Traditionally flood alleviation has been implemented by civil engineering using “hard” solutions such as higher river walls, levees and tunnelling. These still have a significant role to play in the battle with our elements; however, we may be coming to a point where these traditional methods of controlling the path of flood water are no longer sufficiently adequate. This is particularly so if we as a nation are going to manage the effects of climate change. The following discussion considers as an example a single location in the Calder Valley where such solutions may no longer be quite enough: Caldene Avenue in Mytholmroyd, which experienced serious flooding on Boxing Day 2015. However, by careful implementation of Natural Flood Management interventions our existing infrastructure can be supplemented, even future proofed.

The Scottish Environmental Protection Agency (SEPA) have published an excellent handbook on Natural Flood Management (NFM) which is available here for those wishing to learn more:

http://www.sepa.org.uk/media/163560/sepa-natural-flood-management-handbook1.pdf

A real hydrograph to understand Caldene Avenue variables

Our science page goes a long way to explaining the mechanics of fluvial flooding using a diagrammatical hydrograph. The hydrograph here in  Graph 1 illustrates the actual conditions measured at the Caldene Avenue river gauge in Mytholmroyd.   The gauge measures the height of the river at 15 minute intervals and the graph shows the 72 hour period beginning 00.00 hours on 25th December 2015.

The second Graph 2 presents river flow rate in cubic metres per second (m3/s or Cumecs) for the same period, the calculations that convert river level to river flow are relatively straightforward and they form the basis of many of the commercially available river modelling software  programs.

The approach uses the fixed variables relating to the river channel, width, water depth and channel gradient and a formula first developed by Robert Manning in 1889 (and known as Manning’s equation) as follows:

V = (R2/3So1/2)/n

Where:

V = Mean flow velocity (m/s)

R = Area of flow / Wetted perimeter

So = Channel gradient

n = Manning’s roughness coefficient.

Once the flow velocity is calculated the discharge Q, (m3/s) through the channel can be calculated by multiplying the velocity by the area of the flow.

Manning’s roughness coefficient

Manning’s roughness coefficient is a variable that represents the drag on the water from the sides and bottom of the channel. The rougher the surface, the slower the flow, hence a stream containing large cobbles and boulders will have a much higher “n” value than one made from concrete or masonry. Vegetation also plays a role in channel roughness with higher roughness expected in summer than in winter.

As the river channel approaches Caldene Avenue bridge the gradient slackens from around 1 in 155 to around 1 in 890. This slows the velocity, along with the obstruction from the bridge deck which further reduces the channel capacity to convey water.

The channel cross section in this part of the river contains a narrow deeper channel around 5.0 metres wide centrally within a wider channel with an overall width of around 18 metres.   In December 2015 the channel shoulders were covered with vegetation, albeit much reduced from the summertime. Nevertheless, these would have been rougher than the central deeper part of the channel which is lined with cobblestones or setts.  A photograph taken in April 2009 shows broadly how the channel would have looked in December 2015 below (Plate 1):

Plate 1: April 2009

In the Autumn of 2016 the channel vegetation was removed by the Environment Agency (EA) exposing the setts as can be seen in the following photograph (Plate 2). The EA had identified “the benefits in reducing flood risk through management of channel vegetation” in their Action Plan for reducing flood risk in Mytholmroyd available here:

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/525893/Mytholmroyd_Action_Plan_FINAL.pdf

Don’t we want to SLOW the flow?

Reducing the friction coefficient speeds up the flow. That is desirable at this point in the river system, where riverside homes and businesses are under direct threat from the river overtopping its banks. Higher up the catchment the notion of “slowing the flow”, from which our group takes its name, has the effect of flattening the hydrograph as explained on our science page. Clearly this also is a desirable outcome for flood risk because it lessens the problem needing to be dealt with downstream in towns.

