Influence of Sediment Condition on Water Quality in the Izarigawa Dam Reservoir

Influence of Sediment Condition on Water Quality in the Izarigawa Dam Reservoir Ken ichi TAKADA, Tetsuya YABIKI, Makoto NAKATSUGAWA, and Yasuhiro MURAKAMI Civil Engineering Research Institute of Hokkaido, Hiragishi 1-jo 3chome, Toyohira-ku, Sapporo, Hokkaido 062-8602, Japan Abstract: The water environment of dam reservoirs is greatly influenced by sediment inflow from upstream, weather, and hydrological conditions. The Izarigawa Dam was completed in 1980, and sedimentation has been progressing yearly. The volume of accumulated sediment was 840,000 m 3 as of 2002. A sediment shelf formed in the dam reservoir has been advancing and it became conspicuous in 1993, when musty odor occurred from the action of actinomycetes. Nearby residents complained about this musty odor. In view of the above, we compiled existing survey data on transitions of sediment accumulation and hydrological and water quantity conditions of the reservoir toward constructing a hydraulic model. Under the assumption that the inflow conditions are constant, this model was used to calculate the hydraulic conditions of water flow before sedimentation (i.e., immediately after dam completion) and after sedimentation. The results show changes in water flow and temperature, which enabled us to clarify the mechanism of musty odor generation. Toward developing countermeasures against musty odor, we will quantify its causal factors. Keywords: Sedimentation, Water quality, Musty odor, Geosmin, Actinomycetes, Hydraulic analysis for the reservoir 1. INTRODUCTION The Izarigawa Dam Reservoir, at the center of Hokkaido, Japan s northernmost major island, is used for flood control and supplies water to approximately 371,000 nearby residents (Figure 1). Dam basin area is 113.3km 2, and Total reservoir capacity is 15,300,000m 3. The Izarigawa-Dam reservoir serves as a source of local water. When the reservoir water level dropped in the summer of 1993, a musty odor occured in the supply water. This odor has returned every year since. Existing survey data on the reservoir s sediment inflow and water environment were reviewed in an attempt to clarify the factors affecting the reservoir s water environment. 2. ANALYSIS ON EXISTING SEDIMENTATION SURVEY RESULTS Figure 2. Secular change in longitudinal reservoir bed contour In the dam reservoir the sediment forms a shelf (Figure 2), with the reservoir bed being relatively shallow at the upstream and middle parts of the reservoir but rapidly deepening immediately upstream of the dam body.. Altitude(m) 180 175 170 165 160 155 150 145 Izarigawa Dam Figure 1. The position figure of Izarigawa Dam Normal water level E.L.164.30m Limited water level E.L.161.00m :2000 :1981 :1979 0 1000 2000 3000 4000 Distance from a dam(m) 1

3. CHARACTERISTICS OF SEDIMENTATION IN THE DAM RESERVOIR In 1998, the Ishikari River Development and Construction Department carried out a boring survey (Figure 3) on the reservoir s sediment 1)2). As the figure shows, the sediment that had deposited after the completion of the dam consists of grains that are much finer than those of the original riverbed (Figure 4). Over 90% of the sediment grains measure 1 mm or less in diameter. Also, grains in the lower layer are finer than those in the upper layer. Coarse-grained sediment (diameter 0.1 mm or greater) that flows into the reservoir accumulates near the top of the sediment shelf to form a top-set bed. Fine-grained sediment (diameter less than 0.1 mm) accumulates downstream from the head of the delta (i.e., the front edge of the top-set slope of the advancing sediment shelf) as wash load to form a bottom-set bed. In this sedimentation process, a fore-set bed advances on the bottom-set bed. This is why the sediment in the upper layer is coarse-grained and that in the lower layer is fine-grained 3). 