This memo presents information regarding the number of at-risk roadways and buildings in Austin floodplains.Waller Creek Working Grou

Smith, Kevin

Zoun, Reem

English

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 1   Watershed Protection Department  MEMORANDUM  TO: Mapi Vigil, P.E  FROM: Reem Zoun, P.E.; Kevin Smith, GISP.  DATE: July 31, 2014  SUBJECT: Numbers of At-Risk Structures and Roadways in the City of Austin Floodplain   The following memo documents the number of structures located in the City of Austin (COA) 100yr floodplain and at risk of flooding during various storm events identified by the Creek Flood Hazard Mitigation (CFHM) Masterplan process and the Floodplain Management Office (FPO) where CFHM scoring is not yet available.  The memo also identifies the number of roadways at-risk of flooding based on CFHM Masterplan and the City’s Flood Early Warning System (FEWS) Program where CFHM scoring is not yet available. It is notable that the structure and roadway counts are available for the watersheds which have an effective hydraulic model; most but not all watersheds in the City’s Full Purpose Jurisdiction currently have an effective model. Summary A total of 5,799 structures in 46 watersheds are located in the 100yr floodplain in the City of Austin Full Purpose Jurisdiction.   The CFHM Masterplan has identified 5,118 structures in the 100yr floodplain located in 25 watersheds in the City’s Full Purpose Jurisdiction.  Out of the 5,118 structures in the 100yr floodplain, 2,561 are inundated during a 100yr flood; 1,152 during a 25yr flood, 539 during a 10yr flood and 48 are inundated during a 2yr flood.  The FPO identifies 681 structures in 100yr floodplain in 21 additional watersheds. A total of 393 roadways in 26 watersheds are at risk of inundation during a 100yr flood in the City of Austin Full Purpose Jurisdiction.     2  Structures in Floodplain and at Risk of Flooding in the City of Austin A total of 5,799 structures in 46 watersheds are located in the 100yr floodplain in the City of Austin Full Purpose Jurisdiction where a hydraulic model is available.  This number accounts for structures identified by the CFHM Masterplan process and additional structures located by FPO in watersheds that have not yet been scored by CFHM Masterplan. The CFHM Masterplan has identified 5,118 structures in the 100yr floodplain located in 25 watersheds in the City’s Full Purpose Jurisdiction.  The FPO identifies structures in additional watersheds that have not yet been scored by CFHM.  There are 681 structures in 100yr floodplain in 21 additional watersheds identified by the FPO. The following counts were generated for the 25 watersheds in the CFHM 2013 Masterplan: § Number of structures in the 100-yr floodplain in the Full Purpose City limits 1 = 5,118 § Number of structures at risk for flooding in the 100-yr floodplain  2,3,4 = 2,561 § Number of structures at risk for flooding in the 25-yr floodplain 2,3,4 = 1,152 § Number of structures at risk for flooding in the 100-yr floodplain 2,3,4 = 539 § Number of structures at risk for flooding in the 100-yr floodplain 2,3,4 = 48  The counts generated by the 2013 CFHM Masterplan are based on following conditions: 1. Only structures over 400 ft2 were evaluated.  