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JBA Consulting

The existing channel had been modified historically by realignment leading to it being perched above the floodplain (i.e. sits at a higher

level than the valley bottom) and disconnected from the groundwater table. In addition, several weirs were acting to disrupt the

in-channel sediment regime, causing excessive fine sedimentation to occur.

As the river is disconnected from its floodplain, in times of flood when the channel overtops or breaches, the water remains in pools in

the park for long periods of time. This is because the water is unable to flow back into the channel when water levels recede. The

channel is also disconnected from the groundwater table. For a chalk stream, where over 70% of flow is from groundwater, this can have

a big impact on both resilience to low flows and also to the ecology of the channel.

As a result of previous management, such as dredging, the channel is very straight and is over-wide for the amount of flow. This,

combined with the impoundment, means that there is excessive vegetation growth that will readily grow across the entire channel if

maintenance is not carried out.

Challenge

Our catchment and river restoration team were commissioned by the Environment Agency between 2015 and

2017 to undertake an options appraisal and outline design for the feasibility of restoring the River Gade (a

unique chalk stream) within Gadebridge Park in Hemel Hempstead.

River Gade Chalk Stream Restoration

Location: Hemel Hempstead, UK

Solution

We conducted a feasibility study which identified a preferred option for the river restoration in Gadebridge Park.

The project deliverables included a fully costed options appraisal and outline design of three main restoration options and incorporated:

• Geomorphological walkover investigation. This included gathering detailed information of the existing channel in the park, mapping key

morphological problems (such as excessive siltation) and also visiting areas within the wider catchment to find more natural 'analogue sites'

• A Preliminary Ecological Appraisal (PEA) which identified the main ecological constraints (including invasive species) to the project and

potential areas for improvement within the park (e.g. Wetland creation)

Date: 2015 - 2017

Client: Environment Agency

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