Civil Engineering Land Surveying Wetland Science

Transcription

1 Civil Engineering Land Surveying Wetland Science July 5, 2017 Town of Littleton Conservation Commission c/o Amy Green, Conservation Coordinator Shattuck Street Municipal Building 37 Shattuck Street Littleton, MA Re: Discovery Narrative Infiltration Basin 3 Reconstruction Durkee Farm Estates Dear Ms. Green, In response to the letter from Green International Affiliates, Inc. dated June 16, 2017, Hancock offers the following responses: Please note, items not specifically addressed in this response letter either do not require a response or information will be issued under separate cover. 1. Confirmatory test pits were required at each infiltration basin to verify soil texture and to confirm seasonal high groundwater during construction as a condition to Planning Board approval. Two test pits were conducted in Infiltration Basin No. 3 on May 18, These test pits confirm the assumed soil texture and a depth to seasonal high groundwater of 30 to 36 inches. Per Vol. 3, Ch. 1 of the Massachusetts Stormwater Handbook, a groundwater mounding analysis should be provided since less than 48 inches of separation is provided between the bottom of basin and seasonal high groundwater. A mounding analysis was conducted using freely available web software by HydroSOLVE, Inc. ( and calculation worksheets are attached herewith. Groundwater Mounding Criteria for Rectangular Recharge Area: Hydraulic Conductivity (K): Soil properties were determined from USDA NRCS Web Soil Survey. Soil type at the location of Infiltration Basin 3 was listed as Wareham Loamy Fine Sand, a Class A hydrological soil group. The Saturated Hydraulic Conductivity, K sat, was rated at micrometers/sec which equals 26.0 ft/day. Specific Yield (ε): Determined from textural analysis to be loamy sand corresponding to a specific yield of Initial Saturated Thickness (hi): Determined from redoximorphic features and depth of deep observation test holes. Determined to be 30 from bottom (base) of IB#3 (test pit 2017 IB#3 2A). Length of Recharge Area (A): DANVERS, MA 185 Centre Street Danvers, MA Phone: (978) MARLBOROUGH, MA 315 Elm Street Marlborough, MA Phone: (508) CHELMSFORD, MA 34 Chelmsford Street Chelmsford, MA Phone: (978) PRINCETON, MA 206 Worcester Road Princeton, MA Phone: (978) SALEM, NH PO Box 205 Salem, NH Phone: (603)

2 2 The length of the system is approximately 84. Width of Recharge Area (B): The width of the system is approximately Recharge Rate (w): (Recharge volume / bottom of practice area) [ft] x draw down time [days]. Recharge Volume = 4,658 cf Bottom Area = 5,110 sf Rawls Rate = 2.41 in/hr Drawdown Time = 4,658 cf / [(2.41 in/hr)(1 in / 12 ft)(5,100 sf)] = 4.5 hours = days Therefore, Recharge Rate (w) = (4,658 cf / 5,110 sf)( days) = ft/day Time (t): 1 day, data immediately following a twenty four hour event at the start of 72 hour drawdown. Mounding Analysis Result: Infiltration Basin 3: 0.65 (<2 of ESHGW separation provided) 5. We recommend that the pipe/stone wick drawdown device be realigned to discharge into the outlet control structure to prevent short circuiting of water through the pipe bedding surrounding the pipe. The plan has been updated to specify anti seep collars in the location of the pipe within the infiltration basin berm to prevent short circuiting of water though the pipe bedding to allow for the existing outlet control structure to be used. 6. Hancock should review the operation and maintenance of the drawdown device with the Highway Department, as the Highway Department will be responsible for the device following acceptance of the development by the Town. The Operation & Maintenance of the drawdown device will be reviewed with the Highway Department and any changes requested submitted to the Town prior to commencement of reconstruction. 7. We recommend that straw wattles be installed downstream of the drawdown device outlet, between the outlet and the adjacent wetland to prevent erosion and sedimentation. An additional row of straw wattles are proposed at the point of discharge as identified on the plan. 8. To prevent further sedimentation of the infiltration basin, we recommend that all upstream catch basins and associated inlet filter bags (silt sacks) tributary to Infiltration Basin No. 3 be cleaned prior to implementing the restorative actions to the basin bottom outlined in the letter and plan. This activity will remove sediment load that could potentially be later resuspended and conveyed downstream, to the infiltration basin.

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5 6/23/2017 Mound Benearth Rectangular Recharge Area :. Aquifer Test Forum Advanced Software for Pumping Tests Slug Tests Developed and Sold by HydroSOLVE, Inc. Constant Head Tests The World's LEADING Aquifer Test Analysis Software Since 1989! Home News Product Order Support Training Contact More AQTESOLV Home > Aquifer Test Forum > Methods > Rectangular Mound Groundwater Mound Beneath Rectangular Recharge Area by Glenn M. Duffield, President, HydroSOLVE, Inc. Share Hantush (1967) presented the following equations for predicting the maximum height of the water table beneath a rectangular recharge area: h m 2 h i 2 = Z m (t) = (2w/K)νtS*(0.5A/(4νt) 1/2,0.5B/(4νt) 1/2 )..... (1) ν = Kb/ε..... (2) b = 0.5[h i (0) + h(t)]..... (3) where h m is maximum height of mound above aquifer base (i.e., maximum saturated thickness of aquifer beneath recharge area); h i is initial height of water table above aquifer base (i.e., initial saturated thickness of aquifer); K and ε are hydraulic conductivity and storativity (specific yield) of aquifer, respectively; w is constant rate of percolation from rectangular recharge area of length A and width B; b is a constant of linearization; and the function S* is an integral expression (see Hantush 1967). The aquifer is unconfined and assumed to have infinite extent. Aquifer Test Forum» Forum» Methods Pumping Tests + Derivative Analysis + Leaky Aquifers + Skin Effect + Recovery Tests + Step Drawdown Tests Slug Tests Tidal Effects» Calculators Radius of Influence Specific Capacity to T (Approx.) Specific Capacity to T (Exact) Circular Mound Rectangular Mound» Contact If infiltration ends at time t=t 0, Hantush (1967) applied the principle of superposition to compute the decay of the mound as follows: h m 2 h i 2 = Z m (t) Z m (t t 0 )..... (4) Equation (1) is nonlinear owing to the definition of b in Equation (3); however, the solution is readily obtained by successive approximation. Follow Follow Results of Groundwater Mounding Calculation Solution by Successive Approximation Iteration b h m * % Change E E E E 05 K [L/T] h i [L] A [L] B [L] w [L/T] t [T] h m [L] maximum water table rise (h m h i ) at time t = 1 is /2

