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Share This Paper. Figures and Tables from this paper. Figures and Tables. NarasimhaPrasad System simulation for reservoir operation in the Kuttiyadi schemes of Kerala, Proc. Shahul Hameed A , Dr. First Author Publication Year Dr. Girish Gopinath , Dr. Harikumar P S , Dr. George Abe , Dr. Dinesan V P , Dr. Principal Investigator Budget Duration Dr. Principal Investigator Date of Completion Dr. Anitha A B. C Dinil Sony ,.

Abdulla P K. Anitha A B ,. C Dinil Sony. Three methods will be discussed here. The first two are adaptations of HA to accommodate nonstationarity, while the third is a form of CWT analysis with improved frequency resolution. Just as with a i and b i in a normal HA, the coefficients in these polynomials are determined by linear regression here a robust regression. The net result is that there are six coefficients to determine per constituent, though unpublished work has also included atmospheric factors, leading to 8—10 coefficients per constituent.

It has, however, some disadvantages.

For example, the exponents in f a and f b may need to be tuned iteratively, and the generality of the polynomials suggested by Kukulka and Jay [ a , b ] remains unknown. Also, if too many or too few constituents are used, or if LOR is too small, then unrealistic results may be obtained. Care is required in its use. Guo et al. This approach is facilitated by the fact that the flow in the Yangtze River varies annually only by a factor of 4 and usually varies slowly relative to the tidal month, so this binning of data did not lead to excessive fragmentation of the record.

It does not, however, provide any insight into hysteresis effects differences in tidal behavior during periods of rising and falling flow [ Sassi and Hoitink , ] , and the formal statistical properties of the analysis have not been determined. Despite these possible issues, this technique is relatively robust and should be useful in systems that, like the Yangtze, do not exhibit rapid or excessive flow variability. None of the analysis methods described above allows analysis of the variations of multiple tidal constituents within each tidal species on subtidal tidal timescales.

Yet the importance of triad interactions e. We suggest that progress can be made in the area of nonstationary constituent resolution by combining the linearity of wavelet filters and analyses of each tidal frequency on multiple timescales with the traditional tidal analysis idea of inference, i.

Unpublished work shows that once the major frequencies are separated on the fast scale, inference of frequency ratios from the slow scale can be used to resolve closely spaced constituents e. Indeed, progress in understanding river tides will require further elaboration of tidal analysis methods, and this is an area of ongoing research, as suggested by the recent progress cited above. Several broad insights are evident from the above discussion of tidal methods.

Tidal analysis approaches are diverse, and the complexity of nonstationary tides suggests the need for multiple analysis tools. Tidal methodology is evolving quickly, and the rate of innovation has increased dramatically in the last two decades. The distinguishing factor in recent innovations has been the recognition that this apparent noise is often evidence of nonstationarity due to either physical processes [e. In the former case, the nonstationary of tidal records is useful for dynamical analyses, though it complicates tidal forecasting.

Clearly, there is room for improvement in forecasting nonstationary tides, and this could be valuable for ecosystem management, habitat restoration, and navigational safety.

A common thread is that constituent significance estimates used by all usual analysis approaches are less than adequate for short filter windows and strongly nonstationary data. Another issue is the neglect of early tidal observations from circa to , which often recorded only the times and heights of the two or four tidal extrema each tidal day [ Talke and Jay , ]. Finally, we should not rule out the potential application of fundamentally different methods from other disciplines.

For example, medical image analysts have the opposite problem from tidalists; medical images are so dense that there is a need to subsample optimally. One striking finding is that the Nyquist criterion does not directly apply to randomly sampled data, if the underlying phenomena were actually resolved in the original data set. Attempts have also been made to adapt the Huang et al. Numerical models are an alternative means of analysis, offering insight in the physical mechanisms underlying the interaction between river discharge and the tide from the governing shallow water equations [ Dronkers , ]: The interaction of river discharge and the tide occurs as a result of three nonlinear terms in equations, which account for spatial acceleration SA , friction FR , and a gradient in the discharge DG.

Each of these terms contributes to the generation of shallow water tides, represented by angular frequencies that are sums and differences of the tides directly generated by astronomical forcing. Parker [ b ] offers a systematic analysis of the various nonlinear interactions among tidal constituents. In tidal rivers, the generation of M sf and M 4 is particularly important, which can be illustrated by focusing on the Amazon. The M 4 constituent is generated as a result of spatial acceleration and discharge gradients at the continental shelf and peaks at the river mouth.

