Hyporheic zone performances : influential factors and applications in hyporheic restoration

Abstract

Hyporheic zone (HZ), the interstitial region beneath or alongside a streambed with active groundwater and surface water mixing, is important to stream ecosystem. To better understand the uncertainties of the HZ performance and its important applications in hyporheic restoration, this thesis investigated the major influential factors such as streambed hydraulic conductivity (K) and a storm event, developed numerical models for biogeochemical reaction simulation in the HZ, and proposed a method to optimize the design of in-stream structure for hyporheic restoration. The spatial uncertainties of the HZ performance induced by the K were analyzed. Residence time (RT), which represents the duration of a water molecule or a solute remaining within the HZ, was employed as the indicator. From the simulation results with different K distributions at the streambed, both heterogeneity and anisotropy could shorten the mean and median RTs while increasing the range of the RTs. Moreover, K fields arranged in a more orderly pattern had longer RTs than those with random K distributions. These results could facilitate the selectiondesign of the K values and distributions to achieve the desired RT during hyporheic restoration. The spatial and temporal uncertainties of the HZ performance induced by a storm event were analyzed. The impacts of rainfall intensity (RI) and rainfall duration (RD) were investigated, and two indicators, i.e., the influential time (IT) and the influential depth (ID) were proposed. The results revealed that the RI and RD both display logarithmic relationships with the IT and ID, but only between certain thresholds of the RI and RD. In addition, the IT had a linear negative correlation with the groundwater response while the ID was not affected. These results could facilitate the future predictions of the IT and ID during transient events. More accurate numerical models for simulating biogeochemical reactions (i.e., denitrification and nitrification) were developed to evaluate the nitrogen removal in the HZ. Three models were proposed, and the results showed that the third model using pre-determined reaction zones for the biogeochemical reactions and coupling genetic programming had the best performance. Moreover, case studies revealed that the HZ would be more effective in removing nitrogen when the water quality was at a certain level. These results could potentially benefit the HZ management in face of the continual degradation caused by human activities. Finally, a method for optimizing the design of in-stream structures (e.g., weir) was proposed. The objective function for calculating the optimal height of the weir was formulated as the product of variables related to both nitrogen removal amount and ratio in the HZ. The results demonstrated the method’s generally good performance and could enhance the understanding towards the optimal design of in-stream structures for hyporheic restoration. This thesis presents the state of knowledge on the HZ performance including influential factors and applications in hyporheic restoration. It provides improved numerical models for biogeochemical reaction simulation, and offers new insights into the methods for the optimal design of in-stream structure. The present study not only has theoretical significance but also has practical engineering implications.