Estimate of time required for environmentally friendly flushing in Dez dam reservoir

Sediment deposition in reservoirs causes loss of capacity, increased flood risks, degradation of water quality, boosted difficulty in reservoir operation and maintenance, and consequent increase in their associated cost. Besides this, sediment storage can have a significant impact on the ecosystem downstream of large river systems.

The substantial environmental and economic costs of restoring storage capacity by building a new dam are prompting a paradigm shift towards managing existing projects as renewable resources. Potential alternatives can be sub-divided into four general concepts, these include watershed rehabilitation, sediment routeing and bypass, sediment removal and flushing, and compensating for sediment accumulation in a reservoir. Systematic and thorough consideration of technical feasibility, environmental concerns, and economic factors should be made to extend the useful life of reservoirs. The cost and applicability of each strategy will vary from one site to another.

Reducing a sediment yield with a watershed management programme based on environmental concerns is the best alternative for decreasing the rate of reservoir sedimentation. However, use of only erosion control cannot achieve the sediment balance required to stabilise reservoir storage capacity and achieve its sustainable use. Sediment routeing partially preserves the natural sediment-transport characteristics of the river, whereas flushing usually changes these characteristics dramatically. A major disadvantage of sediment routeing is that there is a significant amount of water released during flood events to transport sediments. Sediment routeing is most applicable to small hydrological reservoirs where the water discharged by large sediment-transporting floods exceeds reservoir capacity. This makes water available for sediment release without infringing on beneficial uses. The bypass of flood flow and sediment from entering the reservoir requires certain topographic and flow conditions, and this method is unsuitable for removal of sediment.

Flushing is one of the most economical methods for recovering lost storage without incurring the cost of dredging. Hydraulic flushing can be an effective mechanism for removing sediments, emptying the reservoir through low-level outlets, and allowing natural flows to scour out deposits. On the other hand, flushing also releases large volumes of sediment downstream creating potentially serious problems. Scouring of polluted sediments from the reservoir threatens the downstream water quality and ecology (Sloff 1997), and results in release of a high sediment concentration, which may have a great impact on the downstream biota (Morris & Fan 1997). This led to the development of the ‘environmentally friendly flushing’ technique, detailed in particular, by Fruchard (2008). In the present study which was carried out downstream of Dez dam reservoir, based on the environmentally friendly flushing technique, appropriate hydraulic conditions to decrease the negative impacts of Dez dam flushing operations downstream of the dam have been systematically examined.

This study was carried out at the Dez reservoir which is a large hydroelectric dam located in the south of Iran that was completed in 1963 by an Italian consortium (Figure 1). At the time of construction, the Dez dam was Iran's biggest development project. Dez is a 203 m high double curvature arch dam, and the crest of Dez dam is 352 m above sea level. The original reservoir volume was 3,315 million m³, and the volume of the arrival sediment was estimated at 840 million m³ for a 50-year period. The minimum and maximum water levels of the reservoir operation are 300 and 352 m from sea level, respectively. Although the project has been well-preserved, it is now more than 40 years old and reaching its midlife period. The useful life of Dez reservoir is threatened by a sediment delta, which is approaching the dam's intake tunnels.

The hydrographic working in 2002 showed that sedimentation reduced useful storage of the reservoir of the Dez dam from 3,315.6 × 10⁶ to 2,700 × 10⁶ m³ (19% reduction). The difference between the level of the inlet of the turbine and the bed surface of deposited sediment is 14 m and this difference is decreasing at a rate of 2 m/year. Therefore, sediment management in the Dez reservoir is essential and of considerable importance.

For sediment management in Dez dam reservoir over the last decade, a flushing operation has been conducted to remove the sediment that accumulated immediately upstream of the irrigation outlets, without considering any negative environmental impact downstream. Predictions indicate that 1–1.5 million m³ of fine-grained sediment in the Dez dam reservoir will be discharged into the downstream river annually, in the years to come by performing flushing operations through the dam's irrigation outlets (Samadi-Boroujeni & Galay 2005).

However, based on technical and executive requirements and economic factors, flushing operations are the best alternative for sediment management; the lack of consideration of environmental issues has led to considerable problems downstream. Another issue that must be considered is the major water use area downstream of Dezful re-regulating dam. The re-regulating dam, with a height of 20 m, is located in the vicinity of Dezful city and 31 km downstream of Dez dam. Since the main function of the re-regulating dam is to regularly release water from Dez dam hydropower plant into the downstream river, the useful storage volume is very important for this dam. In recent years, surveys show that 40% of the Dezful re-regulating dam reservoir capacity has already been filled with sediment (Sadeghi 2002). Figure 2 shows the major water use areas (near Dezful re-regulating dam) and intakes, which may be affected by sediment flushing. These include the following:

(1) Domestic and industrial water supply: The communities along the Dez River obtain water from wells or directly from the river for drinking and domestic use, while at Dezful, the river water is filtered and piped to residences and other facilities. High suspended solids will affect the quality of domestic water supplies reducing its use for drinking, washing, and other domestic uses. Where water is filtered, the high suspended solids are likely to increase the costs associated with filtration.

