Pond routing is a critical technique in the design of detention ponds for stormwater management. It helps control runoff discharge from developed areas and minimizes the hydrological impact on downstream systems. This is achieved by ensuring that the post-development runoff discharge, Qout , does not exceed the pre-development discharge, Qpre , even with increased imperviousness due to urbanization. The design must accommodate excess inflow while regulating the outflow.
Introduction
In pre-development conditions, runoff is naturally regulated by pervious land cover, such as soil and vegetation. In contrast, post-development conditions involve impervious surfaces like pavement and buildings, leading to an increase in runoff volume. The objective of pond design is to determine the appropriate pond size and outlet configuration to control the outflow effectively, ensuring that Qout≤Qpre, thereby minimizing downstream impact.
Inflow Hydrograph
The inflow hydrograph represents the variation of runoff discharge over time for a given development. It can be derived using the Rational Method Hydrograph Method (RMHM), which simplifies the process. The hydrograph’s shape depends on storm duration, d, and the time of concentration, tc.
Stage-Storage Relationship
For a detention pond, the relationship between the depth of water and the corresponding storage volume is referred to as the stage-storage relationship. To estimate this relationship, a pond grading plan is used to determine the plan area at various depths.
Stage-Discharge Relationship
The stage-discharge relationship accounts for the outflow from the pond at different water levels. This relationship depends on the behavior of the outlet structures, such as:
1. Weirs: Where the outflow O is calculated as O= CBH^(3/2), with C being the weir coefficient.
2. Orifices: Where the outflow O is calculated as O= CA(2gH)^0.5, with C as the discharge coefficient.
In conjunction with the stage-storage relationship, a storage indication number is derived for each water level to simplify the routing process.
Pond Routing Process
Pond routing determines how the inflow and outflow interact over time to predict water levels in the detention pond. The Modified Puls Method uses the continuity equation.
Using this equation, a routing table is constructed step-by-step:
1. Time is derived from the inflow hydrograph.
2. Storage and outflow are calculated iteratively using the storage indication number.
3. The process is repeated for each time step to determine the pond's behavior over time.
Pond Routing Worked Example
In this worked example, we are required to determine the peak discharge and maximum water level for a pond receiving peak runoff discharge of 6.627 cumec with design storm duration of 30.15 minutes. The time of concentration is found to be 13 minutes. The top and bottom area of the 3m depth pond are 5202 and 3184 meter square. Other than that, a rectangular outlet with the dimension of 0.75m width by 0.6m depth and invert level of 0.6m is provided.
Prior to the pond routing process, it is essential for us to determine the inflow hydrograph, stage-storage and stage-discharge relationships.
Given the key storm data, we can develop an inflow hydrograph using Rational method hydrograph method (RMHM). For our example, type 1 hydrograph is produced. As for the stage-storage relationship, we first determine the layer storage volume, and subsequently the cumulative storage volume corresponding with each stage. When determining the stage-discharge relationship, we have to know the behaviour of outlet at each stage. Basically, the outlet may behave like a weir or an orifice, depends on the water level. Then, for each behaviour we calculate the corresponding outflow passes through it. Once stage-storage and stage-discharge relationships are in place, we can proceed with the calculation of storage indication number.
Next, we are good to proceed with pond routing. To ease the process, it is advisable to construct a routing table. From the routing table, we first identify the largest SIN calculated. Subsequently, we can use it to find the maximum water level and peak discharge. At the end of the routing process, we always plot the inflow and outflow hydrograph on the same axes to visualize and compare them.
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