6 6/23/2017 Mound Benearth Rectangular Recharge Area :. Aquifer Test Forum Return to Groundwater Mounding Calculator Click here for a benchmark for this calculator. Hantush mounding calculations with contouring now available in AQTESOLV. AQTESOLV The World's Most Advanced Software for Aquifer Test Analysis Home News Versions Tour FAQ Demo Upgrades Order! Support Training Contact Search Site Map Privacy Policy Trademark Information Copyright HydroSOLVE, Inc. All Rights Reserved. 2/2

7 ' 39'' W 71 30' 11'' W Saturated Hydraulic Conductivity (Ksat) Middlesex County, Massachusetts ' 19'' N ' 19'' N Soil Map may not be valid at this scale ' 4'' N 42 31' 4'' N Map Scale: 1:3,370 if printed on A landscape (11" x 8.5") sheet. N Meters 270 Feet Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 19N WGS Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey ' 39'' W 71 30' 11'' W /23/2017 Page 1 of 4

8 Saturated Hydraulic Conductivity (Ksat) Middlesex County, Massachusetts MAP LEGEND MAP INFORMATION Area of Interest (AOI) Soils Area of Interest (AOI) Soil Rating Polygons <= Soil Rating Lines > and <= > and <= > and <= > and <= Not rated or not available <= > and <= > and <= > and <= > and <= Soil Rating Points Water Features Not rated or not available <= > and <= > and <= > and <= > and <= Not rated or not available Streams and Canals Transportation Rails Interstate Highways US Routes Major Roads Local Roads Background Aerial Photography The soil surveys that comprise your AOI were mapped at 1:25,000. Warning: Soil Map may not be valid at this scale. Enlargement of maps beyond the scale of mapping can cause misunderstanding of the detail of mapping and accuracy of soil line placement. The maps do not show the small areas of contrasting soils that could have been shown at a more detailed scale. Please rely on the bar scale on each map sheet for map measurements. Source of Map: Natural Resources Conservation Service Web Soil Survey URL: Coordinate System: Web Mercator (EPSG:3857) Maps from the Web Soil Survey are based on the Web Mercator projection, which preserves direction and shape but distorts distance and area. A projection that preserves area, such as the Albers equal-area conic projection, should be used if more accurate calculations of distance or area are required. This product is generated from the USDA-NRCS certified data as of the version date(s) listed below. Soil Survey Area: Middlesex County, Massachusetts Survey Area Data: Version 16, Sep 14, 2016 Soil map units are labeled (as space allows) for map scales 1:50,000 or larger. Date(s) aerial images were photographed: 28, 2014 Sep 12, 2014 Sep The orthophoto or other base map on which the soil lines were compiled and digitized probably differs from the background imagery displayed on these maps. As a result, some minor shifting of map unit boundaries may be evident. Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 6/23/2017 Page 2 of 4

9 Saturated Hydraulic Conductivity (Ksat) Middlesex County, Massachusetts Saturated Hydraulic Conductivity (Ksat) Saturated Hydraulic Conductivity (Ksat) Summary by Map Unit Middlesex County, Massachusetts (MA017) Map unit symbol Map unit name Rating (micrometers per second) Acres in AOI Percent of AOI 32B Wareham loamy fine sand, 0 to 5 percent slopes 51A Swansea muck, 0 to 1 percent slopes 71B 73B 103B 103C Ridgebury fine sandy loam, 3 to 8 percent slopes, extremely stony Whitman fine sandy loam, 0 to 3 percent slopes, extremely stony Charlton-Hollis-Rock outcrop complex, 3 to 8 percent slopes Charlton-Hollis-Rock outcrop complex, 8 to 15 percent slopes % % % % % % Totals for Area of Interest % Description Saturated hydraulic conductivity (Ksat) refers to the ease with which pores in a saturated soil transmit water. The estimates are expressed in terms of micrometers per second. They are based on soil characteristics observed in the field, particularly structure, porosity, and texture. Saturated hydraulic conductivity is considered in the design of soil drainage systems and septic tank absorption fields. For each soil layer, this attribute is actually recorded as three separate values in the database. A low value and a high value indicate the range of this attribute for the soil component. A "representative" value indicates the expected value of this attribute for the component. For this soil property, only the representative value is used. The numeric Ksat values have been grouped according to standard Ksat class limits. Rating Options Units of Measure: micrometers per second Aggregation Method: Dominant Component Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 6/23/2017 Page 3 of 4

10 Saturated Hydraulic Conductivity (Ksat) Middlesex County, Massachusetts Component Percent Cutoff: None Specified Tie-break Rule: Fastest Interpret Nulls as Zero: No Layer Options (Horizon Aggregation Method): Depth Range (Weighted Average) Top Depth: 0 Bottom Depth: 50 Units of Measure: Inches Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 6/23/2017 Page 4 of 4