1D Pits

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Introduction

There are predominantly two types of stormwater pits (drains/gullies) used as inlets to collect overland runoff and transfer that water to the underlying drainage/culvert network; side inlets (side entry pits / kerb inlets) and grated inlets. Although there are various combinations of pits that can incorporate both features, the important thing to remember is that essentially any pit inlet can be modeled if the structure details are known and the performance can be calculated.

Pit types

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Real Life Examples

Some common pit types are shown below. Note: not all variations of side entry pits, grated pits, gully pits (pots) or any similar structure are displayed, the images are typical examples of what can be modelled as a 1D pit structure within TUFLOW.

Gully Pits (Grates)

Also known as Gully Pots, these pit types are commonly found within the UK and are generally a square grate on top of a circular chamber and a riser outlet. The outlet will then feed into a larger culvert that forms part of the larger urban drainage network. The grate variation and sizing can influence the rate of inflow into the pit, therefore it is important to consider this when developing the depth-discharge relationships.

London, UK

Kerbed Entry Pits (Lintels)

Side entry pits are common sights within urban streets in Australia. The pit chamber can vary depending on overall depth, the additional of any haunched riser units and the size of the underlying drainage network that pits are typical directly connected to. Refer to any local drainage guidance on developing depth-discharge relationships.

http://www.lgam.info/side-entry-pit

Linking

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Pit Inlet Depth-Discharge database

Pit inlet depth discharge data can be sourced from a variety of sources. The most common source is typically from suppliers or local agencies who enforce consistent design standards within their jurisdiction.

Sutherland Shire Council

The Sutherland Shire Council Urban Drainage Manual (1992) includes grate and lintel capacity information, derived from Department of Main Roads field tests. The information relevant to TUFLOW modelling is summarised below.

The Sutherland Shire Council guidelines are based on the standardised design shown below:

The guideline states, the capacity of a pit depends on three factors:

• the length of lintel opening
• the area of clear opening of the grate
• the depth of water ponding over the grate
  

The guideline provides information used to derive the below unit area and unit length capacity charts.

A pit inlet curve is derived by

Brisbane City Council

The examples below originate from Brisbane City Council 8000 series standard drawings: https://www.brisbane.qld.gov.au/planning-building/planning-guidelines-tools/planning-guidelines/standard-drawings

Pit Search Radius

Within the *.ecf the following command is required to have TUFLOW connect floating nodes into the 1D network where connectors are not snapped to channel ends:

Pit Search Distance ==xxx
Read GIS Network ==..\model\mi\1d_nwke_*****.MIF

The order of the Pit Search Distance command is important as it can be repeated multiple times with different values that are assigned to the 1d_nwke(s) below the Pit Search Distance command. There is an example on the TUFLOW Forum here that describes the setup of the commands within the .ecf.

To check if the Pit Search Distance is working as expected, import the *_nwk_C_check file to visually see if the pits are automatically connecting to a culvert. The image below is an example of the *_nwk_C_check file and the connections TUFLOW has made to each pit.

Any further questions please email TUFLOW support: support@tuflow.com

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