A Numerical Modeling Study Of The Water Quality Of The Falls Lake Reservoir

As an important type of water body, reservoirs are closely related to the daily freshwater usage of human beings.The Falls Lake reservoir is located in the state of North Carolina, United States of America. It is the main freshwater source for the City of Raleigh, state capital of North Carolina, and surrounding areas.Compared with the historical records,the water management by the local government has improved the water quality of the Falls Lake considerably. However, recent field monitoring data shows that the chlorophyll-a contents in the Falls Lake reservoir frequently exceeds the water quality standard.To prevent possible eutrophication phenomenon and protect the source and quality of water, it is necessary to understand the status of eutrophication and the trend of its variation.Further analysis will be conducted on eutrophication's influence if one is found,both spatially and temporally, on reservoir water quality. This will provide scientific basis for future water quality management.This Ph.D.dissertation is based on a comprehensive analysis of the historical hydrological,water quality and meteorological measurements of the Falls Lake reservoir and the surrounding areas. A three-dimensional eutrophication-hydrodynamic numercial model,HEM-3D,is applied to simulate the water quality of the reservoir in the year 2005,2006 and 2007,with a specific ecological dynamics interest in the matter and energy cycle within the reservoir. This study specifically focuses on the following aspects:1)the characteristics and differences of reservoir water quality in certain dry and wet years as well as the related mechanism;2) the influences on water quality from seasonal thermocline in downstream deep water areas;3)the effect of total suspended solids concentration on water quality; 4) the influence on water quality of point-source as well as non-point-source pollution; 5)the transport of the nutrients and its relationship with water qualities;6) the influence mechanism of every factor on water qualities.The analysis of historical observation over the past 25 years shows that:1)Because of the low water level in dry years,the concentration of nutrients is high and the water quality is low in dry years.The upstream water quality is poorer than downstream and has excessive Chl-a concentration during summers and falls. Due to effective control of the pollution in the reservoir in recent years,the water quality in Falls Lake has been significantly improved.However, the presence of the eutrophication was apparent in the upstream water.2) Statistical analysis shows that inorganic N/P ratio is very close to the Redfield Ratio.Thus the primary production in the water body is N or P-limited rather than solely P-limited, which often occurs in lakes or reservoirs.Considering the limitation in the spacial and temporal distribution of the observation data, a numerical model is applied to simulate water quality of the reservoir. It could compensate the observation measurement loss and provides us insights into the eutrophication in the reservoir. In addtion, it is helpful in understanding and analysis of some relative scientific problems and describing the cycle of energy and matter in reservoir essentially. The simulation results indicated that:1)The influenced area and existence form of the nutrients provided by runoff and bottom dissolution are significantly different.As the runoff input is from upstream,the nutrients proportion appears to be N-limited.Since the residence time of the pollution in the reservoir is relatively long, the influence from the runoff input is mainly in the upstream area. On the contrary, nutrients in downstream area are originated from bottom dissolution.The nutrient proportion is P-limited.The illumination has apparent influence on the primary production, especially in the downstream deep water.2) Seasonal thermocline mainly present in deep water. Its existence and disappearance have significant influence on the trend of the downstream water quality. From late spring to early fall,the presence of thermocline makes the nutrients dissolved from sediment accumulate in the bottom.When seasonal thermocline disappears, strong vertical mixing transports these nutrients to upper layer and provides the nutrients for the algae bloom in late fall and next spring.3)Water exchange of long time scale has positive effect on reservoir water quality improvement. However, when massive water exchange takes place in a short time period and completely mixes the nutrients in the reservoir, the water quality will be degraded instead of increased.4) The experimental modeling of the materials transport processes indicated that the mean age, residence time and turnover time age negatively correlated with water exchange in the reservoir, especially for the residence time and turnover time.In calculation of the mean age using exchange quantity, the water storage must be taken into account.For the same water exchange rate, lower water storage accelerates the transport of the materials in the reservoir. A semi-empirical formula of the mean age, (?)τDis(τ)/Vol(τ)dτ=c,is established.After determining a constant which depends on the features of the reservoir, the pollution transport could be simply and effectively estimated with the runoff and the water storage.5)The re-suspension of the sediment in the reservoir is mainly influenced by the bottom shear stress generated by wind wave.Generally, the suspended particles inhibit primary production.However, when the concentration of nutrients is relatively low and there is a period of massive water exchange, the sediment could enter the upper layer through re-suspension.Its absorption increased the nutrients supplied from the bottom and thus stimulates the primary production.