Flood risk management: Slowing the flow
09 September 2019
Figure 2: On Line flow control structure in the earthen flood storage embankment, Clonakilty.
Flood risk management is an engineering discipline that seeks to mitigate the damage caused when flood flows interact with vulnerable receptors and, a subset of measures – including working with natural processes and flood storage – mitigate the flooding risk by slowing down the response of the catchment to rainfall events, writes Barry O’Connor.
Traditionally this is done by moving flood flows away from the receptors or by moving the receptors out of the way of flood flows.
The former can be done by the dredging of rivers or the construction of flood walls, for example, while the latter can be achieved by relocating vulnerable properties, the provision of flood warnings or by planning and development management.
There is a subset of flood risk management measures that mitigate the risk of flooding by slowing down the response of the catchment to rainfall events. This subset includes working with natural processes and flood storage.
Working with natural processes
Working with natural processes is the term given to techniques that use natural hydrological and morphological processes, characteristics and features to manage the pathways and sources of flood waters.
These techniques use natural features such as floodplains, forests and river banks to impact on flood flows. Working with natural processes is also known as natural flood management or natural water retention.
Working with natural processes can be categorised in three groups of measures: woodland creation, land management and river and floodplain restoration.
Woodland creation can reduce the peak flow in rivers in a number of ways. By planting trees over the general catchment area, the ability of the soil to receive rainwater is improved.
The growing trees break up the structure of the soil and provides pathways for the rainfall to infiltrate the soil. As this rainfall is not running off the land and into watercourses the flow in the rivers is reduced.
This impact is most pronounced for high frequency events when rain is falling on a dry catchment. However research has shown that for less frequent events, that is those with a higher return period, the impact on peak flows diminishes as the soil becomes saturated and can receive no more rainfall.
The planting of trees in the flood plain increases flood plain roughness which slows down the flow. Research has shown that planting 20 per cent of a floodplain with woodland can reduce the peak flow in frequent events by up to six per cent.
Planting of Riparian woodland, that is trees at the edges of rivers, can also reduce peak flows in rivers. It does this in two ways.
First, the presence of trees along the river can intercept overland flows and delay their entry to the river system.
Second, the presence of the trees can increase the physical roughness of the flooded river system, making it less efficient and slowing the flow.
Soil and crop management measures can reduce the runoff from fields and promote infiltration and in-soil storage. Ploughing fields parallel to the contours of the topography instead of down the hill can slow down the surface water runoff.
Ensuring that fields are not left bare during the winter months, when flooding is more likely, increases the roughness of the fields and slows down runoff.
Blocking existing drains can create small storage areas in the landscape and attenuate the peak flows in rivers. Non-floodplain wetlands can be created in the landscape by the construction of ponds where overland flows can be intercepted on their way into watercourses.
These measures can reduce the peak flows seen in rivers in response to frequent rainfall events but when the soil is saturated and the micro storage areas created fill up, the effectiveness of these measures diminish.
The impact that these measures have on peak flows are most noticeable in small catchments. As catchments get bigger and as multiple watercourses contribute to the flows, the impact that these measures have on peak flows reduces.
This is due to the complex interactions and timings of peaks from different sub-catchments that tend to negate the peak flow reductions from the individual sub-catchments.
River and floodplain restoration
Engineering interventions made in rivers in the past may have served to increase the capacity of those channels to carry flows. Obviously, reversing those interventions would serve to slow down the flow.
Reintroducing meanders in a river will increase the length of that river which will lead to flows taking longer to reach their destination thereby slowing the flow.
Removing flood embankments will allow the flood plains to be used for flood storage which can lead to flows being attenuated.
The added advantage of removing embankments is that some of the heavy sediment load carried by flood flows will be deposited on the floodplains as opposed to the river channel where they can affect the capacity of the channel to carry frequent flood flows.
In effect, removing embankments can reduce the frequency of flooding.
Instream structures such as woody debris dams can slow the flow in rivers by creating mini, on-line storage areas in the watercourse. These structures are effective in reducing peak flows for frequent events.