Plate 2: December 2016

Effects of changing the variables

We can see an estimate of the existing channel capacity at Caldene Avenue bridge in Table 1. The only variable to change between the two calculations is Manning’s “n” for the channel shoulders. The bankfull capacities are drawn as a red and a green line on the hydrograph.

 

Case Q (m3/s)
Vegetation Removed (Green) 101
Winter Vegetation (Red) 86

Table 1: Increase in existing channel capacity by vegetation management

Case Q (m3/s)
Vegetation Removed 271
Winter Vegetation 232

Table 2: Increase in channel capacity by raising walls to 1.8 metres above street level and increasing the channel gradient

Case Q (m3/s)
Vegetation Removed 279
Winter Vegetation 242

Table 3: Increase in channel capacity by raising walls to 3.0 metres above street level and leaving the channel gradient as existing

One can see the improvements that have already been made by removal of the vegetation in terms of the increased channel capacity, about a 15% increase is demonstrable, nevertheless there is a way to go to prevent flooding given a similar flooding event as the hydrograph does still break considerably through the green bankfull line. Clearly the intentions, as we already know, are to modify the bridge at Caldene Avenue and we may expect a deepening of the channel here as well to steepen the slackening channel gradient.  These measures will increase the channel capacity. However, to remain in bank, the river walls would have to be raised to around 1.8 metres above the current level of Caldene Avenue to cater for a flood of the magnitude experienced on Boxing Day 2015. The further calculations reported at Table 2 demonstrate this, and 1.8 metres compares with the value given by the EA in the Mytholmroyd plan noted above.  Leaving the channel gradient the same as currently would require the walls to be raised to 3.0 metres as per Table 3.  However, the new bridge deck has to be at a similar level to the current one – as a higher bridge deck would mean traffic could not access it either from Burnley Road or Caldene Avenue.  This must be the reason the river channel is to be widened at this point as raising the walls would have to be accompanied with raising of the bridge deck and the latter is not possible.

Cost comparison between hard engineering and NFM

Altering existing infrastructure such as Caldene Avenue bridge is expensive and disruptive. Slowing the flow higher up the catchment to “squash” the hydrograph below the bankfull capacity can be a far cheaper and greener partial solution. NFM is not the panacea, some civil engineering is clearly required – mitigation is the key message, but NFM can be implemented now, it is civil engineering by the people for the people.   Think of the waste created by the demolition of this bridge and the adjacent retaining walls, the resources used in the construction of the new bridge and of course what of the carbon footprint to all of this!  Not so with leaky woody dams for example, or creation of small attenuation ponds or swales in the fields above the valleys.   As the Table 1 calculations illustrate, supplementation of our existing in bank capacity in the main river channels can be enhanced by vegetation management (reducing roughness) at relatively low cost. Further supplementation using NFM (increasing roughness higher up the catchment) can also play a role at significantly lower cost to society than traditional hard engineered approaches can offer.  Furthermore, NFM interventions implemented widely can futureproof some of our existing and (even newly built) infrastructure against more serious events than that occurring on Boxing Day 2015.

NFM – part of the ‘suit of armour’

This example serves to illustrate the effects on our existing infrastructure of our current and future flood risk and the role Natural Flood Management can play in the flood risk management armoury.   Clearly the EA appear to be softening on the implementation of NFM, given that both the Mytholmroyd and the later Calderdale Catchment Plan identify catchment wide measures including potential Natural Flood Management and upstream storage. However, in my view, the process is too protracted and should be higher up the agenda than it currently is.

Slow The Flow pilot project at National Trust Hardcastle Crags – a first step

It is excellent news that we have a pilot NFM project – but that is all it is, “a pilot”, and currently with limited funding. We cannot simply keep raising river walls, but we can quickly and cheaply implement NFM in our higher catchment. £10m is earmarked for the Mytholmroyd scheme in the aforementioned plan – that could buy a lot of leaky woody debris dams!

©Stuart Bradshaw CEng.