River Gade Chalk Stream Restoration

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.

Client:

Environment Agency

Location:

Hemel Hempstead, UK

Date:

2015-17 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 overwide 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

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 inchannel sediment regime, causing excessive

fine

sedimentation to occur.

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Matthew

Hemsworth

Principal/Lead

Geomorphologist