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.
Hemel Hempstead, UK
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.
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
sedimentation to occur.
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