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;

• Grated inlets
• Lintel side inlets (side entry pits / kerb inlets).

This page of the Wiki describes how pit inlet data is incorporated into a TUFLOW model.

Grates (Gully Pits)

Also known as Gully Pots, these pit types are common 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

Lintels (Kerbed Entry Pits)

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

Pit Inlet Data Sources

Pit inlet depth discharge data can be obtained from a variety of sources. The most common typically being from suppliers or local agencies who enforce consistent design standards within their jurisdiction. Examples from Sutherland Shire Council and Brisbane City Council are provided below.

Sutherland Shire Council

The Sutherland Shire Council Urban Drainage Manual (1992) includes summary tables and graphs documenting pit grate and lintel capacity information. The data was derived from Department of Main Roads field tests. Information relevant to TUFLOW modelling is summarised below.

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

The capacity of a pit depends on three factors:

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

The following graphs summarise grate and lintel flow estimates for a range of water depths and blockage factors.

The above graphs estimate unit length and area flow estimates. These unit values are multiplied by real pit dimensions to define at site depth/flow characteristics. An example is provided below for a single pit location. TUFLOW modelling requires the derivation of a unique depth/flow curve for each pit type within the modelled area.

Brisbane City Council

The pit inlet curve 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

Linking

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 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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