4. SEDIMENT INFLOW SURVEY In 1998, a sediment inflow survey was carried out 1) in the four tributaries entering the dam reservoir (the Rarumanai, Izarigawa, Moichan and Ichankoppe rivers) to reveal sediment transport patterns in these rivers at ordinary and flood levels. Between July and September 1998, seven times surveys were carried out twice at normal water levels, three times during rain, and twice at flood levels. During this period, inflow rates ranged between 3 m 3 /s and 100 m 3 /s (Table 1). Two types of sediment inflow survey were performed: bed load survey, in which sediment flowing near the river bed was measured; and suspended load survey, in which suspended sediment flowing downstream was measured. In the bed load survey, a bed load sampler developed by the Public Works Research Institute was placed at the channel centroid using a truck crane. The sampler was left in place for a fixed period to collect samples that were then measured for (dry) weight and grain density to obtain their volumes. In the suspended load survey, 200 liters of river water was sampled from the channel centroid using a submersible pump hoisted by a truck crane. Samples were manually taken when the rivers were at the ordinary water level. The samples were brought No. Survey date Max.inflow Remarks 1 July 30,10:00-July 31,12:00 3.8 m 3 /s Normal level 2 Aug.16,18:00-Aug.17,0:00 21.7 m 3 /s Raining 3 Aug.20,18:00-Aug.21,0:00 6.8 m 3 /s Normal level 4 Aug.28,10:00-Aug.29,12:00 9.1 m 3 /s Raining 5 Sept.8,11:00-Sept.9,11:00 39.5 m 3 /s Raining 6 Sept.16,3:00-Sep.17,15:00 105.3 m 3 /s Raining 7 Sept.22,18:00-Sept.23,9:00 13.8 m 3 /s Raining 2 Percentage passing(%) Izarigawa River 100 90 80 70 60 50 40 30 20 10 0 Rarumanai River Moichan River Figure 3. Izarigawa Dam and its environs 1.5 km from the dam body Ichankoppe River 0.001 0.01 0.1 1 10 100 Grain size(mm) Figure 4. Sediment grain size distribution in the reservoir and percentage passing Table 1. Sediment inflow survey dates Boring survey spot Sediment inflow survey spot Bed depth 0.00m~0.92m Bed depth 1.00m~1.92m Bed depth 2.00m~2.904m Original riverbed

back to the laboratory and left stationary for over 24 hours to allow the suspended load to settle. After the supernatant liquid was removed, the sediment samples were measured for (dry) weight, sediment content rate, and grain density, to obtain volumes of suspended load. Discharge observations were carried out at the same time as the bed and suspended load surveys. 5. SEDIMENTATION BUDGET To estimate the total volume of sediment inflow, the results 1) of the sediment inflow survey described in Section 4 were used to develop regression equations for the relationship between measured discharge Q and bed load L b, and that between measured discharge Q and suspended load L s (Tables 2 and 3) 3). The bed load (L b1-4 (m 3 /s)) and suspended load (L s1-4 (m 3 /s)) of sediment inflow from the four rivers were estimated by substituting q f1-4 (m 3 /s) of the respective rivers into the estimation equations for these loads. Then, the estimated sediment inflows were multiplied by 3,600 to obtain hourly sediment inflows. This process was repeated for every hour of the day, for 365 days of the year, to estimate annual total sediment inflows for each year in each of the four rivers. Then, for each river, L b1-4 and L s1-4 were added to obtain the total sediment inflow into the dam reservoir. The discharges q f1-4 (m 3 /s) that were used to estimate the sediment inflows of the rivers can be described in relation to the inflow into the dam (Table 4). This was intended to allow the discharge of each river to be easily estimated based on the discharge into the reservoir. This is because a water level - discharge (H-Q) equation provides continuous discharge for the Rarumanai and Izarigawa rivers, whereas continuous water level data do not exist for the other two rivers. Figure 5 indicates that over 90% of the total sediment inflow comes from the Rarumanai and Izarigawa rivers and that suspended load accounts for over 99% of the total sediment inflow. From this, it is presumed that most of the total sediment inflow is fine suspended sediment from the Rarumanai and Izarigawa rivers. Figure 6 shows secular changes, from 1981 through 2001, in annual sedimentation at the dam and estimated annual sediment inflow. In 3 Table 2. Suspended load regression equations River Inflow rate Equation (m 3 /s) Rarumanai River Q 7.0m 3 /s Ls 1 =9.598 10-7 Q f1 4.77 Q>7.0m 3 /s Ls 1 =3.719 10-4 Q f1 1.74 Izarigawa River Q 10.0m 3 /s Ls 3 =1.823 10-5 Q f2 3.24 Q>10.0m 3 /s Ls 2 =4.278 10-4 Q f2 1.98 Ichankoppe River All discharges Ls 5 =1.320 10-4 Q f3 4.29 Moichan River All discharges Ls 6 =1.249 10-3 Q f4 4.11 L s (m 3 /s),q(m 3 /s) Table 3. Bed load regression equations (m 3 /s) River Inflow rate Equation Rarumanai River All discharges L b1 =1.701 10-7 Q 1.22 Izarigawa River All discharges L b2 =3.114 10-11 Q 4.65 Ichankoppe River All discharges L b3 =3.826 10-8 Q 1.55 Moichan River All discharges L b4 =1.315 10-8 Q 1.55 L b (m 3 /s),q(m 3 /s) Table 4. Discharge estimation equations River Equation Rarumanai River q f1 =0.45113Q in 1.07 Izarigawa River q f2 =0.29649Q in 1.03 Ichankoppe River q f3 =0.10354Q in 0.58 Moichan River q f4 =0.19086Q in 0.54 Izarigawa River A=40.6km 2 Suspended load 15,803 Bed load 4 Rarumanai River A=42.5km 2 Suspended load 32,144 Bed load 16 (m 3 /s) q f1 ~q f4 (m 3 /s),q in (m 3 /s) Total sediment inflow Suspended load 50,506 Bed load 22 Unit: m 3 /year Izarigawa Dam Moichan River A=8.3km 2 Ichankoppe River Suspended load 2,527 A=13.8km 2 Bed load 2 Suspended load 32 Bed load 0.13 Figure 5. Estimated suspended and bed loads (1998)

Sedimentation and sediment inflow(m 3 ) 400,000 300,000 200,000 100,000 0-100,000 Annual sedimentation Sediment flushed Suspended load (estimated) Bed load (estimated) 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 Figure 6. Secular changes in annual sedimentation and estimated annual sediment inflow (suspended and bed loads) some of the years covered, the annual sedimentation is zero or negative. This may be partly due to measurement error. The sedimentation and the estimated sediment inflow were on the same order of magnitude, but deviated slightly to greatly in some of the years covered. This may be partly due to void content not being taken into account as well as to possible discharge downstream. However, cross-reference with the results of the sedimentation measurement survey suggests that the general tendencies of the sedimentation budget were verified. 6. SEDIMENTATION AT THE DAM AND ITS IMPACT ON WATER QUALITY Downstream of the Izarigawa Dam, the Ishikari Tobu Regional Water Supply Authority withdraws water and supplies it to the cities of Eniwa, Kita-Hiroshima, Ebetsu and Chitose at a maximum rate of 77,100 m 3 /day. Since 1993, a musty odor in supplied water has returned every summer when the dam reservoir level dropped. The cause of this appears to be actinomycetes, according to the Dam Operation and Management Office 4), Takano et al. 5), and Kobayashi et al. 6). They suspect that actinomycetes propagate upstream of the reservoir, but die in the anaerobic zone at the bed near the dam body. The dead actinomycetes then discharge geosmin. 8/19-9/2 4/28-9/12 7/17-9/4 6/3-9/2 6/26-9/27 6/29-8/21 Figure 7. Secular change in DO and geosmin generation periods at the Izarigawa Dam reservoir (near the dam body, 1996 to 2001) The sediment accumulated in the Izarigawa Dam Reservoir has formed a shelf there. In the center and upstream side of the reservoir, the water is shallow. It is very deep near the dam. Kobayashi et al. 6) proposed measures to improve the water environment in order to prevent the musty odor from occurring. They noted that the lower layer, which tends to become anaerobic, should be maintained in an aerobic environment, that the lower-layer water with musty odor should be discharged, and that the sediment accumulated on the upstream side of the reservoir should be dredged. Figure 7 presents generation periods of DO and geosmin at the dam. The generation period of geosmin is the duration where geosmin is greater than or equal to 5 ng/l, which is the minimum limit of detection. This duration roughly agrees with the duration where DO is smaller than or equal to 3 mg/l (i.e., the lower layer is in an anaerobic state) during summer, which confirms that the present understanding of 4

musty odor generation is proper. This section examines water flow in the presence and absence of the sediment shelf in the dam reservoir and how the water flow affects the occurrence of musty odor. 6-1. Outline of a two-dimensional vertical hydraulic model The model employed in numerical analysis was based on a two-dimensional vertical reservoir model developed by the Public Works Research Institute, Ministry of Construction 7). This control-volume model differentiates the reservoir into boxes of downstream and vertical flows (Figure 8). The hydraulic conditions, suspended solid (SS) balance, and heat balance are calculated for those boxes. The basic equations of the model are as follows: (1) Continuity equation of water v + z = 0 (2) Momentum equation Horizontal direction 1 P + u + v = + D t