Structures were identified based on the SDE 2006 building polygon layer.  Garage apartments, sheds, and other secondary structures over 400 ft2 identified from a visual review were also removed from this count. 2. At-risk structures for different storm events include all structures with estimated flooding depths of > 0 during the respective storm event.   3. Finished Flood Elevations (FFE) for structures are estimated to be 0.5ft higher than 2012 LiDAR data.  Elevations Certificates and survey data for FFE were available for few structures.  4. All structure counts listed above only includes structures in the City’s Full Purpose Jurisdiction.  Table 1 presents the numbers of structures in 100yr floodplain and counts of structures at risk of inundation in each watershed based on CFHM Masterplan.  The same information is presented in graphical form in Figure 1.   A list of 66 watersheds in the City of Austin area and number of structures in 100yr floodplain in full purpose jurisdiction in each of these watersheds where available is included in Table 3 in Appendix A.     3  Table	1	Structures	in	Floodplain	and	at	Risk	of	Flooding	based	on	CFHM	Masterplan	  Watershed	 Structures	in	100yr	Floodplain	Inundated	in	100yr	Flood	Inundated	in	25yr	Flood	Inundated	in	10yr	Flood	Inundated	in	2yr	Flood	Jurisdiction	1 Barton	Creek	1	 42	 19	 8	 3	 1	 Full	Purpose	2 Blunn	Creek	 12	 1	 0	 0	 0	 Full	Purpose	3 Boggy	Creek	 142	 69	 48	 22	 1	 Full	Purpose	4 Bull	Creek	2	 56	 22	 11	 1	 1	 Full	Purpose	5 Carson	Creek	 126	 91	 73	 55	 21	 Full	Purpose	6 Country	Club	East	 12	 1	 0	 0	 0	 Full	Purpose	7 Country	Club	West	 45	 9	 5	 4	 0	 Full	Purpose	8 Dry	Creek	East	 1	 0	 0	 0	 0	 Full	Purpose	9 Dry	Creek	North	 19	 2	 2	 1	 1	 Full	Purpose	10 East	Bouldin	Creek	 67	 16	 6	 0	 0	 Full	Purpose	11 Fort	Branch	 305	 135	 43	 13	 1	 Full	Purpose	12 Harris	Branch	 1	 1	 0	 0	 0	 Full	Purpose	13 Johnson	Creek	 56	 8	 5	 2	 0	 Full	Purpose	14 Little	Walnut	Creek	3	 427	 197	 67	 21	 0	 Full	Purpose	15 Onion	Creek	 876	 829	 406	 203	 0	 Full	Purpose	16 Rinard	Creek	 1	 0	 0	 0	 0	 Full	Purpose	17 Shoal	Creek	 672	 281	 134	 74	 6	 Full	Purpose	18 Slaughter	Creek	 52	 16	 0	 0	 0	 Full	Purpose	19 South	Boggy	Creek	 52	 18	 13	 2	 0	 Full	Purpose	20 Tannehill	Branch	 369	 109	 10	 1	 0	 Full	Purpose	21 Waller	Creek	 368	 122	 44	 21	 1	 Full	Purpose	22 Walnut	Creek	 249	 83	 46	 24	 6	 Full	Purpose	23 West	Bouldin	Creek	 128	 52	 27	 21	 6	 Full	Purpose	24 West	Bull	Creek	 1	 0	 0	 0	 0	 Full	Purpose	25 Williamson	Creek	 1039	 480	 204	 71	 3	 Full	Purpose	 Total	 5118 2561 1152 539 48  	1	Barton	Creek	count	includes	only	structures	along	Gaines	Tributary.	 	2	Bull	Creek	includes	structures	in	West	Bull	Creek	watershed	3	Little	Walnut	Creek	count	includes	structures	in	Buttermilk	Branch. 	 4  Figure	1		Structures	in	Floodplain	and	at	Risk	of	Inundation	by	Watershed	based	on	CFHM	Masterplan	010020030040050060070080090010001100Structure	Flooding	by	Watershed	based	on	CFHM	Masterplan	Structures	in	100	yr	FloodplainInundated	in	100yr	FloodInundated	in	25yr	FloodInundated	in	10yr	FloodInundated	in	2yr	Flood 5  Roadways in at Risk of Flooding in the City of Austin A total of 393 roadways in 26 watersheds are at-risk of inundation during a 100yr flood in the City of Austin Full Purpose Jurisdiction.  