(2) Recreation: A number of locations along the Dez River are used for recreational use, including swimming. These include (1) Dolat Park (upstream of the regulating dam), (2) Ali Kalleh Resort (near the regulating dam), (3) Gasr-e-Ronash (between second and third bridges, east side), and (4) Riverside Park (between second and third bridges, west side). During flushing, high suspended solids makes swimming undesirable and rapidly increasing water levels during flushing constitute a public safety hazard for swimmers and bathers at these parks.

(3) Fish and Fisheries: The Dez River provides a fish habitat and supports commercial (market), subsistence, and recreational fisheries. A list of economically important fish species found along the Dez River downstream of Dez dam are listed in Table 1. Local fishermen indicate that high suspended sediment during flood seasons reduces a catch substantially, because fish abandon their usual locations and cannot be found, and also that the likelihood of damaging nets is higher during high turbidity events. This is consistent with the responses from fishermen the day following flushing in that the fish were not biting or that only small fish were being caught in the nets.

In addition, high suspended solids or turbidity has a significant impact on light penetration through the water column. This reduced light at depth consequently has an impact on aquatic vegetation, including phytoplankton, periphyton (attached algae), and rooted aquatic plants. Consequently, photosynthesis and algal growth will be reduced during and following flushing of the Dez reservoir. Since flushing occurs over a relatively short period and since algal growth can be relatively rapid, the effects on aquatic vegetation are likely minor and of short duration.

(4) Irrigation: Most of the agriculture along the lower portion of the Dez River is supported by irrigation. There are three major pumping stations which supply the main irrigation canals. In addition, there are a number of small pumps operated by individual farmers or a group of individual farmers for watering their fields. Since there are several large and small irrigation pumping operations along the river, the effects of high suspended solids may be significant. In addition, pump maintenance is required more frequently due to the high suspended load.

(5) Livestock watering: Based on information received from the Ministry of Agriculture, there are 240 cattle ranches near the Dez River. All of these raise water buffalos and use the Dez River for watering the buffalo. Although there are high suspended solids in the river, it is not a problem for bathing livestock, but the buffalos cannot drink if turbidity is extremely high.

(6) Industrial water use: There are several industries located along the Dez River or canal system that draw process water from the river or irrigation canals. These industries use a combination of the following processes to treat their water: (1) presedimentation, (2) coagulation and flocculation, (3) clarification (sand filtration), (4) softening, and (5) disinfection. During the 1990s these industries experienced problems with their coagulation and flocculation units and clarifier filters due to high suspended solids from flooding and Dez reservoir flushing.

Following the over-accumulation of sediments behind the Dez dam body, flushing operations were performed for the first time in 1994, through opening of the dam irrigation outlets. Flushing of sediment has been conducted over the last decade to augment irrigation flows as well as to remove the sediment that accumulated immediately upstream of the irrigation outlets. However, these operations were subsequently repeated annually. Accordingly, the Khuzestan Water and Power Authority schedule was provided for the regular flushing operations; three to five flushing operations for each year are completely rehabilitated.

The flushing of the sediment on 17 June 2003 caused a wedge to be head cut upstream from the dam, having a length of 400 m, a width of 90 m, and a maximum depth of 26 m at the dam face (Figure 3). This resulted in approximately 470,000 m³ of fine sediment being flushed in about 4 h. In this paper, attention is paid to the characteristic patterns of water flow and sediment concentration, water quality parameters, sediment quality parameters, and the influence on fish and macro invertebrates.

Figure 4 showed that the day following flushing, turbidity, and total suspended solids (TSS) concentrations near the regulating dam remained high (turbidity ranged from 28–34 NTUs and TSS ranged from 300–360 mg/L) relative to concentrations prior to flushing but diminished by an order of magnitude from those observed shortly after flushing.

The Canadian Council of Ministers of the Environment (CCME 1995) provided guidelines for TSS for the protection of aquatic life. Turbidity and TSS during and 1 day after flushing far exceeded the Canadian guidelines. Consequently, if the flushing of the Dez reservoir were considered an industrial discharge, the effluent would not meet the local standard during flushing.

