BACKGROUND

You have been hired by a developer to help with the design of a major development complex.  The complex will consist of small businesses, shopping areas, multiunit housing, and larger single-family estates.  The proposed development site, which totals 73.3 acres, sits within a small catchment with a creek that drains into an ecologically important stream at the outlet of the catchment. There is also an existing housing development southeast of proposed development

Because of the water-quality limited designation of the stream, the developer needs to design a storm water system that will minimize the impact on it.  One approach is to collect and route stormwater to wetlands adjacent to the development site for natural infiltration into the groundwater system.  Another idea is to route storm water through a storm-sewer network and a holding pond to attenuate the flow into the stream and allow for settling of large particulates.

The developer would like to use the wetlands approach since it is low in initial cost and long-term maintenance.  However, a citizens committee from a nearby existing development is trying to block discharging to the wetlands since they believe the additional discharge to the wetlands will cause the nearby water table to rise, causing basement flooding in their neighborhood.

Figure 1 - Proposed development site and surrounding area.

OBJECTIVE

Your objective is to determine the feasibility of the two approaches.  The major steps of the project involved include:

1. Designing the storm-sewer network (i.e., determining pipe sizes and pipe slopes)
2. Assessing the feasibility of the wetland infiltration approach (assessing water table change at the monitoring well)
3. Designing the holding pond (performing pre- and post-development peak flow analysis and reservoir routing analysis)

Further instructions and retails regarding each major step are provided in the sections below. Detailed tables and figures needed for your analysis are included.

DELIVERABLE

Please assemble a professional-level technical report, written following the guidelines used in CE321.  Major components include:

• Title page
• Executive Summary
• Table of Contents
• Introduction
• Methods
• Results and Discussion
• Conclusions
• Appendix (Supporting Information)

Make sure to document your entire procedure and list all assumptions.  Answer all questions and include all items mentioned in the sections.  Make sure your report is professional and well written.  It must be typed using a word processor.  All illustrations, graphs, figures, and equations must be computer generated. Grammar, spelling, sentence structure, and completeness will all factor into your grade in addition to your ability to solve the problem.

Keep in Mind:

The problem has been formulated such that all the data you need is contained in this report.  When looking at your results, keep in mind if they seem reasonable.  This will help tell you if you are on the right track.  Make sure to list ALL assumptions and to show and explain how you solved the problem.  There is no need to show every calculation but a step by step summary, including equations, variables, and definitions, should be a part of your report.  Be sure to include, in clear form, your spreadsheet analysis.  Using a spreadsheet does not omit the need for a summary of your calculations.

FURTHER INSTRUCTIONS

1. Storm-Sewer Design

Size each pipe for its peak flow using the Rational Method (Q_max=CiA).  Data for each sub-division (I, II, III,…) and the storm-sewer pipes (A,B,C,…) are presented in Tables 1 and 2.

• Use the 10-yr-frequency return for this analysis.
• Time to inlet can be calculated using the velocity extracted from Figure 2 below, and the given length to outlet for each subdivision. You can check your results using an empirical relation (e.g., the Brasby-Williams equation), but please use the time-to-inlet calculated from the graphical approach as your proceed through your calculations.
• Travel-time in the sewers can be estimated assuming a water velocity of 5ft/s.
• Determine the time of concentration to reach each junction (manhole 1,2,3, …). Then use the following relation to determine the intensity in each pipe:

  

  where i= intensity, in./hr; t_c = time of concentration, min; A,B = constants that depend on the return period and climatic factors (see Figure 3).

• Calculate the flow to each junction (or, the flow into each pipe)

• Determine the pipe diameter by using Mannings equation for a pipe flowing completely full. You can use the street slope as an initial guess.
• Make sure you recalculate the velocity when the pipe is sized, and the flow is known. The velocity should be between the design parameters of 1-3 m/s.If all velocities are not in this range, change the slopes to bring them into limits.Comment on new slopes as compared to ground slopes.

It is recommended that you do the calculations using a spreadsheet.  Present your results with a table of the pipes that shows their design size, flow capacity, and slope.  Comment on the design parameters: do they seem reasonable enough?

Table 1- Sub-division data

Table 2 – Storm-sewer data

Figure 2 – Average velocity of overland flow (from U.S. SCS 1975b). Scanned from Gupta (2008): Hydrology and Hydraulic Systems.

Figure 3 – Intensity-Duration Constants for Various Regions. Scanned from Gupta (2008): Hydrology and Hydraulic Systems.

