at the breakpoint is a relative maximum, the second-largest slope. The discharges at the breakpoints corresponded to the minimum flow levels required to maintain the ecological function of the river. The minimum instream flow requirements (MIFRs) of four typical reaches, Zhuba, Daofu, Ganzi and Zumuzu hydrological stations on the West Course of the First Stage Project of the South-North Water Transfer Project (WCFSPSNWTP), are calculated using an improved wetted perimeter method (IWPM). The results show that the MIFRs of Zhuba, Daofu, Ganzi and Zumuzu are approximately 9.06–14.5 m3 s−1, 20.7–43.5 m3 s−1, 38.8–77.2 m3 s−1 and 40.4–59.5 m3 s−1, corresponding to 11.7%–33.9%, 14.2%–37.6%, 12.4%–28.4% and 17.5%–30.2%, respectively of the annual average flow (AAF). These MIFRs can maintain good ecological function in a river according to the criterion furnished by the Tennant method.

The literature has explored various methods for assessing minimum environmental flow. Implementing holistic approaches proves to be prohibitively expensive and impractical for many small and medium projects. Hence, desktop and cost-effective methods are commonly employed without an integrated decision-making system to justify the assessed values. This study introduces a systematic decision-making framework aimed at selecting the most suitable method for assessing the actual needs of river habitats. Employing a fuzzy technique known as the Order Preference Similarity to the Ideal Solution (FTOPSIS), the study considers factors such as physical, thermal, and dissolved oxygen habitat suitability, maximum habitat area, and water demand loss function to determine the most appropriate method among established ones, including the Tennant method, flow duration curve analysis method, wetted perimeter method, and physical habitat simulation method. The results prioritize physical habitat simulation, wetted perimeter by slope method, and flow indices of 70%, 75%, and 80% by flow duration curve analysis method as the optimal approaches for assessing minimum environmental flow. This proposed decision-making system offers a viable platform to explore the applicability of existing cost-effective methods for assessing minimum environmental flow. It also serves as an effective mechanism for reducing negotiations among stakeholders by comprehensively considering all relevant aspects in the environmental management of river ecosystem requirements.

The trade-offs between human water needs and environmental considerations have always been challenging for water resources management and governance. Multi-purpose reservoirs present a particularly challenging decision context where multi-sectoral water and energy demands have to be balanced, while also considering the instream water requirements downstream. A systematic framework to evaluate the trade-offs between demand satisfaction, hydropower production, and satisfaction of minimum environmental flows (MEFs) would help reservoir operators better understand the consequences of various operational choices. In this study, we designed two formulations of a multi-purpose reservoir operation problem; one that prioritized MEF (PF_MEF) releases over demand satisfaction and another that did not (PF_nMEF). We identified Pareto approximate strategies to operate the reservoir for each formulation using the Borg multi-objective evolutionary algorithm considering multiple objectives related to water demand satisfaction, hydropower production, prevention of flood exceedance thresholds, and satisfaction of MEF. We applied the framework to the Nagarjuna Sagar (NS) reservoir in southern India. Reservoir operation strategies were modeled using direct policy search (DPS), where piecewise nonlinear Gaussian radial basis functions (RBFs) are used to condition decisions, and reservoir releases for hydropower in this case, on reservoir storage states. Results show that the Pareto approximate strategies resulting from optimizing for PF_MEF and PF_nMEF attain MEF - MEF in ranges 86-98% and 56- 79%, respectively. However, the ensuing compromises in water demand satisfaction and hydropower production are not considerably higher. Mean volumetric demand deficits and mean annual hydropower production ranged from 99.9 -818.1 Mm3 (48.13-818.8 Mm3) and 3252-3900 Gwh (3394- 3910 Gwh) for PF_MEF (PF_nMEF). Notably, we were able to identify strategies from PF_MEF that attained low values of volumetric demand deficits and high values of hydropower production, indicating that prioritizing MEFs may not necessarily yield compromises for human-related objectives in this case.

