4.2 Implications of flood waves under CUFV condition on water environment
Flood waves enhance the interaction between the surface water and groundwater, and greatly promote the migration and transformation of pollutants in the river ecosystem (e.g. Boutt and Fleming, 2009; Smith et al., 2009). The nitrogen cycle provides a good example: when the river water level rises, it carries abundant oxygen, ammonia, nitrogen, and nitrate into the hyporheic zone, accompanied by the processes of nitrification and aerobic respiration; as the water level falls, groundwater flows back into the river, providing an anaerobic environment for denitrification. During the whole nitrogen cycle, ammonia and nitrate in the river system are converted into N2 and released, thus realizing nitrogen load reduction.
From the hydrodynamic point of view, the main factors affecting nitrogen removal within the hyporheic zone are Q max andRT (e.g. Gu et al., 2012; Naranjo et al., 2015). The former controls the total amount of nitrogen carried into the aquifer through water infiltration, while the latter controls the duration of reaction time of all solutes. Obviously, the greater theQ max and RT are, the better the nitrogen removal effect is. However, according to this study,Q max and RT could not be increased simultaneously under CUFV condition. In such a case, when the flood wave is designed to be flat and wide (i.e. larger T /A ), nitrogen removal is limited by Q max, and vice versa, by RT .
Therefore, the capacity for nitrogen removal of the hyporheic zone first increases and then decreases with an increase of T /A under CUFV condition. There must be an optimal T /A (i.e. inflexion point) that can maximize the nitrogen removal capacity. The optimal T /A may be related to the depth and width of the actual river, which will be studied in our future work. In addition, under CUFV condition, although increasing N reducesQ max and RT , it would improve the nitrogen removal capacity of the hyporheic zone. This was because an increase inN could enhance the chemical reaction by promoting the physical mixing of the surface water and groundwater solutes. Such physical process may play a dominant role, because Q maxand RT decreased slightly with an increase in N , and deserve further investigation.
The study of HE under CUFV condition could be of great scientific and practical significance to the operation of upstream reservoirs, that is, under the premise of constant reservoir capacity, downstream pollutant degradation within the hyporheic zone can be maximized by regulating the reservoir discharge mode.
Conclusions
A systematic study was conducted on HEs driven by flood waves in a riparian zone mainly through numerical simulations. The main conclusions were summarized as follows:
(1) At any time within the duration of the flood wave, the stream-aquifer q is affected by the superposition of water levelh (sine-type) and its change rate v (cosine-type), and is proportional to the polynomial of them:q \(\mathrm{\propto}\)(ω \(\bullet\)h +v ) The weight of influence is determined by \(\omega\) (i.e. 2pi/T), the angular frequency of a flood wave.
(2) The variations of different flood wave parameters essentially reflect the different performances of water level change and its change rate. The maximum aquifer storage and residence time are mainly controlled by the integral of the flood wave over time, which increases as the wave becomes high, wide, round, and less skewed.
(3) Under CUFV condition, the larger T /A , the smaller the maximum aquifer storage (dominated by A ), but the longer the residence time (dominated by T ). With the increase in N , water exchanges more frequently and some water returns to the stream early, leading to the decrease in maximum aquifer storage and residence time.
The study provided important understandings to the HE mechanism driven by surface water level fluctuations. In particular, the investigation of HE under CUFV condition offered a significant guidance to enhance pollutant degradation in hyporheic zones downstream of reservoirs.