Beyond that point, SA and DG contributions have vanished and the effect of friction gradually reduces. At the amplitude peak of M sf and farther inland, friction is the only source of nonlinearity causing the fortnightly variation. It allows investigation of subtidal flow processes. Buschman et al. This work provides field confirmation that in the upstream reaches of an estuary, the subtidal momentum equation essentially represents a balance between the surface level gradient and friction terms. The resulting subtidal momentum balance can be employed to further investigate the main controls on mean elevation, by starting to represent the velocity signal based on the four main species: Consequently, tidal asymmetry can amplify or partially cancel the surface level gradient.

### Addressing Water Challenges

The relative contributions of the three terms is highly dynamic, depending on the discharge. Discharge gauging stations at the end of the catchment, near the river mouth, generally observe water levels or flow velocity modulated by the tide. Such modulations occur at all tidal frequencies, and removing the tidal influence is a nontrivial task. The hydrologic problem is different. Identifying the mean tidal properties from a HA is insufficient, as the river discharge is typically nonstationary. An accurate version of the nonstationary tide must be removed, especially if tidal fluctuations are of comparable magnitude to fluctuations due to river flow.

Several studies have attempted to do this. A rating curve approach was then used to estimate discharges. Because tidal properties were not specifically assessed before and after detiding, it is unclear how successful the detiding exercise was. Given the distance of the station from the mouth, it is to be expected that subtidal variations in water level would be prominent, and that these would affect discharge estimations if such fluctuations were not removed.

The latter is a method frequently used for analysis of nonstationary processes involving fewer frequencies than tidal records; it has been used for detiding of groundwater records [ Heathcote et al. Because the analysis was implemented in the statistical language R , it is also unclear how many of the estimates would have been deemed significant in a conventional robust HA, and how realistic the tidal estimates were for low and high flows. Also, it is unclear whether subtidal variations were properly removed, or even if these oscillations were important, because the station analyzed appears to be fairly close to the ocean.

Interestingly, tidal frequencies can appear in water level records that do not stem from oceanic tidal forcing. Briciu [ ] used a continuous wavelet transform to demonstrate that tidal frequencies can enter hydrologic records in certain geologic circumstances, through the influence of groundwater.

These influences would need to be removed by detiding, if hydrologic records from such locations were to be used to estimate river flow. It has long been known that quadratic interactions between river flow and tides affect tidal propagation in rivers [e. However, Jay and Kukulka [ ] appear to have been the first to use fluctuations in tidal admittance at fluvial stations as a method to estimate flow, defined as the ratio of the amplitude in a tidal river, and the ocean forcing tidal amplitude.

This method is the inverse of the model used by Kukulka and Jay [ a ], which predicted riverine tidal properties from river flow and coastal tidal amplitudes.

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They obtained an approximate theoretical relationship that predicts discharge from the tidal admittance. Moftakhari et al. These flow and sediment input estimates were used to understand secular changes in the systems, and changes in seasonal hydrology, both important properties in understanding ecosystem trajectory. Probably, the largest weakness of this approach is that the Kukulka and Jay [ a ] model on which it is based neglects several factors e. Thus, nonphysical parameters in the model must be calibrated for each system in which it is used.

The advantage of this approach is that a direct analytical result is achieved, so that there is no need for regression analysis to determine parameters. Still, it is necessary to fit the friction parameters to the estuary in the forward model for the tide, which is inverted to estimate discharge. Thus, parameter fitting cannot be entirely avoided.

Also, the theoretical analysis used is applicable primarily to tidal channels with semidiurnal D 2 tides. In summary, the idea of using the fluvial modulation of tides for hydrologic hindcasting is new and promising. The range of applicability of existing methods is unknown, not all of the mechanisms underlying the utility of the method have been explored, and no existing approach is fully satisfactory.

Tidal influences on the river discharge modulate the rates of sediment supply to the coastal ocean, and are a key factor controlling delta formation. The tidal motion counteracts the reduction of sediment transport capacity of a river toward the coast, shifting the deposition focus in seaward direction. Estuaries have often been studied with a single estuarine channel in mind, one which is connected to a river at its upstream end.

