DESIGN PHASE

The preceding phases identify the need for, and establish the viability of, a stormwater pump station. The detailed design phase follows. This section describes the stages in the process, in which the detailed design of the pump station components is performed. Subsequent chapters

provide the criteria and detailed procedures. The discussion is predicated on the assumption that the roadway geometry has been established.

4.7.1.1 Field Survey and Data Development

The data collection needs are similar to those required for normal drainage design of a highway project including establishing:

• utility sizes, locations, and ownership,

• outfall location, cross sections, profile grades, and roughness characteristics,

• right-of-way locations,

• roadway cross-sections, horizontal alignment, and vertical profile,

• drainage area sizes and runoff characteristics,

• collection system sizes, layout, and profile, and

• tie- in elevations.

4.7.1.2 Stage 1: Site Planning

The site planning stage involves evaluating suitable locations for the pump collection system, storage unit (if desired), and pump house. For detailed considerations, refer to Section 5.2 - Considerations for Site Planning.

4.7.1.3 Stage 2: Identification of Design Criteria This stage involves establishing:

• design frequency,

• peak outflow (if appropriate),

• station type (wet-pit station or dry-pit station),

• design philosophy,

• minimum storage volume,

• maximum allowable highwater, and

• discharge velocity.

The criteria must be established prior to proceeding with design of the storage system and pump system. Refer to Chapter 6 for discussion on these and other considerations.

4.7.1.4 Stage 3: Hydrologic Analysis The hydrology stage involves determining:

• drainage boundary and area,

• runoff characteristics,

• design runoff hydrograph(s), and

• cumulative inflow.

The designer may begin hydrologic analysis after establishing the roadway alignment and design criteria. State of the practice hydrologic methods are employed such as appear in the FHWA publication Highway Hydrology.⁵

4.7.1.5 Stage 4: Determination of Storage

This stage comprises designing or evaluating the following for providing storage volume:

• collection system,

• storage unit, and

• wet well.

4.7.1.6 Stage 4a: Collection System Design/Evaluation

The collection system stage establishes the collection system layout, element sizes and shapes, and evaluates the storage in the collection system between a maximum allowable high water and a minimum elevation in the pump station. Some agencies specifically design parts of the

collection system by iteration to provide storage, in lieu of a bona fide storage unit, to minimize the required pump capacity.

4.7.1.7 Stage 4b: Storage Unit Sizing

Many highway agencies design a specific storage unit to meet minimum desired storage volumes or to optimize the required storage volume and pumping capacity. If such a unit provides only the minimum required volume, the sizing process can begin after the roadway alignment and design criteria have been established. If optimal storage is to be provided, the process may proceed as indicated in the flowchart, but several iterations of storage unit sizing and pump configuration may be necessary.

4.7.1.8 Stage 4c: Pit Dimensions and Wet Well Storage One of two approaches is usually taken:

• designer chooses a preliminary pit type and size, determines what pumps are needed, and then ensures clearances meet appropriate specifications for needed pumps, or

• designer ignores wet well storage and establishes pump sizes and number, then sizes the pit based on appropriate criteria.

The approach is usually a matter of choice. Either approach will often result in some leve l of iteration because the pump capacity is dependent on total storage (of which wet- well storage is a component), and wet-well storage is dependent on pit dimensions. Minimum pit dimensions are dependent on pump number and capacity. When storage units are used, the wet well storage is not usually the major part of system storage and minor size adjustments may not alter the pump selection.

4.7.1.9 Stage 5: Trial Pumping Configuration

The design of a typical pump station is an iterative process. The only condition under which a closed solution is likely is the combination of a triangular/trapezoidal runoff hydrograph method and one pump. (A minimum of two pumps is recommended for all highway pump stations.)

The process requires the designer to establish trial types, sizes and numbers of pumps. The subsequent stages will evaluate the performance of the trial configuration. If the configuration proves to be either oversized or undersized, the designer must establish a new configuration and repeat the evaluation.

The trial pump configuration may follow establishment of the design criteria. The following table shows the necessary order of the design process depending on the selected or required approach.

If the design approach is to: Then:

Fix the peak outflow The tria l pump configuration may be established after development of cumulative inflow volume (mass inflow curve) and before evaluating storage.

Optimize storage/pump capacity

The trial pump configuration should follow hydrograph development and trial storage sys tem evaluation. A series of iterations of storage size and pump configuration will be necessary to establish which combination achieves the best balance of pump size, storage unit size, and construction cost Fix the storage volume The trial pump configuration should follow

development of cumulative inflow volume and trial storage system evaluation.

4.7.1.10 Stage 6: Trial System Evaluation

The trial system evaluation stage includes various procedures to determine whether or not the trial pump configuration and storage system achieve the following objectives:

• peak outflow equal to or lower than target peak (if appropriate),

• highest water level in station does not exceed maximum allowable high water for design storm,

• pump configuration is not excessive, and

• selected pumps comply with manufacturer’s specifications, such as cycling time and Net Positive Suction Head (NPSH).

The evaluation includes the following procedures, a suggested order of which appears in the Design Process Flowchart:

• development of inflow hydrograph,

• development of mass curve from inflow hydrograph,

• establishing trial storage unit size,

• selection of initial wet well dimensions,

• development of stage versus storage relationship,

• establishment of usable storage,

• establishment of trial pump switching/sequencing (cut-on, cut-off elevations),

• mass curve routing,

• routing sufficiency checks,

• determination of Total Dynamic Head,

• development of system head curves,

• establishing an operation range,

• establishing power requirements,

• selection of pumps and piping from manufacturers' performance curves and catalogue data,

• performing cycle time checks, and

• design optimization (balancing storage with pump type and size), if desired.