dsear
commented
6 years ago OK some more thoughts on the Confinment Project based on my Skype with Joe and subsequent musings....
TASKS for Geoprocessing (some of which already exist)...
1) Split Channel Centrelines into 500m segments (sensu O'Brien et al in review) and at tributary junctions. 2) Define ALL confining margins (Natural+Anthropogenic+River crossings) 3) Define Erodibility of margins 4) Calculate for each 500m centrline segement the following proportions:
- Confinment
- Anthropogenic confinement and Landscape confinement
- Erodible / Non-Erodible
- Area and Volume of erosion and deposition from GCD output (perhaps in future this could be Air Photo based).
Note that Resource Managers want information on the following risks: A) Bank Erosion (farming issues and infrasrtucture) B) Deposition on Floodplains (farming issues) C) Loss of channel capacity (deposition in channel) D) Damage to infrastructure (this could be a future input based on infrastructure layers - pipelines, cables, property etc). NOTE: WE NEED TO ENSURE WE PROVIDE GUIDANCE ON WHAT IS PROBLEMATIC AND WHAT IS JUST WHAT A RIVER DOES - AND THUS IS PART OF A NATURAL ADJUSTMENT THAT HAS BENEFITS IN TERMS OF DOWNSTREAM DAMAGES.
The remaining issue is what to do about the variability in flood width (which is synonymous with valley width in narrow upland valleys during massive floods). It seems there are some critical changes we need to capture: 1) Constriction - Expansion with large sediment supply in segment(s) upstream of expansion 2) Constriction - Expansion with no immediate sediment supply 3) Expansion - Constriction (with or without sediment supply).
It might simply be a question of auto-extracting 1:1000yr flood polygon widths at c. 250m points along the channel centreline, but the real trick will be how to identify the sequence. Since this is a proof of concept project we might use the Derwent and extract 100m widths (already done) and look at relationship between volume of erosion and deposition and the first differnce in valley width (ie downstream width minus upstream width), negative values are contraction valley width, positive values are expanding valley width. At 100m downstream intervals you get the Figure below
This really does not reveal much control BUT this may be down to how we estimate expansion and contraction. I repeated for 250m and 500m interpolated centreline distances (should perhaps have totalled erosion and deposition over these distances?) and these were similar and equally showed no strong correlations. In effect what this is showing is that the presence and magnitude of erosion or deposition is a function of something other than simjple contraction and expansion as calculated this way....which suggests it may be more the presence of erodible confining margins (supply) as well as slope (power?) and some measure of confinement. In effect a (non-;linear) multiple regression model?! Welcome some thoughts on this one...
joewheaton
So I've made a start at visually (no real confinment stats here) interpreting the sequence of events during the Cumbrian floods and Desmond flooding for a couple of reaches. What seems to be key are: 1) a sequence of constricted flood flow followed by rapid expansion - these occur because of human confinment/constrictions or natural ones, and/or when a valley suddenly changes direction.(Examples 1-4) 2) A large erodible confining margin - hi violumes of sediment supply upstream of a flow expansion results in floodplain deposition and avulsion/channel widening (Example 1,2,4). 3) Changes in channel direction between confining margins where the receiving margin (downstream) is erodible (Example 1)
It is the sequence that is as important as the magnitude of change - ie Constriction into expansion + erodible margin = alot of change. Erodible margin into constriction probably = limited change (transfer of sediment) until expansion then = deposition.. Legacy Infrastructure is important for constriction of flow (Example 4)
I think we're trapping confining margins, erodible and non-erodible margins,anthropogenic confining margins. Are we getting constrictions? What to do about expansion/constriction. How to allocate these attributes in terms of proportionof a 500m segment of channel centreline?