Hydrologists should be able to help you  with a model.

Mirza Arshad Ali Beg

Antonios,

I recommend that you research the concept of the littoral grain size cutoff diameter. Sediment contributed to the littoral system that is smaller than this threshold, even if defined as sand by the Wentworth scale, may not remain on the beach in any significant quantity. The diameter has been shown to vary between about 100 to 200 microns according to the the particular location in question. It can be defined precisely by sampling the grain size distribution at any particular site of interest to determine the smallest grain sizes that remain stable on the beach and/or nearshore profile. It may vary with wave energy with smaller grain sizes being stable on low energy estuarine beaches and only larger sands being stable on ocean beaches.

I recommend the following key paper:

P.W. Limber, K.B. Patsch, G.B. Griggs 2008. Coastal sediment budgets and the littoral cutoff diameter: a grain size threshold for quantifying active sediment inputs

Journal of Coastal Research, 24, pp. 122–133.

Obviously, your question involves estimating the proportion of fluvial input that can contribute to local beaches so you would need to estimate the grain size distribution of the river inputs. Not easy without an extensive sampling programme, but as a rule of thumb it is possible to calculate the suspended load and the bedload inputs separately and then assume that all bedload inputs are sufficiently coarse to contribute to the beach.

One further complication is that lower floodplains and estuaries of rivers may intercept the sediment load before it reaches the coast - especially if there is significant evidence of sand deposition within the estuary. Thus, fluvial input estimates based on gauge data from well upstream of the mouth can be misleading.

Some of these issues are discussed in a previous paper which I also recommend:

BOATENG, I. BRAY, M. and HOOKE, J. 2011. Estimating the fluvial sediment input to the coastal sediment budget: A case study of Ghana, Geomorphology, 138, 100-11. doi:10.1016/j.geomorph.2011.08.028

I wish you success with your research.

Dear Dr Bray,

Thank you very much for your thorough answer. So what you suggest is firstly to exclude sediment material with grain size lower than the littoral grain size cutoff diameter. And then to separate  suspended load from bedload inputs?  If I find a given  sediment materials distribution corresponding to my case-study (the Dyfi estuary in Mid Wales) at the river-mouth all I have to do is to separate suspended load from bedload inputs? 

I don't think your question would have a usable answer in that along with the problem of grain size distribution for the load carried to the ocean mentioned in the other posts, there is also the variable of wave energy.  High wave energy carries much greater load away compared to low wave energy situations.

Antonios,

My recommendation would be to estimate the quantity of both (i) bedload and (ii) suspended load input from the river to the estuary and then to review the literature relating to the Dyfi estuary in order to make judgements about the relative fates of each of these components.

I understand that the estuary is broadly ebb dominant meaning that the ebb current is typically of higher velocity and shorter duration than the flood. It suggests that bedload sediments should become flushed out of the estuary whereas suspended sediments from Cardigan Bay would tend to be drawn into the estuary. This of course suggests that bedload inputs from the river could potentially leave the estuary and supply the adjoining coastline whereas suspended sediments from the river are more likely to be retained within the estuary. These basic relationships can be applied to make a theoretical case for input of coastal bedload sediments from the river Dyfi. Ebb dominance is discussed by the most interesting paper by Brown and Davis (2007) listed by the publisher at:  http://www.crcnetbase.com/doi/abs/10.1201/NOE0415453639-c99

available for download at: http://pages.bangor.ac.uk/~oss062/brofet.pdf

In reality, the situation is more complex than my simplification above. The R. Dyfi clearly has a significant bedload transport capacity - probably mainly when the river is in flood. It obviously transports gravels and deposits them in point bars and riffles along its lower channel. However, can the gravel enter the mid and outer estuary and maybe exit the inlet channel? or is it all deposited in the lower floodplain and upper estuary? Secondly, the Dyfi estuary is very significantly infilled with sandy and silty sediments, but do these signify input from the river? or have they originated from the open coast? Sands can enter the Dyfi inlet and outer estuary driven by storm waves from Cardigan Bay operating in conjunction with a flood tide. During such conditions sands may be pushed further into the estuary beyond the points from which ebb currents may flush them seaward. Hence an ebb dominant estuary may still exhibit a flood tidal delta. Note that key inputs to the estuary from the river and from Cardigan Bay may be controlled by river floods and storms and are likely to be intermittent. To try to make some of these finer distinctions an estuary hydrodynamic model such as Telemac is typically used e.g. Brown and Davies (2007). For a full overview and detailed explanations of all types of estuary model please see the link below.

The discussion above should provide some options for either a basic analysis or a more complex one depending on your requirements. My "feel" for the Dyfi is that gravel cannot pass through the estuary from the river, but sand input from the river can exit the inlet channel and contribute to beaches in Cardigan Bay. A high proportion of suspended sediments from the river are probably trapped within the estuary, however suspended sediments can also be drawn into the estuary from Cardigan Bay, especially during storms when sand would be in suspension. I am unsure of the balance between sand entering and exiting the estuary so its hard to say whether it could be a net source or sink of sediment with respect to neighbouring beaches. I would urge you to be a little skeptical of my ideas, to test them and make your own assessments based on the evidence that you assemble.

Best wishes

http://www.estuary-guide.net/guide/analysis_and_modelling/

Dear Dr Bray,

Thank you very much about your suggestions and comments regarding Dyfi's estuary.