Recently, Canestrelli et al. In many lowland areas, this view is an oversimplification of the actual anabranching planform topology, which features multiple, interconnected channels. Typically, the river discharge, sediments, nutrients, dissolved oxygen, and pollutants in a lowland river are divided between a number of branches, termed distributary channels or distributaries [e. Recent research has focused on the role of tides in the division of flow and sediment over these distributaries [ Buschman et al. Tidal propagation in a network of channels is intrinsically more complex than in a single channel.

Based on results from a linear model introduced by Souza and Hill [ ], Hill and Souza [ ] questioned whether there are preferred tidally induced Lagrangian transport pathways through tidal channel networks. Addressing this question, Buschman et al. A constant river discharge was imposed at the upstream boundary, and a tidally varying water level at the two seaward boundaries. Length, width, and roughness were varied for the two distributary channels. The model results showed that the tide can have a significant influence on river discharge division over delta distributaries.

In a multichannel system, the Stokes transport in one particular channel is not necessarily equal to the Eulerian mean return transport. This implies the mass transport generated by a progressive, or nearly progressive, tidal wave can be returned to sea by a Eulerian mean flow in a different channel. Sassi et al. The rise of the mean surface level in one channel can increase the hydraulic gradient in the neighboring channel, influencing the discharge division ratio.

This mechanism favors the allocation of river discharge to smaller channels that would receive a comparatively small portion of the river discharge in absence of the tide.

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## Hydraulic Engineering and River Basin Development

An unambiguous relation was established between mean elevation, i. On the contrary, when the flow was bidirectional, increasing fluvial discharge decreased tidal velocity amplitudes down to a minimum value, reached at the limit between bidirectional and unidirectional flow.

Leonardi et al. Individual branches can become resonant, or near resonant [ Alebregtse et al. Tides are one of the main factors controlling Holocene delta evolution [ Galloway , ; Orton and Reading , ]. These can be retrieved from a sedimentary record using harmonic analysis in space rather than time [ Mazumder and Arima , ]. Gugliotta et al. They show the rhythmites can be used to record variations in river discharge rather than tides, because the magnitude of river floods is stochastic and does not vary regularly, whereas tides are cyclical and relatively predictable.

The recent developments in methods to process tidal signals influenced by the river discharge, as described in previous sections, may merit the reinterpretation of sedimentary successions. The thickness of the lamina in the records may have a direct relation to tidal admittance, which relates to the river discharge. The planform of a river delta channel network tends to be asymmetrical, and bifurcating channels commonly split their discharges unequally [ Edmonds and Slingerland , , ; Bolla Pittaluga et al.

Based on a numerical model simulating fluid flow, waves, sediment transport and the interaction with morphological changes, Edmonds and Slingerland [ ] showed that asymmetrical configurations are morphologically more stable. Morphological stability here means that the channel morphology evolves back to the initial conditions, when perturbed by adding a mound of sediment in the middle of the channel.

Channel bifurcation is strongly linked to mouthbar development processes, recently reviewed by Fagherazzi et al. Although it is evident that tides exert an influence on these processes, the effect of tides on bifurcation geometry and stability has rarely been addressed. The interaction between river discharge and the tide, leading to differential water level setup [ Sassi et al. The findings of Leonardi et al. This limits the number of deltas that can be considered fully fluvial dominated.

Especially in branching channels, tidal surface elevation variation may poorly represent flow velocity, which has a more direct relation with sediment transport and morphology. Avulsion can be defined as the natural process by which flow diverts out of an established river channel into a new permanent course on the adjacent floodplain [ Slingerland and Smith , ].

Jerolmack and Mohrig [ ] found that in many lowland areas, avulsions have occurred at a distance from the coast that coincides with the backwater length, defined as a characteristic length over which the water level of the receiving basin has an influences on river hydraulics [e. Nittrouer et al. Chatanantavet et al. During low discharge, the flow decelerates in the backwater region, leading to deposition, which can divert the flow out of the river channel.

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The mean water level setup created by the interaction between river discharge and tides has a direct effect on mean surface level profiles, which accumulates in the upstream direction e. This can only be justified in rivers with small tidal velocity amplitudes, which are not easily estimated from surface elevation amplitudes. Given the results of Leonardi et al. The transition between land and sea can be considered among the most complex regions on Earth, because of the interaction of numerous physical, chemical, and biological processes [ Dalrymple and Choi , ; Schwartz , ]. In tidal rivers and the adjacent wetlands, fluxes of organic and inorganic particles and dissolved substances such as carbon, nitrogen, and phosphorus vary at tidal frequencies, complicating these interactions.