A research project in Devon, England, fenced off a 3Ha site and introduced beavers. These beavers constructed 13 dam structures in the catchment which led to a reduction in the peak flow for frequent events of 30 per cent.
However, as these storage areas fill up and the structures become submerged in less frequent events, the peak flow reduction is much diminished.
Multiple functions of working with natural processes techniques include:
• Improve water quality by reducing the amount of sediment and other pollutants washed into watercourses;
• Increase biodiversity by creating new wetland and floodplain habitats;
• Reduce the peak of frequent flood events in smaller catchments.
Frequent flood events generally do not cause damage to flood risk receptors such as houses and infrastructure and it is for this reason that working with natural processes is not normally used to manage flood risk in Ireland.
Indeed, in certain circumstances, working with natural processes could actually increase flood risk. For example, the town of Castleisland in Co Kerry is at risk of flooding from the Anglore stream and the River Maine.
The catchment of the Anglore stream is relatively small and is located near to the town. The catchment of the River Maine is larger and extends far upstream of the town. Under normal circumstances, the peak flow from the Anglore stream passes through Castleisland before the peak from the River Maine.
Theoretically, if natural flood management measures were in place in this catchment that attenuated the peak flow from the Anglore, this could lead to the peaks in the two rivers coinciding and giving rise to a higher peak flow and therefore increasing flood risk.
Flood storage can be an effective measure to reduce the peak flow entering an area where there is a risk of fluvial (river) flooding. This measure works by diverting a portion of the river flow into a storage area for a period of time until the flood has passed and then allowing the stored waters back into the watercourse.
This is done in a controlled manner so that flows are kept below those that cause flooding.
Flood storage is being used in Clonakilty to manage the risk of fluvial flooding in the town as part of the River Fealge (Clonakilty) drainage scheme.
The first of its type in Ireland, this measure will flood an area of pasture land that was not previously prone to flooding thereby protecting the more vulnerable flood risk receptors in the town.
When flows in the river Fealge exceed the capacity of the channel in the town, the excess flow will be stored in the storage area until the flood event has passed.
The flood storage structure consists of a 500m-long earthen embankment which is up to 5m-high and is constructed from locally sourced clay based material.
Where the river passes through the embankment a reinforced concrete structure holds a series of penstocks that control the flow of flood water into the storage area.
This solution is suited to this particular catchment as the design peak flood flow is relatively low. The design flood event in Clonakilty is 25 cubic metres per second.
This is the peak of the flood that has an annual exceedance probability (AEP) of one per cent. While this flow causes a lot of flood damage in Clonakilty it is low when compared to other flood risk towns in Ireland where the design flow can be many hundreds of cubic metres per second.
When complete, the flood storage area in Clonakilty will cover an area of 283,000m2, store 625,000m3 of flood water and protect the town from fluvial flooding.
In choosing this flood risk management measure for Clonakilty, the requirement for more visually intrusive measures has been avoided.
If flood walls were constructed in the town these would have had to have been more than 4m-high to contain flood flows within the narrow channel. In an architecturally rich town this was vital in securing consent to protect the town from flooding.
The Clonakilty scheme is currently under construction and is due to be completed later in the year. When complete, the flood storage area will protect 150 vulnerable homes and 142 commercial properties from fluvial flooding.
Author: Barry O’Connor is a chartered engineer and is the director of the national strategy for angling development at Inland Fisheries Ireland. Before taking up this post he was an associate at Mott MacDonald Ireland where he was the project manager for the Clonakilty flood relief scheme, the Enniscorthy flood defence scheme and the Clonmel flood relief scheme. He was also the project manager for the South Western River Basin District Catchment Flood Risk Assessment and Management (CFRAM) study.https://www.engineersjournal.ie/2019/09/09/flood-risk-management-slowing-the-flow/https://www.engineersjournal.ie/wp-content/uploads/2019/09/a2-2-1024x768.jpghttps://www.engineersjournal.ie/wp-content/uploads/2019/09/a2-2-300x300.jpgElecCork,flooding,project management