Analysis of inflections or breakpoints apparent in relationships between measures of wetted perimeter and discharge can be used to assist in the determination of minimum environmental flows for perennial rivers. This paper suggests refinements and provides an example application of the wetted perimeter method for the determination of cease‐to‐pump limits in a perennial, unregulated gravel‐bed river subject to increasing levels of surface water extraction. HEC‐GeoRAS modelling outputs of riffle wetted area are used to illustrate that the magnitude of the discharge selected to represent 100% habitat availability is of crucial importance to the breakpoint method. Because of the dependence of the technique on this assumption, we suggest that it is prudent to use an upper and lower limiting discharge based on an assessment of the degree of flow variability to develop a flow range around the zone of diminishing return in the wetted perimeter to discharge relationship. For rivers exhibiting a low degree of flow variability, the mean and median daily flows are likely to provide appropriate discharges for representation of 100% habitat availability. For perennial rivers with a higher degree of flow variability and considerable differences between the mean and median daily flows we suggest use of the 50th and 80th flow duration percentiles. Wetted perimeter breakpoint results are also influenced by the degree to which areas of non‐riffle habitat are included in the analysis. Inclusion of excessive pool areas can lead to significant reductions in resultant recommendations for cease‐to‐pump limits or minimum environmental flows. Integration of hydraulic model outputs with GIS for wetted perimeter analysis of riffles provides a useful, rapid, field‐based approach that can assist with determination of cease‐to‐pump limits or minimum environmental flows in gravel‐bed rivers. However, care is needed in its application and interpretation as the technique is prone to numerous subjective choices that have a substantial influence on results. Copyright © 2004 John Wiley & Sons, Ltd.

Adaptive (re)operations facilitate environmental flow maintenance downstream of multi-purpose reservoirs

To maintain river flows necessary to meet social and ecological objectives, instream environmental flows are frequently used as a strategy. The capability of three alternative historical flow approaches to protect against low flows is shown in this study using gage stations in the Shatt Al-Hillah River in Iraq. The extension of the Shatt al-Hillah River is the focus of this research discussion on environmental flow assessment. The available data on discharge in this research were adopted for ten years from 2012-2021. Different flow methods were adopted to establish a minimum environmental flow in the Shatt Al-Hillah River. Three hydrological-based approaches: Tennant, modified Tennant, and low-flow metrics like 7Q10, were used to compare the results of the methods with the provision of a minimal environmental flow. The Tennant method relies on 30 % of the annual discharge rate as the minimum environmental instream flow. The modified Tennant method is based on 30% of the monthly average discharge rate as a minimum environmental instream flow. The 7Q10 method is based on the lowest daily discharge rate in 7 consecutive days for ten years. The results showed that the minimum instream environmental flow given by the Tennant method is 42.26 m3/s while the lowest value of the minimum instream environmental flow obtained from modified Tennant was 37.2 m3/s in January, and the largest value was 47.22 m3/s in August. The one obtained by the 7Q10 method was 50 m3/s in this research.

Hydrodynamic modelling is a powerful tool to gain understanding of river conditions. However, as widely known, models vary in terms of how they respond to changes and uncertainty in their input parameters. A hydrodynamic river model (MIKE HYDRO River) was developed and calibrated for a flood-prone tidal river located in South East Queensland, Australia. The model was calibrated using Manning's roughness coefficient for the normal dry and flood periods. The model performance was assessed by comparing observed and simulated water level, and estimating performance indices. Results indicated a satisfactory agreement between the observed and simulated results. The hydrodynamic modelling results revealed that the calibrated Manning's roughness coefficient ranged between 0.011 and 0.013. The impacts of tidal variation at the river mouth and the river discharge from upstream are the major driving force for the hydrodynamic process. To investigate the impacts of the boundary conditions, a new sensitivity analysis approach, based on adding stochastic terms (random noise) to the time series of boundary conditions, was conducted. The main purpose of such new sensitivity analysis was to impose changes in magnitude and time of boundary conditions randomly, which is more similar to the real and natural water level variations compared to impose constant changes of water level. In this new approach, the possible number of variations in simulated results was separately evaluated for both downstream and upstream boundaries under 5%, 10%, and 15% perturbation. The sensitivity analysis results revealed that in the river under study, the middle parts of the river were shown to be more sensitive to downstream boundary condition as maximum water level variations can reach 8%, 12%, and 15% under 5%, 10%, and 15% changes in the downstream boundary, respectively. The outcomes of the present paper will benefit future modelling efforts through provision of a robust tool to enable prediction of water levels at ungauged points of the river under various scenarios of flooding and climate change for the purpose of city planning and decision-making.