This number accounts for 392 roadways identified by the CFHM Masterplan process and 1 additional roadway located by FEWS in the Cuernavaca Creek watershed.  The roadways counts from CFHM Master Plan are based on COA 100yr future conditions model.   The following counts were generated from the 25 watersheds in the CFHM 2013 Masterplan: § Number of roadway crossings in the 100-yr floodplain in the full purpose City limits = 580 § Number of at-risk roadways during a 100-yr storm in the Full Purpose City limits = 392 § Number of at-risk roadways during a 25-yr storm in the Full Purpose City limits = 309 § Number of at-risk roadways during a 10-yr storm in the Full Purpose City limits = 236 § Number of at-risk roadways during a 2-yr storm in the Full Purpose City limits = 95 § Number of roadway crossings in the 100-yr floodplain in all jurisdictions = 733 § Number of roadway crossings at risk for flooding during a 100-yr storm in all jurisdictions = 499  At-risk roadways for different storm events are computed based on > 0ft of water depth at the upstream face of the roadways during a storm event.  Table 2 presents counts of roadways in floodplain and at-risk of flooding by watershed based on CFHM Masterplan.  Figure 2 shows the total counts of roadways at-risk of inundation in each watershed.   Appendix A includes Table 4 which presents a list of 26 watersheds in the City of Austin area and number of roadways at-risk of inundation in a 100yr storm in Full Purpose Jurisdiction in each of these watersheds.     6  Table	2	Roadways	in	Floodplain	and	at	Risk	of	Flooding	based	on	CFHM	Masterplan				 Watershed	 Roadways	in	100yr	Floodplain	Inundated	in	100yr	Flood	Inundated	in	25yr	Flood	Inundated	in	10yr	Flood	Inundated	in	2yr	Flood	Jurisdiction	1	 Barton	Creek	1	 7	 7	 5	 5	 5	 Full	Purpose	2	 Blunn	Creek	 9	 7	 6	 1	 0	 Full	Purpose	3	 Boggy	Creek	 25	 13	 11	 8	 1	 Full	Purpose	4	 Bull	Creek	2	 45	 26	 17	 10	 5	 Full	Purpose	5	 Carson	Creek	 16	 15	 12	 12	 5	 Full	Purpose	6	 Country	Club	East	 10	 6	 3	 2	 2	 Full	Purpose	7	 Country	Club	West	 22	 12	 11	 7	 5	 Full	Purpose	8	 Decker	Creek	 7	 2	 2	 1	 1	 Full	Purpose	9	 Dry	Creek	North	 15	 13	 9	 7	 4	 Full	Purpose	10	 East	Bouldin	Creek	 16	 14	 13	 11	 2	 Full	Purpose	11	 Fort	Branch	 23	 21	 21	 19	 6	 Full	Purpose	12	 Gilleland	Creek	 3	 1	 1	 0	 0	 Full	Purpose	13	 Harris	Branch	 8	 2	 2	 2	 2	 Full	Purpose	14	 Johnson	Creek	 21	 12	 8	 7	 3	 Full	Purpose	15	 Little	Walnut	Creek	3	 35	 23	 15	 13	 2	 Full	Purpose	16	 Marble	Creek	 2	 1	 1	 1	 1	 Full	Purpose	17	 Onion	Creek	 7	 3	 3	 3	 1	 Full	Purpose	18	 Shoal	Creek	 54	 46	 41	 35	 11	 Full	Purpose	19	 Slaughter	Creek	 22	 4	 3	 3	 3	 Full	Purpose	20	 South	Boggy	Creek	 13	 11	 8	 1	 1	 Full	Purpose	21	 Tannehill	 25	 19	 13	 11	 2	 Full	Purpose	22	 Waller	Creek	 49	 45	 38	 31	 14	 Full	Purpose	23	 Walnut	Creek	 58	 28	 18	 13	 8	 Full	Purpose	24	 West	Bouldin	Creek	 13	 12	 9	 6	 1	 Full	Purpose	25	 Williamson	Creek	 75	 49	 39	 27	 10	 Full	Purpose		 	580	 392	 309	 236	 95		 1	Barton	Creek	count	includes	only	roadways	along	Gaines	Tributary.	 	2	Bull	Creek	includes	roadways	in	West	Bull	Creek	watershed	3	Little	Walnut	Creek	includes	roadways	in	Buttermilk	Branch.    