Overview

Click to read this page in Welsh / Cliciwch i ddarllen y dudalen hon yn Gymraeg

This is an update on the background, the works completed, and the works planned by Natural Resources Wales in the Penyrenglyn area of the Welsh Government Woodland Estate.

Background – Coal tip risk management

The Penyrenglyn coal tip is one of our top priority sites to install drainage works to reduce the risk of landslides.

Many disused coal tips now have drainage systems to reduce water infiltration into the tip material. When rainwater penetrates the top layer of soils and gets into the tip material below, it can increase the risk of landslides.

Penyrenglyn does not yet have a drainage system to control rainwater infiltration and has been categorised as a Category D tip by the Coal Tip Safety Taskforce; commissioned by the Welsh and UK Governments. This categorisation means it has the potential to impact public safety.

We can now share the outline design plans of the proposed network of surface and sub-surface drains which will be installed to capture and channel water across the site and reduce infiltration to the tip. This then reduces the likelihood of impacts to the public. The proposed drainage network contains a variety of different forms of drain which are outlined below.

Works planned – Drainage installation

The outline design proposal has been developed over 2024. The outline design drawings are linked below.

NOTE: these plans represent the current plan for the works and could be subject to change following consultation.

• Outline Design - General Arrangement - Plan Drawing
• Outline Design - Cross Sections and Details

The drainage aims to capture and control the rainfall that lands within the site’s catchment to minimise the amount of water that can infiltrate into the coal tip material.

The drainage system has also been designed to ensure that there will be no increase in flow to the drainage systems downstream of the site.

The drainage system is composed of inclined sub-surface drains, ditches, block stone cascades, and filter drains, which all work in tandem to capture surface run-off and draw water out from the coal tip material.

The inclined sub-surface drains will be drilled directly into the coal tip material. These drains will be wrapped in a membrane to reduce the likelihood of blockage and will act similar to a wick of a candle; they will draw groundwater out from the coal tip material and feed that into the surface drainage measures.

A network of lined ditches will intercept surface water run-off during heavy rainfall events. These features will limit infiltration to the coal tip material below and will carry the flow to the block stone cascades and filter drains.

Figure 1 - Outline design ditch cross section.

The flows will then be carried to the base of the slope through block stone cascades. These features provide a robust open channel which will direct the water captured on the site to the existing drainage networks.

Figure 2 - Outline design cascade cross section.

In some areas the flows will be carried to the base of the slope through a network of filter drains (also known as “French drains”) which will run parallel to an existing forest access track. We will also carry out minor repairs to the track after the drainage work is complete.

Figure 3 - Outline design filter drain cross section.

The purpose of all of these measures is to limit infiltration of rainfall into the coal tip material and control the flow of water from the site through robust drainage measures.

Given the steep nature of the site, it is likely that specialist plant machinery will be required to construct these works. This specialist machinery, commonly referred to as a “spider excavator” will ensure that the drainage measures can be installed safely and efficiently.

Photo 1 - Excavation plant likely to be used during construction.

We are aiming to start construction in 2026, once all the required consents are in place.

Works completed, 1 - Tree felling

In early 2024, we clear felled the Penyrenglyn site. This work was done to deal with a disease that was affecting many of the Larch trees that were in the forest block as well as meeting the woodland management proposals defined within our Forest Resource Plan.

Photo 2 - Felled area on the Penyrenglyn site.

This disease, Phytophthora Ramorum, had killed over half of the trees in the block. These dead trees were a risk to public health as they were prone to being blown over and also risked spreading the disease further.

The felling allowed for surveying of the site to happen through 2024. It will also allow for the drainage measures to be installed around the tip material.

Works completed, 2 - Ground investigation

Over the summer months of 2024, we conducted ground investigation works on the site. This involved minor excavations and the gathering of coal tip material samples for laboratory testing.

Photo 3 - Ground investigation works.

The results of this testing have been used in the development of drainage design and other civil engineering works that will be required to facilitate the development.

Works completed, 3 - Ecological survey

Into the autumn and winter of 2024, we conducted extensive ecological surveys of the site. These surveys were required to investigate the presence, or absence, of various sensitive species of flora and fauna.

Photo 4 - Southern Marsh orchid growing near the site.

The results have allowed us to ensure that the design has the lowest possible impact on nature.

Have your say!

In advance of the planning application, we will consult on the proposed works from Monday 8 September. This will be an opportunity for you to have your say and shape the future of the Penyrenglyn site.

We also plan to hold a public drop-in session on Monday 22 September (3:00pm – 7:00pm) during the consultation period to speak to the community directly about the plans and for you to ask any questions you may have.

The drop-in session will be held at the Penyrenglyn Project / Valleys Kids Hall on Corbett Street (https://w3w.co/guides.repeat.lookout).

Penyrenglyn Project/ Valleys Kids

53-56 Corbett Street

Treherbert

Treorchy

CF42 5ET