The Incomati Estuary, with a length of about 30 km, a width of about 1.5 km, and an average depth of 10 m, is located in the Southeastern part of Africa, north of Maputo Bay, between the Latitudes 24o 00’S and 26o 30’S and Longitudes 29°30′ E and 33° 15′ E. The Incomati River basin as a whole has an area of 46,426 km2, of which 28,745 km2 is located in South Africa, 2,786 km2 in Swaziland and 14,856 km2 in Mozambique. There are several dams upstream, mainly used to supply irrigation water. The river discharge is 200–400 m3 s−1, corresponding to a river discharge of about 700–1000 Mm3, with peak and lower flows during the wet season (November to March) and dry season or winter (June-August), respectively. The morphology of the estuary is characterized by meanders, islands and sand banks, fringed by mangroves. The tides are semidiurnal, with amplitudes varying from 0.5 to 3.5 m during the spring and neap tides, respectively. The predominant winds are trade winds, with average speed of 4 m s−1. The estuary plays an important ecological role and sustains important socioeconomic activities. It is a nursery ground for important fisheries, a tourism attraction, and hosts significant agriculture and livestock activities. The sustainability of the natural resources and the activities in the estuary is threatened by the effects of the dams. The three basin countries agreed in 1991, during the “Piggs Peak Agreement,” to the maintenance of a minimum environmental flow of 2 m3 s−1, which has been violated several times by South Africa. The present work investigates how the hydrodynamics of the estuary, which controls most of the ecological processes, is influenced by the river discharges. Tides, currents and water masses in the estuary are described, based on historical hydrodynamic data. A simple 1-D, depth and width integrated, tidal hydrodynamic model forced by the observed tides at the mouth, was used to reproduce the hydrodynamics of the estuary. In addition, an analytical solution of the advection–diffusion of salt is applied to reproduce the salt intrusion. The model was then used to determine the extent of salt intrusion as a function of the river discharge, and so used to estimate the minimum environmental flow required for a healthy estuarine ecosystem in Incomati. The results indicated the estuary shifts, from a stratified condition in the wet season to vertically-mixed in the dry season. The tidal wave takes about 30 minutes to propagate from the mouth to the head of the estuary. The current varied between 0 and 1.2 m s−1, mostly driven by tides. The density driven circulation is weak, with a density gradient of about 0.10 kg m−3 per km, and a density-driven velocity of about 0.04 m s−1. The hydrodynamic model applied explains 87% of the current observed in the estuary, meaning the currents were mostly tidal driven, with the remaining 13% attributed to other factors such as density gradient and winds. The minimum recommended river flow required to prevent the intrusion of salt water to a distance 20 km upstream is 20 m3 s−1, against the 2 m3 s−1 value in the Piggs Peak Agreement. The results of this study may contribute to a better understanding of the dynamics of the ecosystems in the Incomati Estuary, and in the management of the river for the health of the downstream ecosystems.