7  	Figure	2		Roadways	at	Risk	of	Inundation	by	Watershed	in	COA	Full	Purpose	Jurisdiction	05101520253035404550Roadway	Flooding	by	Watershed	based	on	CFHM	Masterplan	Inundated	in	100yr	FloodInundated	in	25yr	FloodInundated	in	10yr	FloodInundated	in	2yr	Flood8  APPENDIX A  Table	3		Number	of	Structures	in	100yr	Floodplain	in	the	City	of	Austin	watersheds			Watershed	Name	 Structures	in	100yr	Floodplain	(CFHM)	Structures	in	100yr	Floodplain	(FPO)	Comments	1	 Barton	Creek	 42	 		 		2	 Bear	Creek	 		 	0	Mostly	suburban	watershed	with	new	development	conforming	to	current	ordinances	3	 Bear	Creek	West	 		 0	No	mapped	floodplain,	current	floodplain	is	backwater	from	Lake	Austin	4	 Bee	Creek	 		 0	Majority	of	watershed	is	outside	jurisdiction,	with	the	exception	of		next	to	Lake	Austin	steep	terrain	prevents	building	in	Floodplain	5	 Blunn	Creek	 12	 		 		6	 Boggy	Creek	 142	 		 CFHM	Master	plan	rescoring	in	2013	7	 Bohls	Hollow	 		 	0	Outside	COA	ETJ,	majority	is	Barton	Creek	Parks	(BCP)	lands	8	 Bull	Creek	 56	 		 CFHM	Master	plan	scoring	in	2013	9	 Buttercup	Creek	 		 		 Outside	COA	ETJ	10	 Carson	Creek	 126	 		CFHM	Master	plan	rescoring	in	2013/2014	11	 Cedar	Hollow	 		 	0	Outside	COA	ETJ,	almost	entirely	BCP	lands	12	 Coldwater	Creek	 		 4	Majority	of	watershed	is	outside	jurisdiction	and	in	BCP	lands,	with	the	exception	of		next	to	Lake	Austin	13	 Colorado	River	 		 27	 		14	 Commons	Ford	Creek	 		 		Majority	of	watershed	is	outside	jurisdiction,	with	the	exception	of		next	to	Lake	Austin	steep	terrain	prevents	building	in	Floodplain	15	 Connors	Creek	 		 0	 Most	of	watershed	is	BCP	Lands	16	 Cottonmouth	Creek	 		 		 Outside	COA	Full	Purpose	17	 Country	Club	East	 12	 		 		18	 Country	Club	West	 45	 		 		19	 Cuernavaca	Creek	 		 2	Majority	of	watershed	is	outside	jurisdiction,	with	the	exception	of		next	to	Lake	Austin	steep	terrain	prevents	building	in	Floodplain	 9  		Watershed	Name	 Structures	in	100yr	Floodplain	(CFHM)	Structures	in	100yr	Floodplain	(FPO)	Comments	20	 Decker	Creek	 		 0	Majority	of	watershed	is	outside	jurisdiction	21	 Dry	Creek	East	 1	 		Majority	of	watershed	is	outside	jurisdiction	22	 Dry	Creek	North	 19	 		 		23	 Eanes	Creek	 		 4	 		24	 East	Bouldin	Creek	 67	 		 		25	 Elm	Creek	 		 	0	Majority	of	watershed	is	outside	jurisdiction	26	 Fort	Branch	 305	 		 		27	 Gilleland	Creek	 		 		Majority	of	watershed	is	outside	jurisdiction	28	 Harpers	Branch	 		 	0	 No	mapped	floodplain	29	 Harris	Branch	 1	 		 		30	 Harrison	Hollow	 		 	0	Majority	of	watershed	is	outside	jurisdiction	and	in	BCP	lands,	with	the	exception	of		next	to	Lake	Austin	31	 Hog	Pen	Creek	 		 	0	No	mapped	floodplain	with	exception	of	backwater	from	Lake	Austin,	half	of	watershed	is	in	BCP	lands	32	 Honey	Creek	 		 	0	Majority	of	watershed	is	outside	jurisdiction	and	in	BCP	lands,	with	the	exception	of		next	to	Lake	Austin	33	 Huck's	Slough	 		 0	 No	mapped	floodplain	34	 Johnson	Creek	 56	 			35	 Lake	Austin	 		 178		36	 Lake	Creek	 		 14	 		37	 Lake	Travis	 		 0	 		38	 Little	Barton	Creek	 		 	0	 Outside	COA	ETJ	39	 Little	Bear	Creek	 		 	0	 		40	 Little	Bee	Creek	 		 	0	 		41	 Little	Walnut	Creek	 427	 		 		42	 Maha	Creek	 		 	0	 Outside	COA	Full	Purpose	43	 Marble	Creek	 		 32	Majority	of	watershed	is	outside	of	jurisdiction	44	 North	Fork	Dry	Creek	 		 	0	 Outside	COA	Full	Purpose	45	 Onion	Creek	 876	 	3	 		46	 Panther	Hollow	 		 	0	Half	of	watershed	is	BCP	lands,	steep	terrain	prevents	building	in	floodplain	 10  		Watershed	Name	 Structures	in	100yr	Floodplain	(CFHM)	Structures	in	100yr	Floodplain	(FPO)	Comments	47	 Rattan	Creek	 		 1	Email	from	Kevin	S.	on	2014/06/30.	