At Slow The Flow Calderdale, we want to encourage and empower people and organisations to take action on their own property and land, whether that is private or public-facing.
Many small‑scale Natural Flood Management (NFM) and Sustainable Drainage Systems (SuDS) interventions, taken together, can result in a significant amount of water being temporarily stored, or attenuated, during storm events. This contributes to reducing the scale and damaging effects of flood events.
Our experts have identified NFM and SuDS opportunities in a pilot area of the catchment, we hope the map below will inspire many more such interventions!
NFM and SuDS Opportunities Map
This mapping identifies realistic locations on a field / street level of detail, where residents, businesses and organisations in Mytholmroyd can contribute to flood alleviation through natural / sustainable interventions. It gives indications as to what interventions might be appropriate in those locations.
You will find it reads most easily when viewed on a larger screen and by using the ‘full screen’ button in the top right corner of the map.
The mapping contributes to our work with Calderdale NFM Operations Group partners to address actions in the Flood Action Plan, including ‘mapping NFM opportunities’, which is an action in the plan.
Northern Mytholmroyd has been used for the pilot. This is partly because we consider that there are interesting opportunities available, and partly due to the significant urban realm regeneration work being undertaken. This includes a developing neighbourhood plan which incorporates a desire for inclusion of NFM and SuDS.
This mapping should be a useful influence to enable future works to easily and properly consider NFM and SuDS.
We hope ultimately to extend the opportunities mapping work to cover a wider area.
How should the mapping be used?
The map not only acts as a guide and inspiration where land and property owners may not have previously considered that they are able to help, but is available in such situations as:
- A reference tool for use when considering any new or retrofit build projects
- Ensuring appropriate allocation of grant funding (for example future rounds of CMBC’s NFM grant fund, or other funds allocated to flood alleviation)
- An influence, to assist in implementing the emerging local plan, when developing and assessing planning application.
- Potential to combine with other mapping such as CMBC’s online mapping portal, overlaying this information with layers such as designations, local plan information etc. (please contact us if you require this mapping in another format)
- Assisting CMBC’s NFM officer in allocating resources to best effect
Notes on data
Land ownership is generally unknown. The purpose of this mapping is to advise and encourage land owners to consider SuDS and NFM interventions on their land, for the benefit of the whole community.
All suggestions are made without the benefit of survey information, including underground services, levels, ground conditions.
More than one intervention may be appropriate at any given site. Interventions 1, 2, 3 are in order of recommendation.
Downpipes may be diverted to water butts, rain garden planters, ground level rain gardens, or nearby swales/basins/permeable paving as appropriate in each situation. Any overflow should continue into the existing system, as before.
How and why have volumes been calculated?
We have attempted to estimate storage volumes in order to give an indication of the difference that these measures can make to flood alleviation.
Volume calculations are approximate, given the nature of this exercise. It should also be remembered that NFM and SuDS do not just store water, but they Slow The Flow! i.e. They have an additional and complex role that has not been calculated here, in not only lowering the water level, but delaying the lowered flood peak for their sub-catchment.
Urban SuDS (S codes on map)
Urban swale/basin estimated volumes are determined by assuming deepest water level 300mm, shallow slopes, average level 200mm. Allowance for detail design/grading etc, assumed 75% of area apparently available for swale/basin becomes storage capacity. i.e. estimated storage volume = area*0.75*0.2
Trees – estimated volumes are determined by a conservative estimation of number of trees, and an assumption that through interception/uptake/infiltration each tree provides 0.5m³ storage. Where trees are retrofitted into hard surfacing, tree pit is assumed to provide an additional 0.5m³
Permeable surfacing, ground level rain gardens and green/blue roof volumes are determined by assuming 50mm depth capacity (i.e. approx 300mm structural make up with 15% void space)
Retrofit green / blue roofs may require professional advice, to ensure loading and waterproofing are appropriately handled.
Water butts and rain garden planters are assumed to store 0.2m³ per disconnected downpipe. Downpipe numbers where estimates used: 2 per detached house, 1 per semi-detached house, 1/2 per terraced house, 1 per garage
Rural NFM (N codes on map)
Rural basin estimated volumes are determined by assuming 50m³ per hectare of volume, with ponds assumed to be 200m² at 1m depth.
Leaky dam estimated volumes are 3m³ per dam, with dams estimated at 7m intervals along suitable lengths of watercourse.
Hedgerows – estimated at 0.5 m³ storage per 1m length.
Trees – estimated volumes are determined by a conservative estimation of number of trees, and an assumption that through interception/uptake/infiltration each tree provides 0.5m³ storage.
Report on implications of the Opportunity Mapping pilot study
Our report on this mapping (see downloads below) is a rare attempt to quantify the potential benefits of SuDS and NFM on a catchment scale. It shows that, when extrapolated across Calderdale, almost 2 million m³ of water could possibly be attenuated using NFM and SuDS. If implemented widely enough throughout the Calder Valley, they could have a significant impact on flood mitigation. Opportunities should, therefore, be converted into ‘slow the flow’ features as a matter of urgency.
This report shows that residents, land owners, businesses, organisations and developers could substantially reduce flood impacts by implementing SuDS and NFM on their land holdings.