The compliance of environmental flows following the recommendations of the Water Framework Directive (WFD) is a crucial aspect in the management of reservoir water allocation. This issue is especially challenging in catchments with long and recurrent drought periods, such as those affected by the Mediterranean climate. There are still some gaps to be addressed by managers and water authorities. For instance, the correct definition of minimum environmental flow (MEF) to be fulfilled. These MEF values, which are set in the River Basin Management Plan (RBMP), commonly vary slightly throughout the year. Sometimes, this assumption is not accomplished because these values are above those that would have been observed under natural conditions. Therefore, in these regions it is decisive to adapt the MEF requirements to the local hydrometeorological seasonally.      This work proposes the combination of historical streamflow and precipitation data to assess the viability of using seasonal hydrometeorological patterns in the definition of the MEF rates. For that, several monitored water bodies were analysed. Streamflow information from gauging stations and precipitation data were selected in the two main river basin districts in the South of Spain: the Guadalquivir River Basin, and the Andalusian Mediterranean Watersheds River Basin. First, for each case, we reviewed the compliance of the MEF rates analysing when these threshold values were or not achieved at the monthly scale.  In addition, we analysed the seasonal variability in terms of both precipitation and streamflow and compared these outcomes with the seasonal variations of the MEF. This analysis was carried out during 10 hydrological years, from September 2010 to August 2020.According to our results, most of the locations were below 28% of accomplishment during the summer months. This percentage increased when the period analysed was the winter. However, this percentage was below 50% in some locations. On the contrary, only in those locations which are fed by mountain catchments, the accomplishment was fulfilled in 73-100% during the whole year. The joint seasonal analysis of precipitation and streamflow highlights that the MEF established in the RBMP were oversized in most of the cases, overlooking the precipitation patterns.This work showed that a revision of the MEF values set by the RBMP is required. That is especially significant in locations where seasonal variations of the MEF are null or imperceptible. Our outcomes will help to set the basis for the design of a new methodology when defining MEF. Hence, this new approach will consider not only water quantity but also hydrometeorological seasonal variability as the main step to truly address water management from the perspective of the WFD.   Acknowledgments: This work has been funded by the project TED2021-130937A-I00, ENFLOW-MED “Incorporating climate variability and water quality aspects in the implementation of environmental flows in Mediterranean catchments” with the economic collaboration of MCIN/AEI/10.13039/501100011033 and European Union “NextGenerationEU”/Plan de Recuperación, Transformación y Resiliencia.

Environmental flows, sometimes referred to as ecological or instream flow requirements, or compensation flows are defined as the amount and quality of water necessary to preserve ecological functions and values in watercourses. One set of challenges in water resources planning today is to define environment flow requirements (quantity, timing, and quality on a seasonal basis), integrate them in water allocation policies and achieve consensus on this, and translate and incorporate those requirements into the operating rules for flow regulating structures, such as dams, reservoirs, and diversion schemes. In this article, basic concepts of environmental flow are introduced. A minimum environmental flow will have significant benefits toward maintaining a macroinvertebrate population by providing suitable habitat refugee between discharge peaks. A minimum environmental flow, however, would not be expected to fully mitigate the effects of increased hydropeaking. Establishing a minimum environmental flow is partly aimed at mitigating the effects of variable flows. Policies on environmental flows are perhaps the most important direct connection between water resource and wetland resource management. A number of new planning approaches are emerging to quantify and apply environmental flow standards, which have been grouped in three main categories: hydrologic or historical flow record methods, hydraulic and habitat modeling, and holistic methods. The first category consists of approaches where historical flow records are used to develop environmental flow, or instream flow recommendations, based on subjective assessments of ecosystem needs. Hydraulic and habitat rating methods use relationships between habitat condition and discharge to develop instream flow recommendations. The final and more complex method is the holistic approach to the assessment of instream flows and water quality aspects, in which all components or attributes of the ecosystem and their interrelationships are addressed.

In recent years the construction of small hydropower plants has increased in the mountainous part of the Carpathian region. The modern requirements for the ecological flow of rivers are significantly increased. There are no regulations for determining the minimum ecological flow for small mountain rivers in Ukraine. The magnitude of the minimum environmental flow significantly affects the assessment of the hydropower potential of the river. Ecological flow calculations are complicated by the lack of hydrological stations in the upper part of mountain rivers. The purpose of the work is development of a simplified engineering method for determining the minimum ecological flow under limited hydrological data for small mountain river of Ukraine. It was carried out the minimum ecological flow calculation for two sections of the Irshava River in the Tisza basin to estimate the proposed method. For comparison, it was used method of the minimum ecological acceptable flow, wetted perimeter method, BFM, IHA, Tennant. Based on the analysis of European and Ukrainian methods, the simplified engineering method for determining the minimum ecological flow is proposed: for the mountain river it is the mean annual flow of 90% probability.

