Reservoir Consultant's Report

by Mohammad Kheirallah, February 2000

SECTION 3:   SPILLWAY DESIGN FLOOD ANALYSIS

3.1 Introduction

The WSE team completed a Hydraulic & Hydrogeologic analysis to assess the storage capacity and overtopping potential of Arlington Reservoir Dam under test flood conditions. The initial objectives of the analysis were:

  1. To assess the reservoir’s storage/discharge capabilities and overtopping potential for the Probable Maximum Flood (PMF).
  2. To assess the inundation zone affected by the dam failure during the PMF.
  3. At a later stage, we also completed the following analysis:

  4. Inflow Design Flood (IDF), which evaluated flooding conditions immediately downstream of the dam under the 100-year, 500-year, and PMF storm conditions with and without dam breach.

3.2 HEC-1 Analysis

3.2.1 General

As stated before, the dam’s principal spillway crest length is about 9 feet and is controlled by a small taintor gate. Based on our review of existing information, the spillway crest elevation (NGVD) is 153 and 159 with the taintor gate open and closed, respectively. The total drainage area was delineated based on the 7.5-minute USGS quadrangle for the dam, and divided into three sub-basins. Please refer to Figure 5, watershed map. Based on land use characteristics, the Soil Conservation Service (SCS) Soil Survey of the area and Haestad Methods’ Quick TR-55 computer program, a weighted curve number was assigned to each sub-basin. Lag time was estimated for each sub-basin using the SCS method for areas of less than 2,000 acres.

3.2.2 Inputs

The rainfall/runoff process was simulated using the Army Corps of Engineers’ HEC-1 Flood Hydrograph Package computer program. An inflow hydrograph was generated for the 100-year storm using a 24-hour, SCS Type III rainfall distribution, a design rainfall depth of 6.7 inches, and Dimensionless Unit Hydrograph methodology. The 100-year storm was used as a calibrating event. The HEC-1 runoff model was also run under the 500-year flood (total rainfall = 8.2 inches) for comparison purposes. The PMF was generated from a 72-hour Probable Maximum Precipitation (PMP) of approximately 18.06 inches. Incremental runoff values were sequenced and the storm orientated in such a way as to produce maximum runoff potential.

 

The following key input parameters were used in the HEC-1 simulation:

  • Total Drainage Area: 1.75 square miles
  • Rainfall: 72-hour, PMP
  • Runoff Potential: Expressed as SCS Curve Number (CN)

Sub-basin 1 = 64

Sub-basin 2 = 62

Sub-basin 3 = 70

  • Watershed Lag Time: Sub-basin 1 = 2.36 (Time of Concentration - Tc = 3.93 hours)

Sub-basin 2 = 1.20 (Tc = 2.00 hours)

Sub-basin 3 = 1.80 (Tc = 3.00 hours)

3.2.3 Results

As we have previously concluded, Arlington Pond Dam has insufficient capacity to safely pass the PMF without overtopping. Given the large volume of runoff generated under PMF conditions, there is no appreciable peak flow attenuation through the reservoir/dam. The peak PMF inflow/outflow is estimated to be about 3,100 cfs. The maximum pool elevation behind the dam reached 162.3 feet and 162.68 feet, under the ungated and gated scenarios, respectively. Since the crest of the dam embankment is about 162 feet, the resultant water level overtops the dam by about 0.4 and 0.7 feet, respectively. These results assume an initial operating pool level coincident with the spillway weir at an elevation of 153 in the ungated scenario and 159 in the gated scenario (NGVD) at the onset of the test flood. This assumption is consistent with normally accepted engineering practices for spillway design.

 

The result for the 500-year event, however, indicates that the dam’s combined spillway safely passes the flood flow under the ungated condition with a freeboard of 1.7 feet. For the gated scenario, the dam crest is overtopped by about 0.1 feet. Likewise, the ungated spillway has the capacity to pass the 100-year flood with about 2.9 feet of freeboard, but the gated spillway will result in overtopping the dam crest by less than 1 inch. The following table summarizes the spillway HEC-1 analysis results:

HEC-1 Results

 

 

Storm Event

Gate Conditions

Total Rainfall Vol. (in)

Peak Inflow (cfs)

Peak Outflow (cfs)

Maximum Stage (feet-NGVD)

100-year, ungated

6.7

700

450

159.06

100-year, gated

6.7

700

600

162.04

500-year, ungated

8.2

1,000

670

160.31

500-year, gated

8.2

1,000

980

162.14

PMF, ungated

18.06

3,100

3,100

162.37

PMF, gated

18.06

3,100

3,100

162.68

3.2.4 Discussion of the Results

The results of the test flood analyses indicate that Arlington Reservoir Dam’s spillway can pass approximately 572 cfs or 18 percent of the PMF inflow design flood. Thus, the dam will be overtopped for both gated and ungated conditions. The dam will also be overtopped during the 100-year and 500-year floods for the gated conditions. As indicated, these results assume an initial operating pool level coincident with the principal spillway gate crest elevation (159 MSL) at the onset of the test flood. This assumption is only valid during the summer months when the reservoir elevation is maintained at 159 feet for recreational use.

However, if the reservoir is maintained at elevation 153 feet with the gate opened, normal operating procedures for the rest of the year, then the dam will be overtopped only during the PMF. For the 500-year flood, the dam will have 1.7 feet of freeboard, and 2.9 feet during the 100-year flood. Although the peak outflows under these scenarios are large, the impact on the flood elevations in the Mill Brook will be negligible based on a study completed by the CE Maguire, Inc., in 1974.

HEC-1 computer printouts of input/output data are provided in Appendix B.

3.2.5 Spillway Modifications

HEC-1 analysis indicates that the dam is overtopped under the PMF for both gated and ungated conditions, and is overtopped when the dam is gated (i.e., gate closed) for the 100- and 500-year floods. For comparison purposes, we evaluated to what degree the emergency spillway length (crest EL = 159) would have to be increased to pass the various flood scenarios. For the PMF, with the principal spillway gate open (ungated), the emergency spillway needs to be widened from the existing 20 feet to about 180 feet to provide sufficient discharge capacity. The total length of the emergency spillway would need to be modified to about 30 and 60 feet to pass the gated condition for the 100- and 500-year floods, respectively. It is our understanding that the gate is typically closed during the summer recreation period when the water level is maintained at about elevation 159, and is lowered in the late fall. Extreme runoff events are typically associated with hurricane storms occurring between June and November. Thus, the ability to open the spillway gate and allow lowering of the reservoir level to about elevation 153 will be a critical operations component for the long-term safety of the dam.

 

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