Downstream of the tidal freshwater zone, the effect of sea level rise is reinforced by sinking of deltas due to mineral and water extraction, reduced sediment input, and floodplain engineering [ Syvitski et al. The forcing functions of tidal rivers are thus in state of continual change. In general, tidal rivers are expected to become deeper when deltas are sinking, which amplifies the tidal motion and promotes salinity intrusion.

Consequently, the geographic location of the tidal freshwater zone shifts inland. The remainder of this section will address recent approaches to analysis of salinity intrusion and the possibilities of multiscale modeling and monitoring of tidal rivers and wetlands. Differences in the length and depth along alternative pathways from a channel junction to the sea can result in marked differences in the character of the tide in neighboring branches, which then govern the limit of salinity intrusion [ Buschman et al.

This was further illustrated by Jay et al. Regression models offer a straightforward means of monitoring the main factors controlling inundation statistics, which are key to wetland functioning. It is likely they are outperformed by artificial neural network models in terms of skill scores [e. Among the available remote sensing options [e.

Numerical models simulating hydrodynamics, sediment transport, and morphodynamics are in a phase of rapid developments toward generic tools that may address the needs in the context of tidal wetland restoration and taking measures to cope with climate change. Resolution flexibility captures the multiscale nature of the nonlinear development of a tidal wave propagating from deep water into a tidal river.

For example, the horizontal mesh resolution of the model by de Brye et al. One of the main difficulties of simulating hydrologic and geomorphologic processes in tidal wetlands is wetting and drying of cells [ D'Alpaos and Defina , ]. Flexible mesh numerical models have the promise to reproduce sediment transport in tidal rivers and the adjacent wetlands from source to sink [ Pham Van et al. Predicting the fate of tidal rivers and the adjacent wetlands in response to contemporary threats including sea level rise and sinking of deltas will likely remain a challenge for the decades to come.

## Mountain Rivers and Incised Channels - River Dynamics and Integrated River Management - page

Characteristic phenomena in tidal freshwater rivers include tidal bores, fortnightly tides, and water level setup that raises the mean surface levels above the backwater and drawdown profiles that would occur in absence of tides. Empirical models are available to predict tidal river surface levels from river discharge and conditions at sea, using a number of gauge stations along the tidal river as input for parameter estimation.

Historical estimates of river discharge can be inferred from the ratio between tidal amplitudes at sea and in the tidal river, i. Tidal admittance may also be reflected in depositional patterns termed tidal rhythmites, which offers the possibility of retrieving discharge estimates from geological records. In branching tidal channels, the tidal motion can induce a residual flow in response to geometrical differences between branches and bed roughness variation. At river bifurcations subject to tides, differential water level setup from nonlinear interaction between river discharge and the tides tends to cancel the inequality in the division of river discharge over distributary channels.

Tidal velocity amplitudes in distributary channels cannot readily be estimated from tidal elevation amplitudes, because of reflection of tidal energy and the possibility of near resonance behavior. The setup of water levels in a tidal river accumulates in the upstream direction up to the point of tidal extinction and may influence a river's propensity to avulse. Dissimilarities between the geometry of branching tidal channels may cause differences in tidal behavior between the channels. Tidal phase differences between interconnected channels and flow curvature can cause a complex flow structure at channel junctions and marked differences in salinity intrusion between channels.

Multidisciplinary tidal river research addresses multiple contemporary challenges. Delta plains hosting tidal rivers represent regions where the effects of global warming on sea level and discharge are focused. Human measures to mitigate hazards of flood, drought, and loss of biodiversity add to the factors that keep tidal rivers in a state of flux. Joint efforts linking knowledge from hydrology, physical oceanography, and geology are needed to obtain a sharper view on the changes in hydrodynamic and sedimentary tidal regimes, and the associated geomorphological adaptations.

Concrete research priorities for the coming decade are threefold. For many river harbors, the records go back in time at least for a century. In particular, the development of dunes in tidal rivers where the sediment characteristics transition from sand to mud needs to be better understood. New techniques for in situ monitoring [e.

Third, research on the interaction between tidal river hydrodynamics and wetland vegetation deserves priority. Wetland restoration programs and climate change adaptation efforts in deltas can directly benefit from such analyses.