Minimum environmental flows in rivers provide a certain level of protection for the aquatic environment. The relationship between wetted perimeter and discharge can be used to define the minimum environmental flows by the slope method (SM), or curvature method (CM), especially for cases with poor understanding of the aquatic ecosystem. SM and CM derived inconsistent values of minimum environmental flows. It was not clear which method better defined minimum environmental flow. Moreover, the computation and optimization procedures are both time consuming and error‐prone, especially for complicated wetted perimeter–discharge relationships. In this study, flow regulation for rivers was regarded as a multiple criteria decision‐making problem, with the objectives of minimum river discharge and maximum wetted perimeter. Ideal point methods (IPMs) with the scaling coefficient r = 1 (IPM1) and r = 2 (IPM2) were used to solve this model to determine optimal environmental flows. IPM was simple in computation, especially when the wetted perimeter–discharge relationship was given as scattered data pairs. Meanwhile, it was applicable to a wider range of wetted perimeter–discharge relationship than SM and CM. Environmental flows estimated by IMP1 are the same as that by SM. The analytical results for environmental flows using SM, CM, IPM1 and IPM2 were compared for wetted perimeter–discharge relationship expressed as power or logarithmic function. It showed that CM is not a good method to define environmental flows. SM with unity slope and IMP1 were recommended. CM, SM and IPM were examined for the determination of environmental flows in a river in North Xinjiang, China. Environmental flows for different transects of the studying river reach were estimated to be 21% of the mean annual flow by SM or IPM1, which provided the satisfactory wetted perimeter, water depth and average velocity for aquatic organisms. Copyright © 2007 John Wiley & Sons, Ltd.

Environmental issue has been considered more significant in many aspects of engineering decision-making process particularly in river management. There is an increasing effort to conserve functioning of rivers for human use as well as nature, therefore environmental flow assessment has been widely developed. This paper discusses on environmental flow assessment of the Sekampung River, particularly on its middle reach. A new analytical approach based on water-sediment equations was introduced in order to determine a minimum environmental flow at the certain cross section of a river. The result of the new method was then compared with a minimum environmental flow provided by using two hydrological based methods, namely, Tennant and Flow Duration Curve Analysis (FDCA) method. The result shows that the concerned discharge provided by the water-sediment method (3.5 m3/s) is the smallest compare with a minimum environmental flow that is provided by both Tennant (5.7 m3/s) and FDCA method (4.5 m3/s). It is promising that the water-sediment method can be used as a simple approach on preliminary state of environmental flow assessment. The method involves not only water discharge but also its related sediment flow of the river in order to mitigate further ecological and morphological risks.

Environmental flow is vital for maintaining river ecosystem health and ensuring the normal growth of aquatic organisms. The wetted perimeter method is indeed very useful in the assessment of environmental flow due to consideration of stream forms and minimum flow for aquatic life habitat. In this study, a river with obvious seasonality and external water diversion was selected as the typical research object; taking Jingle, Lancun, Fenhe Reservoir, and Yitang hydrological sections as control sections, we improved the existing wetted perimeter method in three aspects: (1) We improved the selection of hydrological data series. The selected hydrological data series should be of a certain length and can well reflect the hydrological changes of wet, normal, and dry years. (2) Different from the traditional wetted perimeter method, which only gives one environmental flow value, the improved method calculates the environmental flow month by month. (3) The improved wetted perimeter method establishes the relationship between native fish survival and environmental flow. Results indicated that the improved wetted perimeter took the survival of the main fishes into consideration, the ratio of the calculated results by the slope method to the multi year average flow was greater than 10%, which can ensure the fishes' habitat is not being destroyed, and the results are more reasonable. Furthermore, the monthly environmental flow processes obtained were better than the annual unified environmental flow value determined by the existing method and are consistent with the natural hydrological situation and water diversion situation of the river. This study shows that the improved wetted perimeter method is feasible for research of river environmental flow with strong seasonal and large variation of annual flow.

Assessing minimum environmental flows in nonpermanent rivers: The choice of thresholds