z ρ 0 mx + D z mz z Vertical direction P = ρg or ys ρ gdy y (3) Equation for suspended solid (SS) balance C C C C C + u + ( v + v ) = D + D t 0 cx (4) Equation for water temperature balance + u + v = D Tx t + D x y Ty + y cy ρ 0 H C w Figure 8. Dimensions of the control-volume model Table 5. Notations and units of the basic equations x: axis representing the downstream direction z: axis representing the vertical direction u: flow velocity component in the x direction v: flow velocity component in the z direction ρ 0 : standard density (kg/m 3 ) P: water pressure (kg/ms 2 ) D mx : eddy viscous coefficient in the downstream direction (m 2 /s) D my : eddy viscous coefficient in the vertical direction (m 2 /s) ρ: flowing water density (kg/m 3 ) g: gravitational acceleration (m 2 /s) v 0 : average sedimentation rate of suspended particulate (m/s) D cx : diffusion coefficient of SS in the downstream direction (m 2 /s) D cy : diffusion coefficient of SS in the vertical direction (m 2 /s) T: water temperature ( ) C w : specific heat of water (J/kg ) H: quantity of heat per unit volume and unit time (5) Kinematic conditions of the free water surface h h vs = + us t (6) Equation of water density (ρ) ρ = f ( C, T ) Table 5 gives explanations on notations and their units. 5

6-2. Application of the model to the Izarigawa Dam Reservoir To confirm the applicability of the model, we performed calculations for the ten days from July 1, 2000. The length of each mesh in the downstream direction was 100 m, and that in the water depth (vertical) direction was 0.5 m (Figure 9). Measured values were assigned to the width of the reservoir. For calculation, we adopted the initial water level of the reservoir measured by the Izarigawa Dam Operation and Management Office. The water temperature and SS were measured at the dam in the vertical direction at noon on July 3. The measured values were assumed to be constant in the horizontal direction. The inflow and discharge volumes measured at the Izarigawa Dam Operation and Management Office were adopted. The volumes of inflow water from the four inflow rivers were estimated from Table 4. Table 6 shows estimation equations for the water temperature of each river, which were formulated from survey data on the relationship between the air temperature and water temperature of each river between 1993 and 1999. The SS concentration of the water from each river was calculated from observed discharge data. These data were collected between July 30 and December 3, 1998, and on days with flood in 2003 (August 9, September 13, and October 22). The data on the days with flood were collected only for the Izarigawa and Rarumanai rivers. Table 7 presents estimation equations for SS load of the water from the four inflow rivers. The weather data that were needed for heat balance calculations were air temperature, wind velocity, humidity, solar radiation, and sunshine duration. While the sunshine duration data were collected at the nearby Tomakomai Weather Station, the other data were collected at the Izarigawa Dam Operation and Management Office. The eddy viscosity coefficient in the horizontal direction was given by Richardson s Four-Thirds Law. That in the vertical direction can be calculated using a function of the local Richardson number, which is expressed by the following equation: ( bri) c D mz = aexp + Where a: 1.0 x 10-4 b: 0.5 c: 1.0 x 10-5 Ri: Richardson number Altitude(m) Figure 9. Cross-sectional profile of the Izarigawa Dam Reservoir with meshes 6 Longitudinal distance (m) Table 6. Estimation equations for temperatures of the water from inflow rivers Equation for water River temperature estimation Rarumanai River T W 1 = 0.5498 T A + 3. 2422 Izarigawa River T W 2 = 0.5008 T A + 3. 3679 Ichankoppe River T W 3 = 0.4474 T A + 3. 8670 Moichan River T W 4 = 0.4485 T A + 3. 9508 T A : Air temperature x=100m z=0.5m Table 7. Estimation equations for SS loads from inflow rivers River Equation for load estimation Rarumanai River Izarigawa River Ichankoppe River Moichan River L: Load (g/s), Q: discharge (m 3 /s) L 1 = 0. 5745 Q L 2 = 0. 4435 Q L 3 = 42. 148 Q L 4 = 6. 5658 Q 2.662 3.1371 3.5808 3.4824

