Wednesday, 3 August 2016

Water plants in Hickling Broad

By Dan Hoare, Environment and Design Supervisor

Surveys of Hickling Broad have demonstrated that less than a fifth of the broad bed is covered by plants.

Hydro-acoustic (sonar) surveys of the broad in mid-June found that detectable plants only covered 17.4% of the whole broad bed – a relatively low amount for such a naturally plant-dominated shallow waterbody.

The map below shows the area where plants were present as green hatchings and the water draught as depth contours. Development of any further plant growth over the summer will be followed closely.

Map showing the area where plants were present as green hatchings and the water draught as depth contours
The Broads Authority has agreement from Natural England for water plant cutting in the marked channel of Hickling Broad if water plants reach high growth levels throughout the Broad and boat access through to Hickling village is impeded.

Both organisations have a duty to protect the special ecological value of this Site of Special Scientific Interest (SSSI), as well as ensuring peoples’ use and enjoyment of the area is maintained. We welcome the pragmatic and practical approach to the challenge of channel management and boating access taken by Natural England and the landowners, Norfolk Wildlife Trust.

The surveys showed several areas where water plants were high and visible to the surface, but these were distinct and confined to areas north of Pleasure Island, on the western margins and in the north bay.

There were also several patchy areas of less dense plants, but with tall (more than 50 cm) straggling growth. The tall plants were typically fennel-leaved pondweed and spiked water milfoil. It is these plant species that have caused issues with some keels and smaller engines getting fouled when travelling outside the marked channel.

The rare stonewort species were also present in the broad, growing close to the sediment in their characteristic billowing beds.

The data has been processed to provide the summary figures in the following table. The survey is carried out on a grid pattern over the broad, which totalled 12.4 kilometres of survey distance, to give a representative sample.


Average water depth (m)
0.68
Maximum water depth (m)
1.58
Average plant height (m)
0.18
Maximum plant height (m)
1.11
Area of broad covered by plants >8 cm (%)
17.4
Plants as % of water volume (PVI) (%)
5.5
 
An example of the graphical output of the survey is shown here where the red line is the surface of the plants present and the black line is the sediment surface. All the data collected is linked to a GPS so can be plotted accurately on maps to show where the plants are located.

Survey graphical output. The red line is the surface of the plants present and the black line is the sediment surface.
There are several ways to keep track of plant growth in the rivers and broads. Hydro-acoustic surveying equipment is just one method used by the Broads Authority. This electronic equipment is set to detect the presence water plants growing beneath the surface of the water, as well as the depth of the sediment at the bed of the river or broad.

4 comments:

  1. "Pragmatic and pragmatic" doesn't sound very accountable. Sounds like the NWT and NE can do what the heck they like with no come back at all, despite the vast amounts of public money they receive.
    Personally, I find the lack of depth in the channel very concerning, both at the broad entrance and between the ridge and the village. I know there were recent reports of a large yacht being pleasantly surprised to make the Pleasure Boat moorings. Looking at those depth markings it sounds like she was lucky.
    I'd love to know the views of both Martin George and Hickling Broad Sailing Club on the situation.
    Paul Howes

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    Replies
    1. While the map is useful in helping boats to keep out of the weediest areas, and we are grateful for the ongoing limited channel dredging, Hickling Broad Sailing Club are currently experiencing difficulties in setting satisfactory courses. For instance, the northerly wind last weekend (when we had our main regatta) made for difficulties for boats tacking past the pinch point opposite Jarvis Point. We are very disappointed that there does not seem to be a will to cut weed or dredge outside the channel. The Upper Thurne Working Group has agreed an minimum "area to be managed for sailing" which covers a rather greater area than just the marked channel and we believe that BA, Natural England and NWT, who all signed up to this, should stand by it.
      Sadly, Martin George died last June.
      Sue Maisey (for Hickling Broad Sailing Club)

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  2. I'd be wary about drawing any conclusions from that screenshot. I'm only drawing a conclusion from what's shown, but the red spikes look more like noise than data (maybe due to cavitation from a propellor, or limitations of the sonar in the shallower water), especially due to the intensity of the return. That aside there looks to be some matter on the bed, could be plants, suspended sediment, etc. You'd only know by taking grab samples,camera work, etc.

    Also, please correct 'draught' to depth in your graphic.

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  3. I think you're a little behind the times Paul, in every respect. See http://tinyurl.com/hzwnmzh and http://tinyurl.com/z2as2f8 and http://tinyurl.com/hqm8haf

    ReplyDelete