Model Results

Conduit Sizing

Initial diameter of all circular conduits is set in the range of 1.75-5.50 feet in this Tutorial. Besides, natural channels C4 and C12 have maximum depths of 4.5 and 2.0 feet respectively. To determine the minimum diameter of the circular conduits which allow no node-flooding and no conduit-surcharge, a number of model simulations are carried out, with 100-year 2-hour design storm and applying Kinematic Wave routing method in an iterative approach. For this purpose, the following steps are undertaken one by one.

1. At first, the model is simulated with initial conduit diameters, and the status report is checked for node flooding and link surcharge. The model shows no node-flooding and link-surcharge at this stage which indicates adequate capacity of the conveyance network. Then, from a hydraulic perspective, these conduits are analyzed to find out if their diameter could be downsized systematically.
2. Downsizing operations are started from upstream to downstream in the Tutorial network which has a dendritic conduit layout. As we go downstream in a dendritic network, the conduit diameter should increase with the increase in the stream order. However, it’s appropriate to follow conduit sizes that are available in the manufacturers’ specifications. In this Tutorial, while downsizing (or upsizing), a diameter difference of 0.25 feet is considered under each iteration.
3. With the reduction of conduit diameter and after a number of iterative simulations, the optimum design conduit sizes have been determined which ensures no node-flooding or link-surcharge. However, note that cross sectional properties of the irregular shaped channels C4 and C12 are not modified in this Tutorial.

The following table lists the design diameters selected for the circular conduits after all iterations are made. In addition, it also provides conduit flow summary status obtained with the finally selected diameter (see Link Flow Summary in the Status Report).

Table 2.8: Design Circular Conduit Diameter and Simulated Flow Summary For A 100-Year 2-Hour Storm And Kinematic Wave Routing

Comparison of Runoff Results among Different Routing Methods

After resizing the conduit diameters, the model has been simulated again in three different scenarios by applying three different routing methods. These methods are:

• Steady Flow
• Kinematic Wave
• Dynamic Wave

Then three outflow hydrographs are prepared using the time series output of Total Inflow at outfall O1 from three scenarios. These outflow hydrographs are plotted together with the Total Inflow at outfall O1 obtained in Tutorial 01 to find the differences. The above operations are made for all storm events i.e. for 2-year, 10-year and 100-year. Note that in Tutorial 01, only overland flow was simulated and no routing was applied.

The following Figure 2.6, Figure 2.7 and Figure 2.8 illustrate the pattern of outflow hydrographs under different storm events. These plots are prepared in Microsoft Excel worksheet using time series result of Total Inflow at outfall O1 from GeoSWMM model results show that no flooding or surcharge condition occurs in the designed conveyance network. Besides, the Steady Flow routing results coincide with the results of Tutorial 02 which had no routing. This is due to the fact that in Steady Flow routing, runoff of a particular subcatchment discharged to its outlet is instantly transferred to the entire site’s outfall (i.e. no lag time). Thus, the effect of channel routing, backwater and storage are ignored here.

On the other hand, both Kinematic Wave and Dynamic Wave routings show slight lag time and reduced peak flow. These effects are more common in Dynamic Wave routing because of consideration for backwater and storage within the conveyance network. Thus, these two routing methods produce flatter shaped hydrographs than those produced under Steady Flow.

Note that, under Steady Flow condition, differences in hydrograph shape may occur between with channel routing and without channel routing if flooding generates due to excessive outflow. Under such flooding events, flow amount exceeding conveyance network capacity is reported as flooding loss. Therefore, the outflow received at the site of the outfall including the conveyance network will be lower than the outflow received excluding any conveyance system. In the latter case, total runoff from contributing all subcatchments is directly discharged to the site outfall.

Effect of Dynamic Wave Routing on the Runoff Results

Table 2.9 compares total runoff volumes, runoff coefficients, and peak discharges at the site outfall O1 computed for the developed model without routing (from Tuutorial 01 post-developed runoff) with the Dynamic Wave routing results (obtained from this tutorial). These values come directly from GeoSWMM's status report. In terms of runoff volumes and coefficients, the results obtained with routing are identical to those found in Tutorial 01 Post-developed runoff where no hydraulic routing was considered. The effects of Dynamic Wave routing are observed in the comparison of peak runoff, which demonstrates the implication of backwater and storage in the conveyance network. In this tutorial, when Dynamic Wave routing is applied, the peak runoff is reduced by 9.3% for 100-yr storm event as per peak total inflow at O1.

Table 2.9: Comparison Of Runoff for Developed Condition With Dynamic Wave Routing And Without Routing

Major Outcomes

The major outcomes from the analysis carried out in this tutorial are presented below:

• A runoff collection system can be represented as a network of links and nodes, where the links are conduits (such as natural canals, grass swales, street gutters, and circular culverts) and the nodes are the points where the conduits join to one another.
• An iterative process that proceeds from upstream to downstream can be used to determine the minimum conduit size needed to prevent flooding under a particular extreme design event.
• Steady Flow hydraulic routing produces outlet discharges identical to those produced without routing unless there is flooding in the drainage system.
• Dynamic Wave and Kinematic Wave routing produce smaller peak runoff discharges than models without routing (Tutorial 01) due to storage and possible backwater effects within the channels. Routing with Dynamic Wave resulted in a decrease of 9.3% for the 100-yr peak outfall flow in the study site.
• Except for flooding, the choice of routing method (Steady Flow, Kinematic Wave or Dynamic Wave) does not affect that much the total volume of runoff that leaves the study area through the outfall.