Modelling of Pit Inlets

In piped stormwater drainage systems, pits serve several purposes. They act as inlets for stormwater, as points where pipes can conveniently change their size, slope or direction, and as inspection and maintenance openings. Stormwater entry into these pits depends on their inlet capacities.

The hydraulic capacity of an inlet depends on its geometry and the characteristics of the gutter flow. Inadequate inlet capacity or poor inlet location may cause:

• under utilisation of the underground system; and

• flooding on traffic lanes resulting in hazard to moving vehicles, or overflow into adjacent properties.

Before discussing the pit inlets, it is important to understand the difference between bypass flow and overflow definition in ILSAX. These two types of flows are shown in Figure 4.6. If overland (surface) flows approaching to a pit is higher than the its inlet capacity, then

bypass flow at the pit occurs. Similarly, at the pit if the sum of approaching pipe flow and inflow from the pit is higher than the reach capacity of the immediately downstream pipe, then overflow occurs from this pit.

Approaching surface flow

Pi

pe

in

flow

Bypass flow

Surface inflow > Pit capacity

Approaching surface flow

Pip

e inf

low

Overflow

Pipe flow > Reach capacity

(a) Bypass flow (b) Overflow Figure 4.6: Bypass Flow and Overflow in a Pipe Reach

Two types of pits are commonly used. They are on-grade and sag pits. On-grade pits are generally located on a slope. Bypass flows from an on-grade pits move away from the pit and travel into the next pit. Sag pits, on the other hand, are located in a depression (or sag), so that water cannot readily escape. If the capacity of the sag pit is not sufficient to accept all flows arriving at the pit, stormwater ponds near the pit until it becomes high enough to cross some barriers such as the crown of a road. Ponded water is released to the pit, when the inlet capacity becomes available. There are several variations of these two common pit types (NAASRA, 1986, O’Loughlin, 1993). For both on-grade and sag pits, there are three types of inlets namely side entry (or kerb-opening), grade inlets and combine inlets. For details of these different variations of these pits, the reader is referred to McIllawraith (1959), NAASRA (1986), O’Loughlin et al. (1992a) and O’Loughlin (1993).

The ILSAX model has two ways of modelling the pit inlet capacity. They are:

• No inlet restriction (Infinite capacity) - In this case, unlimited inlet capacity is assumed. However, if reach capacity is not sufficient to cater for the incoming

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flow, then the overflows at the pit are stored at the upstream end of the reach and released back into the reach when capacity becomes available. This option is available for both on-grade and sag pits.

• Inlet capacity determined by relationships obtained through hydraulic model studies (Finite capacity) - This option is also available for both on-grade and sag pits. When modelling on-grade pits, bypass and overflows can be directed to a pit downstream or directed out of the drainage system. With sag pits, water will pond, up to an user-defined limit. Once water level exceeds the limit, bypass flows can be directed out of the system or directed to a downstream pit. Generally, these relationships have to be determined through hydraulic model studies, since there is no comprehensive theory available to determine them. The inlet capacity can be changed significantly by small differences in dimensions and by features such as depressions and types of grate. For details of these pits and estimation of their capacity parameters, the reader is referred to The Institution of Engineers (1987), O’Loughlin et al. (1992a), O’Loughlin (1993) and Pezzaniti et al. (1999). This method is better than the no inlet restriction method, since it is more closer to the reality.

4.2.4.1 On-grade pits

The relationship available in the ILSAX model to describe on-grade pit inlet capacity has the form of Equation 4.10.

CAPACITY = CAP1+ CAP2 Q + CAP3 Q CAP4 (4.10)

where CAPACITY is the inlet capacity (m3/s),

Q is the flow discharge arriving at the Inlet (m3/s) and

CAP1, CAP2, CAP3 and CAP4 are the factors supplied by users. The relationship given in Equation 4.10 can be used as a linear equation, polynomial or power function, depending on the values of user-defined parameters. The main practical

difficulty is to get the correct values for these four parameters. The ARR87 provides a curve for on-grade pits to estimate these parameters, for pits of 1 m and 2 m sizes.

4.2.4.2 Sag pits

For a sag pit, the following relationship is available in the ILSAX model to compute the inlet capacity:

CAPACITY = VCAP1+ VCAP2.V VCAP3 (4.11)

where CAPACITY is the inlet capacity (m3/s),

Q is the flow rate arriving at the inlet (m3/s),

VCAP1, VCAP2, VCAP3 are the factors supplied by users, and V is the ponded volume (m3).

The user has to develop a relationship for ponded volume (V) and inlet flow velocity (Q) from VCAP1, VCAP2 and VCAP3 can be estimated.

4.2.4.3 Choke factor

When the mathematical relationships are derived from hydraulic model testing, the tests are generally conducted with water, free from debris. However, stormwater flows carry debris loads during storm events. Because of these debris loads, the inlet capacity will be reduced. The choke factor allows for this effect in ILSAX.

For both on-grade and sag pits, the choke factor (CF) simulates the blockage of the pit. If CF is 0, there is no blockage at the pit and the inlet capacity is determined from either Equation 4.10 or 4.11. If it is 1, then there will be complete blockage at the pit and stormwater does not enter the pit at all. Typical values recommended in the ILSAX user manual are 0.2 for an on-grade pit and 0.5 for a sag pit. However, it is understood that CF depends on conditions of the catchment prior to the storm event. These conditions depend

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on the cleaning frequency of road-gutter system, the season of year (i.e. more blockage during Autumn due to fallen leaves), prior rainfall etc. The choke factor is a dynamic parameter for a catchment like AMC. However, the ILSAX model treats this factor as a static factor. Although, an average value of choke factor can be selected for design of urban drainage systems, the most suitable value of the choke factor at the time of analysis should be taken in the analysis (i.e. evaluation of adequacy of the system for different rainfall conditions) of these systems.