b, T'oshassuck River . The discharge from the Moshassuck 

 River (drainage area ■23.8 square miles) for the transposed Sep- 

 tember 1938 flood was obtained in the same manner as for the 

 Woonasquatucket River, The basin was divided into three areas: 

 (1) the three-square mile city area, (2) the Moshassuck River 

 (drainage area = 10,1 square miles ), and (3) its tributary, the 

 V/est River (drainage area = 10,7 square miles), above the city 

 limits. Due to a lack of stream flow records in this basin, no 

 separate unit hydrographs were developed for either the Moshassuck 

 River or the West River, The unit graph developed for the Woonas- 

 quatucket River at Centerdale was used to derive the runoff for 

 both the Moshassuck and West Rivers at the city limits. The rain- 

 fall, runoff and discharge values obtained in this study are 

 summarized in Table C-l;, 



C-8. DESIGN FLOOD FOR LOWER BAY BARRIERS 



For the hurricane protection studies at the Lower Bay Barriers, 

 as vrell as the preliminary investigations at the Fields Point, 

 Conimicut Point, the Middle Bay sites, the transposed September 

 1938 storm was centered over the Blacks tone River, The rainfall 

 distribution was based on the Corps of Engineers' "llaximuni Average 

 Depth-Duration of Rainfall" for New England over a drainage area 

 of ^00 square miles. The residual rainfall was applied to the 

 other basins and to the "local" areas. The total acciimulative 

 rainfall for the 500-square-mile area amounted to 111,2 inches 

 for a 72-hour period and the residual rainfall was 13 .O inches. 

 The inflow hydrograph at the Lower Bay site, including the con- 

 tributions from the various rivers and "local" areas, is shown 

 on Plate C-6, 



a. Blacks tone River , This river, with a drainage area 

 of 539 square miles, flows into the Providence River just below 

 Fox Point, A 6-hour unit hydrograph was developed based on floods 

 of record at the U, S, Geological Survey gaging station at Woon- 

 socket, Rhode Island (drainage area = lil6 square miles). This 

 unit graph was routed to the mouth of the river, using a 6-hour 

 lag. Separate 6-hour unit graphs were developed synthetically 

 for the Ten Mile River (drainage area = 57,6 square miles) and 

 the local area between Woonsocket and the mouth of the Blackstone 

 River (drainage area = 65, U square miles). The three unit graphs 

 were added to give a total 6-hour unit hydrograph at the mouth. 

 An infiltration rate of J. inch per hour was used and the rain- 

 fall excess applied to the unit hydrograph to obtain the maximum 

 discharge. The results of this computation are included in Table 

 C-li. 



C-7 