2. Wetland Infiltration Analysis

The use of an infiltration pond approach is attractive because it promotes natural infiltration and artificially recharge the aquifer; but it is only viable if it does not adversely impact the nearby residential community (i.e., it does not increase the water table in the residential area).

The infiltration pond needs to be designed based on a 24 hour, 100-year (return period) storm event.

• Use the MDOT rainfall intensity-duration curves (Table 3) to determine the effective rainfall depth for the design event.

A groundwater model can be created to assess the impact (or potential water level increase). Develop an unsteady groundwater flow model using MAGNET to simulate potential water level increase in the residential community area in response to stormwater infiltration in the wetlands area.

• Create a model domain that is 6,400 ft in the x- (west-east) direction and 5,000 ft in the y- (north-south) direction. (Use ‘Synthetic case area’ option)
• Save the site map (Figure 1) as a picture and overlay it in the MAGNET modeling environment, using the model domain as the image extent.
• Aquifer properties: assume an average land surface elevation of 7ft and a bottom elevation of -90 ft.; consider the stream to be fully connected to the aquifer and to have an average stage of 0ft; the aquifer is estimated to have a hydraulic conductivity of 125 ft/d and a specific yield of 0.2.
• Assume that the entire storm runoff is drained through the wetland over 7 days.
• The wetland area can be represented as a zone of prescribed flux (depth/time) during the 7 days of infiltration.The wetland area totals about 23 acres. (Hint: use the effective rainfall depth for the design storm event along with information about the wetland acreage, total development acreage, and duration of infiltration to calculate an effective recharge rate to apply during the 7-day infiltration event).
• Set up a 100-day transient simulation (use a time-step of 2 days or less) with a 7-day period of infiltration from the wetland starting on day 3. Assume an initial water table elevation of zero everywhere in the model domain. Use a relatively fine grid (NX=80).
• Assume otherwise ‘dry conditions’ during the transient simulation (recharge=0 in all areas except the wetland).
• You will need to expand your model domain before simulating to ensure that the impacts do not propagate to the 'no-flow' lateral boundaries (otherwise, your results will be inaccurate). Before simulating, expand the western (left), eastern, and southern boundaries by at least 2,500 ft EACH.

Any “significant” change in the water table (>2 in.) warrants further site-specific data collection to improve the accuracy of the model predictions.

MAGNET/MODELING HINTS:

• Use 'Synthetic mode' in MAGNET to create a model domain with the same dimensions as described above.   
• Go to: 'Other Tools' > 'Utilities' > and click "Go to Synthetic Case Area' to access Synthetic mode. (Click OK to prompts that appear).
• Once synthetic model domain appears, go to 'Utilities' > and click 'Geometry unlocked'. Then click anywhere inside the model domain. After answering OK to the prompts that appear, you will be able to click-drag any of the vertices to see the distance between vertices. NOTE: vertices are numbered, and distances are indicated by d##, e.g., d21 is the distance from vertex 1 to vertex 2.
• Once you have the correct dimensions, you can click 'Geometry Locked' to lock-in the shape/geometry (‘Other Tools’ > ’Utilities’ > ‘Geometry Locked’).
• Overlay the provided SiteMap image file included at the top of the problem description (Right-click > Save Picture As…)  
• Go to: 'Other Tools' > 'Utilities' > 'Overlay myImage' and follow the instructions in the Help Page ('?' button)
• Click the 'Use Domain Extent' button to fix the image to the established domain size. (This should be after choosing the image file but before clicking 'Upload'.)
• Use a Zone feature to represent the stream as a prescribed head feature (stream stage given above).
• Assign aquifer properties (elevations, hydraulic conductivity, specific yield) in the Aquifer Properties tab of the Domain attributes Menu (‘Conceptual Model Tools’ > ‘DomainAttr’ > ‘Aquifer Properties’)
• By default, a MAGNET model is a steady-state (time-independent). To make the model transient time-dependent), go to the Simulation Settings tab of the Domain Attributes menu, then click the box next to ‘Modeling Transient Flow’.  Assign a Time-Step and Simulation Length consistent with the information given above.
• The starting head in your model should be zero everywhere; use the options under ‘Initial and Boundary Conditions for Head’ in the Simulation Settings tab to apply this condition.  
• Use another Zone feature to represent the wetland and the 7-day infiltration event.
• After finalizing the Zone geometry, go to the ‘Sources and Sinks Prescribed’ tab and check the box next to ‘Recharge – Quantity & Quality’ to assign the zone as a recharge (infiltration) source.