6-3. Numerical model for water flow and temperature analysis Figure 10 shows calculated and measured values of water temperature at the dam. The calculated results closely reproduce water temperature with respect to elevation above sea level, or in the vertical direction. We assumed that the recreated water flow reproduces the density current. Also, we discuss how changes in the longitudinal cross-sectional profile of the reservoir, for which sedimentation is responsible, affect water flow and temperature. In addition, how changes in water flow and temperature relate to the occurrence of musty odor is discussed. The recent profile (July 2000) was compared with one produced using mesh data compiled from land survey data collected in 1981, when a shelf had not yet formed at the upstream side of the reservoir. The initial conditions of the 1981 profile were the same as those of the recent profile. Elevation(m) 162.0 160.0 158.0 156.0 154.0 152.0 150.0 148.0 146.0 At the Izarigawa Dam, July 10, 2000 Water temperature( ) 5.0 10.0 15.0 20.0 25.0 Measured Calculated Figure 10. Comparison of calculated values of water temperature and measurements taken at the dam 6-4. Flow regime difference and musty odor occurrence Figure 11 shows the flow regimes of the longitudinal cross-sectional profiles of the reservoir in 1981 and 2000. The water flow in the lower layer is less in the 2000 profile. Figure 12 shows the water temperature in these two profiles. The water temperature in the vertical direction ranges more widely in the 2000 profile. Formation of a shelf at the upstream side induced proliferation of actinomycetes and stagnated water flow in the lower layer at the downstream side, as demonstrated by the calculation results. The anaerobic condition was prolonged, and dead actinomycetes caused the production of geosmin. Therefore, formation of a shelf is assumed to be a cause of musty odor. Figure 11. Longitudinal cross-sectional profiles of the Izarigawa Dam Reservoir in 1981 and 2000 for flow regimes (left: 1981 profile; right: 2000 profile;) Figure 12. Longitudinal cross-sectional profiles of the Izarigawa Dam Reservoir in 1981 and 2000 for water temperature (left: 1981 profile; right: 2000 profile;) 7. CONCLUSION In this research, we studied yearly changes in inflow sediment and water environment in the Izarigawa Dam basin. A two-dimensional vertical model that simulates changes in water flow and 7

temperature was constructed and applied to the Izarigawa Dam Reservoir for analysis. Such analysis has indicated that changes in the reservoir bottom profile promote the death of actinomycetes, which creates conditions optimum for the production of geosmin in high concentrations. We will study hydrological conditions (e.g., precipitation and water inflow volume) and land use in the basin upstream of the Izarigawa Dam Reservoir, quantify conditions promoting actinomycetes proliferation and death, and examine other causes of musty odor and countermeasures to them. 8. REFERENCES 1 Hokkaido Development Bureau Ishikarigawa Development and Construction Department (1998): Report on Examination of Measures against Sedimentation in the Izarigawa Dam (in Japanese). 2 Hokkaido Development Bureau Ishikarigawa Development and Construction Department (1998): Report on Survey of the Present Condition of the Izarigawa Dam Reservoir (in Japanese). 3 Sediment and Turbid Water Meeting, New Technology and Energy Subcommittee, Committee of Civil Engineering for Energy Equipment, Japan Society of Civil Engineers (2001): Issues of Sediment in Dam Reservoirs and Turbid Water and Required Efforts. 4 Hokkaido Development Bureau Ishikarigawa Development and Construction Department (1999): Report on Survey of the Water Environment in the Izarigawa Dam Reservoir (in Japanese). 5 Takano, K., Ichikawa, H., Sato, S., Itoh, Y. (1994): Characteristics of Actinomyctes Isolated from Sediment and Surface Water in Dammed up Lake Izarigawa (in Japanese). 6 Kobayashi M., Kano A., Tachibana H., Masuzuka Y., Inazawa Y. (2002): Production of Odorous Substances in the Izarigawa Dam Reservoir (in Japanese). 7 Dam Hydraulic Engineering Research Team, Public Works Research Institute, Ministry of Construction (1987): Numerical Model for Analysis of Cold Turbid Water and Eutrophication (Part 2), p. 443, Document of Public Works Research Institute. 8 Matsuo N., Yamada M., Munemiya I. (1996): Relationship between Freshwater Red Tide and Water Flow Characteristics Upstream from Dam Reservoirs, Proceedings of International Conference on Coastal Engineering, Vol. 40, pp. 575-58. 8