Floodplain	from	Upper	Brushy	creek	study.		48	 Rinard	Creek	 1	 		Majority	of	watershed	is	outside	jurisdiction	49	 Running	Deer	Creek	 		 0	Majority	of	watershed	is	outside	jurisdiction,	with	the	exception	of		next	to	Lake	Austin	50	 Shoal	Creek	 672	 		CFHM	Master	plan	rescoring	in	2013/2014	51	 Slaughter	Creek	 52	 		 		52	 South	Boggy	Creek	 52	 		 		53	 South	Brushy	Creek	 		 0	Majority	of	watershed	is	outside	jurisdiction,		with	new	development	conforming	to	current	ordinances	54	 South	Fork	Dry	Creek	 		 	0	 Outside	COA	Full	Purpose	55	 St.	Stephens	Creek	 		 11	 		56	 Steiner	Creek	 		 	0	Majority	of	watershed	is	outside	jurisdiction,	with	the	exception	of		next	to	Lake	Austin	57	 Tannehill	Branch	 369	 		 CFHM	Master	plan	scoring	in	2013	58	 Taylor	Slough	North	 		 0	No	mapped	floodplain,	current	floodplain	is	backwater	from	Lake	Austin	59	 Taylor	Slough	South	 		 1	No	mapped	floodplain,	current	floodplain	is	backwater	from	Lake	Austin	60	 Ladybird	Lake	 		 25	 		61	 Turkey	Creek	 		 25	Half	of	watershed	is	BCP	lands,	steep	terrain	prevents	building	in	floodplain	62	 Waller	Creek	 368	 		 		63	 Walnut	Creek	 249	 		 		64	 West	Bouldin	Creek	 128	 		 		65	 West	Bull	Creek	 1	 		 		66	 Williamson	Creek	 1039	 		 			 	 	 	 			 Subtotal	 5118	 681	 			 	 	 	 			 Total	 		 5799	 		** Structures in Buttermilk Branch are included in the count for Little Walnut Creek 	 11  Table	4		Number	of	Roadways	at	Risk	of	Flooding	During	a	100yr	Flood	in	the	City	of	Austin	Full	Purpose	Jurisdiction			 Watershed	 Roadways	at	Risk	of	Inundation	in	a	100yr	Storm	(CFHM)	Roadways	at	Risk	of	Inundation	in	a	100yr	Storm	(FEWS)	Comments	1	 Barton	creek	 7	   Only	includes	roadways	along	Gaines	Tributary.	2	 Blunn	Creek	 7	     3	 Boggy	Creek	 13	   CFHM	Master	plan	rescoring	in	2013.	4	 Bull	Creek	 26	   CFHM	Master	plan	scoring	in	2013.	Includes	roadways	in	West	Bull.		5	 Carson	Creek	 15	   CFHM	Master	plan	rescoring	in	2013.	6	 Country	Club	East	 6	     7	 Country	Club	West	 12	     8	 Cuernavaca	Creek	 		 1	 LWC	on	River	Hills	Road.	9	 Decker	Creek	 2	     10	 Dry	Creek	North	 13	     11	 East	Bouldin	Creek	 14	     12	 Fort	Branch	 21	   CFHM	Master	plan	rescoring	in	2013.	13	 Gilleland	Creek	 1	     14	 Harris	Branch	 2	     15	 Johnson	Creek	 12	     16	 Little	Walnut	Creek	 23	   Includes	roadways	in	Buttermilk	Branch.	17	 Marble	Creek	 1	     18	 Onion	Creek	 3	     19	 Shoal	Creek	 46	   CFHM	Master	plan	rescoring	in	2013.	20	 Slaughter	Creek	 4	     21	 South	Boggy	Creek	 11	     22	 Tannehill	 19	   CFHM	Master	plan	scoring	in	2013.	23	 Waller	Creek	 45	     24	 Walnut	Creek	 28	     25	 West	Bouldin	Creek	12	     26	 Williamson	Creek	 49	     		 Subtotal	 392	 1	 			 	 	 	 			 Total	 		 393	 		**	Roadway	counts	are	within	the	COA	full	purpose	jurisdictions.		

Numbers of At-Risk Structures and Roadways in the City of Austin Floodplain

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Numbers of At-Risk Structures and Roadways in the City of Austin Floodplain

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