• Under this option, check the box next to ‘Transient’ to make the recharge time-varying, then click the ‘Transient’ button to enter the recharge data.
• In the sub-menu that appears, make sure the Starting Date is the same as the Starting Date set in the Simulation Settings tab, then use the table provided to add recharge data for the first ten days of simulation (see additional instructions in the sub-menu).
• Note that you do not need to add recharge data for all simulation time-steps. MAGNET will apply the recharge value of the last entered data point for all remaining time-steps (e.g., if you apply a recharge of 0 on day 11, the remaining days will also have a recharge of 0 if no other data are provided).

• A monitoring well should be added to the location indicated on the site-map so that the water table elevation can be monitored during transient simulation (‘Conceptual Model Tools’ > ‘Wells’ > ‘Draw Well’> check box next to ‘Monitoring Well’ after placing the well).
• Simulate (‘Simulation Tools’ > ‘Simulate’) and monitor the head at the monitoring well location (‘Analysis Tools’ > ‘Analysis’ > click ‘Display Charts’).Use the information provided in the monitoring well chart to determine the maximum change in the water table during transient simulation.

Table 3 – Intensity-duration-frequency estimates for mid-Michigan climatic zone.

3. Holding Pond Design

You will need to redirect the subdivision runoff to a holding pond.  You will need to size the holding pond so that post-development peak flow to the stream is the same as pre-development peak flow.  To do this, you will use the stormwater runoff from the development as an inflow to the pond.  The outflow will be controlled by one or more drain-pipes at the bottom of the pond. 

The holding pond needs to be designed based on a 24 hour, 100-year (return period) storm event. To compare pre- and post-development peak flow to the stream, we must need to compute the runoff hydrograph for both conditions for the design event (24hr, 100 yr. storm). Include all hydrographs in your report.

A. Unit hydrograph:

• A creek drains the entire catchment under natural conditions. Use the historical creek flow data (see Table 4) to create a unit hydrograph for the catchment (pre-development).
• The unit hydrograph (UH) derivation first requires base flow separation (you may assume a constant), and then computing net runoff depth (or effective rainfall depth that contributes to runoff).Then, UH = measured runoff hydrograph / effective rainfall depth.
• The unit hydrograph is applied to create a pre-development hydrography for the design event (24hr, 100yr. storm) – see below.

B. Design Storm Rainfall Intensity:

• Use the MDOT rainfall intensity-duration curves (see Table 3 above) to determine the effective rainfall depth for the design event.
• Distribute the overall rainfall depth across the design event duration (24hr) using the Type II NRCS rainfall distribution data provided in Table 5.
• Calculate the rainfall intensity in each 1-hour interval.

C. Pre-Development Hydrograph:

• Apply the Unit Hydrograph for every 1hr rainfall “event” from the 24hr storm. In other words, 1hr Hydrograph = UH*(rainfall intensity for that hour)
• Apply the lagging method to the different 1hr hydrographs, adding them together to get the overall (24hr) hydrograph for the design event.
• Add back baseflow to obtain the total flow hydrograph for a 24hr, 100 yr. event under natural conditions.

D. Post-Development Hydrograph:

• The Rational Method can once again be used to compute peak flow in the catchment during post-development conditions for the design event (24hr, 100yr storm).
• Instant routing can be assumed from the catchment to the entry of the detention pond; in other words, the inflow hydrograph to the pond can be calculatedas 1-hr intervals of “constant flow”, where the inflow rate is Q_max = CiA
• Again, analyze the Type II rainfall distribution data to get the rainfall intensity for each hour of the 24 design event.

E. Pond and Outlet Drain Design:

Size the pond and outlet drain-pipes.  Recall that the maximum flow for the design storm with pre-development conditions cannot be exceeded by discharge from the pond.

• Perform a reservoir flood routing through the pond using the following equation for flow out of the pond.

  

  where Qo is the outflow from the pond, C is the discharge coefficient (use 0.6), A is the cross-sectional area of the outlet pipe, g is the acceleration due to gravity, and h is the depth of the water in the pond.

• Assume a straight-sided (vertically-speaking), circular holding pond (S=Ah, A = area of the pond) and determine the radius of the pond such that the outflow is no more than the pre-development peak runoff.Assume the outflow elevation is the same as the base of the pond.
• The detention pond depth should be reasonable not to be a hazard
• Follow the reservoir storm routing method discussed in class.
• Set up your spreadsheet so the storage component is a function of the diameter of the pond.The outflow hydrograph should be part of your report.

Table 4 – 1-hr storm Event runoff hydrograph data for small creek, just upstream of its junction with the stream.

Table 5 - standard 24-hour NRCS rainfall distributions