Table 2 lists the concentrations of dissolved oxygen (DO), biological oxygen demand (BOD), chemical oxygen demand (COD), ammonia, nitrate, nitrite, total phosphorus, and phosphate. Reduction of oxygen levels downstream is due to the release of oxygen-depleted hypolimnetic water, the oxygen demand exerted by organic sediment, and increased turbidity, which shades primary producers in the river (Morris & Fan 1997). Francis-Floyd (2011) noted oxygen depletion refers to low levels of DO and may result in fish mortality, and a concentration of 5 mg/L DO is recommended for optimum fish health.

At the dam site, this method is a modification of the partial draw-down flushing technique. In addition to a bottom outlet, a mid-depth outlet is required. During flushing water is released from both the mid-depth outlet and the bottom outlet simultaneously (Figure 5). The hypolimnic water from the bottom outlet, which is high in suspended sediment mixes with the water from the mid-depth outlet, which is low in suspended sediment. Thus, the flushing flow created during the event will have much lower concentrations of suspended sediment than would otherwise be the case (Fruchard 2008).

Actually, this technique also improves downstream oxygen concentration and minimises temperature changes and pollution: water released from near the bottom of a stratified reservoir is usually cold, oxygen-depleted, and high in hydrogen sulphide and other pollutants, whereas water released from a mid-depth is more oxygenated, warmer, and less concentrated in heavy metals and pollutants (Davis 1975; McCartney et al. 2001).

In 1997 flushing of the Dez reservoir resulted in a substantial fish kill from which, according to the Dezful Environment Department, the fish stocks have yet to fully recover (Dezab Consulting Engineers in Association with ACTRES International 2004). During this episode, the spillway was not employed to dilute the suspended solids.

During flushing on 17 June 2003, the maximum TSS concentration measured downstream of the regulating dam was 7,860 mg/L utilising a duration of exposure of 4 h (Figure 4).

For determining a time required for environmentally friendly flushing in Dez dam reservoir, based on studies by MacDonald & Newcombe (1993) and Newcombe (1986, 1997), the graphs illustrating the relationship between concentration, duration, scale of the severity (SEV), average historical outflow from dam, and volume of sediment flushing are provided in Figures 6,⁷⁸–9.

Since no field information was available, the dry density of clay and silt immediately after deposition was assumed to be 0.5 and 1.0 t/m³, respectively. The ultimate density of clay and silt was assumed to be 1.25 and 1.36 t/m³, respectively.

The categories most representative of the fish community of the Dez River were adult freshwater non-salmonids and eggs and larvae of salmonids and non-salmonids. The severity indexes for this event would not result in mortality, as was consistent with our observations, but would result in sub-lethal effects, which would affect angling and the condition of fish. Based on fish species downstream of Dez dam and water quality analyses before and after flushing on 17 June 2003, the SEV was 7.65 for adult non-salmonids and 8.11 for eggs and larvae. These severity indexes would result in moderate habitat degradation for adults and major physiological stress for eggs and larvae. As most fish in the Dez River spawn in March or April, the most conservative of the eggs and larvae or the adult graphs should be employed from March through June (to include the larval period) and the adult graphs throughout the rest of the year.

Estimate of days of flushing to pass a wedge of 470,000 m³ for various concentrations and SEV = 7.65 and SEV = 8.11 are shown in Tables 3 and 4. Based on results, the acceptable time required for a single environmentally friendly flushing in Dez dam reservoir for a concentration of 8,000 mg/L downstream, for SEV = 8.11 is 4.7 days and for SEV = 7.65 is 2.7 days. In addition, the number of flushes to pass a wedge with volume = 470,000 m³ for SEV = 8.11 is 5 and for SEV = 7.65 is 8.

In this paper, the time required for environmentally friendly flushing in Dez dam reservoir, reviewed for a various scenarios, was estimated. Results showed that the acceptable concentrations of sediment downstream of Dez dam reservoir during flushing operation should be less than 8 g/L during 3.5 h on average. In addition, the results showed that wedge or turbidity flushing needs to be confined to relatively short-time intervals if the SEV of ill-effect for fish is to be maintained. Rapid assessment techniques for suspended solids concentrations should be developed so that the duration of flushing can be managed to prevent the severity index from exceeding 8 and avoid long-term impacts on the fish community. Finally, based on observation, timely warnings to industry, municipalities, irrigation pumping stations, and aquaculture facilities that flushing will occur are recommended to allow the parties to prepare for the event and potentially reduce their costs.