Hydraulic Modeling

G&D is experienced in the development and application of computer hydraulic models, using a variety of software tools, to evaluate sanitary sewer collection systems for the purposes of capacity analysis, guiding I/I control programs, and capital planning.

Establishing Modeling Baseline

G&D begins any modeling project by first establishing baseline sewer modeling assumptions, evaluation criteria and performance metrics. Examples include default system model parameters such as pipe roughness values as a function of pipe material and age; model inputs such as the 2-year and 10-year storm hyetographs; and performance metrics such as degree of backwater, if any, allowed. Finally, model calibration goals should be established a priori.

Modern software tools include a variety of model parameters that can be adjusted to accurately simulate the wet weather response of a sanitary sewer collection system to a given storm event. These may include parameters that represent the physical processes of antecedent soil moisture content, rainfall induced infiltration, and groundwater infiltration. Therefore, the calibration process is complex and must be consistent and the resultant parameter values documented clearly for the client. This results in the adoption of the various assumptions and criteria by the project team before the analyses begin.

Temporary Flow Monitoring to Support Computer Modeling

G&D has extensive experience conducting temporary flow monitoring studies to support hydraulic model calibration. Flow metering data is often used to develop hydrologic/hydraulic model parameters and to calibrate the model. Flow monitors are often located on the trunk sewers, which offer significant utility in model verification. However, supplemental temporary flow and rainfall monitoring is suggested to provide greater resolution and isolate areas by land use, pipe material, age, condition, and/or in areas of chronic problems including major connections to trunks to develop the model parameters themselves. Prudent site selection is critical to project success. G&D reviews available system data and performs site visits at the candidate metering sites as discussed elsewhere.

Collection System Model Calibration

The process of calibration is an on-going effort with any sanitary sewer system model, as the system is constantly changing and expanding. G&D is experienced in starting and documenting the process for a client. G&D will define the on-going data management concerns and approaches, and set up a plan for the long-term allocation of resources that a viable sewer model requires. The goals of calibration are to:

1. Deliver a technically sound and reliable simulation tool, in place and fully operational within the organization;
2. Integrate the model into the organization to support the required operations;
3. Produce a plan to sustain the modeling resource well into the future to support future modeling activities at minimal expense.

G&D estimates the distribution of the wastewater demand based on population distribution in the areas of study, socioeconomic level, and existence of commerce, industries, and public services, present and future. We disaggregate demands by sub basin, identifying areas with the highest discharges. Model calibration will account for the accumulation of silt and debris. Pump operating parameters will be input to the model including wet well dimensions, pump capacity, number of pumps, and operating levels. In addition, meeting with operations staff will provide valuable insight to the true functioning of the network.

G&D considers multiple often conflicting objectives when calibrating a sanitary sewer collections system model to support wet weather engineering studies:

1. Dry and wet weather flow volume and peak flow rate within 10 percent of the observed flow with similar shaped hydrographs.
2. Emphasis is placed on matching peak flow rates when the model is to be used to determine available capacity.
3. Greater depth of flow accuracy anticipated during un-surcharged dry weather flow.
4. Timing of minimum and maximum flows should be similar.
5. Multiple storm events simulated

Model Evaluation of System Performance

Model verification runs are often performed for historical storm events for which permanent flow monitoring data is available. The difference between the simulated and observed peak flows and volumes will be reported.

G&D often utilizes GIS coverages to summarize the results of the modeling analysis.

One of the first uses of the calibrated collection system model will be to perform a stress test to determine the maximum capacity of various elements of the system and to identify hydraulic bottlenecks. Model nodes are 'loaded' with synthetic hydrographs that increase linearly with time (i.e., ramp functions), which will ultimately cause the system to fail via capacity-related overflows. The flow in the system prior to failure indicates its maximum capacity. In some instances, hydraulic bottlenecks can be remedied cost-effectively.

The hydraulic model may also be used to evaluate the conveyance capacity of the various systems under both dry and wet weather conditions. This task will identify capacity shortfalls in the existing system under existing conditions. Capacity problems may be indicated by high flows, high depths of flow, surcharge, backwater, or the accumulation of silt/debris. Average dry weather velocities of less than 1.5 fps can be indicative of possible silt/debris buildup. Wet weather analyses will consider historic storm events of record, synthetic design storm events, and continuous simulations to evaluate the effect of back-to-back storm events on critical infrastructure. The following are identified during the wet weather analysis:

1. Pipes less than 75 percent full during peak flow (sufficient capacity)
2. Pipes less than 100 percent full (threatened)
3. Surcharges pipes (failing)
4. Overflows (failed)

Hydraulic Model Documentation

G&D may prepare a technical report documenting the flow monitoring, model development, and calibration processes and summarizing the results of project in the final documentation, which will highlight those manholes and pipes that exhibited capacity constraints under existing conditions. Capacity constraints include both surcharged sewers and flooded manholes. Results will be presented graphically illustrating the degree of constraint. Documentation will present a variety of data including:

1. Model assumptions and approach
2. Calibration parameters and results
3. Normal dry weather diurnal patterns for weekdays and weekends
4. Dry and wet weather response metrics
5. Volume and duration of capacity-related SSO's predicted by the model under various wet weather conditions
6. Recommended procedures for long-term model maintenance