High Flow and Low Flow Channels – Baptism of Fire

There are several reasons I am carrying out so much habitat enhancement, river restoration and work on the river channels on the estate. Mainly this is to try and repair the river after historic dredging, provide some much needed habitat for fish of all ages / life cycles (from fry, to juveniles to mature fish), to increase the native through  more successful spawning and increase survival rates. However, part of the work is also designed to reduce flood risk to both me and the village downstream.

Previously, as described in Improving the River – Introduction and Rivers Week – Day One with Volunteers, the bank and river channel were deeply incised and trapezoidal. This unfortunately meant that when we had some rainfall, the river rose rapidly in the vertical banked channel and spilled out quickly onto the field and paddocks. It also meant than when the river dropped, pools of standing water (and occasionally trapped fish) were left in the fields and paddocks. This was mainly due to the dredged material being deposited on the banks, creating artificially high banks between the fields and the channel, resulting in the water not being able to flow back into the river channel as the water level drops.

There was also an issue with too much water flowing downstream which meant the village downstream had issues with flooding. Naturally the water would have been held back by woody debris and the flood plain, as natural rivers are not dredged and so consequently can spill out onto the flood plain. This then stores the excess water, allowing the river to release water downstream at a slower speed. However, due to historic dredging, this river channel was no longer connected to the flood plain. If you look at the photo below, you can see what I mean in terms of an incised channel and high vertical banks in the background, and the newer bank profile in the foreground.

10995555_10153093853690781_6691877370849286071_n

My answer to this was to create a two stage channel, to allow the river to rise at a slower rate, slow down the speed at which it flows downstream and also allow it to drain back into the channel if it did spread out onto the fields. The work should also reduce the number of occasions it does spill out onto the fields / flood plain. The two stage channel looks something along the lines of the diagram below.

two stage ditch enlargement

I wanted a slightly different type to the one in the diagram above, by narrowing the over-wide low flow channel, but creating a gently sloping river bank leading up to the fields. This was done with volunteers and excavators as I’ve previously scribed about.

I will admit to being slightly apprehensive when, in July just gone, we had a deluge of rain for 24 hours after a prolonged dry period. I had hoped we would get a long, continual gentle rain for a few days to soften the ground and allow the river to come up fairly slowly. Unfortunately we didn’t! This lead to the river rising rapidly and I hoped the 2-month old new banks (fortunately now well established with vegetation) would  cope. I needn’t have worried!

As you can see from the photos below, the high/low flow channels I constructed worked brilliantly in storing the water, slowing the velocity, providing habitat for small and juvenile fish out of the main flow, and allowing the river to spread out up the channel gradually. The woody debris also provided slack spots for larger fish, and assisted in slowing the discharge of water downstream.

DSC_0001a DSC_0003 DSC_0004a DSC_0005a DSC_0006a DSC_0011 DSC_0013a DSC_0025

The river has since receded and is back to it’s normal summer levels. However I am hopeful that the couple of days of higher flows has encouraged some of the Barbel and Chub from downstream to move up into the new river channel. This hope may be a little optimistic, but the fact that one of the rods lost a decent sized fish when ledgering in the weir pool (at the top of the new channel) a couple of days ago means I may not be wrong. I didn’t see the fish that was lost, but I’m told it was likely to be a Barbel.

All the best,

ACC

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s